CA2497728C - Control of particulate flowback in subterranean wells - Google Patents

Abstract

The addition of fibrous materials in intimate mixture with particulates for fracturing and gravel packing decreases or eliminates the flowback of proppant and/or formation fines while stabilizing the sand pack and lowering the demand for high polymer loadings in the placement fluids. Preferred fibers include glass, aramide, nylon and other natural and, synthetic organic and inorganic fibers and metal filaments.

Description

CONTROL OF PARTICULATE FLOWBACK IN
SUBTERRANEAN WELLS

This is a divisional application of copending application 2,119,316, filed March 17, 1994.

This invention relates to the art of recovery of hydrocarbon values from subterranean formations, and more particularly, to a method and means for controlling the transport of particulate solids in conjunction with the flow of hydrocarbon fluid from the formation into a wellbore.

BACKGROUNU OF THE INVENTION

Transport of particulate solids during the production of fluids from a wellbore has long presented a problem in the recovery of fluid mineral values from subterranean formations. Such transported particulate solids cannot only clog the wellbore thereby limiting or cornpletely stopping fluid production, they represent a significant wear factor in well production equipment including pumps and seals used in the recovery process.
Finally, these particulate solids must be separated from the valuable fluids adding further expense to the processing of these fluids for use.

Particulates which are available for transport in fluids flowing to the wellbore may be present due to an unconsolidated nature of the subterranean formation and/or as a result of various well treatments placing particulate solids into the formation or the near wellbore area such as by fracturing and gravel packing.
Several means have been attempted in order to limit or eliminate flowback of particulate proppant materials placed into the formation in a fracturing process. One means showing re.asonable effectiveness has been to gradually release fracturing pressure once the fracturing opera=tion has been completed so that fracture closure pressure acting against the proppant builds gradually allowing proppant matrix to stabilize before fracturing fluid flowback and well production can carry significant quantities of the proppant out of the fractures and back to the wellbore. It has also been common to use so-called "resin-coated proppant", that is, particulate proppant materials having an adherent coating bonded to its outer surface so that the proppant particles are bonded to each other, a process which further reduces the possibility of proppant flowback.
Since fracturing treatments may employ thousands or even millions of pounds of proppant, the use of expensive, resin-coated proppants is generally limited to use in the final stages of a fracturing treatment for economic purposes.

in unconsolidated formations, it is common to place a filtration bed of gravel in the near-wellbore area in order to present a physical barrier to the transport of unconsolidated formation fines with the production of wellbore fluids. Typically, such so-called "gravel packing operations" involve the pumping and placement of a quantity of gravel and/or sand having a mesh size between 10 and 60 U.S. Standard Sieve Series mesh into the unconsolidated formation adjacent to the wellbore.

It is sometimes also desirable to bind the gravel particles together in order to form a porous matrix through which formation fluids can pass while straining out and retaining the bulk of the unconsolidated sand and/or fines transported to the near wellbore area by the formation fluids. The gravel particles may constitute a resin-coated gravel which is either pre-cured or can be cured by an overflush of a chemical binding agent once the gravel is in place. It has also been known to add various binding agents directly to an overflush of uncoated gravel particles in order to bind them together to form the porous matrix.

It has been known in the past to utilize fibrous materials in well, treatment fluids in order to prevent or limit fluid loss into a porous formation or vugular zone. In this regard, glass, asbestos, cotton or cellulosic fibers have been pumped in a slurry into a wellbore or formation in order to create a mat of fibrous material over areas of high fluid loss. As the mat builds, flow of well treatment fluids into these high fluid loss areas is inhibited or prevented. Such fluid loss agents are typically pumped in conjunction with drilling fluids or in nonproppant-carrying pad solutions preceding a proppant-laden fracturing fluid.
The primary intent of using such fibrous materials in these fluids is for arresting or inhibiting fluid flow of fluids in certain desired areas of the wellbore or formation.

SUMMARY OF THE :CNVENTION

The present invention provides a method and fluid for treating a subterranean formation and a resultant porous solid pack which inhibits the flow of both deposited and natural formation particulates and fines back through the wellbore with the production of formation fluids.

In accordance with the invention, a method of treating a subterranean formation penetrated by a wellbore comprises the steps of providing a fluid suspension including a simultaneous mixture of particulate material and a fibrous material, pumping the fluid suspension including the mixture of the particulate material and fibrous material through the wellbore and depositing the particulate and fibrous material mixture in the subterranean formation.

Further in accordance with the invention, the above-described method comprises providing a particulate material having a size in the range of 10 to 100 mesh and a fibrous material selected from a group consisting of natural and synthetic organic fibers, glass fibers, ceramic fibers, carbon fibers, inorganic fibers and metal fibers and mixtures of these fibers.

Still further in accordance with the invention, a well treatment fluid comprises a fluid suspension including a simultaneous mixture of a particulate material and a fibrous material.

Still further in accordance with the invention, a means for inhibitir.Lg particulate transport in subterranean wells comprises a porous pack including a particulate material having a size ranging from about 10 to about 100 U.S. mesh in intimate mixture with a fibrous material.

This invention provides a means and method whereby flowback of particulate materials either pumped into a wellbore with a well treatment fluid or present as a result of unconsolidated formation fines is prevented or inhibited by the presence of fibers in an intimate mixture with a particulate material.

This invention provides a means to control the flowback of particulate material in subterranean fluid production without the use of complicated and expensive resin formulations and procedures utilizing such resins to form a porous, consolidated bed of particulate.

In one specific aspect, the invention provides a method of treating a subterranean formation penetrated by a wellbore, comprising the steps of:_ providing a fluid suspension comprising a mixture of a particulate material and a fibrous material, wherein said fibrous material has a length of at least 2 mm and a diameter of about 3 to 200 microns; pumping the fluid suspension comprising the mixture of the particulate material and the fibrous material through the wellbore; and depositing the mixture of particulate material and fibrous material in the subterranean formation.

In a further specific aspect, the invention provides a fluid for treatment of a subterranean formation comprising a viscous liquid having an intimate mixture of a particulate material and a fibrous material suspending therein, wherein the fibrous material has a length of at least 2 mm and a diameter of about 3 to 200 microns.

In a still further aspect, the invention provides a method of treating a subterranean formation penetrated by a wellbore, comprising the steps of: providing a fluid suspension comprising a mixture of a particulate material and an alkaline resistant fibrous glass material; pumping the fluid suspension comprising the mixture of the particulate material and the alkaline resistant fibrous glass material through the wellbore; and depositing the mixture of the particulate material and the fibrous glass material in the subterranean formation.

This divisional application in some aspects provides:

In a subterranean formation penetrated by a wellbore, a porous pack comprising a particulate material having a size in the range of 10 to 100 U.S. mesh in intimate mixture with a fibrous material having a length of at least 2 mm and a diameter of about 3 to 200 microns, wherein the particulate material is a fracture proppant selected from the group consisting of sand, resin-coated sand, ceramic beads, glass microspheres, synthetic organic beads and sintered minerals.

A method of inhibiting flowback of proppant from a subterranean formation into a wellbore penetrating the subterranean formation, comprising the steps of: providing a fluid suspension comprising a mixture of a particulate material and a fibrous material; pumping the fluid suspension through the wellbore; and depositing the mixture of particulate material and fibrous material in the 6a subterranean formation, wherein the step of pumping is carried out at a pressure less than fracturing pressure for the subterranean formation and the step of depositing comprises depositing the particulate material and fibrous material mixture in a near-wellbore zone.
6b These and other aspects of the invention are accomplished througlh the manner and form of the present invention to be*.described hereinafter in conjunction with various examples of.preferred embodiments illiustrating the present invention. It will be understood that the examples presented are solely for the purpose of illustration and should not in any way be 1.0 construed as a limitation of the overall concept of the invention.

In the treatment of subterranean formations, it is common.to place particulate materials as a filter medium and/or a proppant in the near wellbore area and/or in fractures extending outwardly from the wellbore. In fracturing operations, proppant is carried into fractures created when hydraulic pressure is applied to -these subterranean rock formations to a point where .20 fractures are developed. Proppant suspended in a viscosified fracturing f2uid is then carried outwardly awaLy from the welibo.re within the fractures as they are created and extended with continued pumpinflr.
Ideally, upon release of pumping pressure, the proppant materials remain in the fractures holding the separated rock faces in an open position forming a channel for flow of formation.fluids back to the wellbore Proppant flowback is the transport of proppant sand back into the wellbore with the production of formation fluids following fracturing. This undesirable result causes undue wear on production equipment, the need for separation of solids from the produced fluids and occasionally also decreases the efficiency of the fracturing operation since the proppant does not remain within the fracture and may limit the size of the created flow channel.

Currently, the primary means for addressing the proppant flowback problem is to employ resin-coated proppants, resin consolidation of the proppant or forced closure techniques. The cost of resin-coated proppant is high, ranging from $.20 to $.40 per pound and is therefore used only as a tail-in in the last five to twenty percent of the proppant sand placement. Resin-coated proppant is not always effective since there is some difficulty in placing it uniformly within the fractures and, additionally, the resin coating can have a deleterious effect on fracture conductivity. Resin coated proppant also interacts chemically with common fracturing fluid crosslinking systems such as guar or hydroxypropyl guar with organo-metallics or borate.

This interaction results in altered crosslinking and/or break times for the fluids thereby affecting placement.
Additionally, these chemicals can dissolve the coating on the resin-coated proppant making their use ineffective despite their high cost.

In accordance with the present invention, the difficulties of using resin-coated proppants are overcome by incorporating an amount of fibrous material in intimate mixture with conventional proppants. The fibers act to bridge across constrictions and orifices in the proppant pack and to stabilize the proppant pack with no or minimal effect on proppant conductivity.
While not wishing to be limited by theory, it appears that the fibers are dispersed within the sand and, at the onset of sand production frorn the fracture, the fibers become concentrated into a mat or other three-dimensional frantework which holds the sand in place thereby limiting further proppant flowback with the fluid productiori.

As used in this specification, the term 'intimate mixture" will be understood to mean a substantially uniform dispersion of components in a mixture.

Similarly, the term "simultaneous mixture" will be understood to mean that the mixture components are blended in the iiiitial steps of the process, i.e., prior to pumping.

Fiber length, thickness, density and concentration are important variables in the success of preventing proppant flowback. In accordance with the invention, the fiber length ranges upwardly from about 2 millimeters and fiber diameter ranges of from about 3 to about 200 micron.s. There appear to be no upper limits on the length of the fibers employed from a stabilization standpoint. However, practical limitations of handling, mixing, and pumping equipment currently limit the practical use length of the fibers to about 100 millimeters. Fibrillated fibers can also be used and the diameters of the fibrils can be significantly sinaller than the aforementioned fiber diameters. The fiber level used in the proppant pack can range from 0.01% to 50% by weight of the proppant sand. More preferably, the fiber concentration ranges from 0.1% to 5.0$ by weight of proppant.

The modulus or stiffness of the fiber appears to be important in determining performance. Modulus is a measure of the resistance to deformation of a material and is a material property rather than a sample phenomena. Stiffness is a sample specific number which depends on both the material and its dimensions. As a general rule, fibers with a modulus of about 70 GN/sq. m or greater are preferred. This,includes materials like E-glass, S-glass, AR-glass, boron, aramicles and graphitized carbon fibers. Organic polymers other than the aramides usually have lower modulus values than this. Therefore, in order for organic polymers, such as nylon, to be useful in this application larger diameter fibers are required to provide equivalent performance to that of E-glass and stiffer materials.

In:the materials listed above, E-glass is a comnercially available grade of glass fibers optimized for electrical applications, S-glass is used for strength applications and AR-glass has improved alkali resistance. These terms are comnon in the glass fiber _ 1 -industry and compositions of these types of glass are universally understood.

A wide range of dimensions are useful. Length and diameter have been discussed above. An aspect ratio (ratio of length to diameter) in excess of 300 is preferred. The fiber can have a variety of shapes ranging from simple round or oval cross-sectional areas to more complex trilobe, figure eight, star shaped, rectangular cross-sectional areas or the like.

Most commonZy, straight fibers are used. Curved, crimped, spiral-shaped and other three dimensional fiber geometries are useful. Likewise, the fibers may be hooked on one or both ends. They may be of a composite structure, for example a glass fiber coated with resin to increase fiber-fiber adhesion.

The materials from which the fibers are formed is not a key variable provided that the fibers do not chemically interact with components of the well treatment fluids and are stable in the subterranean envirornment. Thus, the fibers can be of glass, ceramic, carbon, natural or synthetic polymers or metal filaments. Mixtures of these fibers may also be advantageously employed. Glass, carbon and synthetic polymers are preferred for their low cost and relative chemical stability. The density of the fibers used is preferably greater than one gram per cc to avoid separation by flotation in the fluid/particulate slurry.
Preferably, the fiber density is in the range of 1 to 4 grams per cc, closely mimicking the density of the particulate materials employed.

Glass fibers are particularly preferred due to their relatively low cost, easy availability and high stiffness. Because of the fact that placement fluids and subterranean formation fluids tend to have an alkaline pH, it is most preferred to use an alkaline resistant glass (hereinafter AR-glass) having a high zirconium conterit. The use of more common, commercially available silica glasses is possible within the scope of this invention but, the solubility of these glasses in an alkaline medium, particularly at elevated temperatures, may affect the long term stability of the fiber/proppant mixture over its lifetime in the wellbore.

A number of different proppants can be used in this invention. Sized sand and synthetic inorganic proppants are the most common. Examples include 40/60 sized sand, 20/40 sized sand, 16/20 sized sand, 12/20 sized sand and similarly sized ceramic proppants such as "CARBOLITETM"
proppants.

The proppant can be resin coated sand or ceramic proppant. Resin coated sand is used in some cases as a substitute for more expensive ceramic proppants because both are claimed to be more crush resistant than sand.
The addition of fibers would aid in the control of proppant flowback or serve the other purposes described herein.

The combination of resin coated sand and fibers .would provide a stronger pack than either system alone.
This may be useful in itself. In addition, the fibers could allow use of more highly precured resin coated proppants thereby minimizing the deleterious interaction of curable resin coated proppant with typical fracturing fluid components.

The preferred job execution practice is to mix the fibrous material throughout the entire batch of proppant to be pumped during the job. This may be accomplished by adding the fiber to the proppant before it is mixed with the fluid, adding the fiber to the fluid before it is mixed with the proppant or by adding a slurry of fibers at some other stage, preferably before the slurry is pumped downhole.

In certain cases, it may be preferred to pump the slurry of proppant and fiber only during a portion of the job, for example as the last 10-25% of the proppant into the fracture as a"tail-in" to control flow back in the most economical manner or for other reasons. The slug could also be pumped at other stages, for example to provide an absorbed scale inhibitor to be pumped to the front of the fracture.

In certain cases, it may be desired to pump small slugs of the slurry of proppant and fiber in between slugs of slurry of proppant or to pump small slugs of a slurry of fiber between slugs of proppant slurry. This could conceivably be used to control flow dynamics down the fracture, for example by providing more plugflow-like behavior. Pumping of small slugs of slurry of fiber as the tail-in is one example of this general procedure.

The slurry of a mixture of proppant and fibers is useful for various reasons in the entire range of reservoir applications from fracturing to sand control.
This especially includes the newer technologies of frac-and-sand-pack and high permeability stimulation. In these applications formation permeabilities are typically higher than those for classical fracturing, extending into the lOmd to 2 darcy range. As a result, the fractures are shorter (e.g. 10-200 ft) and wider (e.g. 1/2-2 inches) than classical fractures. Control of flowback of proppant on these types of jobs can reduce or elimir.Late the need for costly hardware such as gravel pack screens in the hole and simplify job design.

The selection of fiber can be based on chemical as well as physical. reasons. For example, in gravel packing and related applications where it is anticipated that the resulting pack-in-place will be treated with acid mixtures containing hydrofluoric acid, carbon fibers will be preferred over glass fibers when long life of the fibers is desired.

The opposite may also be desired. Use of carbon fibers through the first 90% or so of the job followed by glass fibers in the tail-in would result in a pack which could be treated with solutions of hydrofluoric acid to dissolve the glass, allow flowback of a small portion of the sand at the face of the fracture and improve well productivity. Pumping alternate slugs of proppant/fiber slurries containing the different fibers could be followed by treatment with acid to produce fracture with high permeability zones (where the glass fibers were) but with stable proppant/fiber pack zones (where the carbon fibers were) to keep the fracture open.

Beyond the advantages of avoiding proppant flowback, additional advantages have been noted in the use of fibrous niaterials in the well treatment fluid.

The presence of fibers in the fluid slows the settling rate of the solid materials in the fluid thereby permitting the use of lesser amounts of polymeric gelling material in the placement fluid. This feature offers the advantages of less cost, greater retained permeability, a need for lower concentrations of breaker and avoidance of chemical interaction with the treatment fluid components.

The fluid loss properties of the fibers are also available when fibers are incorporated into a proppant carrying fracturing fluid. In areas of high fluid loss, the fibers and sand will concentrate into a mat thereby limiting additional fluid loss in these =areas.
Fibers also offer an opportunity to place well treatment chemicals in a dispersed form within the proppant pack. Thus, porous or hollow or dissolvable fibers can be filled or formed with various materials such as polymer breakers, scale inhibitors, and/or paraffin and asphaltene inhibitors which can be slowly released within the pack.

The following examples will illustrate several formulations incorporating fibers. It will be understood that the presentation of these examples is solely for the purpose of illustrating the invention and should not be considered in any way a limitation on the scope or applicability of the concept of the present invention.

EXAMPLE 1 ( CONR'ROL) :

The Ieakc,ff rate of a borate-crosslinked guar fracturing fluid was determined in the following manner:
A fracturing fluid was prepared from synthetic seawater containing 30 lb/1000 gal of a polymer slurry, 1.0 gal/10o0 gal surfactant, 0.5 gal/1000 gal bactericide and 0.25 gal/1000 gal antifoaming agent. Approximately 2000 ml of this fluid was crosslinked with a borate crosslinking agrent, poured into a large baroid cell and heated to 200 F for 30 minutes. Using 1000 psi pressure, a fluid leakoff test was performed with a one inch sandstone core having a low permeability (0.8 millidarcy). Results are presented in Table A.

EXAMPLES 2-5:

In a manner similar to example 1, the behavior of fiber/fracturing fluid mixtures were determined. All tests were performed identical to example 1 but included 2.og of glass fibers- (1/2" long and 16 microns in diameter) that were added to the fluid prior to crosslinking. Other modifications to example 1 were:
EXAMPLE 2 contains 30 lb/1000 gal of a polymer slurry.

EXAMPLE 3 contains 25 lb/1000 gal of a polymer slurry.
EXAMPLE 4 was prepared using 2% KC1 tap water, 30 lb/1000 gal polymer slurry, 1.0 gal/1000 gal surfactant, 0.5 gal/1000 gal bactericide and 0.25 gal/1000 gal arntifoaming agent. No crosslinker was added to the system.

EXAMPLE 5 is identical to Example 3 but a sandstone core having a permeability of 100 millidarcy was used.

The data are presented in Table A. These data demonstrate that the fibers dramatically decrease the leakoff rate urider fracturing conditions.

TABLE A: IEIAROFF VOLUMES AS A FUNCTION OF TIM$
EX.1 EX. 2 EX. 3 EX. 4 SX. 5 1 min. 0.4 ial 0.3 ml 0.6 ml 0.8 ml 6.6 ml 4 min. 1.2 ml 0.6 ml 0.9 ml 1.0 ml 7.6 ml 9 min. 2.1 ml 0.6 ml 1.1 ml 1.7 ml 8.2 ml 16 min. 2.9 ml 0.6 mi 1.1 al 2.2 al 8.8 al min. 3.6 ml 0.6 ml 1.4 ml 2.7 ml 9.4 ml 36 min. 4.4 ml 0.6 ml 1.5 ml 3.1 ml 10.1 ml EXAXPLE 6: ( CorMRoL) The leakoff rate of a particulate carrying fluid was measured. The fluid contained tap water and 80 lb/1000 gal. of hydroxyethylcellulose. The particulate was a sized calcium carbonate (1-500 microns) which was added at a concentration of 0.5 lbs/gallon of fluid.
Approximately 250 mis of this fluid was blended and added to a large baroid fluid loss cell preheated to 175 F.
After 15 minutes, 500 psi of nitrogen pressure was applied to force the fluid against a one inch sandstone' core having a permeability of 250 millidarcy.

Results are presented in Table B.

MUMPLEs: 7-10 The tests were repeated using glass fibers alone and in combination with the calcium carbonate particulate material. The particle loading remained constant at 0.5 lbs/gallon of fluid. The fibers were added to the fluid at the time of the calcium carbonate addition. The fiber was added as a function of weight percent of the initial calcium carbonate material.

BXAMPLE 6: 100% Calcium Carbonate; 0ti Fiber EXAMPLE 7: 99% Calcium Carbonate; 1% Fiber EXAMPLE 8: 95% Calcium Carbonate; 5% Fiber EXAMPLE 9: 90$ Calcium Carbonate; 10% Fiber EXAMPLE 10: 0% Calcium Carbonate; 100% Fiber TABLE B LEAKOFF VOLUMES AS A FONCTION OF TIME
TIME EX. 6 EX. 7 EX. 8 EX. 9 EX. 10 1 min. 110 87 76 171 30-112 4 sain. 117 90 79 174 31 9 min. 118 93 81 175 31-1/2 16 min. 119 94 83 176 37 25 min. 118 94 83 176 37 36 min. 118 94 83 176 38 Example 10 (fibers alone) showed no miqgration into the core. Particulate systems (Example 6) always shov some migration into the core.

The data demonstrate superior leakoff control by the fibers. An additional advantage of fibers is no particulate migration into gravel pack or formation, therefore, less damaqe.

The following examples illustrate the ability of f-ibrillated fibers to stabilize proppant packs:

FsXAMPLE 11: (CONTROL): 200grams 20/40 mesh sand in 105 ml aqueous guar solution was poured into a 25 mm diameter glass column fitted with a bottom valve. Permeability of the pack was 380 darcies. The sand readily flowed through the 1/8 inch diameter valve when it was opened.
EXAMPLE 12: In a similar manner, the Fxample 11 was repeated but 2g polyacrylonitrile fibrillated fiber was mixed with the same slurry before it was poured into the column. The pack permeability was 120 darcies. The pack did not flow out when the valve was opened. It was also stable when the valve was completely removed leaving a 1/4 inch diameter hole directly under the sand pack.

This illustrates the ability of fibrillated fibers to consolidate a sand pack.

EXAMPLE 13: Fibers Stabilize Sand Pack: A 30 lb/1000 gallon uncrosslinked guar solution was made. The composition of this fluid was the same as in Example 1.

Fifty ml of this fluid were mixed with 0.s grams of 12ma long, 16 micron diameter glass fibers. They were mixed with a Hamilton Beach stirrer at low speed for 15 seconds. 100 grams of 20/40 proppant sand were added to the mixture and mixed by hand in a closed 4 oz. jar by gentle shaking. The resulting mixture was poured into a vertical glass column 12 mm in diameter with a"T"

section at the bottom. The left end of the "T" had a screen installed and the right end did not. First, water was flowed down the colu=nn and out the left side of the "T" to clean the guar from the sand/fiber and make a pack. The pernneability of the pack was then measured.
It was 278 darcies.

Next, the water flowed left to right across the "T" .
This washed the sand and fiber from the "T" section. The sand/fiber pack in the column section remained stable.

The water direction was then changed to flow down the column and out the right side of the "T". This created a pressure drop across the sand/fiber pack, and no screen prevented the sand from moving with the flow.
The pressure drop was increased (by increasing the flow rate) until the sand/fiber pack failed and flowed out of the vertical section of the column. The pressure drop across the sand/fiber pack recpired to do this xas in excess of 275 kPa (40 psi). Almost none of the sand in the sand/fiber pack flowed out of the vertical section of the column until the sand pack "failed."

EXAMPLE 14: A 30 lb/1000 uncrosslinked guar solution was mixed with the proppant sand (50 mi solution with 100 grams sand) following the same procedure as in Example 13 but WITHOUT the fiber. This mixture was put into the column and the guar was cleaned out of the sand pack in the same manner as in Example 13. The permeability of the sand pack was 250 darcfes. The sand pack failed under an unmeasurably low pressure.

These examples (13 and 14) illustrate that mixing fibers with the proppant sand caused the formation of a stable pack in the column. The fibers held the sand in place against a much higher force (pressure) than the sand without filbers. Also, the fibers had a negligible effect on the permeability of the sand pack.

EXAMPLE 15: Nylon Fibers: Fifty snl of a 30 lb/1000 gallon guar solution were mixed with 0.2 qrams of 20mm long, 64 micron dianeter, nylon polyamide fibers. The mixing was done in a similar manner to that of Example 13. This mixttzre was poured into the column and tested as described in Example 13. The permeability of the sand/fiber pack was 200 darcies. The sand/fiber pack failed at a drawdown pressure across the pack of 265 kPa.

EXAMPLE 16: Sand Pack Stabili$ation With High Viscosity Fluids: 1 gram of 32mm long, 16 micron diameter glass fiber was mixed with a solution of corn syrup and water having a viscosity of 600 centipoise. The mixing was done in a Hamilton Beach stirrer at low speed for 10 seconds. 100 grams of 20/40 proppant sand was then mixed with the fiber and solution. The 3aixture was poured into the column described in Example 13. In this case, the 600 centipoise corn syrup solution was flowed through the 15 column. The sand/fiber pack permeability was 352 darcies. The pressure drop across the sand/fiber pack was increased with the flow direction out of the right side of the "T" (no screen). The pressure drop across the sand pack was raised to 400 kPa without pack failure.

This example illustrates that the fibers cause the sand pack to be stable even with high viscosity fluids flowing through them. High viscosity fluids flowing through the sand would occur if a.guar gel was flowed back through the fracture during clean-up.

EXAMPLE 17: Settling: A 30 lb/1000 gallon guar/borate crosslinked gel was made. The composition was that of the guar solution in Example 13. 12 mm, 16 nicron diameter glass fibars (0.8 weight 4 of sand) and 20/40 proppant sand were added to a quantity of the gel such that the sand concentration was .10 ].b/gallon of gel. The sand and fiber were added to the guar solution prim to thi ;. gel cross<l:fnker soluti=.. The f iber van adds& Awl t3:e solution, arid dispersed wit3: a Haailton Beach ai:cer.
This was added to the sand in a closed j ar and gently mixed by shaking. The composition of the crosslinker solution was 0.3 grams boric acid, 0.6 grams sodiwa hydroxide, 1.2 grams sodium gluconate, 0.5 ml triethanol amine, and 0.6 grams sodiua thiosulfate for 500 m2 o!.
guar solution. The resulting mixture was placed in a heated closed column and further mixed by inverting the column once per minute. The mixture was heating to 66 degrees Celsius and,the column was oriented in the vertical. The mixture ran to the bottoa of the colmon.
The settling of the sand and fiber in the guar gel wera observed as a function of time at 66 degrees Celsius.
Percent settling was calculated as follows:

-t settling = 100 X (total height-sand height)/maximum liquid height.

Total height is the height of sand plus gel liquid.
Sand height is the height of the top of the sand layer. Maximum liquid height is determined with sand and water in the same amounts.

After 315 minutes the settling for the sand and fiber was 17%. There was no tendency of the sand and fibers to phase separate during the settling.

E7CAMPLE le : The experiment of Example 17 was repeated with 1.3% of tthe glass fiber based on the sand weight.
in this case, after 260 minutes the settling was 144.
FXAMPLE 19: The sand alone in the fluid of Example 17 settled 60$ in 300 minuteB. By comparison with Examples 17 and 18, this example shows that the glass fibers reduce the settling rate of the sand in the gel.

EXAMPI,E 20: Interaction with Borate Gel: Six liters of a 30 lb/1000 gallon uncrosslinked guar solution were mixed with 47.6 gram of 12 mm long, 16 micron diameter glass fibers. The fiber level was based on 8 lb/gallon sand loading. No sand was added to the fiber/solution mixture. The fiber/solution mixture was allowed to sit approximately one half hour after mixing. Two fifty ml samples were removed. The fibers were filtered from one of the fifty mi samples. The Fann35 viscosity of each sample was measured at 70 degrees F. The sample with fibers had viscosities of 51 and 30 cp. at 170 and 510 sec'1 rate respectively. The filtered sampls had = viscosities of 42 and 24 cp respectively. The viscosities of the filtered sample were well within specifications for this guar solution. The solution with fibers had a slightly higher viscosity.

Next borate crosslinkar solution (composition in Example 17) was added to both solutions. The time to gelling was measured for both by "hang lip" methods. The filtered solution had a "hang lip" time of 4 minutes, 44 seconds. The sample with fiber had a "hang lip" time of 4 minutes 27 seconds. Both these crosslinking times are within specifications for these guar gels.

This example illustrates that the preferred glass fibers do not affect the viscosity and the "hang lip" gel times of the borate crosslinked guar gel. This illustrates that the glass fibers do not affect the guar gel chemistry or viscosity significantly.

EXAMPLE 21: Interaction With Zirconate Gel: The same mixing procedure as in Example 20 was followed with a 50 lb/1000 gallon hydroxypropyl guar solution. The 12 nnt glass fibers were added to, then filtered out of one aliquot of the solution. This aliquot and another aliquot that had not been exposed to the fibers were crosslinked with a 4.5 lb/1000 gallon zirconium solution.
The solution was 40% zirconium crosslinker, 24% high temperature stabilizer, and 36% water. Crosslinking hang lip times were 9:19 minutes for the sample not exposed to the fibers, ani 10:13 minutes for the sample exposed to the fibers. Again, the fibers do not affect the crosslinked gel chemistry.

EXAMPLE 22: Conductivity. Conductivity testing was done with 20/40 mesh proppant. The fluid was a 30 lb/1000 gallon uncrosslj.nked guar solution. The composition was 17m1 of 2% KC1 water, 0.12 ml guar slurry, 0.02m1 fluorocarbon surfactant, and 0.005m1 defoamer. The fluid was mixed with 63 grams of 20/40 proppant. The test was done in a conductivity cell at 250 F and 5000 psi closure stress. The coriductivity after 23 hours of flowback was 157 darcies.

The test was repeated with the same quantities of fluid and proppant. In addition, 0.5 grams (0.8%) of 12 mm long, 16 micron diameter glass fibers were mixed with the proppant and fluid. The conductivity after 24 hours of flowback was 153 darcies.

This example illustrates that the fibers have a negligible effect on proppant pack permeability.
EXAMPIE 23: Slot Flow. The fiber/sand pack stability was tested in a slot geometry. 5 liters of 30 lb/1000 gallon uncrosslinked guar solution were made (34 ml guar slurry, 5 mi surfactant, and 1.25 ml defoamer and 5000 ml of tap water). This was mixed by recirculating the fluid through a holding tank and centrifuge pump for 15 minutes. 5000 grams of 20/40 sand was then added and allowed to disperse for appro3cimately 1 minute. 50 grams of 12 mm long, 16 micron diameter glass fiber were added to the mixture. The resulting slurry was pumped into the slot.

The slot is approximately 5-1/2 feet long, 1/4" wide and 6" high. The surfaces are smooth, with the front surface being clear to allow observation. A screen was placed over the exit port so that the sand could not flow out of the slot. The slurry was pumped into the slot from the other iand. In this geometry, a pack of sand and fibers built up against the scYeen, while the fluid was allowed to fiow through the screen to a holding tank. A
6" long sand/fiber pack was built up against the screen.

The guar fluid was then washed from the pack with water. The screen was removed from the end of the slot, leaving the pack with an open 1/4" x 6" face. Water was flowed through the pack to test its strength. The water flow was increased until a 6 psi pressure drop was .20 supported by the pack. At this point the pack began to fail and sand flowed out of the slot.

EXAMPLE 24: Slot Flow, Rough walls, Glass fibers: The same slurry as in Example 23 was again tested in the slot geometry. In this example, the walls of the slot were roughened. This was done by adhering a layer of 20/40 sand to the wa].ls of the slot with rubber cement. in this geometry, a 22" sand/fiber pack was obtained and the strength of the pack exceeded 15 psi drawdown (upper limit on pump).

EXAMPLE 25: Slot With Gas Flow: A similar slurry as used in Example 23 was used in this example. In this example we used a 10 lb/3000 gallon guar solution. This slurry was pumped into the slot with rough walls and the screen as descrjlbed in Example 24. The guar solution was washed from the sand/fiber pack with water. Then the pack was dried with air flowing through it for 3-1/2 hours. The screen was removed and the test for pack strength was performed. The pack length was 18". The air flow rate was increased to 13 psi drawdown across the pack. The pack did not fail.

The pack was then further dried at low air flow rate for an additional two hours. The test was repeated. The sand/fiber pack did not fail with flow up to an 11 psi drawdown across the pack.

This example illustrates that the sand/fiber pack is resistant to gas flows as well as water flows.

EXAMPLE 26: Slot Flow With 1/2" aramide fibers:
'XCVLARTM" polyaramide fibers were tested in the slot geometry with rough walls. The fluid was a 20 lb/1000 gallon uncrosslinked guar solution similar to Example 23.

The aramide fibers were 12 mm long and 12 microns in diameter. The slurry mixture was 4 liters of fluid, 4 kg of 20/40 proppant sand, and 12 grams of "R~LM fiber (0.3 wt. % of sand).

The sand/fiber slurry was pumped into the rough walled slot with the screen at one end as was described in Examples 23 and 24. The resulting sand pack was 14.5"
long. The fluid was washed from the sand fiber pack with water. The screen was removed and the water was again flowed through the pack. The pack began to fail at 3 psi drawdown.

EXAMPLE 27: Slot Flow, 1" Nylon Fibers: We tested 1"
long nylon fibers in the rough walled slot. The fibers were 64 microns in diameter. The slurry was 5 liters of 30 lb/1000 gallon uncrosslinked guar solution, 5 Itg of 20/40 proppant sand, and 15 grams of nylon fiber. The sand/fiber pack length was 6". The pack began to fail at less than 1 psi drawdown.

Examples 23-27 illustrate that fibers stabilize a proppant pack iin a fracturing geometry even with smooth walls and no closure stress.

EXAMPLE 28: Slot Flow: The fiber sand pack strength was tested. A 30 lb/1000 gallon uncrosslinked quar solution with the same composition as Example 23 except that 2*
RC1 water was used. 20/40 proppant was added to the fluid at 12 pounds per gallon. 12 mm long, 16 micron diameter glass fibers were also added at i; of the proppant level.

The slurry was loaded into a 5-1/4" by 5-1/4" by 1/4" slot. The walls of the slot were lined with Springwall sandstone. A closure stress of 250psi was applied. The cell was heated to 210 F. The fluid was washed from the gel with a 1t RC1 solution flowing at a slow rate (30m1/min). The brine was then washed from the . Y.. ... ~ .,,o-.,.. .- -- ~. CA 02497728 2005-11-08 cell with a saturated nitrogen gas;f.low. The cell was then heated to 225 F. The test was not performed with the nitrogen flow at increasing drawdown across the pack.
The pack was stable at 20 psi/ft. with a closure stress ranging from 100 to 200 psi.

EXAMPLE 29: Slot Flow, NO FIBERS: The same experiment as in Example 28 was performed with proppant without fibers.
At 250 psi closure stress, 1/4" slot, 225 F, the proppant pack failed at less than 0.2 psi/ft.

These examples demonstrate the ability of fibers to stabilize a proppant pack under representative downhole conditions.

EXAMPLE 30: Yard Test: The glass fibers were tested in a yard test. The 12 ma long, 16 micron diameter glass fibers were added at a 1* level to the sand in a simulated fracture job. The fibers were added by hand into a fracturing fluid blender with the 20/40 proppant.
This mixture was combined with the 30 lb/1000 gallon crosslinked fracturing fluid in the blender. It then flowed through a triplex pump, a tree saver, a variable choke with 1000 psi drawdown, and 300 yards of 3 inch pipe.

The pumpiizg schedule was:

1 ppg proppant at 6bbls/min.
1.5 ppq proppant at 6 bbls/min.
2 ppg proppant at 6 bbls/min.
3 ppg proppant at 8 bbls/min.
4 ppg proppant at 8 bble/min.

Samples of the mixture were taken at the exit of the pipe. The glass fibers were well mixed with the proppant and fluid, although some fiber breakage was apparent.

The example demonstrates that fiber/sand slurries can be pumped with conventional pumping equipment and that the fibers are stable enough to survive this treatment.

EXAMPLL 31: Perforation Packing: The ability of fibers to keep sand in a reservoir over a 1/40 perforation vas tested. A model perforation 1/4" in diameter and 3" long with'a 75 cubic inch reservoir at the outlet was used for the tests. The reservoir was equipped with a 20 mesh screen at the other side from the perforation. Slurry could then flow into the reservoir through the perforation and out through the screen. 4.5 L of a 20 ibm/1000 gal/hydroxyethyl cellulose (EEC) solution was prepared (135 g NH4C1 (3 wtt), 28.3 mL HEC solution and dry caustic to raise the pH to 8) . This was mixed by recirculating the fluid through a holding tank and a centrifuge pump. The fluid was hydrated for ca. 30 min.
13.5 g Aramide staple, 1/2" long, was mixed in and 28696.5 q 20/40 sand were added to the mixture (5 iwgal proppant, 0.5 wt% fiber based on the proppant). The resulting slurry was pumped into the reservoir through the 1/4" perforation. A pack of sand and fibers built up against the screen, while the fluid was allowed to flow through the screen into the holding tank.

After packing the performation, the lines, the holding tank and the pump were cleaned and filled with water. The flow direction, was reversed and water was pumped from the screen side through the packed perforation. No proppant was produced through the 1/49 hole even by increasing the flow rate till a pressure drop across the pack of 15 psi was reached and kept for several minutes. The water flow was turned off and on several times. That did not produce sand either.

EXAMPLE 32: The same perforation was packed with 20/40 sand and 12 mm long, 16 micron diameter glass fiber using a 30 ibm/1000 gallon uncrosslinked guar solution. 4.5 L
fluid were prepared (9 0 q RCl (2 wtt), 4. 5 mL surfactant, 1.125 mL defoamer, 30.6 aL guar slurry) and hydrated for 30 min. 27 g glass fiber were added and after one minute 2,700 g 20/40 proppant (5 ibm/gal, 1 wt* fiber based on proppant). The packing and water flow were done as in Example 31.

The packed perforation was kept for 10 days. Within this time water was flown through it ca. 5 times, each time turning the pump on and off several times. The pack was stable and produced one teaspoon proppant at the most.

EXAMPLE 33: The same setup as in Example 31 except for a 1/2" perforation. This time polypropylene fibers (1/2"
long, 4 denier) and 30 ibm/1000 gal HEC were used.

Fluid: 4.5 L. 135 g NI34C1, 42.5 aL HEC solution, caustic to raise the pH to s.

Proppant: 2,696.5 g 20/40 sand (5 ibm/gal) Fiber: 27 q polypropylene, 1/2" long, 4 denier (1 wtt based on proppant) Packing and flowing water through worked well, no sand production even over 1/2" hole.

Examples 31 through 33 illustrate that different types of fibers may be used to hold sand in place in the formation beyond the perforation tunnels. This is applicable to gravel packing, where gravel is placed outside of the perforations to stabilize subterranean formation sands.

EXAMPLE 34: Stabilization of Different Types of Proppant: Column experiments were performed using the fluid compositiori (30 lb/1000 gallon guar solution), and procedure as in Example 13. 50 ml aliquots of fluid were mixed with 100 grams each of various proppants and 1 gram (or 1.6 grams) each of 12 mm long, 16b micron diameter glass fibers. The proppants were 20/40 "CARBOLITE7'M", 20/40 "ACFRAC SB ULTRATM" curable resin coated sand, and 20/40 "ISOPACTM" light weight gravel.

The "CARBOLITE" proppant has approximately the same density as sand, but is more spherical. The "SB ULTRA' has approximately the same density and sphericity as sand, but has a polymer coating. The "ISOPAC' -- - ----~-lightweight gravel i-s'much less dense than sand, is more spherical, and has a polymer coating.

The results of the column tests are shown in Table C.

TABLE C. Strengths of Various Glass Fiber/Proppant Packs Fiber Level CARBOLITE SB ULTRA ISOPAC
St. % sand 2t >225 kPa >250 kPa 55 kPa 1.6$ >250 kPa >250 kPa Examples 13 and 34 illustrate that the'coating and sphericity of the proppant do not affect the ability of the fiber to strengthen the pack. Low density proppants ("ISOPAC") may require greater amounts of fiber for pack strength.

EXAMPLE 35: The procedure of Example 31 was repeated except that the pack was made in such a way that the half of the perforation model closest to the perforation hole was filled with an identical sand/fiber.mixture while the back half of the perforation was filled with sand. The pack was tested in the same way. No sand was produced.

~... _ This example demonstrates that the proppant/fiber slurry may be used as a tail-in or pumped in stages between slugs of proppant slurry.

EXAMPLE 36: Proppant/fiber pack strength tests were performed in a rectangular cell with inside dimensions of 12.7 cm long, 3.8 cm wide, and..2.5 cm thick. The cell was open at both ends. A perforation type geametry was set up in the cell by creating a restriction 0.63 cm on all inside dimensions. The cell was set up with a screen at the outlet. A slurry containing 500 ml o.f 30 lb/1000 gallon guar solution in water (composition in Example 1), 500 grams or 20/40 sand, and water (composition in Example 1), 500 grarns of 20/40 sand, and 1.25 grams of 7 micron by 0.63 cm carbon fiber was pumped into the cell and formed a pack against the exit screen. The guar was washed from the pack and then the screen was removed from the exit port. A closure stress of 500 psi was applied to the face of the pack. Water was flowed from the inlet to outlet through the length of the pack. The proppant and carbon fiber pack resisted the flow of water up to 35 kPa (about 5 psi) before the pack filed and flowed through the restriction.

EXAMPLE 37: The same test as above was performed by 5 grams of AR grade glass fibers (20 micron diameter, 1.27 cm long) were added to the sand and carbon fiber sluriy.

The resulting pack held a drawdown of 135 kPa (about 18 psi) without failing.

EXAMPLE 38: The same test as above was performed with a slurry of 500 ml 30 lb/1000 gallon guar solution, 500 grams 20/40 sand, and 5 grams AR grade glass fibers (20 micron diameter, 1.27 cm long). The pack failed at a drawdown of 36 kPa (5 psi).

EXAMPLE 39: The same test as above was performed with a slurry of 500 ml 30 lb/1000 gallon guar solution, and 50 grams 20/40 sand without fiber added. The pack failed immediately with the onset of water flow, and no measurable pressure drop was maintained across the pack.

EXAMPLES 36 - 39 show that carbon fibers can be used to stabilize the pack and that mixtures of fiber can result in stronger packs than a single fiber type.

EXAMPLE 40: Proppant fiber pack strength tests were performed in a disk shaped cell. The diameter of the disk is 15.2 cm, and the thickness is 1.2f/-0.05 cm.
the cell has inlet and output openings 10.2 cm across.

A screen was placed across the outlet. A slurry containing 1000 inl of 50 lb/ 1000 gallon guar solution, 1000 grams of proppant, 15 grams of AR glass fibers (20 micron diameter, 12.7 mm long) was pumped into the cell and formed a pack against the screen. In each test the proppant size was varied. The guar was washed from the pack, and then the screen was removed. Closure stress of 1000 psi was applied to the faces of the disk. The excess pack was cleaned from the cell, so that the pack was perpendicular to the flow direction from inlet and outlet. This resulted in a pack length from inlet to outlet of 11.4 cm. Water was then flowed through the pack until it failed and proppant flowed out of the cell. This coincided with a relaxation of the closure stress.

PROPPANT PACK STRENGTH
20/40 60 kPa (8.5 psi) 12/20 21 kPa (3 psi) 16/30 21 kPa (3 psi) The same procedure was followed as in example 40 except that no fiber was added to the 20/4.0 sand pack.
The pack failed at the onset of water flow' and no pressure drawdown was maintained.

The results show that the fibers will strengthen different proppant sizes.

EXAMPLE 41: 500 ml of a 50 pound per 1000 gallon borate crosslinked guar gel were prepared. The gel contained 3 grams guar, 10 grains potassium chloride 0.5 mL

surfactant, 0.25 mL bactericide, 0.125 mL antifoata agent, 0.5 mL stabilizer (iron control), 0.6 grams, oxygen scavenger, 0.6 grams boric acid, 1.5 grams sodium hydroxide, and 3 grams sodivm gluconate. 500 grams of 20/40 US mesh brady sand and 7.5 grams AR grade glass fiber (20 microns diameter, 12.7 nun length) were mixed into the gel.

The:resulting slurry was poured into a metal tube 22.1 uan inside diameter, aad 127 uun in length. The ends of the tube were capped, and it was then heated to l50 C
for 24 hours. These conditions were sufficient to "break" the gel. The tube was cooled, opened and a washer with 12.7 mm hole was fitted into one end of the tube. The tube was connected to a water source such that the washer was at the outlet end of the tube.
Effectively the slurry mixture was held from sliding out of the tube by the washer, but water could flow through the slurry sand pack.

The water flow was initiated at a low flow rate to wash the broken gel from the sand pack. No sand flowed out the tube with the water. The water flow rate was then increased. No sand flowed until the flow rate reached 7.6 L/min. which corresponded to 381 kPa drawdown across the pack. At this point the sand pack failed and ran out of the tubing through the washer.

EXAMPLE 42: The same experiment as above was run with the crosslinked gel and sand, but without the AR glass fibers. The sand pack flowed out of the tube through the washer at very low flow rate during the cleaning of the broken gel from the pack.

While the invention has been described in the more limited aspects of preferred embodiments hereof, including numerous examples, other embodiments have been suggested and still others will occur to those skilled in the art upon a reading and understanding of the foregoing specification. It is intended that all such embodiments be included within the scope of this invention as limited only by the appended claims.

Claims (12)

CLAIMS:

1. In a subterranean formation penetrated by a wellbore, a porous pack comprising a particulate material having a size in the range of 10 to 100 U.S. mesh in intimate mixture with a fibrous material having a length of at least 2 mm and a diameter of about 3 to 200 microns, wherein the particulate material is a fracture proppant selected from the group consisting of sand, resin-coated sand, ceramic beads, glass microspheres, synthetic organic beads and sintered minerals.

2. The porous pack as defined in claim 1, wherein the fibrous material is selected from the group consisting of natural organic fibers, synthetic organic fibers, glass fibers, carbon fibers, ceramic fibers, inorganic fibers, metal fibers and mixtures thereof.

3. The porous pack as defined in claim 1 or 2, wherein the pack is located adjacent the wellbore.

4. The porous pack as defined in claim 1 or 2, wherein the pack is located in fractures extending radially outwardly from the wellbore.

5. The porous pack as defined in any one of claims 1 to 4, wherein the fibrous material comprises hollow fibers having a well treatment chemical contained within an interior volume of the fibrous material.

6. The porous pack as defined in claim 5, wherein said well treatment chemical is selected from the group consisting of polymer breakers, corrosion inhibitors, scale inhibitors and chemical tracers.

7. A method of inhibiting flowback of proppant from a subterranean formation into a wellbore penetrating the subterranean formation, comprising the steps of: providing a fluid suspension comprising a mixture of a particulate material and a fibrous material; pumping the fluid suspension through the wellbore; and depositing the mixture of particulate material and fibrous material in the subterranean formation, wherein the step of pumping is carried out at a pressure less than fracturing pressure for the subterranean formation and the step of depositing comprises depositing the particulate material and fibrous material mixture in a near-wellbore zone.

8. The method as set forth in claim 7, wherein the step of pumping is carried out at a pressure in excess of fracturing pressure for the subterranean formation and the step of depositing comprises depositing the particulate material and fibrous material mixture in fractures created in the subterranean formation during the step of pumping.

9. The method as set forth in claim 7 or 8, wherein the fluid suspension is a mixture of sand and a fibrous material selected from the group consisting of natural organic fibers, synthetic organic fibers, glass fibers, carbon fibers, ceramic fibers, inorganic fibers, metal filaments and mixtures thereof.

10. The method as set forth in claim 9, wherein the fluid suspension is a mixture of sand and an alkaline resistant fibrous glass material.

11. The method as set forth in claim 9, wherein the fluid suspension is a mixture of sand and carbon fibers.

12. The method as set forth in claim 9, wherein the fluid suspension is a mixture of sand, alkaline resistant glass fibers and carbon fibers.