CA1267747A

CA1267747A - Gel and process for preventing carbon dioxide break through

Info

Publication number: CA1267747A
Application number: CA000459113A
Authority: CA
Inventors: Burton Burns Sandiford; Roger Carroll Zillmer
Original assignee: Oxy USA Inc
Current assignee: Oxy USA Inc
Priority date: 1984-06-25
Filing date: 1984-07-18
Publication date: 1990-04-10
Anticipated expiration: 2007-04-10
Also published as: US4673038A

Abstract

GEL AND PROCESS FOR PREVENTING CARBON DIOXIDE BREAK THROUGH
ABSTRACT
A 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, an aldehyde, and water, and which requires contacting with a brine which has absorbed substantial amounts of carbon eioxide before the gel-forming composition will form a gel. The gel-forming composition is useful for retarding the flow of carbon dioxide and other fluids in subterranean formations. For example, a method is provided for preventing the loss of carbon dioxide to nonproductive parts of an oil reservoir. Such method is particularly useful in carbon dioxide flood operations to increase the sweep efficiency of the oil recovery process and in cyclic carbon dioxide injection operations for increasing the fluidity of the reservoir oil.

Description

GEL AND P~OCESS FOR PREVENTING CARBON DIOXIDE BREAK THROUG~

Technical Field This invention relates to gels, methods of forming gels, and uses for gels. A polyvinyl alcohol based-aldehyde hydrogel, or gel, is provided - which is useful for immobilizing large volumes of earth. The gel can be 10 used for reducing the permeability of soils and subterranean formations to the flow of carbon dioxide and other fluids, including waters or brLnes. The gels of this invention are particularly valuable in retarding the flow and 105s of carbon dioxide in hydrocarbon production from a ~ellbore.
15 Related APplications The subject mat~er of this application is related to that of Canadian Patent Application Serial Number 459JO31~ filed July 7,1984 for "Gel for Retarding Water Flow".

20 Background of the Invention l~e-recovery of hydrocarbons~ especially oil~ frequently involves the injection of fluids into the reservoir to either force or drive the hydrocarbons from one location to another, as in flooding operations, or even more basically stated, to improve the flow of the hydrocarbons to 25 the production well as in various stimulation operations. Carbon dioxide and other fluids, including water and steam, are frequently injected for such purposes, particularly for the recovery of oil.
A discussion of the problems encountered with the use of injected carbon dl'Dxide is presented in an article entitled Reservoir Application 30 of MobiLity Control Foams in C02 Floods, of the Society of Petrcleum Engi-neers/U. S. Department of Energy paper SPE/DOE 12644, pp 159 to 167.
Foams and surfactants are frequently used for retarding the formation of viscous fingers during carbon dioxide floods.
A known method of reducing the flow of water i5 described in U.S.
35 Patent No, 3,762,476 wherein a first aqueous polymer solution selected from the group consisting of polyacrylamide, a partially hydrolyzed polyacrylamide, a polysaccharide, a carboxymethylcellulose, a polyvinyl alcohol, and polystyrene sulfonate, is injected into a subterranean formation. Thereafter, a complexing ionic solution of multivalent 40 cations and retarding anions, and which also comprises aluminum citrate, q~

7'~7 is injected into the subterranean formation. The multivalent cations are selected from the group consisting of Fe(II), Fe(III), Al(III), Ti(IV),Zn(lI), Sn(IV), Ca(II), Mg(II), Cr(III), and the retarding anions are selected from the group consisting of acetate, nitrilotriacetate, tartrate, citrate, phosphate. Brine is then injected followed by a second slug of an aqueous polymer solution which can be the same or different from the first aqueous polymer solution. In any event, the complexing ionic solution of multi~alent cations and retarding anions is capable of gelling both ehe first and second aqueous polymer solution.
Water produced from a wellbore can come from the infiltration of naturally occuring subterranean water as desrribed above, or the water c~n come from in~ected water put into the formation in those hydrocarbon recovery processes which utilize waterflooding. U.S. Patent No.
4,~98,~37 discloses a method for forming a hydroxymethylated poly-15 acryla~ide gel, in situ, to reduce the permea~ility of a thusly treatedzone ~here the w~terflood method of oil recovery is employed. In this case the gel was ~ormed in situ by the injec~ion of an aqueous poly-acrylamide ~olution and an aqueous formaldehyde solution.
Although polyacryla~ide-based gels can be effective in retarding 20 water production or flow in some subterranean formations, poly-scrylamide-based gels will not be stable or effective in all formations.
In g~neral, polyacrylamide-based gels will work satiæfactorily in formations having a temperature below about 65C. Above sbout 65C, polyacrylamide-based gels become very sensitive to hardness of the 25 brines, especially where hardness is above about lO00 ppm. The hardness of the water becomes a more detrimental factor the higher the temper-ature, thus for ve~y hot regions even low hardness levels can render many gels ineffective. FormationR which have a higher temperature, hardness, or total`dissolved solids content above the aforementioned ranges usually 30 are not capable of being successfully treated with polyacrylamide-based polymers except for a relatively short period of time.
In many hydrocarbon producing wells temperatures of 80C or higher are often encountered. Formation waters frequently have hardnesses which exceed 1000 ppm. It is therefore desirable to develop a gel which can be 35 used to retard or block the flow of water in subterranean formations having a temperature of 65C or higher, and a water hardness of 1000 ppm or higher.
In other flooding operations, rather than water, other fluids can be used. Some fluids which are frequently used are carbon dioxide and 40 steam. Carbon dioxide is also used in other treating methods such as ~i77~'7 "Push and Pull" operations, sometimes referred to as "cyclic carbon dioxide injection" or "Huff and Puff" operations, where a production well is injected with carbon dioxide for several days and then produced for a month or so result in channels being formed which if not blocked will 5 result in an inefficient carbon dioxide treating operation due to loss of the gas into channels which drain into nonproductive parts of the reservoir. Because many of the existing gels degrade rapidly at elevated temperatures, polymers such as polyacrylamides are generally not satisfactory. Other fluids such as steam can also be used in push and 10 puLl operations.
Flooding operations using carbon dioxide as the drive fluid frequently experience a loss of drive fluid to nonproductive parts of the reservoir because of greaeer ability of the gas to dissipate into such channels as opposed to liquids. Loss of drive gases in carbon dioxide 15 flooding operations and carbon dioxide in C02 stimulation methods is more difficult to prevent because the flow channels responsible for such tos-se~ can be very small in diameter or width thereby making it very drfficult to fill such channels with a blocking agent. Some viscous plugging substances, even though they may have the desired stability at 20 higher temperatures, are not able to penetrate and effectively fill narrow channels, particularly as such channels become more distant from the wellbore.
Thus there is a need for plugging agents which can be formulated to penetrate deeply into the formation. The use of this invention addresses 25 this problem and provides polyvinyl alcohol based gels which can be ta;lor made to the particular problem at hand and which can overcome many of the shortcomings of prior art plugging agents and gels.
Polyvinyl alcohol gels have been used to protect well casings from corrosion. U.S. Patent No. 2,832,414 tiscloses such a method wherein an 30 aqueous solution of a water soluble polyvinyl alcohol which i8 capable of forming a gel if maintained in a quiescent state, is pumped into the annular space between the casing and the wall of the bore hole. After allowing the polymer to remain quiescent over a period of time a gel is formed. The thusly formed gel prevents the intrusion of formation water 35 into the annular space thereby reducing corrosion of the metal casing.
Apparently, no crosslinking agent is employed and for that reason is not believed that this particular gel would be useful for plugging channels or fractures on a permanent bases. Furthermore, in Patent ~o. 2,832,414 the gel is used to fill a relatively large but stagnant cavity compared 40 to the volume of a typical channel in a subterranean formation associated ;'4~7 with loss o carbon dioxide.
Studies of the macroscopic changes in polyvinyl acetate gels that occur upon removal from swelling equilibrium with isopropyl alcohol were reported in the Journal of Colloid and Interfsce Science, Vol. 90, No. 1, 5 November 1982, pages 34 to 43. These studies were conducted using films of gels having variou~ degrees of crosslinking and polymer concentration.
The polyvinyl acetste gels were formed from precursor polyvinyl alcohol gels that were crosslinked with glutaric dialdehyde ~hich we~e then converted to acetate gels by polymer homologous acetylation.
U. S. Patent No. 3,265,657 discloses a process for preparing an aqueous polyvinyl alcohol composition, which remains fluid for at least a few seconds after preparation and spontaneously gels thereafter. The gel i8 formed by contacting a gelable fluid aqueous polyvinyl alcohol ~olution with a hexavalent chromium compound and a reductive agent to 15 convert Cr (VI) to Cr (III). The compositions are used to produce foams -s~itab1e ~ insulating, acoustical, and packaging materialo. The gels are cross1inked wieh chromium, not an aldehyde.
~ _S. Patent ~o. 3,658,745 discloses a hydrogel which is capable of significant e~pansion upon cooling in water'and reversible shrinking upon 20 heating-which comprises a crosslinked acetalated hydrogel formed by reac~ing a polyvinyl alcohol previousiy dissolved in water and a mona1dehyde and 8 dialdehyde. The hydrogels are alleged to have sufficient crosslinking to prevent imbibition of macromolecular materials ~uch as-proteins but not the imbibition of micromolecular materials such 25 as low molecular weight water solutes. T'hese hydrogels are alleged to be useful for dialytic purification when pure water is added to the macro-~olecular solution after each cycle. Apparently these particular hydrogels are able to absorb and desorb water and microsolutes with alternat~ cooling and heating cycles. Apparently a major amount of 30 shrinkage of these gels occurs upon slight heating ~uch as from 1~ to 37C which indicates that these gels would have little value for blocking carbon dioxide and other fluids, including water, in ~ubterranean formations, especially at temperatures of 37C or higher.
Summary of the Invention By the term "aldehyde" as used herein is meant a monoaldehyde, a dialdehyde, a polyaldehyde, and any of the former whether substituted or unsubstituted. Preferably the aldehyde contains two functional groups such as dialdehyde or a substituted monoaldehyde as used herein is meant to include unsatusated carbon-carbon bond as well 85 substitution of 40 functional groups. Nonlimiting examples of substituted monoaldehyde are acrolein and acrolein dimethylacetal. Polyaldehydes can be used and may in some cases be more desirable, however, polyaldehydes are not as available commercially as dialdehydes and as a consequence use of polyaldehydes may not be practical.
Non-limiting examples of dialdehyde crosslinking agents are glyoxal, malonsldehyde, succinaldehyde, glutaraldehyde, adipaldehyde, terephthaldehyde. Non-limiting examples of dialdehyde derivatives are glyoxal bisulfite addition compound ~a2 HC(OH)S03CH(OH)S03, 10 glyoxal trimeric dihydrate, malonaldehyde bisdimethylacetal,

2,5-dimethoxytetrahydrofuran, 3,~-dihydro-2-methoxy-2H-pyran, and furfural. Acetals, hemiacetals, cyclic acetals, bisulfite addition compounds, shiff's bases or other compounds which generate dialdehydes in water, either alone or in response to an sdditional agent such as an acid 15 or a conditiGn such as heat are also meant to be included in the term "aldekyde" as ~ed a~d claimed herein.
~ on-limiting examples of monoaldehyde with a second functional group io addition to the aldehyde group are acrolein and acrolein dimethylacetal.
~on-limiting examples of polyaldehydes are polyacrolein dimethyl-acetal, addition products of acrolein for example, ethylene glycol plus acrolein, and glycerol plu5 acrolein.
By the-tenm "acidic catalyst" or "crosslinking catalyzing substance"
as used herein is-meant a substance which is a proton donor or a 25 substance which in its environment will form or become a proton donor.
All acids are operable as an acidic catalyst in the gel systems described herein, for example, Bronsted acids such as mineral and carboxylic acids, or Lewis acids. Non-limiting examples of a Lewis acid are zinc chloride, ferrous chloride, stannous chloride, aluminum chloride, barium fluoride, 30 ant sulfur trioxide. Some of these chemicals hydrolyse in water to produce metal oxides or hydroxides and HCl or HF. The rate of hydrolysis - of many Lewis acids is dependent on temperature and the other dissolved compounds in the solution. The rate of production of the acidic catalyst, HCl, from some of the above Le~is acids determines the rate of - 35 gel formation.
A telayed action catalyst is a substance which is not acidic in and of itself, but which generates an acidic catalyst slowly on interaction with water at the temperature of interest. For example, the rate of generation of the acid in oil well usage is usually cortrolled by the reservoir temperature experienced during the in-situ gel formation. In ,7~4~7 many applications the rate of acidic catalyst generation or release can be controlled by the gel-forming fluid formulation to range from a few minutes to a few days or more.
The acid catalyst can be a two component system, for example, a two 5 component delayed action catalyst can comprise a first component which will react with a second component, to form an acidic catalyst. A
non-limiting example of such a two component delayed action catalyst is sodium persulfate and a reducing agent. In such a delayed catalyst system the sodium persulfate reacts with the reducing agent to produce 10 sulfuric acid. In another two component delayed action catalyst system the reaction product of the two components can react with water to form the acidic catalyst.
The acidic catalyst and/or delayed action catalyst must, of cou~se, have some solubility in water. However, in ~ome oil field usages the 15 partial solubility of the acidic catalyst in the oil product can be aa~s~tageous if treatment is to include subterranean zones containing both oil-snd water. Ihe fraction of the acidic catalyst or delayed action catalyst which dissolutes in oil will, of course, not be available to catalyze the gel formation reaction in such zones of high oil content;
20 consequently such oil-water zones will not be blocked by gel formation to the same extent 8S those zones with little or no oil present.
-Nonrlimiting example5 of delayed action catalysts are methyl formate, ethyl formate, methyl acetate, ethyl acetate, glycerol monoacetate or acetin and glycerol diacetate or diacetin.
Laboratory tests conducted on core samples have shown that diacetin hydroLysis more slowly than methyl formate at all temperatures including the higher temperatures. Therefore, where subterranenan formations having higher temperatures are encountered, diactin or acetin because of their slower rate of hydrolysis are used to provide a longer time for 30 crosslinking reactions to occur and hence provide a longer time for the gelling forming fluids to penetrate into the pores of such subterranean zones before gelation occurs. Non-limiting examples of delayed action catalyst and their acidic catalyst product are:
Delayed Action Catalyst Acidic Catalyst Product Methyl formate Formic acid Glycerol diacetate - Acetic acid Sodium persulfate Sulfuric acid Sodium dodecyl sulfate Sulfuric acid Methyl methane sulfonate Methylsulfonic acid Sodium triiodide/sodium Hydroiodic acid 7'~ 7 bisul~atelwater Therefore, delayed action acidic catalysts can be esters 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 5 and acids such 85 sulfones, xanthates, xanthic acids, thiocyanates, and the like. In some of these examples, hydrolysis produces an acidic catalyst which speeds the crosslinking reaction and an alcohol which does not ~ffect gel formation. An example of & delayed action acidic catalyst i8 methyl formate which is influenced by the environment with respect to 10 the rate of formation of acid. For example, the higher the temperature, the faster methyl formate will hydrolyze and generate formic acid.
By the term "Bronsted acid" as used herein i9 meant a chemical which can ~ct as a source of protons. By the term "Lewis acid" as used herein is meant a chemical that can accept an electron pair from a base. By the 15 term "delayed action acid" as used herein is meant any acidic catalyst whic~ ~akes availabLe or generates donor proton over a period of time or a~ter an initial period of time either as a consequence of its character-ist~c or the characeeristics of the environment in which it is used.
By the term -"gel" as used herein is meant a chemically crosslinked 20 three-dimensional elastic network of long-chain molecules with a certain amount of immobilized solvent (diluent) molecules.
By the term "PVA based substance" or "PVA" (frequently referred to herein as the "first substance") as used herein is meant long-chain ~olecules selected from the group consisting of polyvinyl alcohols~ poly-25 vinyl alcohol copolymers, and mixtures thereof.
By the term "PVA-aldehyde gel" as used herein is meant a chemically crosslinked three-dimensional elastic network of long-chain molecules selected from the group consisting of a polyvinyl alcohol, a polyvihyl alcohol copolymer, and mixtures thereof, crosslinked with an aldehyde, 30 and containing a certain amount of immobilized and chemically bound water molecules.
By the term "PVA-glutaraldehyde gels" as used herein is meant a cbemically three-dimensional elastic network of various PVA based substances crosslinked with glutaraldehyde, and containing a certain 35 amount of immobilized and chemically bound water molecules.
All of the above mentioned acidic catalysts are effective crosslinking catalyzing substances for PVA-aldehyde gel systems.
Non-limiting examples of polyvinyl alcohol copolymers are polyvinyl alcohol-co-crotonic acid, polyvinyl alcohol-co-acrylic acid, polyvinyl 40 alcohol-co-methacrylic acid, polyvinyl alcohol-co-vinylpyridine, and polyvinyl alcohol-co-vinylacetate, the latter of which is frequently present in small amounts in commercial grade polyvinyl alcohols.
~ y the expre~sion "carbon dioxide break through fingers" as used herein is meant nonproductive reservoir channels having high permeability 5 to the flow of carbon dioxide and/or formation brines. In general, the fingers permit the carbon dioxide to be channeled into nonproductive areas of the reservoir thereby substantially lowering the efficiency of the carbon dioxide injection operation. Such fingers frequently contain substantial amounts of absorbed carbon dioxide.
It has been discovered that improved gels can be produced which are more stable and effective at elevated temperatures by using a high concentration of glutaraldehyde as the crosslinking agent for forming the gel. It has also been discovered that by using a relatively higher c~ncentration of glutaraldehyde that an acidic catalyst or cTosslinking 15 cataly~ing substsnce is not required. This discovery offers a very di&tinct sdvaQt~æe o~er other PVA aldehyde gel systems in that it permits the ~el-forming co p~sition to be used in subterranean formations having high arkalin~ty w~ere the alkaline material increase~ the p~ of the gel-forming c posi*i~n to levels which, if not basic, is sufficiently high 20 that gelation will not occur or occurs only after a very long period of time which is often too long to be of commercial ~alue for retarding the flow of fluids. This invention also permits the gel-forming composition to penet-rate ~n-depth, i.e., to relatively greater distances from the welLbore before the gel is formed than would be possible in the same gel 25 systems promoted with an acidic catalyst. The higher glutaraldehyde -concentration somehow produces a slightly acidic condition as the ~el-forming composition penetrates into the formation thereby enabling both better control and greater in-depth penetrstion. Accordingly, there is provided_a process for retarding the flow of carbon dioxide-containing 30 substance selected from the group confiisting of carbon dioxide, carbonic acid, and mixtures thereof, in carbon dioxide break-through fingers in a subterranean formation, the process comprising introducing an effective amount of a gel-forming com?osition into a subterranean formation, the gel-forming composition being operable, when contacting carbon dioxide 35 break-through fingers containing brine which has absorbed substantial amounts of carbon dioxide, to form a gel in said fingers which is operable for retarding the flow of said carbon dioxide-containing sub-stance in the fingers, the gel-forming composition comprising i. an aqueous solution comprising a PVA based substance or first substance selected from the group consisting of polyvinyl alcohol, .

7~74~
_ 9 _ a polyvinyl alcohol copolymer, and mixtures thereof, and ii. an amount of an aldehyde which is operable for effecting gelation of the gel-forming composition in the fingers after contacting the gel-forming composition with a brine which has absorbed substantial 5 amounts of carbon dioxide, but which is inoperable for effecting gelation of the gel-forming composition in flow passages containing brine which has not absorbed substantial amounts of carbon dioxide, or which is free of effective amounts of crosslinking catalyzing substances, and iii. wherein, before contacting gel-forming composition with a 10 brine containing substantial amount of absorbed carbon dioxide~ the gel-forming composition is substantially free of effective amounts of crosslinking catalyzing substances which are operable for promoting substantial crosslinking reactions between the first substance and the aldehyde; and allowing the gel-forming composition to contact the brine 15 cont~ining substan~ial amounts of absorbed carbon dioxide and to form a gel in-the finger~ of the subterranean formation which is effective for retarding the flow of said carbon dioxide-containing substance in the fingers.
ln one embodiment the aldehyde is glutaraldehyde~ In another 20 embodi~ent the amount of aldehyde is from about 0.01 to about 2 per~ent of the weight of the gel-forming composition. In another embodiment, the amount of aldehyde is at least about 2% of the stoichiometric amount required to react with all of the crosslinkable sites of the first substance. In still another embodiment the amount of the aldehyde is not 25 sufficient to cause substantially complete gelation of the gel-forming composition while the acidity of the gel-forming composition is higher than a p~ of about 6. In yet another embodiment, the amount of aldehyde iB not sufficient to cause substantially complete gelation of the gel-form~ng composition while the acidity of the gel-forming composition

3 is higher than a pH of about 5.
In another embodiment the amount of the PVA based substance is from about ~.5 to about 5X of the weight of the gel-forming composition. In a preferred embodiment the amount of the first substance is about 2.5% of the gel-forming composition and the aldehyde is glutaraldehyde which 35 provides about 0.1% of the weight of the gel-forming composition. In a further embodiment the subterranean formation in which the gel-forming compositio~ is injected has an average formation temperature of at least about 65C.
In still another embodiment the gel-forming composition is at least 40 about 65 weight percent water. In yet another embodiment the gel-forming 1~j7~;'L1 ~7 composition is at least about 93 weight percent brine. In another embodiment the first substance has an average molecular weight of at least 30,000, pre~erably at least lO0,000. Preferably the first substance is polyvinyl alcohol.
In still another embodiment, the process further comprises preventing the introduction into the subterranean formation of an effective amount of a crosslinking catalyzing substance under conditions which are operable for causing substantial mixing of the cro~slinking catalyzing substance with the gel-forming composition, wherein the crosslinking catalyzing substance i9 not a brine which has absorbed carbon d;oxide but is operable for promoting substantial crosslinking reactions between the first substa~ce and the aldehyde.
There is also provided a gel-forming composieion comprising i. a first substance selected from a group consisting of 15 pQly~inyl alcohol, a polyvinyl alcohol copolymer, and mixtures thereof~
ii. ~ater~ and iii~ an a~Qunt of an aldehyde which is operable for forming a gel with the first substance and the water when the acidity of the gel-forming composition has a 6ufficiently low pH, but which i8 not operable 20 for for~ing a gel ~hen the pH is 6 or higher. In a further embodiment the gel-form~ng composition is cause to have such sufficiently low pH by contacting it with an effective amount of a reservoir brine having effecti~e amounts of absorbed carbon dioxide sufficient for catalyzing, in the gel-forming composition~ a crosslinking reaction between the first 25 æubstance and the aldehyde, the gel-forming composition being free of efertive smounts of crosslinking catalyzing substances operable for promoting a crosslinking reaction in t~e gel-forming co0position between the first substance and the aldehyde. In a further embodiment the aldehyde is glutaraldehyde. In another further embodiment the gel-30 forming compo6ition will not form a gel when the acidity of the gel-forming composition has a pH of 5 or higher.
In yet another embodiment water is at least about 65 percent of the weight of the gel-forming composition. In ~till another embodiment the PVA based substance is from about 1.5 to 5 percent of the weight of the 35 gel-forming composition. In yet another embodiment the aldehyde is from about 0.03 to about 2 percent of the weight of the gel-orming compo~ition. In still another embodiment the water is provided by a brine, and the brine is at least about 93 percent of the weight of the gel-forming composition. In yet another embodiment the amount of the

4~ aldehyde is at least about 2 percent of the stoichiometric amount .

~ ~7~74'`~

required to react with all of the crosslinkable sites of the first substance. In one embodiment the Eirst substance is polyvinyl alcohol.
In yet another embodiment, the first substance has an average molecular weight of at least 30,000. In a further embodiment the first substance

5 has an average molecular weight of at least 100,000.
There is also provided a gel formed by reacting (a) a gel-forming composition comprising a first substance selected from the group consisting of polyvinyl alcohol, a polyvinyl alcohol copolymer, and mixtures thereof, water, and an amount of an 10 aldehyde which is operable for forming a gel with the first substance and the water when, the acidity of the gel-forming composition has a sufficiently low pH, but which is not operable for forming a gel when the pH is 6 or higher, with (b) an effective amount of carbon dioxide sufficient to lower 15 the acidity of the gel-forming composition to a pH less then about 6. In a further embodiment the first substance is polyvinyl alcohol having an a~erage Lecular weight of at least about 30,000. In yet another eMbQdiment the aldehyde is glutaraldehyde. In yet another embodiment water i~ at least about 65 percent of the weight of the gel-forming 20 -composition used to form the gel.
In general, the gel-forming composition is formulated so that it will no~ gel unless it is in contact with an acidic brine such as that occuring at carbon dioxide break-through points in the subterranean formation unless there is also present an effective amount of a cross-25 linking catalyzing substance. In a further embodiment the gel-forming composition will not gel except in channels containing a brine having a p~ below about 6. Preferably the gel forming composition is formulated so that it will not gel in brines having a p~ o about 5 or higher.
These ge~ formulations, therefore~ are designed so that they will not gel 30 in flow channels containing brines of higher acidic p~'s.
This embodiment is therefore useful in carbon dioxide floods, or cyclic carbon dioxide injection, in which the efficiency has been reduced due to fingering of carbon dioxide through the reservoir. By forming a gel in the carbon dioxide break-through fingers, the efficiency of the 35 operation is greatly improved.
This process will also apply to producing wells that are being used for carbon dioxide injection ~or purposes of reducing the viscosity of the oil. Absorption of the carbon dioxide by the oil causes the oil to swell and thereby lower its viscosity. This oil can then be produced 40 more easily. Nonetheless~ by first blocking those channels or fingers '7~'7 which cause a serious 10s8 of carbon dioxide into the nonproducing strata, the efficiency of recovering oil by lowering its viscosity through carbon dioxide absorption can be greatly increased.
In still further embodiments of the above de~cribed gels, the water 5 used to form the gel has a hardness of at least about 1000 ppm. In further embodiments the water has a hardness of at least about 3000 ppm, or 6000 ppm, or higher. In other further embodiments of the above described gels, the water used to form the gel has a total dissolved - solids content of at least about 30,000 ppm. In a still further 10 embodiment such water has a total dissolved solids content of at least about 80,000 ppm.
In the embodiments of this invention the various aldehydes, which are operable for crosslinking, crosslink with the polyvinyl alcohol or polyvinyl alcohol copolymer principally through formation of acetals.
15 Gels formed in this way are adaptable to the hardness of the water from which they are ormed or exposed. These gels are also more stable at high ~emperatures tkan polyacrylamide based gels or gels made from biopolymers or p~ly~inyl alcohols gelled by other crosslinking agents such as borate.
Because of the a~aptability and compatibility of these gels to water hardness or total dissolved solids content, these gels can be prepared using formation water, brackish water, sea water or usually any other available source of w~ter conveniently at hand. Because the largest ingredient used to formulate the above described gels is principally 25 water, substantial economic advantage is provided by this invention which permits gels to be formed with the cheapest source of available water.
~owever, the adv~Dtages of this invention are not limited merely to eco~omic advantages because these gels also provide substantial technical advantages over other gels. ~or example, in many of their uses these 30 gels are subjected to the infusion~of severely contaminated water into the gelling mass prior to reaching its gelation point. Where such contaminated water infusion occur6 in many other gelling fluids the gelation thereof is destroyed or so severely harmed that such other gels, if in fact they do gel, would be rendered ineffective for their intended 35 use~
Due to their stability at elevated temperatures, the above described gels can also be formed and used in formation~ having an average in-situ temperature of about 80~C or higher, and in some embodiments where the average in-situ temperature is 125C or higher.
The above described methods of forming a gel in situ in subterranean formations can be practiced using all of the gels provided by this invention.
The principles of this invention can also be used where the subterranean carbon dioxide-conveying and/or carbonic acid-conveying zone 5 i8 under the subterranean hydrocarbon-producing zone; or where the subterranean carbon dioxide/carbonic acid-conveying zone surrounds the subterranean hydrocarbon-producing zone; or where at least part of the carbon dioxide/carbonic acid-conveying zone coincides with at least part of the subterranean hydrocarbon-producing zone.
In one embodiment of this invention which is directed to carbon dioxide flood operations, it frequently is desirable to treat the carbon dioxide injector wells with a polymer gel-forming solution to control the carbon dioxide flow profile. In this embodiment such treatment prevents nonproductivE channeling o carbon dioxide at the injector well and/or 15 controls and/or redirects carbon dioxide flow through regions of varying peT~eabili~y~ 5ince in this embodiment the polymer is injected as a relati~ely low viscosity aqueous phase it penetrates preferentially the region of highest permeability. Accordingly, after formation of the gel in high permeability regions, such regions are converted to low 20 permeab;lity to further retard carbon dioxide/carbonic acid flow thereby causing, ~pon further carbon dioxide injection, a carbon dioxide sweep of previously inacce~sible areas in the formation which usually have relatively low permeability. By extending the carbon dioxide flow to such previously inascessible regions, more hydrocarbons can be recovered 25 than would be recovered in the absence of such polymer treatment.
The gels of this invention have improved resistance to heat and are stable in hard water. These characteristics make these gels particularly u~eful for many oil field applications. For oil field application, the stabilit~ and durability of the gels of this invention are an important 30 advantage.
Accordingly, one objective of this invention i5 to provide a means of controlling carbon dioxidelcarbonic acid flow in the nonproductive parts of the reservoir. The process is especially useful in formations having temperatures 80C or higher, or where the formation waters involved are 35 saline or hard.
- Another object of this invention is to provide a gel which can be formulated using hard water and water containing a high level of dissolved solids such as sea water and formation water encountered in deep off-shore hydrocarbon fields.
4~ Another object of this invention is to provide a gel which is stable 77~

at high temperatures and in psrticular more stable than other gels at such high temperatures.
Description of the Preferred Embodiment An oil well having an average in-situ temperature of 65C (150F) or 5 higher, and also having a high permeability to carbon dioxide, and in particular experiencing a loss of carbon dioxide to nonproductive parts of the reservoir, is treated by injecting a polyvinyl alcohol-glutaraldehyde-water mixture into the wellbore and from the wellbore into the reservoir. The mixture contains about 2.5% polyvinyl alcohol having 10 an average molecular weight of 125,000 or higher, about 0.1%
glutaraldehyde, and the remainder a brine having a tota] dissolved solids content of ab~ut 50,000 ppm and a hardness of about 5000 ppm. The polymer uill undergo crosslinking and gel in situ in a period of time ranging between se~eral hours to several days depending upon, in part, 15 the average in situ temperature. The following examples demonstrate how the gel-s of this invention can be tested and used for reducing the permeabi}ity of saadstone materials to carbon dioxide and/or carbonic acid.
Example ~o. 1 This example demonstrates how to determine the proper gel-forming composition for a reservoir experiencing carbon dioxide break through in a carbon dioxide flooding operation. Preferably a reservoir brine i5 ~sed to prepare the gel-forming composition; however, if desired a syn-thetic brine which simulates the reservoir brine can be used. A
25 useful formulation for a simulated brine is 4.5% NaCl, 0.4Z CaC12, and O.lX MgC12. The gel-forming composition is prepared by adding about 2.5%
polyvinyl alcohol having an average molecular weight of about 125,000 to the brine and heating the mixture for 45 minutes at 95C to completely dissolve the polymer in the brine. ~The brine-polymer mixture can then be 30 allowed to cool to room temperature. Just before injection, about 0.1%
glutaraldehyde is sdded to the polyvinyl alcohol-brine mixture to produce the gel-forming composition.
A 60 centimeters t60 cm) long, 5 cm diameter high pressure core holder is packed with crushed reservoir rock to form a packed test core 35 sample which is then saturated with brine and heated to 70C. Brine is pumped through the core sample at the rate of about 30 cm per day or one foot per day (1 FPD) and the pressure drop across the core sample determined. Mineral oil having a viscosity of 10 centipoise (lQcp) at 25C, is then pumped through the core sample at a rate of 30 cm per day 40 until no more brine is displaced therefrom. More brine is then pumped '77~7 through the core sample at 30 cm per day, until no more mineral oil is displaced therefrom and the pressure drop measured. Brine saturated with carbon dioxide is then pumped through the core sample, at 30 cm per day and the pressure drop determined. Thereafter the freshly mixed gel-5 forming composition is pumped into the core sample at a rate of 30 cm perday simultaneously with the flow of carbon dioxide saturated brine, and the pressure drop monitored. The gel point occurs when the pressure drop rapidly increases.
Example ~o. 2 A producing well, having an average formation temperature over 65C, is prepared for treatment by running tubing down the wellbore to the formation depth. As a precaution, about 16 cubic meters (100 barrels) of formation brine is injected into the reservoir to displace any brine, vhich may have absotbed substantial amounts of carbon dioxide and could 15 act as a crosslinking catalyzing substance, awày from the wellbore.
Abou~ 160 c~bic meters of the above-described gel-forming composition is injected through the tubing into the formation, or alternatively the gel-for~ing compositio~ is injected until the pumping pressure increases rapidly. This step is then followed by injecting additional formation 20 brine into -the reservoir to displace the gPl-forming composition deeper into the formation. The well is shut in for about 48 hours and thereafter production resumed. It is expected that a before-treatment production of 10 cubic meters per day (lO CMPD) of oil and 50 CMPD of water will be improved about one month after treatment with the gel-25 forming composition to a production of about 20 CMPD of oil and 20 CMPDof water.
In all of the above illustrative examples it is to be understood that the gel-forming composition will not gel until it is in contact with a brine whi`ch has absorbed substantial amounts of carbon dioxide. Thus 30 effective amounts of other acidic catalyzing sub~tances which can promote crosslinking of the polymer and aldehyde are to be excluded from the system.

Unless otherwise specified herein, all percents are weight percents.
The gels, the methods of forming the gels, and the processes for re-tarding the flow of carbon dioxide andtor carbonic acid have some degree of flexibility. For example, if the environment in which the gels are to be used has a relatively high temperature, gel time can be slowed by using a smaller amount of the aldehyde or glutaraldehyde. Similarly, if 40 the environmental temperature is relatively low, gelation can be speeded ~i77~'7 by the use of larger amounts of the aldehyde. It is permissible to use the formation brine of the subterranean zone as the water part of the gel-forming composition since the gel will form even with hard water.
Other variations of formulations, ~ethods and processes will be apparent 5 from this invention to those skilled ;n the art.
The foregoing disclosure and description of the present invention is illustrative and explanatory thereof and various changes in gel formation procedures and gel composition as well as the uses and applications of 10 such gels to for~ them in situ in subterranean ~ones and to retard, block or redirect carbon dioxide flow in subterranean zones may be made within the scope of the appending claims without departing from the spirit of the invention. For example, many gel formulations can be produced and ma~y ~ethods for forming such gels in situ in subterranean formations lS will be apparent to one skilled in the art from this invention. For example, any number ~f sequential injection steps of the gel-forming compQsitions can be ~ade. Furthermore, the necessary concentrations, amounes and sequence-of injection of the gel-forming compositions can be tailored to suit the particular well or subterranean formation being 20 treated.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for retarding the flow of a carbon dioxide-containing substance selected from the group consisting of carbon dioxide, carbonic acid, and mixtures thereof, in carbon dioxide break-through fingers in a subterranean formation, said process comprising:
(a) introducing an effective amount of a gel-forming composition into a subterranean formation, said gel-forming composition being operable, when contacting carbon dioxide break-through fingers containing brine which has absorbed substantial amounts of carbon dioxide, to form a gel in said fingers which is operable for retarding the flow of said carbon dioxide-contain-ing substance in said fingers, said gel-forming composition comprising i. an aqueous solution comprising a first substance selected from the group consisting of polyvinyl alcohol, a polyvinyl alcohol copolymer, and mixtures thereof, and ii. an amount of a second substance which is operable for effecting gelation of said gel-forming composition in said fingers after contacting said gel-forming composition with a brine which has absorbed substantial amounts of carbon dioxide, but which is inoperable for effecting gelation of said gel-forming composition in flow passages containing a brine which has not absorbed substantial amounts of carbon dioxide and which is free of effective amounts of crosslinking catalyzing substances, said second substance being 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, and iii. wherein, before contacting said gel-forming composition with a brine containing substantial amounts of absorbed carbon dioxide, said gel-forming composition is substantially free of effective amounts of crosslinking catalyz-ing substances which are operable for promoting substantial crosslinking reactions between said first substance and said aldehyde; and (b) allowing said gel-forming composition to contact said brine containing substantial amounts of absorbed carbon dioxide and to form a gel in said fingers of said subterranean formation which is effective for retarding the flow of said carbon dioxide-containing substance in said fingers.

2. The process of claim 1, wherein the said second substance is glutaraldehyde.

3. The process of claim 1, wherein said amount of the said second substance is from about 0.01 to about 2 percent of the weight of said gel-forming composition.

4. The process of claim 3, wherein the said second substance is glutaraldehyde.

5. The process of claim 1, wherein said gel-forming composition is at least about 65 weight percent water.

6. The process of claim 5, wherein the said second sub-stance is glutaraldehyde.

7. The process of claim 1, wherein said gel-forming composition is at least about 93 weight percent brine.

8. The process of claim 7, wherein the said second substance is glutaraldehyde.

- 18a - 71440-4

9. The process of claim 1, wherein said amount of the said second substance is at least about 0.7% of the stoichio-metric amount required to react with all of the crosslinkable sites of said first substance.

10. The process of claim 9, wherein the said second substance is glutaraldehyde.

11. The process of claim 1, wherein said amount of the said second substance is not sufficient to cause substantially complete gelation of said gel-forming composition while the acidity of said gel-forming composition is higher than a pH
of about 6.

12. The process of claim 11, wherein the said second sub-stance is glutaraldehyde.

13. The process of claim 1, wherein said amount of the said second substance is not sufficient to cause substantially complete gelation of said gel-forming composition while the acidity of said gel-forming composition is higher than a pH
of about 5.

14. The process of claim 13, wherein the said second substance is glutaraldehyde.

15. The process of claim 1, further comprising preventing the introduction into said subterranean formation of an effective amount of a crosslinking catalyzing substance under conditions which are operable for causing substantial mixing of said crosslinking catalyzing substance with said gel-forming composition, wherein said crosslinking catalyzing substance is not a brine which has absorbed carbon dioxide but is operable - 18b - 71440-4 for promoting a crosslinking reaction between said first substance and the said second substance.

16. The process of claim 15, wherein the said second substance is glutaraldehyde.

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

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

19. A process for retarding the flow of a carbon dioxide-containing substance selected from the group consisting of carbon dioxide, carbonic acid, and mixtures thereof, in carbon dioxide break-through fingers in a subterranean formation and recovering oil therefrom, said process comprising:
(a) introducing an effective amount of a gel-forming composition into a subterranean formation, said gel-forming composition being operable, when contacting carbon dioxide break-through fingers containing brine which has absorbed substantial amounts of carbon dioxide, to form a gel in said fingers which is operable for retarding the flow of said carbon dioxide-containing substance in said fingers, said gel-forming composition comprising i. an aqueous solution of polyvinyl alcohol having an average molecular weight of at least 30,000, and ii. an amount of glutaraldehyde which is operable for effecting gelation of said gel-forming composition in said fingers after contacting said gel-forming composition with a brine which has absorbed substantial amounts of carbon dioxide, but which is inoper-able for effecting gelation of said gel-forming composition in flow passages containing a substance which when contacting said gel-forming composition does not cause said gel-forming composition to have a pH about 6 or higher, iii. wherein said gel-forming composition introduced into said subterranean formation is at least about 65 weight percent water, and wherein, before contacting said gel-forming composition with a brine containing substantial amounts of absorbed carbon dioxide, said gel-forming composition is substantially free of amounts of crosslinking catalyzing substances which are operable for promoting a crosslinking reaction between said polyvinyl alcohol and glutar-aldehyde;
(b) preventing the introduction into said subterranean formation of an effective amount of a crosslinking catalyzing substance under conditions which are operable for causing substantial mixing of said crosslinking catalyzing substance with said gel-forming composition, wherein said crosslinking catalyzing substance is not a brine which has absorbed carbon dioxide but is operable for promoting a crosslinking reaction between said polyvinyl alcohol and glutaralde-hyde;
(c) allowing said gel-forming composition to contact said brine containing substantial amounts of absorbed carbon dioxide and to form a gel in said fingers of said subterranean formation which is effective for retarding the flow of said carbon dioxide-containing substance said fingers; and (d) after said gel is formed in said fingers, recovering oil from said subterranean formation.

20. The process of claim 19, wherein said polyvinyl alcohol has an average molecular weight of at least 100,000.

21. The process of claim 19, wherein said water of said gel-forming composition mentioned in step (a), part iii, is water contained in a brine and wherein at least about 93 weight percent of said gel-forming composition is said brine.

22. A gel-forming composition for plugging carbon dioxide break-through fingers of a subterranean formation, wherein the gel-forming composition comprises:
i. a first substance selected from the group consisting of polyvinyl alcohol, a polyvinyl alcohol copolymer, and mixtures thereof, ii. water, and iii. an amount of a second substance which is selected from the group consisting of aldehydes, aldehyde generating substances, acetals, acetal generating substances and mixtures thereof and is operable for forming a gel with said first substance and said water through the formation of acetal crosslinkages when said gel-forming composition is contacted with an effective amount of a reservoir brine having effective amounts of absorbed carbon dioxide sufficient for catalyzing, in said gel-forming composition, a crosslinking reaction between the said first substance and the said second substance, said amount of said second suhstance not beiny operable for forming a gel when said gel-forming composition has a pH of 6 or higher, said gel-forming composition being free of effective amounts of crosslinking catalyzing substances operable for promoting a crosslinking reaction in said gel-forming composition between said first substance and said second substance, but which is not operable for forming a gel when said pH is 6 or higher.

23. The gel-forming composition of claim 22, wherein said second substance is glutaraldehyde.

- 20a - 71440-4

24. The gel-forming composition of claim 22, wherein said gel-forming composition will not form a gel when the acidity of said gel-forming composition has a pH of about 5 or higher.

25. The gel-forming composition of claim 22, 23 or 24, wherein said water is at least about 65% of the weight of said gel-forming composition.

26. The gel-forming composition of claim 22, 23 or 24, wherein said first substance is from about 1.5 to about 5%
of the weight of said gel-forming composition.

27. The gel-forming composition of claim 22, 23 or 24, wherein said second substance is from about 0.01 to about 2% of the weight of said gel-forming composition.

28. The gel-forming composition of claim 22, 23 or 24, wherein said water is provided by a brine, and wherein said brine is at least about 93% of the weight of said gel-forming composition.

29. The gel-forming composition of claim 22, 23 or 24, wherein the amount of said second substance is at least about 0.7% of the stoichiometric amount.

required to react with all of the crosslinkable sites of said first substance.

30. The gel-forming composition of claim 22, wherein said first substance is polyvinyl alcohol.

31. A gel-forming composition comprising i. polyvinyl alcohol having an average molecular weight of at least about 30,000, ii. water, and iii. an amount of glutaraldehyde which is operable for forming a gel with said polyvinyl alcohol and water when said gel-forming composition is contacted with an effective amount of a reservoir brine having effective amounts of absorbed carbon dioxide sufficient for catalyzing, in said gel-forming composition, a crosslinking reaction between said polyvinyl alcohol and said glutaraldehyde, said amount of glutaraldehyde not being operable for forming a gel when said gel-forming composition has a pH of 6 or higher, said gel-forming composition being free of effective amounts of crosslinking catalyzing substances operable for promoting a crosslinking reaction in said gel-forming composition between said polyvinyl alcohol and said glutaraldehyde, and wherein said water is at least about 65% of the weight of said gel-forming composition.

32. The gel-forming composition of claim 31, wherein said gel-forming composition will not form a gel when the acidity of said gel-forming composition has a pH of about 5 or higher.

33. The gel-forming composition of claim 31, wherein said first substance is from about 1.5 to about 5% of the weight of said gel-forming composition.

34. The gel-forming composition of claim 31, wherein said glutaraldehyde is from about 0.01 to about 2% of the weight of said gel-forming composition.

35. The gel-forming composition of claim 31, wherein said water is provided by a brine, and wherein said brine is at least about 93% of the weight of said gel-forming composition.

36. The gel-forming composition of claim 31, wherein said polyvinyl alcohol has an average molecular of at least about 100,000.

370 A gel formed by reacting (a) 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. water, and iii. an amount of an aldehyde which is operable for forming gel with said first substance and said water when the acidity of said gel-forming composition has a sufficiently low pH, but which is not operable for forming a gel when said pH is 6 or higher, with (b) an effective amount of a carbon dioxide-containing substance selected from the group consisting of carbon dioxide, carbonic acid, and mixtures thereof, sufficient to lower the acidity of said gel-forming composition to a pH less than about 6.

38. A gel formed by reacting (a) a gel-forming composition comprising i. polyvinyl alcohol having an average molecular weight of at least about 30,000, ii. water, and iii. an amount of glutaraldehyde which is operable for forming a gel with said polyvinyl alcohol and water when the acidity of said gel-forming composition has a sufficiently low pH, but which is not operable for forming a gel when said pH is about 6 or higher, and wherein said water is at least about 65 percent of the weight of said gel-forming composition, with (b) an effective amount of a reservoir brine having substantial amounts of absorbed carbon dioxide sufficient to lower the acidity of said gel-forming composition to a pH less than about 6.

39. The process of claim 19, wherein said polyvinyl alcohol is about 2.5% by weight of said gel-forming composition, and wherein said amount of said glutaraldehyde is about 0.1% by weight of said gel-forming composition.

40. The process of claim 39, wherein said subterranean formation has an average formation temperature of at least about 65°C.

41. The gel of claim 37, wherein said carbon dioxide containing substance is a reservoir brine.

42. The gel of claim 37, wherein said amount of aldehyde is not operable for forming a gel when said pH of said gel-forming composition is greater than 5, and wherein said amount of said carbon dioxide containing substance is sufficient to lower the acidity of said gel-forming composition to a pH of 5 or lower.

43. A process for retarding the flow of carbon dioxide-containing gas in nonproductive high gas permeable flow channels in a subterranean formation, said process comprising:
(a) introducing a gas selected from the group consisting of carbon dioxide and gases containing carbon dioxide into a subterranean deposit containing nonproductive high gas permeable flow channels for a period of time sufficient for said flow channels to sorb a predetermined amount of said gas thereby forming a sorbed substance which can form an acidic substance selected from the group consisting of carbon dioxide, carbonic acid, and mixtures thereof;
(b) after said flow channels have sorbed said predetermined amount of said gas, stopping the flow of said gas into said subterranean formation;
(c) after stopping the flow of said gas into said subterranean formation, introducing an effective amount of a gel-forming composition into said flow channels for retarding the flow of said gas in said flow channels, said 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, wherein said first substance is from about 0.5 to about 5% of the weight of said gel-forming com-position, and ii. an effective amount of a second substance which is operable for effecting gelation of said gel-forming composition when said gel-forming composition has a pH equal to a predetermined pH value or less within a predetermined period of time, said effective amount of said second substance being inoperable for effecting gelation of said gel-forming composition within said predetermined period of time when said gel-forming composition has a pH which is higher than said predetermined pH value, wherein prior to being introduced into said flow channels said gel-forming composition has a pH higher than said predetermined pH
value and is free of an effective amount of a crosslinking catalyzing substance which is operable for effecting gelation of said gel-forming composition at a pH which is higher than said pre-determined pH value within said predetermined period of time, said second substance being 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 said second substance is from about 0.01 to about 2% of the weight of said gel-forming composition, wherein the average molecular weight of said first substance is at least about 30,000, and wherein said gel-forming composition is at least about 93% by weight water, and further wherein said predetermined pH
value 19 from about 3 to about 6;
(d) allowing said sorbed substance to form an effective amount of said acidic substance;
(e) allowing said thusly formed effective amount of said acidic substance to be sorbed by said gel-forming composition in said flow channels, wherein said thusly formed effective amount of said acidic substance is sufficient to lower the pH of said gel-forming composition in said flow channels to at least said predetermined pH
value;
(f) allowing said gel-forming composition in said flow channels, after it has sorbed said thusly formed effective amount of said acidic substance, to form a gel in said flow channels, wherein said predetermined amount of said gas referred to in step (a) is sufficient to enable said effective amount of said acidic substance to be formed from said sorbed substance and sorbed in said gel-forming composition in said flow channels and also sufficient to enable said gel to be formed in said flow channels within said predetermined period of time, said predetermined period of time being from about 15 minutes to about 5 days after said gel-forming composition is introduced into said flow channels, thereby retarding the flow of said carbon dioxide gas in said flow channels; and (g) after said gel is formed in said flow channels, injecting a substance into said subterranean formation to flush out gel-forming composition in said formation which has not gelled.

44. A process for retarding the flow of carbon dioxide-containing gas in nonproductive high gas permeable flow channels in a subterranean formation, said process comprising:
(a) introducing a gas selected from the group consisting of carbon dioxide and gases containing carbon dioxide into a subterranean deposit containing nonproductive high gas permeable flow channels for a period of time sufficient for said flow channels to sorb a predetermined amount of said gas thereby forming a sorbed substance which can form an acidic substance selected from the group consisting of carbon dioxide, carbonic acid, and mixtures thereof;
(b) after said flow channels have sorbed said predetermined amount of said gas, stopping the flow of said gas into said subterranean formation;
(c) after stopping the flow of said gas into said subterranean formation, introducing an effective amount of a gel-forming composition into said flow channels for retarding the flow of said gas in said flow channels, said 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, wherein said first substance is from about 1.5 to about 3% of the weight of said gel-forming composition, and ii. an effective amount of a second substance which is operable for effecting gelation of said gel-forming composition when said gel-forming composition has a pH equal to a predetermined pH value or less within a predetermined period of time, said effective amount of said second substance being inoperable for effecting gelation of said gel-forming composition within said predetermined period of time when said gel-forming composition has a pH which is higher than said predetermined pH value, wherein prior to being introduced into said flow channels said gel-forming composition has a pH higher than said predetermined pH
value and is free of an effective amount of a crosslinking catalyzing substance which is operable for effecting gelation of said gel-forming composition at a pH which is higher than said predetermined pH value within said predetermined period of time, said second substance being 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 said second substance is from about 0.03 to about 2% of the weight of said gel-forming composition, wherein the average molecular weight of said first substance is from about 30,000 to about 1,000,000, and wherein said gel-forming composition is at least about 95% by weight water, and further wherein said predetermined pH value is from about 3.5 to about 5.5;
(d) allowing said sorbed substance to form an effective amount of said acidic substance;

(e) allowing said thusly formed effective amount of said acidic substance to be sorbed by said gel-forming composition in said flow channels, wherein said thusly formed effective amount of said acidic substance is sufficient to lower the pH of said gel-forming composition in said flow channels to at least said predetermined pH
value;
(f) allowing said gel-forming composition in said flow channels, after it has sorbed said thusly formed effective amount of said acidic substance, to form a gel in said flow channels, wherein said predetermined amount of said gas referred to in step (a) is sufficient to enable said effective amount of said acidic substance to be formed from said sorbed substance and sorbed in said gel-forming composition in said flow channels and also sufficient to enable said gel to be formed in said flow channels within said predetermined period to time, said predetermined period of time being from about one hour to about 4 days after said gel-forming composition is introduced into said flow channels, thereby retarding the flow of said carbon dioxide gas in said flow channels; and (g) after said gel is formed in said flow channels, injecting a substance into said subterranean formation to flush out gel-forming composition in said formation which has not gelled.

45. A process for retarding the flow of carbon dioxide-containing gas in nonproductive high gas permeable flow channels in a subterranean formation, said process comprising:
(a) introducing a gas selected from the group consisting of carbon dioxide and gases containing carbon dioxide into a subterranean deposit containing nonproductive high gas permeable flow channels for a period of time sufficient for said flow channels to sorb a predetermined amount of said gas thereby forming a sorbed substance which can form an acidic substance selected from the group consisting or carbon dioxide, carbonic acid, and mixtures thereof;
(b) after said flow channels have sorbed said predetermined amount of said gas, stopping the flow of said gas into said subterranean formation;
(c) after stopping the flow of said gas into said subterranean formation, introducing an effective amount of a gel-forming composition into said flow channels for retarding the flow of said gas in said flow channels, said 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, wherein said first substance is from about 1.5 to about 3% of the weight of said gel-forming composition, and ii. an effective amount of a second substance which is operable for effecting gelation of said gel-forming composition when said gel-forming composition has a pH equal to a predetermined pH value or less within a predetermined period of time, said effective amount of said second substance being inoperable for effecting gelation of said gel-forming composition within said predetermined period of time when said gel-forming composition has a pH which is higher than said predetermined pH value, wherein prior to being introduced into said flow channels said gel-forming composition has a pH higher than said predetermined pH
value and is free of an effective amount of a crosslinking catalyzing substance which is operable for effecting gelation of said gel-forming composition at a pH which is higher than said predetermined pH value within said predetermined period of time, said second substance being 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 said second substance is from about 0.03 to about 1% of the weight of said gel-forming composition, wherein the average molecular weight of said first substance is from about 100,000 to about 1,000,000, and wherein said gel-forming composition is at least about 96% by weight water, and further wherein said predetermined pH value is from about 4 to about 5;
(d) allowing said sorbed substance to form an effective amount of said acidic substance;
(e) allowing said thusly formed effective amount of said acidic substance to be sorbed by said gel-forming composition in said flow channels, wherein said thusly formed effective amount of said acidic substance is sufficient to lower the pH of said gel-forming composition in said flow channels to at least said predetermined pH
value;
(f) allowing said gel-forming composition in said flow channels, after it has sorbed said thusly formed effective amount of said acidic substance, to form a gel in said flow channels, wherein said predetermined amount of said gas referred to in step (a) is sufficient to enable said effective amount of said acidic substance to be formed from said sorbed substance and sorbed in said gel-forming composition in said flow channels and also sufficient to enable said gel to be formed in said flow channels within said predetermined period of time, said predetermined period of time being from about 2 hours to about 3 days after said gel-forming composition is introduced into said flow channels, thereby retarding the flow of said carbon dioxide gas in said flow channels; and (g) after said gel is formed in said flow channels, injecting a substance into said subterranean formation to flush out gel-forming composition in said formation which has not gelled.

46. A process for retarding the flow of a carbon dioxide-containing gas in nonproductive high permeable flow channels in a subterranean formation, said process comprising:
(a) introducing a gas selected from the group consisting of carbon dioxide and gases containing carbon dioxide into a subterranean deposit containing nonproductive high gas permeable flow channels for a period of time sufficient for said flow channels to sorb a predetermined amount of gas thereby forming a sorbed substance which can form an acidic substance selected from the group consisting of carbon dioxide, carbonic acid, and mixtures thereof;
(d) after said flow channels have sorbed said predetermined amount of said gas, stopping the flow of said gas into said subterranean formation;
(c) after stopping the flow of said gas into said subterranean formation, introducing an effective amount of a gel-forming composition into said flow channels for retarding the flow of said gas in said flow channels, said gel forming composition comprising i. an aqueous solution comprising of polyvinyl alcohol, wherein said polyvinyl alcohol is from about 2 to about 3% of the weight of said gel-forming composition, and ii. an effective amount of glutaraldehyde which is operable for effecting gelation of said gel-forming composition when said gel-forming composition has a pH equal to a predetermined pH value or less within a predetermined period of time, said effective amount of said glutaraldehyde being inoperable for effecting gelation of said gel-forming composition within said predetermined period of time when said gel-forming composition has a pH which is higher than said predetermined pH value, wherein prior to being introduced into said flow channels said gel-forming composition has a pH higher than said predetermined pH
value and is free of an effective amount of a crosslinking catalyzing substance which is operable for effecting gelation of said gel-forming composition at a pH which is higher than said predetermined pH value within said predetermined period of time, wherein said glutaraldehyde is from about 0.03 to about 1% of the weight of said gel-forming composition, wherein the average molecular weight of said polyvinyl alcohol is about 125,000, and wherein said gel-forming composition is at least about 96% by weight water, and further wherein said predetermined pH value is from about 4 to 5;
(d) allowing said sorbed substance to form an effective amount of said acidic substance;
(e) allowing said thusly formed effective amount of said acidic substance to be sorbed by said gel-forming composition in said flow channels, wherein said thusly formed effective amount of said acidic substance is sufficient to lower the pH of said gel-forming composition in said flow channles to at least said predetermined pH
value;
(f) allowing said gel-forming composiiton in said flow channels, after it has sorbed said thusly formed effective amount of said acidic substance, to form a gel in said flow channels, wherein said predetermined amount of said gas referred to in step (a) is sufficient ot enable said effective amount of said acidic substance to be formed from said sorbed substance and sorbed in said gel-forming composition in said flow channles and also sufficient to enable said gel to be formed in said flow channels within said predetermined period of time, said predetermined period of time being from about 3 hours to about 2 days after said gel-forming composition is introduced into said flow channles, thereby retarding the flow of said carbon dioxide gas in said flow channels; and (g) after said gel is formed in said flow channels, injecting a substance into said subterranean formation to flush out gel-forming composition in said formation which has not gelled.

47. The process of claim 43 wherein said predetermined pH value is from about 3.5 to about 5.5.

48. The process of claim 43 wherein said predetermined pH value is from about 4 to about 5.

49. The process of claim 43 wherein said predetermined pH value is from about 3.5 to about 4.5.

50. The process of claim 43 wherein said predetermined pH value is from about 4.5 to about 5.5.

51. The process of claim 43 wherein said predetermined pH value is from about 4.5 to about 5.

52. The process of claim 43 wherein said first substance is polyvinyl alcohol, and said second substance is glutaraldehyde.

53. The process of claim 44 wherein said predetermined pH value is from about 4 to about 5.

54. The process of claim 44 wherein said predetermined pH value is from about 4.5 to about 5.

55. The process of claim 44 wherein said first substance is polyvinyl alcohol, and second substance is glutaraldehyde.

56. The process of claim 45 wherein said first substance is polyvinyl alcohol, and said second substance is glutaraldehyde.

57. The process of claim 43 wherein said predetermined amount of gas referred to in step (a) is sufficient to enable said gel to be formed in said flow channels within the period of time from 4 hours to about 1 day.

58. The process of claim 44 wherein said predetermined amount of gas referred to in step (a) is sufficient to enable said gel to be formed in said flow channels within a period of time from 4 hours to about 1 day.

59. A process for retarding the flow of a carbon dioxide in carbon dioxide break-through fingers in a subterranean formation, said process comprising:
(a) introducing a gas selected from the group consisting of carbon dioxide and gases containing carbon dioxide into a subterranean deposit containing carbon dioxide break-through fingers;
(b) after said carbon dioxide break-through fingers have sorbed a predetermined amount of said gas, stopping the flow of said gas into said subterranean formation;
(c) after stopping the flow of said gas into said subterranean formation, introducing an effective amount of a gel-forming composition into said subterranean formation and into said carbon dioxide break-through fingers, said gel-forming composition being operable, when contacting carbon dioxide break-through fingers containing brine which has absorbed substantial amounts of carbon dioxide, to form a gel in said fingers which is operable for retarding the flow of said gas in said fingers, said 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, and 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 the first substance through the formation of acetal crosslinkages, which is operable for effecting gelation of said gel-forming composition in said fingers after contacting said gel-forming composition with a brine which has absorbed substantial amounts of carbon dioxide, but which is inoperable for effecting gelation of said gel-forming composition in flow passages con-taining a brine which has not absorbed substantial amounts of carbon dioxide and which is also free of effective amounts of other crosslinking catalyzing substances, and wherein said first substance is from about 1.5 to about 5% of the weight of said gel-forming composition, and wherein said second substance is from about 0.01 to about 2% of the weight of said gel-forming composition;
(d) allowing said gel-forming composition to contact said brine containing substantial amounts of absorbed carbon dioxide; and (e) allowing a gel to form in said fingers of said subterranean formation which is effective for retarding the flow of said carbon dioxide-containing substance in said fingers.

60. The process of claim 59 wherein said gel-forming composition is at least about 97 weight percent water and at least about 65 weight percent H2O.

61. The process of claim 59 wherein said water is a brine.

62. A gel formed by contacting (a) 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. water, and iii. 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 the first substance through the formation of acetal crosslinkages, which is operable for forming a gel with said first substance and said water when said gel-forming composition has a pH between about 3.5 and about 5.5, but which is inoperable for forming a gel when said pH is higher than 5.5, with (b) an effective amount of a carbon dioxide-containing substance selected from the group consisting of carbon dioxide, carbonic acid, and mixtures thereof, which is operable to lower the pH of said gel-forming composition to a pH between about 3.5 and about 5.5, and operable for causing said gel-forming composition to gel within a period of time from about 3 hours to about 2 days after contacting said carbon dioxide-containing substance with said gel-forming composition, and wherein the amount of said first substance used to form said gel is from about 1.5 to about 5% of the weight of said gel, and wherein the amount of said second substance used to form said gel is from about 0.01 to about 2% of the weight of said gel.

63. 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. water, and iii. an amount of a second substance which is selected from the group consisting of aldehydes, aldehyde generating substances, acetals, acetal generating substances and mixtures thereof and is operable for forming a gel with said first substance and said water through the formation of acetal cross-linkages when said gel-forming composition is contacted with an effective amount of a reservoir brine having effective amounts of absorbed carbon dioxide sufficient for catalyzing, in said gel-forming composition, a crosslinking reaction between the said first substance and the said second substance, said amount of said second substance not being operable for forming a gel when said gel-forming composition has a pH of 6 or higher, said gel-forming composition being free of effective amounts of crosslinking catalyzing substances operable for promoting a crosslinking reaction in said gel-forming composition between said first substance and said second substance, but which is not operable for forming a gel when said pH is 6 or higher.

64. The gel-forming composition of claim 63, wherein said second substance is glutaraldehyde.

65. The gel-forming composition of claim 63, wherein said gel-forming composition will not form a gel when the acidity of said gel-forming composition has a pH of about 5 or higher.

66. The gel-forming composition of claim 63, 64 or 65, wherein said water is at least about 65% of the weight of said gel-forming composition.

67. The gel-forming composition of claim 66 wherein said first substance is from about 1.5 to about 5% of the weight of said gel-forming composition.

68. The gel-forming composition of claim 66 wherein said second substance is from about 0.01 to about 2% of the weight of said gel-forming composition.

69. The gel-forming composition of claim 66 wherein said water is provided by a brine, and wherein said brine is at least about 93% of the weight of said gel-forming composition.

70. The gel-forming composition of claim 66 wherein the amount of said second substance is at least about 0.7% of the stoichiometric amount required to react with all of the cross-linkable sites of said first substance.

71. The gel-forming composition of claims 63, 64 or 65, wherein said first substance is polyvinyl alcohol.