Enhanced proppant transport for hydraulic fracturing

Abstract

The present disclosure relates to compositions and methods for recovery of hydrocarbons from subterranean formations. The compositions may be dry blends of synthetic and naturally derived polymers. The blend compositions may also be produced as high activity solvent-based fluidized polymer suspensions. Either in dry or liquid forms, the blend compositions provide higher proppant carrying capacity in comparison to conventional solutions, as well as improved breakability and crosslinking capacity.

Claims

1. A composition comprising: mineral oil, from 10-90% by weight of polyacrylamide; and from 10-90% by weight of guar.

2. The composition of claim 1, wherein the polyacrylamide is a dry powder.

3. The composition of claim 2, wherein the dry powder has a particle size below about 500 μm.

4. The composition of claim 1, wherein the guar has a particle size below about 500 μm.

5. The composition of claim 1, wherein the composition comprises about 25% to about 75% by weight of the polyacrylamide.

6. The composition of claim 1, wherein the composition comprises about 40% to about 70% by weight of the polyacrylamide.

7. The composition of claim 1, wherein the guar comprises a minimum solution viscosity of about 3,500 cP at 1%.

8. The composition of claim 1, wherein the polyacrylamide has a weight average molecular weight of at least about 2,000,000.

9. The composition of claim 1, further comprising a liquid carrier.

10. The composition of claim 9, wherein the liquid carrier comprises a glycol.

11. The composition of claim 1, further comprising a friction reducer.

12. The composition of claim 11, wherein the friction reducer is a water-soluble polymer.

13. The composition of claim 12, wherein the water-soluble polymer is selected from the group consisting of polyacrylamide, polyethylene oxide and any combination thereof.

14. The composition of claim 9, further comprising a cross-linking agent.

15. The composition of claim 9, further comprising an organophilic clay.

16. The composition of claim 9, further comprising a surfactant.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1 is a graph comparing polymer shear sweeps for suspensions 1-3 and control 3 at atmospheric pressure and temperature ranging from ambient to 180° F.

(2) FIG. 2 is a graph comparing friction reduction for suspensions 1 and 3, control 1 and guar in tap water.

(3) FIG. 3 is a graph comparing crosslinked viscosity for suspension 1, control 1 and control 3 at ambient temperature.

DETAILED DESCRIPTION OF THE INVENTION

(4) The fracturing fluid may contain one or more types of proppant. Suitable proppants include those conventionally known in the art including quartz, sand grains, glass beads, aluminum pellets, ceramics, resin coated ceramics, plastic beads, nylon beads or pellets, resin coated sands, sintered bauxite and resin-coated sintered bauxite.

(5) The composition of the present invention includes at least one naturally-derived polymer. Naturally-derived polymers occur in nature and can be extracted or chemically modified to improve functionality. They are often water-based polysaccharides. In one embodiment, the naturally derived polymer may be cellulose or starch, or guar and derivatives thereof. In one embodiment, the naturally derived polymer is a guar source. In accordance with the principles of the invention, the guar source may include any grade of guar gum. In an aspect, the guar source may be guar pod harvested from the endosperm of leguminous seeds. Typically the guar source may be the endosperm, also called the guar split, which constitutes approximately 30-40% of the seed. The guar source may further be derived from the remainder of the seed, referred to as the hull (approximately 15%) and the inner germ (approximately 45%). For instance, the guar source may be the refined guar split, which is the polished fibrous layers that are removed from the husk. The guar source may further be guar gum that is produced from refined guar split by softening, flaking, pulverizing and sieving.

(6) In an aspect, guar is in a powder form. Typically, powders having a size of between about 60 mesh and about 400 mesh, more typically between about 100 to 325 mesh. The guar may have a particle size below 500 μm (micron), preferably below 300 μm and most preferably below 200 μm.

(7) Suitable guar derivatives include carboxyalkyl guars and hydroxyalkyl guars. Preferred are carboxymethyl guar, hydroxypropyl guar, hydroxyethyl guar, hydroxybutyl guar and carboxymethylhydryoxypropyl guar. Preferably the hydroxyalkylated guar has a molecular weight of about 1 to about 3 million. In an aspect, the degree of substitution of the carboxylated guar is typically between from about 0.08 to about 0.18. In an aspect, hydroxypropyl content of the hydroxyalkylated guar is typically between from about 0.2 to about 0.6.

(8) The carboxyalkyl guar can be obtained in many ways, including a) using premium grade guar as the starting material to which the anionic groups are chemically added; and/or b) selecting processing parameters that provide better uniformity in placing the anionic substituent on the guar polymer backbone; and/or c) additional processing steps, including ultrawashing to remove impurities and refine the polymer. Preferred polymers include those guars having randomly distributed carboxymethyl groups.

(9) Guar derivatives may also be suitable for the dry compositions of the present invention. In an embodiment, guar may be chemically modified to produce guar derivatives such as hydroxypropyl guar, carboxymethyl guar, carboxymethyl Hydroxypropyl guar and cationic guar.

(10) Naturally-derived cellulosic derivatives suitable for use include carboxymethyl cellulose, hydroxyethyl cellulose and carboxymethyl hydroxyethyl cellulose; while naturally-derived starch derivatives suitable for use include carboxymethyl starch and hydroxyethyl starch.

(11) Without limitation, useful polysaccharides for the practice of this invention may have average molecular weights typically in the range of from about 200,000 to about 3,000,000.

(12) In another aspect, the preferred guar, guar derivatives and cellulose derivatives have a solution viscosity of at least 3,500 cP (centipoise); preferably from about 4,000 to about 5,000 cP and most preferably higher than 5,000 cP at 1%.

(13) The composition of the present invention includes at least one synthetic friction reducer. The synthetic friction reducer may be a water-dispersable acrylamide polymer or polyethylene oxide. It has been discovered that the acrylamide polymer enhances a fluid's high temperature endurance. The acrylamide polymer may be a homopolymer or a copolymer of acrylamide monomers with one or more different monomers. As used herein, the term homopolymer is meant to encompass polymers having less than about 0.1% by weight of any other monomers. The acrylamide homopolymer is a non-ionic polymer made of acrylamide monomers. With respect to the acrylamide copolymers, the other non-acrylamide monomers may be selected to provide the acrylamide polymer ionic properties. For example, in the acrylamide-acrylate copolymer, the acrylate segments are anionic. Examples of suitable non-acrylamide monomers include acrylate monomers, such as sodium acrylate, potassium acrylate and ammonium acrylate. Examples of acrylamide copolymers also include acrylamido methylpropane sulfonate (AMPS)-acrylamide copolymer. The copolymers may be block or random copolymers. The non-acrylamide monomers may makeup from about 0.1% to up to about 50% or more of the copolymer, more particularly from about 5% to about 15%. Acrylamide-acrylate copolymers may also be formed by hydrolysis of an acrylamide homopolymer typically conducted with heat under alkaline reaction conditions. As used herein, the expression “polyacrylamide” or “acrylamide polymer” are meant to include both acrylamide homopolymers and copolymers of acrylamide with other monomers unless stated otherwise or as is apparent from the context.

(14) The polyacrylamide may have a weight average molecular weight of from greater than about 2 million; preferably greater than about 5 million; and more preferably greater than about 15 million. The polyacrylamide may be used in the fluids of the invention in an amount of from about 0.1% to about 5% by weight of the fluid. In certain applications, the polyacrylamide may be used in an amount of from about 0.03% to about 0.4% by weight of the fluid. The polyacrylamide may be added in liquid form, such as dispersed in mineral oil, glycol, water, or other carrier. The polyacrylamide may also be added in solid or particulate form.

(15) In an embodiment, the synthetic friction reducer is polyethylene oxide. Polyethylene oxide (PEO) is a straight-chained, high molecular weight polymer. The weight average molecular weight of the polyethylene oxide is between from about 1 million to about 20 million, more preferably between from about 2 million to about 10 million. Typically the amount of PEO in the fracturing fluid is between from about 10 ppm to about 400 ppm, more typically between from about 20 ppm to about 100 ppm.

(16) Typically the weight ratio of naturally-derived polymer to synthetic friction reducer in the dry composition is between from about 3:1 to about 1:3; more typically between from about 2:1 to about 1:1.

(17) Previous attempts to combine guar or guar slurry with standard polyacrylamide emulsions have not produced satisfying results. Conventionally, the guar polymer would hydrate in the emulsion package of polyacrylamide and form a pasty mixture; or attempts to blend guar powder with standard granular polyacrylamide friction reducer were unsuccessful because the particle size and density difference between the naturally derived polymer and synthetic friction reducer did not produce a homogeneous blend. Instead, separation of the components occurred.

(18) By contrast, it has been discovered that when polyacrylamide is ground to a given particle size distribution, below 500 μm, preferably below 300 μm and most preferably below 200 μm; homogeneity issues are resolved. The dry composition of the present invention can be put into suspension at very high solids content with no hydration issues.

(19) One advantage of the composition is that it may be more easily stored and transported to a well site. In addition, preparation of the fracturing fluid is simplified, as the dry composition delivers two constituents in a single form, thereby reducing the number of additive streams into a fracturing fluid. Furthermore, the number of operations on location is reduced due to the reduced number of streams. The dry composition also provides a reduction in volume and weight of the treatment as compared to additives in liquid form.

(20) Another advantage of the dry composition is stability. It is not subject to freezing, thereby facilitating use in colder climates. In addition, the concentration of the components in the dry composition will not change due to evaporation of solvent, which is particularly beneficial for oilfields in warmer climates.

(21) Still another advantage of the dry composition is improved activity in terms of crosslinkability, proppant loading, viscosity and breakability. Crosslinking activity, particularly for zirconium and titanium crosslinkers, has been shown to decrease with time in solution. The dry composition of the present invention reduces the time that the crosslinker is in solution prior to being combined with the two polymers, thereby maintaining a higher and more consistent level of activity.

(22) A cross-linking agent may be used with the fluids. The cross-linking agents used include boron or Group IV transition metal compound cross-linking agents. The cross-linking agent may include zirconium, titanium and hafnium cross-linking agents and combinations thereof and may include organometallic compounds. In particular, boron, organo-zirconium and titanium crosslinking agents are useful. The cross-linking agent may be included in the fluid in an amount of from about 0.1% to about 1.5% by weight of the fluid, more particularly from about 0.01% to about 1.5% by weight of the fluid, more particularly, from about 0.02% to about 0.3% by weight of the fluid.

(23) The fracturing fluid may also contain other conventional additives common to the well service industry such as corrosion inhibitors, surfactants, demulsifying agents, scale inhibitors, asphaltene inhibitors, paraffin inhibitors, gas hydrate inhibitors, dispersants, oxygen scavengers, biocides and the like.

(24) Suitable surfactants will act as surface active agents and function as emulsifiers, dispersants, foamers or defoamers. In some embodiments of the invention, the surfactant is an anionic surfactant. Examples of suitable anionic surfactants include, but are not limited to, anionic surfactants such as alkyl carboxylates, alkyl ether carboxylates, alkyl sulfates, alkyl ether sulfates, alkyl sulfonates, alpha olefin sulfonates, alkyl phosphates and alkyl ether phosphates. Examples of suitable anionic surfactants also include, but are not limited to, cationic surfactants such as alkyl amines, alkyl diamines, alkyl ether amines, alkyl quaternary ammonium, dialkyl quaternary ammonium and ester quarternary ammonium compounds. Examples of suitable ionic surfactants also include, but are not limited to, surfactants that are usually regarded as zwitterionic surfactants and in some cases as amphoteric surfactants such as alkyl betaines, alkyl amido betaines, alkyl imidazolines, alkyl amine oxides and alkyl quarternary ammonium carboxylates.

(25) The compositions of the present invention are suitable for use in fresh water, brackish water and hard brine environments.

(26) Conventionally, proppant loading for slickwater treatment is only up to 2 ppa (pounds of proppant added). However, with the present invention, a proppant loading of at least about 3 ppa can be achieved. In an aspect, the amount of proppant in the fracturing fluid may even be greater than about 4 ppa.

(27) Moreover, in conventional slickwater treatments, the liquid suspension made from the dry composition of the present invention for treating the wellbore has a viscosity only up to about 12 cPs at 4 pptg (pounds per thousand gallons) polymer loading. With the composition of the present invention, viscosity of greater than about 13 cP can be obtained. In an aspect, viscosity greater than about 15 cP can be obtained. The increase in viscosity is desirable for providing higher carrying/suspending capacity. The higher the viscosity, the more proppant can be suspended in the liquid, particularly under laminar flow conditions.

Example 1

(28) It was discovered that stable high polymer active content suspensions in mineral oil could be prepared with combined polymers to deliver multifunctional properties. The suspension composition of this invention consists of benzene, toluene and xylene-free mineral oil, organophilic clay, surfactant and a combination of powdered guar gum and polyacrylamide polymers. Table 1 provides the compositions of the suspensions tested.

(29) TABLE-US-00001 TABLE 1 Detailed Suspensions Composition Ingredients Suspension 1 Suspension 2 Suspension 3 Mineral Oil 45.5 45.5 45.5 Organophilic clay 2 2 2 Surfactant 1 0.5 0.5 0.5 Surfactant 2 2 2 2 Dry fine powder 25 30 37.5 Polyacrylamide Guar gum 25 20 12.5

(30) The suspension examples were prepared with SHELLSOL D80 mineral oil available from Shell Chemicals; surfactant 1 (TDA-9, C-13 alcohol ethoxylate available from Sasol); surfactant 2 (LUMISORB, a sorbitan sesqueoleate available from Lambent Technologies): organophilic clay (CCOC 882, available from Imperial Group); dry polyacrylamide (FLOJET AN943VHV, available from SNF); and guar (35-45 cPs grade). The suspension is prepared by adding the organophilic clay into the oil while mixing, followed by addition of the surfactants. Then the polymer blend of guar and polyacrylamide is added to the mixture and homogenized.

(31) The hydration profile of the various suspensions was assessed by means of “linear gel viscosity at 3 min mark after the polymer addition”, corresponding to the hydration of the polymer after mixing in a WARING blender for 2.5 minutes at 1500 rpm and stabilization on a Grace 3600 viscometer set at 300 rpm for 30 seconds. The evaluation was conducted in tap water at 4.0 gptg (gallons per thousand gallons) polymer and viscosity measured at 3 minute mark after the initial addition (0.5 minutes following the 2.5 minutes of mixing). The hydration profiles are illustrated by the viscosity data in Table 2.

Example 2

(32) For this evaluation, suspension examples representative of the composition of this invention were compared to commercially available polyacrylamide emulsions-based friction reducers.

(33) Data in Table 2 indicates clearly that the suspensions of this invention synthesized as described in Example 1 provide much higher linear gel viscosity than conventional compositions. Controls 1-3 are anionic polyacrylamide based friction reducers commercially available. Furthermore, the suspensions of this invention do not require any particular hydration equipment to achieve such high viscosities. The highest viscosity is achieved with a combination of 50/50 of guar/polyacrylamide combination. Suspensions 1 and 3 were evaluated for their friction reduction capacity. Results in FIG. 2 also show that fast hydration and good friction reduction levels were achieved with lower loadings (0.3 gptg) vs current solutions at 0.5 gptg. As expected, the low loading (0.30 gptg) is causing an increase in shear out, the suspension 1 is much better for both inversion/hydration rate and max percent of friction reduction.

(34) TABLE-US-00002 TABLE 2 Comparative Linear Gel Viscosity Profile in Tapwater Suspen- Suspen- Suspen- Control Control Control Ingredients sion 1 sion 2 sion 3 1 2 3 3 min 17 16.4 14.7 11.9 9.2 10.1 apparent viscosity @ 511 s.sup.−1, cP (4 gptg, 2.5 min hydration @ 1500 rpm)

Example 3a

(35) The comparative samples were further tested for their crosslinking capacity. The gel solutions of Table 2 were crosslinked with 2 gptg BXL-411 instant and self-buffered boron crosslinker, available from Univar Inc.

(36) The gels made with the suspensions 1-3 of this invention produced strong and lipping gel, while gels with controls 1-3 did not crosslink. Results in FIG. 3 show comparative crosslinked viscosities at ambient temperature on Grace 5600 HPHT viscometer. It is clear that superior crosslinking capacity of suspension 1 in comparison to controls 1 and 3, conventional synthetic friction reducers. The controls did not crosslink, while suspension 1 achieves initial viscosity of about 200 cP, which remained above 100 cP after one hour. This indicates that this invention provides multiple functionalities that would allow enhanced proppant transport and placement.

(37) Furthermore, after hydration, the gel solutions (4 pptg of suspensions 1-3) were subject to API shear sweeps (method RP 39). After a baseline viscosity was observed for 10 minutes at ambient temperature, a shear sweep was performed, followed by raising the gel temperature to 90° F., 145° F., and 175° F., with shear sweeps performed once the test temperature was reached. The resulting data (FIG. 1) indicate to what degree the fluids remained non-Newtonian as the temperature increased. Some thermal thinning of the fluid takes place, but it is important to highlight the fact that all the three suspensions fully hydrate with no particular hydration equipment or conditions.

Example 3b

(38) Crosslinking capacity of the compositions was also tested under high temperature conditions. A gel of suspension 1 was prepared as described in Example 1, with a concentration of 25 pounds polymer blend/1000 gallons fracturing fluid. These gels were then buffered to a pH of 9-10 and cross-linked with a high pH buffer, surface and delayed boron crosslinkers available from Univar (BES-y and BXL-411).

(39) The fluid viscosity was then measured using a Grace 5600 HTHP viscometer. The temperature was ramped from ambient to 180° F. over a period of ten minutes. Shear was measured at 100/s with periodic API (American Petroleum Society) shear sweeps. The gel produced was a strong lipping gel with a consistent viscosity over 100 cP at 180° F. Control fluids did not gel with the addition of buffer and cross-linker. API shear sweeps indicated an average n′=0.55 and K′=2.72 (both showing strong non-Newtonian behavior at high temperature). Control fluids yielded n′>1 above 130° F. and K′<0.7. These results provided in Table 3 below indicate increased proppant carrying capacity.

(40) TABLE-US-00003 TABLE 3 High Temperature Crosslinking Capacity Coefficient Visc. Kv (lbf- Determi- K' (lbf- K'Slot Visc. at Visc. at at 170 Shear Cycle Time Temp. s{circumflex over ( )}n/ nation s{circumflex over ( )}n/ (lbf-s{circumflex over ( )}n/ 40 (1/s) 100 (1/s) (1/s) Ramp No. (min) (° F.) n' 100 ft.sup.2) (R {circumflex over ( )}2) 100 ft.sup.2) 100 ft.sup.2) cP cP cP 1 1 13.7 183 0.6252755 1.319125 0.836 1.2596   1.401232 186.36 135.57 112.76 2 1 23.7 182 0.533543  2.658727 0.658 2.500145 2.86566  245.52 160.13 125.02 3 1 33.7 180 0.54435   2.436904 0.743 2.294692 2.623631 233.93 154.08 120.99 4 1 43.7 179 0.443515  3.951964 0.894 3.674958 4.29097  263.75 158.39 117.9  5 1 53.7 179 0.730751  0.957225 0.674 0.923426 1.004999 178.22 139.26 120.72 6 1 63.7 179 0.41797   3.828364 0.82  3.549101 4.162276 232.83 136.59 100.3  lbf = pounds-force visc. = viscosity n = power law index K = consistency

Example 4

(41) The yield point and plastic viscosity of the suspensions were also tested. Comparative gels were made as described in Example 1, with three blends of the inventive compositions (70/30, 65/35, 60/40 guar/friction reducer) as well as conventional treatments of xanthan or guar gum. Xanthan is known for its capacity to suspend proppant and cuttings at very low shear rate, as indicated by the yield point. Samples were tested for linear gel viscosity using a Fann 35-type viscometer at ambient temperatures, at 300 and 600 rpm. These results provided in Table 4 below are useful in calculating fluid yield point and plastic viscosity. The yield point and plastic viscosity of the compositions was calculated to be 6-8 pounds/100 feet and 6-7 cP, where guar alone or xanthan alone each gave 3-5 pounds/100 feet and 5-6 cP.

(42) TABLE-US-00004 TABLE 4 Yield Points and Plastic Viscosity Plastic Yield 15 # Hydrations 300 rpm 600 rpm Viscosity Point Guar Reading  7.8 cP  6.3 cP 5 cP 3 lb/100 ft.sup.2 Dial  8 13 Xanthan Reading 11.5 cP  8.3 cP 6 cP 5 lb/100 ft.sup.2 Dial 11 17 FR Visc. (60/40) Reading 13.9 cP 10.3 cP 7 cP 7 lb/100 ft.sup.2 Dial 14 21 FR Visc. (65/35) Reading 13.5 cP 10.0 cP 6 cP 8 lb/100 ft.sup.2 Dial 14 20 FR Visc. (70/30) Reading 13.1 cP  9.8 cP 7 cP 6 lb/100 ft.sup.2 Dial 13 20 FR Visc. = friction reducer viscosity

Example 5

(43) The compositions' tolerance to salt was also measured. Comparative samples were prepared as in Example 1, with the addition of a given amount of various salts representative of oilfield conditions. After hydration, the fluid viscosity was measured at ambient temperature using a Fann 35-type viscometer at 511/s. As provided in Table 5 below, the inventive compositions showed the highest tolerance to two of three brines tested.

(44) TABLE-US-00005 TABLE 5 Viscosity Differences visc., cP @ Percent 511/s 2000 mg/l Difference in Viscosity Fluid GPT tap water NaSO.sub.4 3.5% NaCl 1% CaCl.sub.2 A 3.0 12 −30% −57% −83% A 5.0 20 −35% −59% −87% A 3.0 5.4  0% −33% −59% A 5.0 9.4 −11% −45% −66% 60/40 3.0 11.8 −14% −22% −47% 60/40 5.0 21.6  −9% −23% −46% Fluid A = conventional friction reducer GPT = gallon per thousand gallons

Example 6

(45) The comparative crosslinked samples may be further evaluated for their breakability. To a gel solution, 0.75-2.0 gptg standard breaker (enzyme, acid or oxidizer) would be added. The viscosity reduction profile would be followed over time. It is anticipated that the solution viscosity of the solutions prepared with suspensions 1 to 3 of the present invention would show a higher viscosity reduction ratio as compared to controls 1 to 3.

Example 7

(46) The comparative gel solutions (which may or may not be crosslinked) may be further evaluated for the capacity to increase proppant carrying capacity. The proppant carrying capacity may be assessed by means of static settling of various amounts of proppant in various gel solutions over a period of time and compared side by side with the incumbent controls. It is anticipated that gel solutions made with suspensions 1 to 3 would exhibit lower proppant settling rates in comparison to controls 1 to 3. It is further anticipated that the gel solutions of suspensions 1 to 3 would tolerate and suspend higher proppant loading (3-4 ppa (pounds of proppant added)) in comparison to controls 1 to 3 that are anticipated to be limited to no more than 2 ppa.

(47) For the present invention, each numerical value should be read once as modified by the term “about” (unless already expressly so modified), and then read again as not so modified unless otherwise indicated in context. Concentration ranges listed or described herein include any and every concentration within the range, including the endpoints. For example, “a range of from 1 to 10” is to be read as indicating each and every possible number along the continuum between about 1 and about 10. Thus, even if specific data points within the range, or even no data points within the range, are explicitly identified, it is to be understood that inventors appreciate and understand that all data points within the range are considered to have been specified, and the inventors have disclosed and enabled the entire range and all points within the range.

(48) It is understood that modifications to the invention may be made as might occur to one skilled in the field of the invention within the scope of the appended claims. All embodiments contemplated hereunder which achieve the objects of the invention have not been shown in compete detail. Other embodiments may be developed without departing from the spirit of the invention or from the scope of the appended claims. Although the present invention has been described with respect to specific details, it is not intended that such details should be regarded as limitations on the scope of the invention, except to the extent that they are included in the accompanying claims.