CA1258734A

CA1258734A - Gel and process for preventing loss of circulation and combination process for enhanced recovery

Info

Publication number: CA1258734A
Application number: CA000459115A
Authority: CA
Inventors: Roger C. Zillmer; Burton B. Sandiford
Original assignee: Oxy USA Inc
Current assignee: Oxy USA Inc
Priority date: 1984-06-25
Filing date: 1984-07-18
Publication date: 1989-08-22
Also published as: US4643255A

Abstract

GEL AND PROCESS FOR PREVENTING LOSS OF CIRCULATION, AND COMBINATION PROCESS FOR ENHANCED RECOVERY

ABSTRACT

A rapid setting gel-forming composition is provided comprising a first substance selected from the group consisting of a polyvinyl alcohol, a polyvinyl alcohol copolymer, and mixtures thereof, and aldehyde, and water, which is useful in preventing the loss of circu-lation fluids in well drilling, completion of workover operations. A
combination process is also provided using a slower setting gel-forming composition for retarding the flow of waters or brines in high permeable non-productive channels in combination with a subsequent acidizing step for increasing the permeability of low porous structure in subterranean formations. The combination process is particularly useful in waterflood operations to increase the sweep efficiency of the oil recovery process while improving the flow oil and drive water in the productive parts of the reservoir.

Description

~ 25~734 , GEL AND ~ROCESS FOR PREvENTING LOSS OF CIRCULATION
AND COMBINATION PRocEss FOR ENHANCED RECOVERY

Technical Field This invention relates to gels, and processes for for~ing and using th~ ~ls. A polyvinyl alcohol based-aldehyde hydrogel~ or gel, is 10 provided which is useful for immobilizing large volumes of earth or ~tcr. One of the gels can be used for reducing the loss of drilling, co~pletion or ~or~over fluids from a wellbore. Other gels can be used for reducing the permeability of subterranean formations to the flow of f1uids, wateFs ~r brines. The various gels of this invention are 15 parcicularly valuable in retarding the flow of fluids, waters or brine~
in h~drocar~on production from a wellbore.
Rel~ted Applications The subject ~ttFr of this application is related to that of ~m~ n Patent Application Serial Numbers 459,031> filed July 17, 1~8~ for "Gel for Retarding Water Flo~" and 459,111 filed July 18, 19S4 for "Gel and Process for Retarding Fluid Flow".

B~ckground of_the Invention The recovery of hydrocarbons, both liquid and gaseous, from subter-25 ranean ~ones has frequently resulted in the ~imultaneous production ofl~rge quantities of water or brines. In some cases, even ehough 6ubstantial flows of hydrocarbons have been shown, water production is so ~reat and water disposal costs so high, that hydrocarbon production is not econ~mical. Such water production has in some cases been disposed of 30 in an abandoned or dry well by separating such water from the hydro-c~rbons and reinjectin~ the 5eparated water into such wells. Where a disposal well is not available nor near the producing ~ell, pipelining the water product over a long distance to a di~posal 3ite can become so costly that it renders the well noncommercial. Even if a disposal well 35 is close by, the disposal cost can still be very expensive. Therefore ie is desirable to find a ~ay to reduce or shut off the flow of water while pcrmitting hydrocarbon production to continue.
It i8 ~ell known that the production of large amounts of water from hydroc~rbon producing wells is a major expense item i~ the overall 40 hydrOcnrbon reCovery cost. It is not uncommon for an oil well to produce .~t ~

- 2 - ~2~8734 an effluent which is 60 - 99% by volume water and only l - 40X by volume oil. In such situa~ions, the mAjor part of the pumping energy is ex-pended in lifting water from the well, a cost which the producer would like to avoid if possible. The effluent must then be subjected to a 5 costly separation procedure to recovery water-free hydrocarbons. The foul water ~eparated therefrom alse presentA a troublesome snd expensive di~po~al problem. Consequently, it i8 desirable to decrease the volume o water produced from hydrocsrbon wells. It is, of course, desirable ~o be able to achieve this objective and at the same time not materially 10 affect the hydrocarbon recovery rate. However, where the volume of water is Yery high, e.~., 80 to 99~ vater, and only 1 - 20X oil, even substan-tial reduction in hydrocarbon production can be tolerated if water production can be sub~tantially reduced.
One 8uch ~ethod of reducing the flow of water has been described in 15 U.S. Patent No. 3,?62,476 wherein a first aqueous polymer solution ~elected from the group consisting of polyacrylamide, a p~rtially hydr~lyzed ~olyacr~Iamide, a polysaccharide, a csrboxymethylcellulose, a polyvin~l alcohol, and polystyrene sulfonate, i8 injected into a subterranean for~a~ion. Thereafter, a complexing ionic solution of 20 multivalent cstions and re~arding anions, aad which al90 compri8es aluminum citrate, is injected into the subterranean formation. The ~ult`ivalent catio~ sre selected from the group consi3ting of Fe~II), Fe(III), Al~III) r Ti( I~),Zn(II)~ Sn(I~), Ca(II), MgtII~, Cr(III), and ~he retarding anions are ~elected from~the group con~isting of acetate, 25 nitrilotriacetate, tartrate, citrate, pho~phate. Brine is thea injected followed by a ~econd slug of an aqueou3 polymer solution ~hich can be the same or different from the first aqueous polymer solution. In any event, the c~mplexing ionic 601ution of multivalent cations and retardin8 snions is capabre of gelling both the fir~t and ~econd squeou~ polyMer 801ution~
Uater produced from A wellbore can come from the infiltration of na~urally occuring ~ubterranean water a~ described above, or the water oan come irom injected water put into the formation in tho~e hydrocarbon recovery processes ~hich utilize waterflooding~ U.S. Patent NoO
4,098,337 disclose~ a method for for~ing a hydroxymathylated polyacry-35 lamide gel, in situ, to reduce the per~eability of ~ thusly treated zonewhere the waterflood method of oil recovery is ~mployed. In thifi case the gel was formed in situ by the injection oX an aqueous polyacrylamide solution and an aqueous formaldehyde solutionD
In waterflood operation~ it c~n be de~irable to treat the water 40 injector well~ with 3 polymer gel Pon~ing ~olution to control snd/or ~2~ 73~

redirect the water flow profile. Such t~eatment can prevent channeling of water at the injector ~ell andlor control or redirect the water flow through re~ions of varying permeability.
Although polyacrylamide-based gels can be effective in retarding 5 ~ater produc~ion cr flow in some subterranean formations, polyacrylamide-based gels will not be s~able or effective in all formations. In general, polyacrylamide-based gels uill w~rk satisfactorily in formations having a temperature below about 65~C.
Above about 65C, polyacrylamide-based gels become very sen~itive to lO hardne~s of the brines, especially where hardness is above ~bout 1000 pp~. The hardness of the water becomes a more detrimental factor the higher the temperature9 thus for very hot regions even low hardness levels csn render many gels ineffecti~e. Formations which have a higher temperature, hardness, or toeal dissolved solids content above the afore-15 mentioned ranges usually are not capable of being successfully treated~ith polyacrylamide-based polymers to retard the flow of water.
In ~any hydrocarbon producing wells temperatures of 80~C or higher are ~-t~ encoun~ered. Formation ~aters frequen~ly have hardnesses which exceed`lOOO ppm. It i8 therefore desirable to develop a gel which can be 20 used to retard or blocX the flow of water in 6ubterranean formations having a temperature o 65C or higher~ and a ~ater hardness of 1000 ppm or higher.
In other flooding operations, rather than water, other fluids can be used. 50me fluids which are used flre carbon dioxide and steam~ Because 25 Of the high ~emperature required in steam flooding or other steam stim-ula~ion methods, many of the gels used for blocking water are not ~uitable or satisfactory for blocking s~eam. Other steam treating method~ such as "Push and Pull" operations, ~ometimes referred to as "cyclic ~team injection" or "Huff and Puff" operations, where a 30 production well i8 steamed for ~everal days and then produced or a month OS 60 result in ~team channels being formed which if not blocked will re~ult in an inefficient steaming operation due to 10s8 of steam into channels which drain into nonproductive parts of the reservoir. Again because ~any of the existing gels degrade rapidly at steam temperatures, 35 polymers such as polyacrylamides are generally not satisfactory. Other fluits such as carbon dioxide can also be used in push and pull operations.
Flooding operations u~in~ carbon dioxide and other gases as the drive fluid frequently experience a los3 of drive iluid to nonproductive par~s 40 of the reservois because of Breater ability of gases to dis~ipate into ~587~3~

such channel 8S opposed t~ liquids. Loss of drive gases in flooding operations and ~team in stimulation methods is more difficult to prevent because the flow channels responsible for such los~es can be very small in diameter or width thereby making it very difficult to fill such 5 channels wi~h a blocking agent. Some Vi8CoU5 plugging substances, even thou~h they may have the desired ~tability at higher temperatures, are not able to penetrate and effectively fill narrow channels, particularly a~ such channel6 become more distant from the wellbore.
Thus there is a need for plugging fluids which can be formulated to 10 penetrate deeply into the formation. The use of thi~ invention addresses this problem and provides polyvinyl alcohol based gels which can be tailor made to the particular problem at hand and which can overcome many of the shortcomings of prior art plugging agents and gels.
~olyvinyl alcohol gel~ have been used to protect well ca~ings from 15 cQrrosion. ~.S~ ~atent ~o. 2,832~414 disclo~s such a method wherein an ~queous ~lut~o~ ~f a wa~er soluble polyvinyl alcohol which is capable of for~ing a gel if malntained in ~ quie3cent st&te~ is pumped into the annular space betw~en the casing and the wall of the bore hole. After allo~7ng the polymer to remain quie3cent over a pe~iod of time a gel i9 20 formed. The thusly formed gel prevent~ the intrusion of formation water into the snnular ~pace ~hereby reducing corro~ion o~ the metal casing.
Apparently, no crosslinking agent i6 employed and for that reason is not believed thst this particular gel would be useful for plugging channels or fractu~es on a permanent ~sses. Furthermore~ in Patent No. 2,ô32,414 25 the gel is used to fill a ~elatively large but stagnant cavity compared to ehe volume of a typical channel in a ~ubterranean formation associated with hydro~arbon production from ~ wellbore.
Studies of the macroscopic change~ in poly~inyl acetate gels that occur upQn removal from swelling equilibrium with i~opropyl alcohol were 30 reported in the Journal of Colloid and Interface Science, Vol. 90, ~o. 1, November 1982, pages 34 to 43. The~e ~tudies were conducted using films of gels having various degrees of cro~linking and polymer &oncentration.
The polyvinyl acetate gels were formed from precur~or polyvinyl alcohol gel~ that ~ere crosslinked with glutaric dialdehyde which were then 35 converted to acetate gel~ by polymer homologous acetylation.
~ .S. Patent No. 3,265~657 disclose~ a proces~ for preparin~ an aqueou~ polyvinyl alcohol composition, which remains fluid for at least a few seconds after prepsration and spontaneously gels thereat2rO The gel is formed by contacting a ~elable fluit ~queous polyvinyl alcohol ~olu-40 tion with a hexavslent chromium compount a~d a reductive agent to convert 7~

CR(VI) to Cr(lII). The compositions are used to produce foams suitableas insulatin&, acougtical, and packaging materials. The gel~ are cro~slinked with chromium, not an aldehyde.
UOS. Patent ~o. 3,658,745 disclose~ a hydrogel which is capable of 5 si~nifican~ expan~ion upon cooling in water and reversible ~hrinkin~ upon heating ~hich comprises a crofislinked acetalated hydrogel formed by re~cting ~ polyvinyl slcohol pre~iously dissolved in uater ~nd a monaldehyde and a dialdehyde. The hydrogels are alleged to have suffi-cient crosslinking ~o prevent imbibieion of macromolecular ~aterials auch 0 a8 proteins but no~ the imbibition of micromolecular material~ such as low molecular weight water Rolutes. These hydrogels are alleged to be useful for dislytic purification when pure water is added to the macro-~olecular solution after each cycle. Apparently these particular hydro-gel$ ~re able to absorb and desorb water snt microsolutes with alternate i5 covliug ~nd heating cycle~. Apparently a m~jor amount of shrinka8e of the~e gel5 occurs up~n slight heating ~uch as from 12 to 37C which i~aicates that these gels would bave little value for blocking water in subterranean for~ations, especially at temperaturea of 37C or higher.
Summary of the Invention By ~he term "aldehyde" as used herein is meant a monoaldehyde, a dialdehyde~ a polyaldehyde, and any of the former whether ~ubstituted or un~ub~tituted. Preferably the aldehyde contain~ two func~ional groups such as dialdehyde or a ~ubseituted monoaldehyde as uBed herein is mean~
to include unsaturated carbon-carbon bond 8S well a~ substitution of 2S functional groups. ~onlimiting exEmples vf subQtitu~ed monoaldehyde are acrolein and acrolein dimethylacetal. Polyaldehyde~ can be used and may i~ some case~ be more desirable, however, polyaldehydes are not as aYailable co~mercially as dialtehydes and as a consequence use of polyaldehydes may not be practical.
Non-limiting examples of dialdehyde cro~slinking agents are ~lyoxal, malonaldehyde, ~uccinaldehyde, glutaraldehyde, adipaldehyte9 terephthaldehyde. Non-limiting exampleB of dialdehyde derivati~es are glyoxal bisulfite sddieion compound Na2 ~C(OH)S03C~(OH)S03~
35 glyoxsl trimeric dihydrate, malonaldehyde bisdimethylacetal, 2,5-dimethoxytetrahydrofuran, 394-dihydro-2-methoxy-2H-pyran, and furfursl. Acetals, hemiacetals, cyclic acetal~, bisulfite addition compounds, ~hiff'~ bsses or o~her compounds which genera~e dialdehydes in water, either nlone or in response eo sn addition~l agent 8uch as an acid 40 or ~ condition such ~ heat are also meant to be included in the term - 6 _ ~ ~S8~
"aldehyde" as used and claimed herein.
Non-limiting example~ of monoaldehyde with a second functional group in ~ddition to the aldehyde group are acrolein and acrolein dimethyl-acetal.
Non-limiting examples of polyaldehydes are polyacrolein dimethylacetal, ~ddition products of acrolein f~r example9 ethylene glycol plus scrolein, and glycerol plus acrolein.
By the term "acidic cataly3t" or "crosslinking catalyzing substance"
as used herein is meant a substance ~hich i8 a proton donor or a 10 substance which in its 2nviro~ent w;ll form or become a proton donor.
All acids are operable as an acidic caealyst in the gel systems described herein, for example~ Bron3~ed acids ~uch as mineral and carboxylic acids, or Lewis acids. Non-limiting ex~mples of a Lewis acid are zinc chloride, ferrou~ chloride, stannous chloride, aluminum chloride, barium fluoride, 15 and sulfur trio~ide. Some of these chemicals hydroly~e in water to prodnc~ ~etAl o~ide~ or hydroxides and ~Cl or HF. The rate of hydrolysi~
of many ~e~is ~cids is dependent on temperature ~nd the other disfiolved compoun~s in the solu~ion. The rate of production of the acidic catalyse, HCl, fr~a some of the above Lewis acids determine~ the rate of 20 gel formatIon.
A delayed action c~taly~t is a substance which is not acidic in and of itself, but which generates an acidic catalyst 810wly on interaction with water at the tempera~ure of interest. For example, the rate of generaeion of t~e acid in oil well usage i~ u3ually controlled by the 25 reservoir ~emperature experienced during the in-situ gel formation. In msny applications the rate of ~cidic catalyst generation or release can be controlled by the gel-forming fluid formulation to r~nge from a few ~inu~es to a few day~ or more.
The àcid catalyEt can be a two component system, for example, a two 30 component delayed action catalyst can comprise a first component which will react ~ith a second component, to form an acidic catalyst. A
non-limiting example of such a t~o component delayed action catalyst i~
sodium per6ulfate and a reducing agent. In ~uch ~ delayed cataly~t system ~he sodium persulfate resct~ ~ith the reducing agent to prDduce 35 sulfuric ~cid~ In another two component delayed action catalyst 3ystem the reaction product of ~he two components can react with wster to form the acidic catalyst.
The acidic c~talyst and/or delayed action cataly~t must, of course, have so~e solubility in ~ater. However~ in some oil field u~age~ the 40 partisl 601ubility oi the acidic cstalyst in the oil product can be 37~

advantag~ous if treatment is to include subterranean zones containing both oil and water. The fraction of the acidic catalyst or delayed action catalyst which dis~olutes in oil will, of course, not be available to caealyze the gel format;on reaction in such zones of high oil content;
5 consequently such oil-water zbnes will not be blocked by gel formation to che same extent as those zone~ with little or no oil present.
Non-limiting e~amples of delayed action cataly~ts are methyl formate, e~hyl iormate, meehyl acetate, ethyl acetate, glycerol monoacetate or 2cetin and glycerol discetate or diacetin.
Laboratory tests conducted on core samples have 3hown that diacetin hydrolysis more 810wly than methyl formate at all temperatures including the higher temperature~. Therefore, where subterranenan formations ha~ing higher temperatures are encountered, diactin or acetin because of eh~ir slower raee of hydrolysis are used to provide a longer time for 15 cros~linking reactioas to occur and hence provide a longer time for the geLli~g forming fluids $~ penetrate into the pore~ of such ~ub~erranean ~ones before gela~ion occurs. Non-limiting examples of delayed action catalyst and thæ~r acidic catalyst product are:
Delayed Action Catalyst Acidic Catalyst Product ~ethyl formate Formic acid ~Glycerol diacetate Acetic acid ` Sodium persulfate Sulfuric acid Sodium dodecyl sulfate Sulfuric acid ~ethyl methane sulfonate Methylsulfonic acid Sodium triiodide/sodium Hydroiodic acid bisulfate/~ater $herefore, delayed action acidic catalysts can be ester~ which slowly hydrolyze in water, the rate of hydrolysis being dependent on temperature and initial pH. Other delayed action catalysts are the analogs of esters 30 and acid~ Ruch as sulfones, xanthates, xanthic acids, thiocyanates, and the like. In some of these example6~ hydrolysis produce~ an acidic catalyst ~hich speeds the crosslinking reaction and an alcohol which does not affect gel formation. on example o a delayed action acidic catalyst i8 methyl formate which i8 influenced by the en~ironment with respect to 35 the ra~e of formation of acid. For example, the higher the temper~ture, the faster methyl formate will hydroly~e and generate formic acid.
By the term "Bron~ted acit" a~ used herein i~ meane a chemical which can act a~ a ~ource of protonR. By the term "LewiR acid" as used herein i8 meant a chemical that can accept an electron pair frcm a base. By the 40 term "delayed action acid" a8 used herein i~ meant any acidic cataly6t ~LX587;~

which makes a~ailable or generates donor proton over a period of time or after an initial perind of time either a8 a con~equence of its character-istic or the eharacteristics of the environment in which it i5 used.
By the term "gel" ~s used herein is meant a chemically crosslinked 5 three-dimensional elsstic network of long-chain molecules with a certain ~mount of immobilized solvent tdiluent) molecule~.
By the term "PVA based substance" or "PVA" (frequently referred to hçrein ss the "first substance") as used herein is ~eant long-chain molecules selected from the group con~isting of polyvinyl alcohols, 10 polyvinyl alcohol copolymers, and mixtures thereof.
By the term "PVA-aldehyde gel" as used herein is meant a chemically crossli~ked three-dimensional ela~tic net~ork of longchain molecules ~elected from the group consisting of a polyvinyl alcohol, a polyvinyl nlcohol copoly~er, and m;xtures thereof, cro~slinked with an aldehyde, 15 and containing a certain amourt of immobilized and chemically bound water mol~cule~.
By the term nPVA-glutaraldehyde gels" a~ u~ed herein is meant a che~ically three-di~essional elastic network of various PVA based sub-stan~es crossli~ked ~ith glutaraldehyde and containing a certain amount 20 of immobilized and chemically bound water molecules.
~ 11 of the above mentioned acidic cataly~ts are effective cro~slinkin~ cataIyzing substances for PVA-aldehyde and PVA-glutaralde-hyde gel systems.
Non-limiting examples of polyvinyl alcohol copolymers are polyvinyl 25 alcohol-co-crotonic acid, polyvinyl alcohol-co-acrylic acid, polyvinyl alcohol-co-methacrylic ncid, polyvinyl alcohol-co-vinylpyridine, and polyvinyl alcohol-co-vinylacetate, the latter of ~hich is freque~ntly present in small amounts in commercial grade polyvinyl alcohols.
A problem which ~ometime~ occurs in the oil field i~ the 108~ of 30 circulation of ~pecial fluids such as drilling, completion and workover f1uids into the subterranean for~ation. Loss of circulation fluids into ~ the formstion can cause damage to the drill bit cauaed by overhea~ing and lsrge decrease in drilling rate either of which ean cause large increases in the cost of drilling, collap~e of ~he formation at the wellbore which 35 ea~ damage the wellbore beyond repair, or in-depth plugging of the f ormr ~tion which can damage the reservoir to such an extent that the reservoir ~ay have to be abandonet.
In order to stop or retard the 1098 of circulation fl~id~ into the reservoir it is de~irsble to plug the flow passage~ responsible for such 40 losses very quickly. Cements and ~ilicates are frequen~ly u~ed, however, 1~5~
g because of the flow properties of cement and sili~ates completely - effective plugging is not alway~ achieved. The large particles in cement prevent it from penetrating much beyond 8 çentimeters (B cm) int~ the low flow rate channels. ~hereas in high flow rate channels the cement often 5 does not stop the loss of circulation fluid~ probably becau~e the cement did not ~et, which ~ould be because of dilution due to formation water infusion or merely because the fa~t flow rate prevented ~etting. Cement plug8 near the wellbore are frequently short circuited by the circulation fluid shortly after the resumption of drilling, completion, or workover 10 operation. Thus, ~here i6 a need for a ~ystem that ~ill plug both low flow and hi~h flow rste channels adjacent a wellbore and not allow circulation fluids to pass.
The PVA-aldehyde gel systems of this invention can penetrate the for~ation for dista~ce~ much greater than 8 cm whether the formation be a 15 saDd-like or carbonate-type matrix, and also stick to the matrix after gelation The gel ~ime~ of PVA-aldehyde gel sy3~em~ can be varied from a couple of minutes to day~. However~ in most casea drilling, completion or workover opera~ions is very costly~ Consequently time delays are avoided ~herever possible. Los~ of circulation fluid problemY theref~re 2~ need to be corrected rapidly. Fast ~etting pluggin~ agents for use close to the wellbore are a long ~ought solution to the problem.
In serious lost circulation cases often 20 cubic meters or 120 barrels or more of circulation fluid can be lost in 10 minutes.
In our invention, there is provided rapid setting PV~-aldehyde gels 25 formulated ~o that they are gelled within a period of time no greater than 12 minutes~ und preferably from im~ediately to about 10 minutes after formulation. These gels can be partially formulated at the surface but completely formulated in the wellbore preferably at or ~ear the point of lost circulation. Our gel-forming compositions are ~elled in the 30 presence of an acidic catalyst, whieh in combination with the amoun~ of aldehyte, causes a rapid ~Ptting of the gel to occur. In one embodiment the acidic catalyst i8 added to the gel-forming mixture either in the wellbore or preferably at or near the point of lost circulation. In snother embodiment the ~ldehyde i~ the la~t ~omponent of the gel-forming 35 composition to be added to the mixture and it i~ added st the wellbore near the point of lo~t circulation. The use of such rapid 3etting PVA-aldehyde gel ~ystem~ offers additional advantages of ea~e of wellbore clean-up as oppo~ed to cements, and ~rester ele~ated te~perature sta-bility over other gel ~ystems such as polyacrylamide ba~ed ~el~ which in 40 general are not stable at temperatures of 65C or higher. Our PVA-~25~3 ~3~

aldehyde gel-forming compositions also have the sdvant~ge that they can be formul~ted using formation brine rather than merely fresh ~ater. This compatibility i6 an important advantage in locations where fresh water is not readily available.
Accordingly, there is provided a proceæs for reducing the loss of sirculation fluids into flow passages of a ~ubterranean formation during a well drilling9 completion or workoYer operations, the circulntion fluids being ~elected from the group consisting of drilling fluids, completion fluids and workover fluid6, the process comprising ~topping 10 the injection of the circulation fluid into the wellbore; introducing tnto the flow pa~sages, an effective amoun~ of a gel-forming compo~ition comprising (i~ ~n aqueous solution compri~ing a first substance selected from the group consisting of polyvinyl alcohol, a polyvinyl slcohol c~poly~er, and mixtures thereof~ (ii) an amount of an aldehyde, and (iii) 15 an a~ount of a cro~slinking catalyzing substance, which in combination with the amount of the aldehyde i8 operable for effecting gelation, 8t the temperature ~f s~e subterranean formation, of the gel-forming composition i~ a period of time no greater ~han about 12 minutes after being introauced i~to the subterranean formation; and allowing the gel-20 forming composition to flo~ into the flow passages and to form 8 gel therein within such period of time thereby reducing the 1085 of circulation 1uid upon re~uming well drilling, completion or workoYer operation. ~In fi preferred embodiment the amounts o~ A crosslinking catalyzing subst~nce and aldehyde are operable for effecting gelation, at 25 the temperature of the subterranean formation, of the gel-forming composition in a period of time from about zero to sbout 10 minutes after being introduced into the ~ubterranean formation.
In one embodiment of our rapid ~etting gel-forming composi~ion, the amount o~ the aldehyde i8 from about 0.03 to 4 percent of the weight of 30 the gel-fonming composition, and the pH of ehe gel-for~ing composition i8 no greater than ~bo~e 5 .5 . Preferably the pH of ~he gel-forming co~position is no greater thsn about 5. In another embodiment of the amount of the aldehyde i8 no more than about 2 percent of Ehe stoichiometric amount required to react with all of the crosslinkable 35 si~es of the first substance. In another embodiment the aldehyde is glu~araldehyde. I~ yet another embodiment ehe gel-forming compo~ition is at least about 64 weight percent wa~er. In ~till another embodiment the gel-forming composi~ion i8 at least about 91% by weight brine. In another embodiment the amount of the PV~ ~ased sub~tance or first 40 substance is from about 105 to about 5% of ehe ~eight of the gel-forming ~ 37~
composition. In still another embodiment the fi~st substance has an average molec~lar weight of at least about 30,000, preferably at least about 100,000. Pr~fer~bly the first subatance i9 polyvinyl alcohol~ In a preferred embodiment the gel forming composition is About 3% by weight 5 polyvinyl alcohol and about 2X glutaraldehyde. ~his composition is useful in treating a ~ell having a ~ubterranean témperature at least as high as 80C.
In a further embodiment, prior to introducing the rapid ~etting gel-forming composition into the wellbore, a 6hort-term plugging agent is 10 introduced into the wellbore and into the flow pa~sages to temporsrily plug the flow passages until a gel i8 formed in the flow pas~ages from ~he gel-forming co~position. Examples of ~hort-term plugging agents are diatomaceous earth, ground up nut ~hells, wax beads, and mixtures the~eof. In another further eQbodiment, cement is introduced into the 15 wellbore ~d from the wellbore into the ~ubterranean formation after introducing the-rapit ~etting gel-forming composition into the wellbore.
In ano~her further embodiment, a silicate i8 introduced into ~he ~ellbore snd ~r~m the wellbore into the subterranean formation after introducing the rapid setting gel-forming composition into the wellbore. Preferably 20 a ~ilicate followed by a cement i~ introduced into the wellbore after introducing the gel-forming composition into the wellbore.
The above proce~ses are especially useful for reducing the loss of circulation fluids in well~ having a severe loss circulation problem.
Where the loss circulation occurs in fractures having extremely high 25 permeability, it is desirable to precede the introduction of the gel-forming composition into the wellbore with a temporary plugging agent such as diatomaceous eartht ground up nut ahell~, wax beads or other substances to initially reduce the permeability in 6uch severe fractures 80 that the gel-forming composition will have an opportunity to set up

3~ and form a gel in s~ch fractures. In general, the gel-forming compositions used for reducing the loss of drilling fluid will be relatively quick ~etting. Thi6 i6 achieved~ for example, by having a relatively high glutaraldehyde concentration in the gel-forming composition with a high pH. The PVA-aldehyde gel~ as described are 35 stable st high temperature~ for long periods of time and offer a definite atvantage over ~any other polymes based gela which are not effective in formationa having a high temperature. By having the gel set up in a period of time no greates than about 12 minutes after it come~ in con~act with the formation, these proce3~es offer a definite adv~ntage over 40 cements which will not penetr~te 88 deeply into the formation b~t require l~S87;~

a longer time to s~t up. In some wells having 6evere fractures the use of cement by itself i3 ineffective because the cement is lost before it has a chance ~o set up. In such situations the loss of ~irculation fluid i8 only partially corrected. However, in our invention described above, 5 the gel-forming co~position setc up rapidly and the los& of circulation fluid is ~reatly reduced.
There i~ also provided a gel, which is esp~cially useful for reducing the loss of eirculation fluids bu~ is also useful for other purposes which require a rapid setting gel, formed by reacting, in the presence of 10 an effective amount of an acidic catalyst~ the components of a gel-fonming composition comprising i. a first substance selected from the group consisting of polyvinyl alcohol, a polyvinyl alcohol copolymer, and mixtures thereof, ii. an effective amount of an aldehyde, and iii. ~ater~ wherein the water provides at least about 64 per~ent of t~e weight of the ~el, and wherein the amo~D~ of the acidic cataly~t in combination with the amount of ~he aldehy~e is operable for effectin~ gelation, at a predetermined temper~ture, of the- gel-forming composition in 8 period of time ~o 20 greater than about 12 minutes after the gel-forming compo~ition i~
formed. Preferably the amount of the acidic catalyst in combination ~ith the amount of aldehyde is operable for effecting gelation, at a predeter~ined tPmperature, of the gel-forming composition in a period of time from about zero to about 10 minutes after the ~el-forming 25 composition is formed.
In one embodiment the amount of aldehyde i8 from about 0.03 to about 4Z of the weight of the gel-forming compo~;tion. In another e~bodiment the amount of ~he altehyde iB at least about 2Z of the ~toichiometric ~mount required to react with all the crosslinkable 8ite8 of the PVA-30 based subs~ance or first substance. In a preferred embodiment thealde~ydP i~ glutsraldehyde. In yet another embodiment the amount of the scidic catalyst is fiufficiant to maintain ~he p~ of the gel-forming composition at a v~lue no greater than about 4. In yet another embodiment the amount of the PVA-ba~ed Pubsesnce or fir~t substance is 35 from about l.S to about 5X of the gel-forming composition. In another embodiment the first cub~tance has an average molecular weight of at least about 30,000 preferably at least about 100,000. In yet another embodiment the fir6t substance i~ polyvinyl alcohol. I~ yet another embodiment the water of the gel-~orming composition is provited by a 40 brine, and tha brine ia at least about 91 p2rcent of the weight of the ~5~3~7;~

gel-forming composi~ion.
Another problem which frequently occurs in enhanced oil recovery operations ~uch a8 flooding or Btimulating i8 the lo~ of injected fluids into nonproductive and usually high petmeability pa~t~ of the reservoir 5 ~hile the productive parts of the reservoir remain largely inacces~ible because of their usually low permesbility. Thi~ invention ~ls~ provides a combination process in which the high permeability channels which are usually nonprotuctive are treated to retard the flow of fluid~, especially water or brines, ~hile the low permeability ~ones which are 10 usuaIly oil-bearing ~re acidized to increa3e the permeability thereof to the flow of oil or drive or stimulaeing fluid~. Our particular combina~ion of steps provides a relatively easy and effective way to trea~ reservoirs experiencing both inefficient loss of fluids to nonproductive area~ while at the same time improving the recovery from 15 the productive oil-bearing a~eas of the reservoir.
~ ccordin~ly ~us p~ocess provides a method to penetrate the Qon~roductive high permeability channels for relatively large distance~
from the wellbore and effec~ively block the flow of water or brines therein so that better u~e of the drive or 6timulating fluids can be 20 achieved. ~hi8 is then closely coordinated with an acidizing process to improve flows in desired areas such that as a result of our combination process the efficiency ~nd profitability of the enhanced oil recovery operation is improved. Accordingly, there i9 al80 provided a process for retarding the flow of water in high permeability channels in a 25 subterranean formation and increasing the permeability of low permeability oil-bearing porou~ structure in the subterranean formation comprising introducing into the fiubterranean fonmation a predetermined amount of a gel-forming composition which when gelled in the high permeability channel~ i~ operable for retarding the flow of ~ater 30 therein, the gel-forming composition comprising (i~ an ~queou~ solution comprising a first ~ubstance selected from the group consi~ting o~
polyvinyl alcohol, a polyvinyl alcohol copolymer, and mixtures thereof, and (ii~ an effective amount of an aldehyde sufficient to form a gel with said aqueou~ ~olution, when in the presence of an effec~ive am~unt of a 35 cro~slinking catalyzing substance; and allowing the gel-formin~
co~position to form a gel, in the presence of an effectiv~ am~unt of the cro~slinking catalyzing substance, in the high permeabilit~ channels which iB effective for retarding the flow of ~ater therein; ~fter LntroducinR the gel-forming compos;tion into the hi~h permeability 40 chsnnels, introducing into the subterranean formation a predetermined 37~4 amount of an acidizing ~ubstance which is operable for penetrating the low permeability porous ~tructure and dissolving flow inhibiting deposits therein; and sllowing the acidizing ~ubstance to dissolve the flow inhibiting deposit~ thereby increasing the permeability of the low 5 permeability porous ~tructureO In one embodiment of our combination process the acidizing substance i8 introduced into the subterr~nean formation within a period of time from about zero to about one day after the gel is formed in the high permeability channels, In another embodiment the acidizing ~ubYtance is introduced into the subterranean 10 formation after the gel i8 formed in the high permeability channels. In a further embodi~ent the acidizing 6ubstance is introduced into the 6ubterra~ean formation within a period of time from about zero to about 10 hours after the gel i8 formed in the high permeability channels. In ~nother embodi~ent af~er introducing into the subterranean formation the 15 predetermined amount of the gel-forming compo~ition, an effective amount of a cro~s1inkin~ catalyzing ~ubstance is introduced into the subter-ranesn formation which i8 operable to cause the gel-forming composition in the ~igh per~eabi1ity ch~nnel~ to gel therein. In yet another em-bodime~t ~he cros-slinking catalyzing ~ubgtance and the acidizing 20 6ubst~ce have tke ~ame composition.
In a further embodiment of our combination process the amount of aldehyde is fro~ about 0.01 to about 4% of the weight of the gel-forming composition. In another embodiment the ~mount of aldehyde is at least O.7X of-the stoichio~etric amount required to react ~ith all of the 25 crossLinkable sites of the first substanoe. In a preferred embodiment the aldehyde is glutaraldehyde. In yet another embodiment the PYA-based substance or irst substance is from about 1.5 to about 5X of the weight of the gel-forming mixture. In another embodiment the first substance has sn average molecular weight of at least 30,000, and preferably at 30 least 100,000. In yet another embodiment, the first substance is polyvinyl alcohol. ln another embodiment the gel-forming composition i8 at least about 64% bg weigbt water. In another embodiment the water of the gel-forming composition i6 provited by a brine, and the brine is at lea~t about 91 percent of the weight of the gel-forming composition. In 35 a preferred embodiment the gel-forming compo~ition i8 about 2.5~ by weight polyvinyl alcohol and about O~lZ by weight glutaraldehyde. This gel-forming composition i~ particulsrly useful in 3ubterranean for~ation~
havin~ d formation temperature at les3t a8 high a~ ~bout 65C.
In another e~bod;ment of our combined process~ the aldehyde is 40 glutaraldehyde and the amount of gluearsld2hyde i~ operable for promoting - 15 - lX5~3~
crosslinking of the fir~t subgtance and glutaraldeh-Jde under weakly acidic conditions and a separately provided acidic catalyst i5 not requi~ed. In a further embodiment, other than glutaraldehyde and acidic products produced in the gel-forming composition fr~m the glutaraldehyde, the gel-forming çomposition i~ ~ubstantially free of effective amounts of crosslinking catalyzing ~ubstances which are operable for promoting sub-stantial acidic catalysis of a croszlinking reaction bet~/een the first substance and glutaraldehyde; and wherein the gel is formed in the sub-~erranean formation without the neces3ity of contacting ~he gel-forming composition with any additional effective amounts of ~ crosslinking catalyzing substance. This particular gel-forming composition and method of fonming is more fully described in our copending serial number y59~
In a further embodiment of our co~bined proce~s further comprises, 15 after formin~ ehe gel in the high permeability channel~ and dissolving the ~lo~ i~hibiting depo~it~ in the low permeability porous structure, recoverng oil from the ~ubterranean formation. In a further embodiment oi~ is recovered by water flooding.
In ~ preferred embodiment of our combinstion process the crosslinking 20 catalyzing ~ubstance is a delayed action caealyst such as an ester which 810wly hydrolyzes as it moves sway from the wellbore into the formation.
The e~ter is selected 80 that it will slowly form a weak or~anic acid as it peoetrates into the formation. In this embodiment the weak organi acid al~o react~ wi-th the flow inhibiting deposits in the low 25 permeability channels. The combination of selectin~ the crosslinking catalyzing substance which allows the gel-forming composition to slowly gel thereby enabling in-depth plugging of the high permeability channels, and which also allows in-depth acidi~ing of the low permeability channels~ is a partîcularly ~seful combination. This embodiment has the 30 advantage over msny prior art processes in that only two compositions are involved; næmely, the gel-forming composition and the crosslinking catalyzing 3ubstance which ~180 serve~ as the acidizing substance. These t~o ~ubstances csn be initially premixed~ or simultaneously injected, or injected in alternate slugs, into the formation. If separately but 35 simultaneously injec~ed, ~fter the high permeability zones are plugged, the injection of ~he gel-for~ing co~position i~ terminated while i~jection of the acitizing sub~tance i8 continued. The adv~ntage of not having several different formulation~ to pU~p into ehe well or to premix st v~rious times greatly fscilitates the u~e of thi~ me~hod for enhsncea 40 oil recovery. Having only two co~po~itions for injection al~o reduces .

the chances for operating error. The method is particularly valuable in remote locations where providing several compositions and ~everal ~torage tanks would be difficult. For example, jungle locations are very dif~i-cult to operate in because the general lack of utilities tenders complex 5 proce~ses susceptible to prolonged down times for relatively minor break downsO Thus there i8 a need for 8 relatively simple method of enhancing oil recovery. In all of these embodiments there i8 the additional ad~antages of being able to use the formation brine as a ~ource of water for the injected mixtures, and the fitability of the gels at elevated 10 temperatures.
In still further embodi~ent~ of the above described gels, the water used to form the gel has a hardneYs of at least about 1000 ppm. In further embodimenes ehe water has a hardness of at least about 3000 ppm, or 6000 pp~ or higher. In other further embodimen~s of the above 15 described gel~ the water used to form the gel has a total dissolved sol;~s content of at least about 30,000 ppm. In a still further embod~ment ~uch ~ater ha~ a total dissolved ~olids content of at least abou~ 80,000 ppm.
In-the embodLme~t~ of this invention the various crosslinkable 20 aldehydes and glutaraldehyde crosslink with the polyvinyl alcohol or poly~inyl alcohol copolymer through formation of acetsl3. It has been found that gels formed in this way are adaptable eo the hardness of the water from which they are formed or exposed. These gels are also more ~table at high-temperaturea than polyacrylamide based gels or gels made 25 from biopolymers or polyvinyl alcohols gelled by other crosslinking agents such as borate.
8ecause of the adaptability and compatibility of these gela to water hardness or total dissolved solids content, these gels can be prepared using formation water, brsokish water, ~ea ~ater or usually any other 30 available source o~ water conveniently at hand. Because the largest ingsedient used to formulate the aboYe described gels i9 principally water2 substantial economic advantsge is provided by this invention which permits gels to be fonmed with the cheapest aource of available water.
However, the ad~sntages o~ this invention sre not li~ited ~erely to 35 economic advantage~ because these gels also provide substantial technical advantages oYer other gels. For example, in many of their uses ehese gel~ are ~ubjected to the infusion of se~erely contaminated water into the gelling ma8B prior to reaching its gelation point~ Where ~uch conta~inated water in~u~ion occur~ in ~any other gelling fluids the 40 gelation thereof iB destroyed or ~o se~erely harmed that such other gel~, ~xs~

if in fact they do gel, ~ould be rendered ineffective for their intended use.
Due to their stability at elevated temperatures, the above described gels can also be formed and used in formations having an average in-situ 5 temperature of about 80C or higher, and in some embodiments where the average in-situ temperature i8 125C or higher.
~ he above described methods of forming a gel in ~itu in subterranean formation~ be be practices u~ing all of the ~els provided by this invention.
The principles of this invention can be uQed where the ~ubterranean water-conveying zone i8 under the sub~erranean hydrocarbon-producing ~one; or where the subterranean water-conveying zone surrounds the subterrsnean hydrocarbon-producing zone; or where at least part oE ~he ~ubterranean ~ater-conveying zone coincides with at least part of the - 15 subeerrsnean hydrocarbon-producing ~one.
In one emboai~e~e of this invention directed to a water flood operations, it freyuently i8 desirable to treat the water injector wells with ~ polymer ge~-~orming solution to control the water flow profile.
I~ thi-~ embodimeD~ ~uch treatment prevents channeling of ~ater at the 20 injector well and~nr controls ~nd/or redirects water flow through region~
of varyin~ permeability. Since in this embodiment the polymer i8 injected as a relatively low vi~cosity aqueous phase it penetrate~
preferentially the region of highest permeability to water. Ac~ordingly, after-formation of the gel in high permeability regions, such regions are 25 converted to lo~ permeability to further retard water flow thereby causing, upon further water injection, a water sweep of previously inaccessible areas in the formation which usu~lly have relatively lo~
permeability. By extending the water flow to ~uch previously inaccessible regions, more hydrocarbons can be recovered than would b~
30 recovered in the absence of such polymer treatment.
The gels of this invention have improved resistance to heat and are ~table in hard ~ater. These char3cteri~tics make the~e gels particularly useful for many oil field applications nuch ~s ~ater mobility control.
These gels can be advantageou~ly u3ed in other har~h environment~ ouch a~
35 solar p~nd constr~ction ~here they can be used to consolidate loose soil ant tc retard or stop the leakage of brine through the pond floor, or to prevent convective flow from lower intervals eontaining hotter ~ater into upper interv~l~ containing oooler water. For oil field application, no other gels ~re known which exhibit the ~tabili~y and durability of the 40 gels of this invention especially in high temperature reservoir~.

87;~

Accordingly, one objec~ive of thi~ invention i5 to provide a means of controlling water movement in oil well8 and ~ubterranea~ formations especially in fonmation~ having temperatures 80C or higher, or where the waterR involvet are saline or hard.
Still ~nother object of this invention is to provide a gel which can be fonmulated using hard water and water containing ~ high level of dissol~ed solids ~uch a~ sea water and formation wat~r encountered in deep Dff-shore hydrocarbon fields.
Another obiect of this invention is to provide & gel ~hich i9 quick 10 set~ing ~o that it can be used to stop the lo~s of circulating fluids su~h as drilling~ completion and workover fluids.
Another object of ~his invention iR to provide a gel which i~ stable at high temperatures and in particular more stable than other gels at ~uch hi8h temperatures.
15 Description of the Preferred Embodiments I~ o~e embodLmen~ ~he gel-forming mixture i8 first formed at or near ~e point o lost circulation in the wellbore by injecti~g one of the compouen-ts of the gel-for~ing mixture separately into the formation adjacent or near the poin~ of 108t circulation. For example, the acidic 20 catalyst by itself or mixed ~ith the aldehyde, or the aldehyde by itself can be injected into the formation through a separate tubing run down the wellbore adjacent to, or near, the point of lost circulation. The aqueou~ ~olution ~f PVA based substance can be introduced directly into the ~ellbore or in another separate tubing the outlet of which is adja-25 cent to the point of loss circulation. The two separate streams meet andmi~ for the first time adjacent to, or near, the point of lost circu-lation. In the fast setting gel-forming composition of this invention isolation of any one of the three components, i.e., aldehyde, acidic catalyst, or PVA based substance, from the other two components ~ill 30 prevent the initiation of the crosslinking and hence gelation reaction.
Example No. 1 This example demonstrates how to deter~ine the relation~hip between the aldehyde concentration and the pH of the gel-forming composition and the gel time thereof in a particul~r reservoir rock associated with a 35 well e~periencing a loss of circulation fluid~ Reservoir injection water ~IW) or reservoir brine i~ preferably used to produce the gel-forming composition. Polyvinyl alcohol having an average molecul~r wei~ht o~
about 125,000 is added to RIW to p~oduce a 3X concentration and the mixture heated to 95aC for 45 minute~ to completely di~olYe ~he polymer.
40 The pH af the mixture is adjusted to 5.0 by the addition of 12X ~Cl ~ 19 - ~ ~5~7;~
solution.
A high pressure core holder i3 packed with crushed reservoir rock to form a 60 centimeters (60 cm) long~ 5 cm diameter test core sample. The test core ~ample is saturated with Rl~ ~nd heaeed to 90C in preparation 5 for a flow test. RIW, at 90~, is injected into the core 6ample at a rate of 30 cm per day and the pressure drop across the core sample mea-aured. Mineral oil, at 90C, having a viscosity of 10 centipoise ~10 cp) ~t 25C, is then pumped throu~h the core sample at 30 cm per day until no ~ore RIW is displaced therefrom. Additional RIW, at 90C, is then pumped 10 through the core sample at 30 cm per day until no more mineral oil is displaced therefrom and the pressure drop measured.
~ he gel-forming compositinn is prepared by mixing 24 parts of the thusly prepared 3Z polymer solution with one part by weight of a 50%
aqueo~s glutaraldehyde solution (commercial grade) in a mixing ~ee 15 located ae the inlet of the high psessure core holders. The gel-forming composi~iDn is ther~fter injected into the core 6ample immediately after its f~r~ation. Tbe injection rate is 30 cm per day until the gel point is reached which is iudicated by a rapid increa~e in pressure~ The actual ~el ~ime i~ co~pared to the nominal gel time in neutral rock. The 20 variation of gel-tsme from nominal i8 an indication o the influence of the reservoir rock on crosslinkin~ reaction rates.
Example No. 2 Preferably after determining the effective sldehyde concentration and pH of the gel-forming composition in core samples of crushed reservoir 25 ro~c~, for e~ample a~ described in ~xample No. 1, treatment of a well experie~cing a loss of circulation fluid can be conducted with a gel-forming composition ~hich i~ effective for stopping the 1089 of circu-lation fluid. For example, in 8 well having a temperature of about 90C
and experiencing a loss of circulation fluid to a 30 cm interval at the 30 2700 meter (2700 m) depth, the end of the drill pipe i8 set adjacene the 30 cm interval at the 2700 m depth. Sepsrste tubing is alRo see opposite ~he 30 cm interval.
A 3% polyvinyl alcohol RIW mixture havin~ its pH adjusted to 5.0 is injected into the wellbore at ~ rate of about 20 cubic meters per hour 35 (20 CMPH) until 160 cubic ~eter~ i~ injected~ Simultaneou~ly with the injection of the poly~er mixture~ a 50X ~queous glutaraldehyde ~olution (co~mercial grade) i8 injected into ~he separate tubing st a rate which corre~pond6 to the effective glutarsldehyde concentrate determined in a core ~a~ple of crushed seservoir rock, or alternatively ~t ~ predeter-40 Lined rate ~hich corresponds to a predetermined ratio of glutaraldehyde ~ 25~

to polyvinyl alcohol. For example the glutaraldehyde can be injected at a rate of about 0.83 CMPH.
If after injertion of 80 cubic meters of the gel-forming ~omposition the pressure has not increased substantially, then to the remaining 80 S cubic meters of polymes mi~ture, is added about 0.04 cubic meters of a bridging agene~ Nonlimiting examples of bridging agent~ are diatomaceous earth~ wax beads, crushed ~alnut shells, and other plugging agents.
At any point in the plugging operation, when the injection pressure increa~es rapidly indicating that plugging of the 30 cm interval has been 10 completed~ the injection of gel-for~ing aqueous ~olutions into the formation is ~topped and the gel-forming mixture is displaced into the formation with no more than about 0~5 cubic meters of brine of over dis-plscement.
The gel~forming composition i8 formulated so that i~ will ~el within 15 5 mi~ute~ afeer enterin8 the lost circulation ineerval. The first part of th~ formed gel is pushed away from the wellbore in-depth into the intervsl. This process is repeated with subsequently form~d gel segments uneil ~ufficient gel is formed in the entire interval and the injection pressure~ for the aqueous ~ixtures increase rapidly. Even though the 20 mi~t~re i9 designed to gel rapidly, several hours of injection will proba~ly be required in order to inject all of the mixtures necessary to completely plug the interval. In most cases9 about 80 to 160 cub;c meters of gel-forming composition is required. After plugging the lost circu~ation interval, the drilling operation can be resumed.
25 Example No. 3 This example demonstrates how to determine the relationship between the gel-forming composition and the pH and gel-time thereof in a parti-cul~r reser~oir rock associated with a well experiencing water channeling in 8 water flood operation. Reservoir injection water (RIW) or reservoir 30 brine i8 preferably used to produce the gel-forming composition. Poly-vinyl slcohol having an average molecular weight of about 125,000 is added to RIW to produce a 2.5% concentration and the mixture heated to 95C for 45 minutes to completely diasolve the polymer.
A reservoir test core sample 60 centimeters (60 cm) lon~, and 0.8 cm 35 in dismeter i8 wr~pped with Teflon t~pe and satur~ted with RIW. A 0.4 cm hole i8 trilled in the core along it~ axis thereby producing an ~nnu-lsr core sfimple of reservoir rock. A porous distribution disk is sealed to one end of the snnular core aample with Teflon tspe. The central cylindrical cavity of the annular core 6a~ple i8 then p~cked with crushed 40 reservoir rock snd packed snnular core sample with attached di~tribution ~5~7;~

disk is inserted into a tightly fitted heat ghrinkable Teflon~ tube and the tube sealed. The packed central column represents a ~one of high permeabili~y and the annular core a zone of low permeability. Thé core unit is then mounted in an o~erbusden cell, saturated by RIW and heated 5 to 90C in preparation for a flow ~e~t. RIW, st 90C, is injected ineo the core sample at a rate of 30 cm per day and the pre~ure drop ~cross the core sample mea~ured. ~ineral oil, at 90~C, having a viscosity of 10 centipoise (10 cp) at 25C, i~ then pumped through the core ~ample at 30 cm per day until no more RIW i~ displaced therefrom. Additional RIW, at 0 90C~ i8 then pumped through the core sample at 30 cm per day until no more mineral oil is displaced therefrom and the pre~sure drop measured.
Ihe gel-formin~ compositio~ is prepared by mixing 99 parts of the thusly prepared ~.5X polymer solution with one part by weight of a 5~%
aqueous glutaraldehyde ~olution (com~ercial grade) and the pH adjusted to 15 4Ø ~y ~he addition of 12% of ~Cl ~olution~ Pre~e~ably the amount of glutaraldehyde and the pH of the composition i8 designed to gel in about 3 ho~rs.
Ihe thusly formed gel-forming comRosition, at 90JC~ is then injected into the packed core ~ample at a rate of 150 cm per day until the gel 20 poin~ i~ resched which i8 indicated by a rapid increase in pre~ure drop across the core sample. At this point the packed column of cru~hed reservoir rock has been plugged while the annular core sample has not.
RIW, at 90DC, is injected into the core sample at a rate of 30 cm per day ana the pres~ure drop meaQured. The ratio of the pressure drop~ across ~5 the core sample beore treatment with the gel-forming composition and after tre~tment and gelation is an indication of the effectiveness of the plugging procedure. Accordingly, the higher ~uch ratios are more effectiYe in the plugging operation.
~he low perme~bility of the annu]ar core sample is now increased by 30 injecting a 3% ~Cl aqueous solution into the core s3mple at a steady flow rate until a 6ignificant tecrease in pressure drop occur~ , at 90C, i8 then injected at a rate of 30 cm per day and the pre~3ure drop again meaRured. The effectivenes3 of the acidizing step is indicated by the reduction in pressure drop acros~ the sample.
35 Example No- 4 Preferably after deten~ining the effecti~e gel-forming compo~ition ~nd pH thereof in core samples a6 de~cribed in Example No. 3, treatment of a reservoir experiencing ~ater channeling in waterflooding can be conducted with a gel-for~ing composition which is effective for reducing 40 water flow in high permeability channelsO The reservoir has a ~L~S~73~

temperature of 90C, an average permePbility of 200 millidarcies ~200 md) and a porosity of 20%, and before treatment is experiencing a RIW
injection rate of 32 cubic meeers per day (32 CMPD) at an injection surface pres~ure of 70 kilograms per gquare centimeter gauge (70 kscmg) 5 or 1000 psig. Before treatment, approximately 50% of the RIW is entering the bottom 25 cm of a 47~ cm interval, 75% of the bottom 50 cm of the interval, and about lOOZ of the bottom 100 cm o the interval.
~ 2.5~ polyvinyl alcohol aqueous solution, formulated a~ in Example No. 3, is heated to 95C in an in-line heater snd ~tored in an insulated 10 tank for at lea~t 45 minutes to completely dissolve the polymer. The gel-forming compo~ition i~ prepared as in Example No. 3 by mixing 99 parts of the polymer solution with one part by weight of a 50X aqueous glutaraldehyde solution and the pH adjusted'to 4Ø About 100 cubic ~eters of the gel-formiDg mixture i8 injected into the injection well at 15 a seeady rate ~er a period of three days. The gel-forming composition is then displaced int~ the reservoir preferably wieh no more than about o~ ~ubic'meter o BIW over di~placement. The well iR then shut in for ~bout t~o tays ~hich is then followed by acidizing. Acidizing is accomplished by injecting 7.6 cubic meters or 2000 gallons of 3X HC1 20 golution. The acidi~Ing solution i~ followed by resumption of the water-flood operation.
It is expected that the combined process of retarding water flow in ~he high permeabili~y channels with the gel-forming composition and incre~sing the per~eability of the low permeable channels with the 25 subsequent acidizing step will reduce the injection surface pressure to about 35 kscmg9 incre se the RIW'injection rate to about 160 oubic meters per day~ and provide an improved injection profile in the 470 cm interval such that about lOZ of the RI~ enter the bottom 25 cm of the interval~
20~ the b`ottom 50 cm, 25% the bottom 100 cm, 50X the bottom 200 cm, and 30 100% ~he total interval.

Unless otherwise ~pecified herein, all percents are weight percent~.
The gels, the methods of forming the gels, and the proesse~ for preventing 10~8 circulation and retarding the flow of fluid~ have ~ome 35 degree of flexibility. For example, if the environment in which the gela are to be used has a relatively high te~perature, gel ~ime can be ~lowed by using 8 smaller a~ount of acidic catalyst and aldehyde. Similarly, if the environmental temperature i8 relatively low, gelation can be ~peeded by the u~e of larger a~ounts of acidic catalyst and aldehyde. It i6 40 permi89ible to use the formation brine of the 8ubterranesn zone as the ~5~'7;~
- 23 ~
~ater part of the gel-forming composition since the gel will form even ~ith hard water. Other variations of formulations, methods and processes will be apparent from thi~ invention Co those skilled in the art.
The foregoing disclosure and de~cription of ~he present invention is 5 illustratiYe snd explanatory thereof znd various changes in gel formation procedures and gel composition as well a5 the uses and application~ of ~uch ~els to form them in situ in subterranean formations and to retard or block fluids in subterranean formations may be made within the scope of the appendin~ claims without departing from the ~pirit of the 10 invention. For example, ~any gel formulation~ can be produced and many ~ethods of forming such gels in situ in subterranean deposits ~ill be apparent to one ~killed in the art from this invention. For example, the necessary concentrations, amounts and sequence of injection of the gel forming fluids can be tailored to 8Uit the particular well or 15 ~ubterranean fo~ation bei~g treated.

Claims

Claims What is claimed is:

1. A process for reducing the loss of circulation fluids into flow passages of a subterranean formation during well drilling, completion, or workover operations, said circulation fluids being selected from the group consisting of drilling fluids, completion fluids and workover fluids, said process comprising:
(a) topping the injection of a circulation fluid selected from the group consisting of drilling fluids, completion fluids and workover fluids into a wellbore;
(b) introducing into said flow passages, an effective amount of an acidic gel-forming composition comprising i. an aqueous solution comprising a first substance selected from the group consisting of polyvinyl achohol, a polyvinyl alcohol copolymer, and mixtures thereof, ii. an amount of an aldehyde, and iii. an amount of a crosslinking catalyzing substance, which in combination with said aqueous solution and said aldehyde is operable for effecting gelation, at the temperature of said subterranean formation, of said gel-forming composition in a period of time no greater than about 12 minutes after being introduced into said subterranean formation; and (c) allowing said gel-forming composition to enter into said flow passages and to form a gel therein within said period of time mentioned in step (b), thereby reducing the loss of said circulation fluid upon resuming well drilling, completion or workover operation.

2. The process of claim 1, wherein said aldehyde is glutaraldehyde.

3. The process of claim 1, wherein said amount of aldehyde is from about 0.03 to about 4 percent of the weight of said gel-forming composition, and wherein the pH of said gel-forming composition is no greater than about 5.5.

4. The process of claim 3, wherein said aldehyde is glutaraldehyde.

5. The process of claim 3, wherein the pH of said gel-forming composition is no greater than about 4.

6. The process of claim 1, further comprising prior to introducing said gel-forming composition into said wellbore, the step of introducing a short term plugging agent into said wellbore and into said flow passages to temporarily plug said flow passages until a gel is formed in said flow passages from said gel-forming composition.

7. The process of claim 6, wherein said short term plugging agent is selected from the-group consisting of diatomaceous earth, ground-up nut shells, wax beads, and mixtures thereof.

8. The process of claim l, wherein said gel-forming composition is at least about 64 weight percent water.

9. The process of claim 8, wherein said aldehyde is glutaraldehyde.

10. The process of claim 1, wherein said gel-forming composition is at least about 91 weight percent brine.

11. The process of claim 10, wherein said aldehyde is glutaraldehyde.

12. The process of claim 1, wherein said amount of said aldehyde is at least about 2% of the stoichiometric amount required to react with all of the crosslinkable sites of said first substance.

13. The process of claim 12, wherein said aldehyde is glutaraldehyde.

14. The process of claim 1, wherein said first substance has an average molecular weight of at least 30,000.

15. The process of claim 1, wherein said first substance has an average molecular weight of at least 100,000.

16. The process of claim 1, wherein said amounts of said crosslinking catalyzing substance and said aldehyde are operable for effecting gelation, at the temperature of said subterranean formation, of said gel-forming composition in a period of time from about zero to about 10 minutes after being introduced into said subterranean formation.

17. The process of claim 16, wherein said aldehyde is glutaraldehyde.

18. The process of claim 3, wherein said amounts of said crosslinking catalyzing substance and said aldehyde are operable for effecting gelation, at the temperature of said subterranean formation, of said gel-forming composition in a period of time from about zero to about 10 minutes after being introduced into said subterranean formation.

19. The process of claim 18, wherein said aldehyde is glutaraldehyde.

20. The process of claim 1, further comprising the step of introducing cement into said wellbore and from said wellbore into said subterranean formation after introducing said gel-forming composition into said wellbore.

21. The process of claim 1, further comprising the step of introducing a silicate gel-forming composition into said wellbore and from said wellbore into said subterranean formation after introducing said gel-forming composition into said wellbore.

22. A process for reducing the loss of circulation fluids into flow passages of a subterranean formation during well drilling, completion or workover operations, said circulation fluids being selected from the group consisting of drilling fluids, completion fluids and workover fluids, said process comprising:
(a) stopping the injection of a circulation fluid selected from the group consisting of drilling fluids, completion fluids and workover fluids into to a wellbore;
(b) introducing into said wellbore, and from said wellbore into said flow passages, an effective amount of an acidic gel-forming composition comprising i. an aqueous solution of polyvinyl alcohol having an average molecular weight of at least 30,000, ii. an amount of glutaraldehyde from about 0.03 to about 4 percent of the weight of said gel-forming composition, and iii. an amount of a crosslinking catalyzing substance, which in combination with said aqueous solution and glutaraidehyde, is oper-able for effecting gelation, at the temperature of said subterranean formation, of said gel-forming composition in a period of time no greater than about 12 minutes after being introduced into said subterranean formation, wherein the pH of said gel-forming composition is no greater than about 5.5 9 an wherein said gel-forming composition is at least about 64 weight percent water;
(c) allowing said gel-forming composition to enter into said flow passages and to form a gel wherein within said period of time mentioned in step (b), thereby reducing the loss of said circulation fluid upon resuming well drilling, completion or workover operation.

23. The process of claim 22, wherein said polyvinyl alcohol has an average molecular weight of at least 100,000.

24. The process of claim 22, wherein the pH of said gel-forming composition is no greater than about 4.

25. The process of claim 22, wherein said amounts of said crosslinking catalyzing substance and said glutaraldehyde are operable for effecting gelation, at the temperature of said subterranean form-ation, of said gel-forming composition in a period of time from about zero to about 10 minutes after being introduced into said subterranean formation.

26. The process of claim 22, further comprising prior to introducing said gel-forming composition into said wellbore, the step of introducing a short term plugging agent into said wellbore and into said flow passages to temporarily plug said flow passages until said gel-forming composition forms a gel in said flow passages.

27. The process of claim 22, further comprising the step of introducing cement into said wellbore and from said wellbore into said subterranean formation after introducing said gel-forming composition into said wellbore.

28. The process of claim 22, further comprising the step of introducing a silicate gel-forming composition into said wellbore and from said wellbore into said subterranean formation after introducing said gel-forming composition into said wellbore.

29. The process of claim 28, further comprising the step of introducing cement into said wellbore and from said wellbore into said subterranean formation after introducing said silicate gel-forming composition into said wellbore.

30. A gel formed by reacting, in the presence of an effective amount of an acidic catalyst, the components of a gel-forming composition comprising i. a first substance selected from the group consisting of polyvinyl alcohol, a polyvinyl alcohol copolymer, and mixtures thereof, ii. an effective amount of an aldehyde, and iii. water, wherein said water provides at least about 64% of the weight of said gel, and wherein said amount of said acidic catalyst in combination with said amount of said aldehyde is operable for effecting gelation, at a predetermined temperature, of said gel-forming composition in a period of time no greater than about 12 minutes after said gel-forming composition is formed.

31. The gel of claim 30, wherein said aldehyde is glutaraldehyde.

32. The gel of claim 30, wherein said amount of said acidic catalyst in combination with said amount of aldehyde is operable for effecting gelation, at a predetermined temperature, of said gel-forming com-position in a period of time from about zero to about 10 minutes after said gel-forming composition is formed.

33. The gel of claim 32, wherein said aldehyde is glutaraldehyde.

34. The gel of claim 30, wherein said amount of said first substance is from about 1.5 to about 5% of the weight of said gel-forming composition.

35. The gel of claim 34, wherein said aldehyde is glutaraldehyde.

36. The gel of claim 30, wherein said amount of aldehyde is from about 0.03 to about 4 percent of the weight of said gel-forming composition.

37. The gel of claim 369 wherein said aldehyde is glutaraldehyde.

33. The gel of claim 30, wherein said amount of said aldehyde is at least about 2% of the stoichiometric amount required to react with all of the crosslinkable sites of said first substance.

39. The gel of claim 38, wherein said aldehyde is glutaraldehyde.

40. The gel of claim 30, wherein said first substance has an average molecular weight of at least 30,000.

41. The gel of claim 40, wherein said asldehyde is glutaraldehyde.

42. The gel of claim 30, wherein said first substance has an average molecular weight of at least 100,000.

43. The gel of claim 42, wherein said aldehyde is glutaraldehyde.

44. The gel of claim 30, wherein said first substance is polyvinyl alcohol.

45. The gel of claim 44, wherein said aldehyde is glutaraldehyde.

46. A gel formed by reacting, in the presence of an effective amount of an acidic catalyst, the components of a gel-forming composition comprising 1. a polyvinyl alcohol having an average molecular weight of at least 30,000, ii. an effective amount of glutaraldehyde, and iii. water, wherein said water provides at least about 64% of the weight of said gel, and wherein said amount of said acidic catalyst in combination with said amount of glutaraldehyde is operable for effecting gelation, at a predetermined temperature, of said gel-forming composition in a period of time no greater than about 12 minutes after said gel-forming composition is formed.

47. The gel of claim 46, wherein said amount of said acidic catalyst in combination with said amount of said glutaraldehyde is operable for effecting gelation, at a predetermined temperature, of said gel-forming composition in a period of time from about zero to about 10 minutes after said gel-forming composition is formed.

48. The gel of claim 46, wherein said amount of glutaraldehyde is from about 0.03 to about 4 percent of the weight of said gel-forming composition.

49. The gel of claim 46, wherein said polyvinyl alcohol has an average molecular weight of 100,000.

50. The process of claim 1, wherein said first substance is from about 1.5 to about 5% of the weight of said gel-forming composition.

51. The process of claim 22, wherein said polyvinyl alcohol is from about 1.5 to about 5% of the weight of said gel-forming composition.

52. The process of claim 46, wherein said polyvinyl alcohol is from about 1.5 to about 5% of the weight of said gel-forming composition.

53. The process of claim 22, wherein said polyvinyl alcohol is about 3% of the weight of said gel-forming composition, wherein said amount of said glutaraldehyde is about 2% of the weight of said gel-forming composition, and wherein the temperature of said subterranean formation is at least as high as 80°C.

54. The process of claim 46, wherein said polyvinyl alcohol is about 3% of the weight of said gel-forming composition, wherein said amount of said glutaraldehyde is about 2% of the weight of said gel-forming composition, and wherein the temperature of said subterranean formation is at least as high as 80°C.

55. A process for reducing the loss of circulation fluids into flow passages of a subterranean formation during well drilling, completion, or workover operations, said circulation fluids being selected from the group consisting of drilling fluids, completion fluids and workover fluids, said process comprising:
(a) stopping the injection of a circulation fluid selected from the group consisting of drilling fluids, completion fluids and workover fluids into a wellbore;
(b) introducing into said flow passages, an effective amount of an acidic gel-forming composition comprising:
i. an aqueous solution comprising a first substance selected from the group consisting of polyvinyl alcohols, polyvinyl alcohol copolymers, and mixtures thereof, ii. an amount of an aldehyde, and iii. an amount of a crosslinking catalyzing substance, which in combination with said aqueous solution and said aldehyde is operable for effecting gelation, at the temperature of said subterranean formation, of said gel-forming composition in a period of time no greater than about 12 minutes after being introduced into said subterranean formation; and (c) allowing said gel-forming composition to enter into said flow passages and to form a gel therein within said period of time mentioned in step (b), thereby reducing the loss of said circulation fluid upon resuming well drilling, completion or workover operation.

56. A process for reducing the loss of circulation fluids into flow passages of a subterranean formation during well drilling, completion or workover operations, said circulation fluids being selected from the group consisting of drilling fluids, completion fluids and workover fluids, said process comprising:
(a) stopping the injection of a circulation fluid selected from the group consisting of drilling fluids, completion fluids and workover fluids into a wellbore;
(b) introducing into said wellbore, and from said wellbore into said flow passages, an effective amount of an acidic gel-forming composition comprising:
i. an aqueous solution of a first substance selected from the group consisting of polyvinyl alcohols, polyvinyl alcohol copolymers and mixtures thereof, wherein said first substance is from about 0.1 to about 5% of the weight of said gel-forming composition, ii. an amount of a second substance selected from the group consisting of aldehydes, aldehyde generating substances, acetals, acetal generating substances, and mixtures thereof capable of crosslinking with said first substance through the formation of acetal crosslinkages, wherein total aldehyde content of said gel-forming composition is from about 0.03 to about 10% of the weight of said gel-forming composition, and iii. an amount of a crosslinking catalyzing substance, which in combination with said aqueous solution and second substance, is operable for effecting gelation, at the temperature of said subterranean formation, of said gel-forming composition in a period of time no greater than about 12 minutes after being introduced into said subterranean formation, wherein the pH of said gel-forming composition is no greater than about 5.5; and (c) allowing said gel-forming composition to enter into said flow passages and to form a gel therein within said period of time mentioned in step (b), thereby reducing the loss of said circulation fluid upon resuming well drilling, completion or workover operation.

57. A process for reducing the loss of circulation fluids into flow passages of a subterranean formation during well drilling, completion or workover operations, said circulation fluids being selected from the group consisting of drilling fluids, completion fluids and workover fluids, said process comprising:
(a) stopping the injection of a circulation fluid selected from the group consisting of drilling fluids, completion fluids and workover fluids into a wellbore;
(b) introducing into said wellbore, and from said wellbore into said flow passages, an effective amount of an acidic gel-forming composition comprising:
i. an aqueous solution of a first substance selected from the group consisting of polyvinyl alcohols, polyvinyl alcohol copolymers, and mixtures thereof, wherein said first substance is from about 0.1 to about 5% of the weight of said gel-forming composition, ii. an amount of a second substance selected from the group consisting of aldehydes, aldehyde generating substances, acetals, acetal generating substances, and mixtures thereof capable of crosslinking with said first substance through the formation of acetal crosslinkages wherein total aldehyde content of said gel-forming composition is from about 0.03 to about 10% of said gel-forming composition, and iii. an amount of a crosslinking catalyzing substance, which in combination with said aqueous solution and second substance, is operable for effecting gelation, at the temperature of said subterranean formation, of said gel-forming composition in a period of time no greater than about 12 minutes after being introduced into said subterranean formation, wherein the pH of said gel-forming composition is no greater than about 5.5, and wherein said gel-forming composition is at least about 60 weight percent H2O; and (c) allowing said gel-forming composition to enter into said flow passages and to form a gel therein within said period of time mentioned in step (b), thereby reducing the loss of said circulation fluid upon resuming well drilling, completion or workover operation.

58. A process for reducing the loss of circulation fluids into flow passages of a subterranean formation during well drilling, completion or workover operations, said circulation fluids being selected from the group consisting of drilling fluids, completion fluids and workover fluids, said process comprising:
(a) stopping the injection of a circulation fluid selected from the group consisting of drilling fluids, completion fluids and workover fluids into a wellbore;
(b) introducing into said wellbore, and from said wellbore into said flow passages, an effective amount of an acidic gel-forming composition comprising:
i. an aqueous solution of a first substance selected from the group consisting of polyvinyl alcohols, polyvinyl alcohol copolymers, and mixtures thereof, wherein said first substance is from about 0.1 to about 5% of the weight of said gel-forming composition, ii. an amount of a second substance selected from the group consisting of aldehydes, aldehyde generating substances, acetals, acetal generating substances, and mixtures thereof capable of crosslinking with said first substance through the formation of acetal crosslinkages, wherein total aldehyde content of said gel-forming composition is from about 0.03 to about 10% of the weight of said gel-forming composition, and iii. an amount of a crosslinking catalyzing substance, which in combination with said aqueous solution and second substance, is operable for effecting gelation, at the temperature of said subterranean formation, of said gel-forming composition in a period of time no greater than about 12 minutes after being introduced into said subterranean formation, wherein the pH of said gel-forming composition is no greater than about 5.5, and wherein said gel-forming composition is at least about 85 percent brine; and (c) allowing said gel-forming composition to enter into said flow passages and to form a gel therein within said period of time mentioned in step (b), thereby reducing the loss of said circulation fluid upon resuming well drilling, completion or workover operation.

59. A process for reducing the loss of circulation fluids into flow passages of a subterranean formation during well drilling, completion or workover operations, said circulation fluids being selected from the group consisting of drilling fluids, completion fluids and workover fluids, said process comprising:
(a) stopping the injection of a circulation fluid selected from the group consisting of drilling fluids, completion fluids and workover fluids into a wellbore;
(b) introducing into said wellbore, and from said wellbore into flow passages, an effective amount of an acidic gel-forming composition comprising:
i. an aqueous solution of a first substance selected from the group consisting of polyvinyl alcohols, polyvinyl alcohol copolymers, and mixtures thereof, said first substance having an average molecular weight of at least 30,000, wherein said first substance is from about 1.5 to about 5% of the weight of said gel-forming composition.

ii. an amount of a second substance selected from the group consisting of aldehydes, aldehyde generating substances, acetals, acetal generating substances, and mixtures thereof capable of crosslinking with said first substance through the formation of acetal crosslinkages, wherein total aldehyde content of said gel-forming composition is from about 0.03 to about 4 percent of the weight of said gel-forming composition, and iii. an amount of a crosslinking catalyzing substance, which in combination with said aqueous solution and second substance, is operable for effecting gelation, at the temperature of said subterranean formation, of said gel-forming composition in a period of time no greater than about 12 minutes after being introduced into said subterranean formation, wherein the pH of said gel-forming composition is no greater than about 5.5; and (c) allowing said gel-forming composition to enter into said flow passages and to form a gel therein within said period of time mentioned in step (b), thereby reducing the loss of said circulation fluid upon resuming well drilling, completion or workover operation.

60. A process for reducing the loss of circulation fluids into flow passages of a subterranean formation during well drilling, completion or workover operations, said circulation fluids being selected from the group consisting of drilling fluids, completion fluids and workover fluids, said process comprising:

(a) stopping the injection of a circulation fluid selected from the group consisting of drilling fluids, completion fluids and workover fluids into a wellbore;
(b) introducing into said wellbore, and from said wellbore into said flow passages, an effective amount of an acidic gel-forming composition comprising:
i. an aqueous solution of a first substance selected from the group consisting of polyvinyl alcohols, polyvinyl alcohol copolymers, and mixtures thereof, said first substance having an average molecular weight of at least 30,000, wherein said first substance is from about 1.5 to about 5% of the weight of said gel-forming composition, ii. an amount of a second substance selected from the group consisting of aldehydes, aldehyde generating substances, acetals, acetal generating substances, and mixtures thereof capable of crosslinking with said first substance through the formation of acetal crosslinkages, wherein total aldehyde content of said gel-forming composition is from about 0.03 to about 4 percent of the weight of said gel-forming composition, and iii. an amount of a crosslinking catalyzing substance, which in combination with said aqueous solution and second substance, is operable for effecting gelation, at the temperature of said subterranean formation, of said gel-forming composition in a period of time no greater than about 12 minutes after being introduced into said subterranean formation, wherein the pH of said gel-forming composition is no greater than about 5.5, and wherein said gel-forming composition is at least about 64 weight percent H2O; and (c) allowing said gel-forming composition to enter into said flow passages and to form a gel therein within said period of time mentioned in step (b), thereby reducing the loss of said circulation fluid upon resuming well drilling, completion or workover operation.

61. A process for reducing the loss of circulation fluids into flow passages of a subterranean formation during well drilling, completion or workover operations, said circulation fluids being selected from the group consisting of drilling fluids, completion fluids and workover fluids, said process comprising:
(a) stopping the injection of a circulation fluid selected from the group consisting of drilling fluids, completion fluids and workover fluids into a wellbore;
(b) introducing into said wellbore, and from said wellbore into said flow passages, an effective amount of an acidic gel-forming composition comprising:
i. an aqueous solution of a first substance selected from the group consisting of polyvinyl alcohols, polyvinyl alcohol copolymers and mixtures thereof, said first substance having an average molecular weight of at least 30,000, wherein said first substance is from about 1.5 to about 5% of the weight of said gel-forming composition.

ii. an amount of a second substance selected from the group consisting of aldehydes, aldehyde generating substances, acetals, acetal generating substances, and mixtures thereof capable of crosslinking with said first substance through the formation of acetal crosslinkages, wherein total aldehyde content of said gel-forming composition is from about 0.03 to about 4 percent of the weight of said gel-forming composition, and iii. an amount of a crosslinking catalyzing substance, which in combination with said aqueous solution and second substance, is operable for effecting gelation, at the temperature of said subterranean formation, of said gel-forming composition in a period of time no greater than about 12 minutes after being introduced into said subterranean formation, wherein the pH of said gel-forming composition is no greater than about 5.5, and wherein said gel-forming composition is at least about 91 weight percent brine; and (c) allowing said gel-forming composition to enter into said flow passages and to form a gel therein within said period of time mentioned in step (b), thereby reducing the loss of said circulation fluid upon resuming well drilling, completion or workover operation.

62. A process for reducing the loss of circulation fluids into flow passages of a subterranean formation during well drilling, completion or workover operations, said circulation fluids being selected from the group consisting of drill fluids, completion fluids and workover fluids, said process comprising:

(a) stopping the injection of a circulation fluid selected from the group consisting of drilling fluids, completion fluids and workover fluids into a wellbore;
(b) introducing into said wellbore, and from said wellbore into said flow passages, an effective amount of an acidic gel-forming composition comprising:
i. an aqueous solution of polyvinyl alcohol, wherein said polyvinyl alcohol is from about 0.1 to about 5% of the weight of said gel-forming composition, ii. an amount of glutaraldehyde from about 0.03 to about 10 percent of the weight of said gel-forming composition, and iii. an amount of a crosslinking catalyzing substance, which in combination with said aqueous solution and glutaraldyhyde, is operable for effecting gelation, at the temperature of said subterranean formation, of said gel-forming composition in a period of time from about one second to no greater than about 12 minutes after being introduced into said subterranean formation, wherein the pH of said gel-forming composition is no greater than about 5.5; and (c) allowing said gel-forming composition to enter into said flow passages and to form a gel therein within said period of time mentioned in step (b), thereby reducing the loss of said circulation fluid upon resuming well drilling, completion or workover operation.

63. A process for reducing the loss of circulation fluids into flow passages of a subterranean formation during well drilling, completion or workover operations, said circulation fluids being selected from the group consisting of drilling fluids, completion fluids and workover fluids, said process comprising:
(a) stopping the injection of a circulation fluid selected from the group consisting of drilling fluids, completion fluids and workover fluids into a wellbore;
(b) introducing into said wellbore, and from said wellbore into said flow passages, an effective amount of an acidic gel-forming composition comprising:
i. an aqueous solution of polyvinyl alcohol, wherein said polyvinyl alcohol is from about 0.1 to about 5% of the weight of said gel-forming composition;
ii. an amount of glutaraldehyde from about 0.03 to about 10 percent of the weight of said gel-forming composition, and iii. an amount of a crosslinking catalyzing substance, which in combination with said aqueous solution and glutaraldehyde, is operable for effecting gelation, at the temperature of said subterranean formation, of said gel-forming composition in a period of time from about one second to no greater than about 12 minutes after being introduced into said subterranean formation, wherein the pH of said gel-forming composition is no greater than about 5.5, and wherein said gel-forming composition is at least about 60 weight percent H2O; and (c) allowing said gel-forming composition to enter into said flow passages and to form a gel therein within said period of time mentioned in step (b), thereby reducing the loss of said circulation fluid upon resuming well drilling, completion or workover operation.

64. A process for reducing the loss of circulation fluids into flow passages of a subterranean formation during well drilling, completion or workover operations, said circulation fluids being selected from the group consisting of drilling fluids, completion fluids and workover fluids, said process comprising:
(a) stopping the injection of a circulation fluid selected from the group consisting of drilling fluids, completion fluids and workover fluids into a wellbore;
(b) introducing into said wellbore, and from said wellbore into said flow passages, an effective amount of an acidic gel-forming composition comprising:
i. an aqueous solution of polyvinyl alcohol wherein said polyvinyl alcohol is from about 0.1 to about 5% of the weight of said gel-forming composition, ii. an amount of glutaraldehyde from about 0.03 to about 10 percent of the weight of said gel-forming composition, and iii. an amount of a crosslinking catalyzing substance, which in combination with said aqueous solution and glutaraldehyde, is operable for effecting gelation, at the temperature of said subterranean formation, of said gel-forming composition in a period of time from about one second to no greater than about 12 minutes after being introduced into said subterranean formation, wherein the pH of said gel-forming composition is no greater than about 5.5, and wherein said gel-forming composition is at least about 85 weight percent brine; and (c) allowing said gel-forming composition to enter into said flow passages and to form a gel therein within said period of time mentioned in step (b), thereby reducing the loss of said circulation fluid upon resuming well drilling, completion, or workover operation.

65. A process for reducing the loss of circulation fluids into flow passages of a subterranean formation during well drilling, completion or workover operations, said circulation fluids being selected from the group consisting of drilling fluids, completion fluids and workover fluids, said process comprising:
(a) stopping the injection of a circulation fluid selected from the group consisting of drilling fluids, completion fluids and workover fluids into a wellbore, (b) introducing into said wellbore, and from said wellbore into said flow passages, an effective amount of an acidic gel-forming composition comprising:
i. an aqueous solution of polyvinyl alcohol having an average molecular weight of at least 30,000, wherein said polyvinyl alcohol is from about 1.5 to about 5% of the weight of said gel-forming composition, ii. an amount of glutaraldehyde from about 0.03 to about 4 percent of the weight of said gel-forming composition, and iii. an amount of a crosslinking catalyzing substance, which in combination with said aqueous solution and glutaraldehyde, is operable for effecting gelation, at the temperature of said subterranean formation, of said gel-forming composition in a period of time from about one second to no greater than about 12 minutes after being introduced into said subterranean formation, wherein the pH of said gel-forming composition is no greater than about 5.5; and (c) allowing said gel-forming composition to enter into said flow passages and to form a gel therein within said period of time mentioned in step (b), thereby reducing the loss of said circulation fluid upon resuming well drilling, completion or workover operation.

66. A process for reducing the loss of circulation fluids into flow passages of a subterranean formation during well drilling, completion or workover operations, said circulation fluids being selected from the group consisting of drilling fluids, completion fluids and workover fluids, said process comprising:
(a) stopping the injection of a circulation fluid selected from the group consisting of drilling fluids, completion fluids and workover fluids into a wellbore;
(b) introducing into said wellbore, and from said wellbore into said flow passages, an effective amount of an acidic gel-forming composition comprising:

i. an aqueous solution of polyvinyl alcohol having an average molecular weight of at least 30,000, wherein said polyvinyl alcohol is from about 1.5 to about 5% of the weight of said gel-forming composition.
ii. an amount of glutaraldehyde from about 0.03 to about 4 percent of the weight of said gel-forming composition, and iii. an amount of a crosslinking catalyzing substance, which in combination with said aqueous solution and glutaraldehyde, is operable for effecting gelation, at the temperature of said subterranean formation, of said gel-forming composition in a period of time from about one second to no greater than about 12 minutes after being introduced into said subterranean formation, wherein the pH of said gel-forming composition is no greater than about 5.5, and wherein said gel-forming composition is at least about 64 weight percent H2O; and (c) allowing said gel-forming composition to enter into said flow passages and to form a gel therein within said period of time mentioned in step (b), thereby reducing the loss of said circulation fluid upon resuming well drilling, completion or workover operation.

67. A process for reducing the loss of circulation fluids into flow passages of a subterranean formation during well drilling, completion or workover operations, said circulation fluids being selected from the group consisting of drilling fluids, completion fluids and workover fluids, said process comprising:

(a) stopping the injection of a circulation fluid selected from the group consisting of drilling fluids, completion fluids and workover fluids into a wellbore;
(b) introducing into said wellbore, and from said wellbore into said flow passages, an effective amount of an acidic gel-forming composition comprising:
i. an aqueous solution of polyvinyl alcohol having an average molecular weight of at least 30,000, wherein said polyvinyl alcohol is from about 1.5 to about 5% of the weight of said gel-forming composition, ii. an amount of glutaraldehyde from about 0.03 to about 4 percent of the weight of said gel-forming composition, and iii. an amount of a crosslinking catalyzing substance, which in combination with said aqueous solution and glutaraldehyde, is operable for effecting gelation, at the temperature of said subterranean formation, of said gel-forming composition in a period of time from about one second to no greater than about 12 minutes after being introduced into said subterranean formation, wherein the pH of said gel-forming composition is no greater than about 5.5, and wherein said gel-formation composition is at least about 91 weight percent brine; and (c) allowing said gel-forming composition to enter into said flow passages and to form a gel therein within said period of time mentioned in step (b), thereby reducing the loss of said circulation fluid upon resuming well drilling, completion or workover operation.

68. The process of any one of claims 55 to 57, wherein the pH said gel-forming composition lo no greater than about 4.

69. The process of any one of claim 55, further comprising prior to introducing said gel-forming composition into said wellbore.
the step of introducing a short term plugging agent into said wellbore and into said flow passages to temporarily plug said flow passages until a gel is formed in said flow passages from said gel-forming composition.

70. The process of claim 69, wherein said short term plugging agent is selected from the group consisting of diatomaceous earth, ground-up nut shells, wax beads, and

71. The process of any one of claims 55 to 57, wherein said first substance has an average molecular weight of at least 100,000.

72. The process of any one of claims 55 to 57, further comprising the step of introducing cement into said wellbore and from said well-bore into said subterranean formation after introducing said gel-forming composition into said wellbore.

73. The process of claim 55, further comprising the step of introducing a silicate gel-forming composition into said wellbore and from said wellbore into said subterranean formation after introducing said gel-forming composition into said wellbore.

74. The process of claim 73, further comprising the step of introducing cement into said wellbore and from said wellbore into said subter-ranean formation after introducing said silicate gel-forming compo-sition into said wellbore.

75. The process of any one of claims 62 to 64, wherein said first sub-stance is polyvinyl alcohol.

76. The process of any one of claims 56 to 58, wherein said amount of said second substance 18 at least about 27 of the stoichiometric amount required to react with all of the crosslinkable sites of said first substance.

77. The process of any one of claims 56 to 58, wherein said amounts of said crosslinking catalyzing substance and said second substance are operable for effecting gelation, at the temperature of said subterranean formation, of said gel-forming composition in a period of time from about zero to about 10 minutes after being introduced into said subterranean formation.

78. The process of any one of claims 62 to 64, wherein said amounts of said crosslinking catalyzing substance and said glutaraldehyde are operable for effecting gelation, at the temperature of said subter-ranean formation, of said gel-forming composition in a period of time from about zero to about 10 minutes after being introduced into said subterranean formation.

79 A gel formed by reacting the components of an acidic gel-forming composition comprising i. an aqueous solution of a first substance selected from the group consisting of polyvinyl alcohol, a polyvinyl alcohol copolymer, and mixtures thereof, wherein said first substance is from about 0.1 to about 5% of the weight of said gel-forming composition, ii. an amount of a second substance selected from the group consisting of aldehydeu, aldehyde generating substances, acetals, acetal generating substances, and mixtures thereof capable of crosslinking with said first substance through the formation of acetal crosslinkages, wherein total aldehyde content of said gel-forming composition is from about 0.03 to about 10% of said gel-forming composition, and iii. an amount of a crosslinking catalyzing substance, which in combination with said aqueous solution and second substance, is operable for effecting gelation of said gel-forming composition in a period of time no greater than about 12 minutes after being formulated, wherein the pH of said gel-forming composition is no greater than about 5.5, and wherein said gel-forming composition 18 at least about 60 weight percent H2O.