WO2021175760A1

WO2021175760A1 - Method of fracturing subterranean formations

Info

Publication number: WO2021175760A1
Application number: PCT/EP2021/054994
Authority: WO
Inventors: Daniel Barrera-Medrano; Faissal-Ali El-Toufaili; Jack F TINSLEY; Brent Busby; Tobias Joachim ZIMMERMANN; Anna-Corina SCHMIDT; Dennis Loesch; Markus OSTERMAYR
Original assignee: Basf Se
Priority date: 2020-03-06
Filing date: 2021-03-01
Publication date: 2021-09-10

Abstract

Method of fracturing subterranean, oil- and/or gas-bearing formations penetrated by at least a wellbore by injecting an aqueous fracturing fluid into a wellbore at a rate and pressure sufficient to penetrate into the formation, and to initiate or extend fractures in the formation using an aqueous fracturing fluid comprising a polyacrylamide friction reducer, wherein the aqueous fracturing fluid is prepared in a 2-step process by mixing a polyacrylamide inverse emulsion or a liquid dispersion polymer comprising polyacrylamides with an aqueous fluid thereby obtaining an aqueous premix having a concentration from 2 to 19.9 % by weight of polyacrylamides, allowing the aqueous premix to ripen for a certain time and further diluting the ripened aqueous premix in a second step with an aqueous liquid thereby obtaining an aqueous fracturing fluid comprising 0.002 % by weight to 0.12 % by weight of a polyacrylamides.

Description

Method of Fracturing Subterranean Formations

The present invention relates to a method of fracturing subterranean, oil- and/or gas-bearing formations penetrated by at least a wellbore by injecting an aqueous fracturing fluid into a wellbore at a rate and pressure sufficient to penetrate into the formation, and to initiate or extend fractures in the formation using an aqueous fracturing fluid comprising a polyacrylamide friction reducer, wherein the aqueous fracturing fluid is prepared in a 2-step process by mixing a polyacrylamide inverse emulsion or a liquid dispersion polymer comprising polyacrylamides with an aqueous fluid thereby obtaining an aqueous premix having a concentration from 2 to 19.9 % by weight of polyacrylamides, allowing the aqueous premix to ripen for a certain time and further diluting the ripened aqueous premix in a second step with an aqueous liquid thereby obtaining an aqueous fracturing fluid comprising 0.002 % by weight to 0.12 % by weight of a polyacrylamides.

The productivity of oil or gas wells often is insufficient because the permeability of the subterranean formation from which oil and/or gas is produced is too low. It is known in the art to enhance the productivity of such oil and/or gas wells by fracturing. In course of fracturing, an aqueous fracturing fluid is injected into the well at a pressure sufficient to penetrate into the subterranean formation and to generate new fractures or fissures in the subterranean formation and/or to extend existing fractures of fissures thereby enhancing its permeability. A fracturing fluid usually comprises so-called proppants. Proppants are small hard particles, typically having dimensions in the range from 0.1 mm to 2.5 mm, such as for example, naturally-occurring sand grains which are deposited in the fractures by the fracturing fluid and remain in the created fractures after the fluid is removed, thereby keeping the fractures open and allowing hydrocarbons to flow more easily from the formation to the production wellbore.

Because the density of proppants, such as sand particles, usually is significantly higher than that of aqueous fluids, the suspension of the proppants in the fracturing fluid and the transport by the fluid requires particular attention. It is necessary to avoid that proppants settle in course of transport, for example in the wellbore or in pipe which transport the fracturing fluid.

Basically, two different techniques are known in the art for avoiding such settlement of proppants. One method comprises increasing the viscosity of the fluid using a suitable viscosifier, such as guar gum. Another method comprises flowing the fluid at very high speed. The latter method, also known as slickwater fracturing, is often preferred, but the high speeds employed create turbulent flow with excessive pumping pressure. Necessary pressures often are too high for the field equipment and the net effect is that the required flow rate is not achieved. For example, typical injection rates for slickwater hydraulic fracturing can reach about 15 m³ per minute, but turbulence can restrict it to 1,5 m³ per minute. To avoid this problem, it is known in the art to employ friction reducers that dampen the turbulent eddies, creating pressure drops approaching those of laminar flow. Examples of friction reducers comprise high molecular weight polymers, for example high molecular weight polyacrylamides. Such polyacrylamide friction reducers may be applied in various kinds, for instance as powders or as diluted aqueous solutions.

It is widely distributed to employ polyacrylamides for use as friction reducer as inverse emulsion, such as disclosed for instance by US 5,067,508 or CA 2 864 159 A1. Inverse emulsions of polyacrylamides for used as friction reducer are commercially available. In such inverse emulsions or water-in-oil emulsions a discontinuous aqueous phase comprising water-soluble polymers such as polyacrylamides is dispersed a continuous organic phase not miscible with the aqueous phase. The inverse emulsions comprise suitable surfactants which stabilize the aqueous droplets in the continuous organic phase. Several processes of using inverse emulsions as friction reducers and suitable formulations of inverse emulsions for that purpose are disclosed in US 8,841,240 B2, US 9,315,722 B1, US 2012/0214714 A1, US 2015/0240144 A1, US 2017/0121590 A1, WO 2016/109333 A1, or WO 2017/143136 A1.

It is also known in the art to remove the water completely or at least partially from such inverse emulsions thus obtaining a dispersion of particles of water-soluble polyacrylamide (co)polymers in a hydrophobic oil phase. Such dispersions are also known as “Liquid Dispersion Polymers”, also abbreviated as LDP and usually their water contents is less than 5 wt. %. The polymer contents of LDPs may be up to more than 50 wt. %. Liquid dispersion polymers and their manufacture are disclosed for example in DE 24 19764 A1 , US 4,052,353, US 4,528,321, US 6,365,656 B1, or US 6,833,406 B1. US 2014/0131039 A1 discloses the use of such liquid dispersion polymers for mineral oil production.

For use as friction reducer, the inverse emulsion may be metered into a flowing aqueous fluid in which it is intended to act as friction reducer, for example into a flowing aqueous fracturing fluid. In order to be effective as friction reducer, the inverse emulsion needs to become inverted so that the polyacrylamide friction reducers can become released from the discontinuous aqueous phase. Said step is typically carried out by adding so-called “inverting surfactants”. Such inverting surfactants enable conversion or at least speed up the step of inversion. Liquid dispersion polymers may be used as friction reducer basically in the same manner.

Such a process of inverting inverse emulsions or liquid dispersion polymers may be a 1-step process. It is also known in the art to apply 2-step processes or multi-step processes to invert such inverse emulsions or liquid dispersion polymers.

US 2011/0118153 A1 discloses an enhanced oil recovery method in which an aqueous solution of polyacrylamides is used as flooding medium. The method comprises mixing an inverse emulsion of polyacrylamides with water. For mixing an apparatus comprising two static mixers is used, one of them mounted in the main injection line and one of them in a bypass to the main injection line. The method comprises pre-diluting an inverse emulsion with water in the first static mixer mounted on said bypass of the main injection water line, thereby obtaining a mixture comprising at least 0.5 % by weight of polyacrylamides, preferably from 0.5 % by weight to 2 % by weight, and diluting the mixture obtained with additional water in the second static mixer mounted on the main water injection circuit, thereby obtaining a diluted mixture for injection having a polyacrylamide concentration from 0.05 % by weight to 0.3 % by weight. The mixing process is a quick process: The overall residence time in the apparatus described above is from about 2 s to 10 s.

US 2014/0131039 A1 discloses an enhanced oil recovery method wherein the injection fluid comprising at least water and a polyacrylamide is made by mixing a liquid dispersion polymer with water. Mixing may be carried out in a 2-step process comprising a pre-dilution step thereby obtaining a polyacrylamide concentrate and thereafter further diluting the concentrate with additional water in a second step. Static mixers may be used for mixing.

The concentrate obtained from pre-dilution may have a concentration from 0.51 % by weight to 5 % by weight of polyacrylamides.

US 2017/0158948 A1 discloses a method of preparing an inverted polymer solution for use as friction reducer in hydraulic fracturing comprising providing a liquid polymer solution and inverting it in an aqueous fluid, thereby obtaining an inverted polymer solution having a concentration from 0.005 % by weight to 1.5 % by weight of (co)polymer such polymer. The inversion may be a one-step process but also a multi-step process. In one embodiment in the first step a concentrate having a concentration of up to 1.5 % by weight of polymer, for example from 0.5 to 1.5 % by weight may be formed, which is further diluted with additional liquid second step. The document also discloses a process of fracturing in which such an inverted polymer solution is used as friction reducer.

US 2019/0002754 A1 discloses a method for hydrogen recovery which comprises preparing an inverted polymer solution, e.g. an inverted polyacrylamide solution from a liquid polymer or an inverse polymer emulsion by mixing it with an aqueous liquid. The inversion may be a 2-step process, wherein in the first step a concentrate having a concentration of up to 1.5 % by weight of polymer, for example from 0.5 to 1.5 % by weight is be formed, which is further diluted with additional liquid second step.

WO 2017/187150 A1 discloses a formulation for use in a fracturing fluid comprising a fluid comprising an oil phase and particles of a water-soluble polymer. The fluid comprising an oil phase preferably is an inverse emulsion. So, the formulation preferably is an inverse emulsion in which additionally particles of a water-soluble polymer are dispersed. The document does not disclose ripening a mixture of the formulation and water before use.

There is still a need for improving the performance of friction reducers in slickwater fracturing processes. Surprisingly, is has been found that pre-diluting inverse emulsions of polyacrylamides or liquid dispersion polymers and allowing the mixture to ripe for a certain time before using them as friction reducers significantly increases their performance as friction reducers in slickwater fracturing processes.

Accordingly, the present invention relates to a method of fracturing subterranean, oil- and/or gas-bearing formations penetrated by at least a wellbore comprising at least the steps of

(1) providing an aqueous fracturing fluid comprising at least an aqueous base fluid and 0.002 % by weight to 0.12 % by weight of a polyacrylamide friction reducer, and

(2) injecting said aqueous fracturing fluid into the wellbore at a rate and pressure sufficient to penetrate into the formation, and to initiate or extend fractures in the formation, wherein at least a part of the aqueous fracturing fluid injected additionally comprises a proppant and the amount of the friction reducer relates to the total of all components of the aqueous fracturing fluid except the proppants, and wherein step (1) is carried out as follows:

(1-1) providing a composition (C) comprising at least 20 % by weight of polyacrylamides, relating to the total of all components of the composition, wherein the composition (C) is selected from

• a water-in-oil emulsion (C-l) comprising a polyacrylamide in its aqueous phase, and

• a liquid dispersion polymer composition (C-l I) comprising an organic, hydrophobic liquid and particles of polyacrylamides dispersed therein, wherein the amount of water in the composition is less than 10 % by weight, relating to the total of all components of the composition (C-l I),

(1-2) mixing the composition (C) with an aqueous liquid, thereby obtaining an aqueous polyacrylamide premix (P), wherein the concentration of polyacrylamides in the premix (P) is from 2 % to 19.9 % by weight, relating to the total of all components of the premix (P),

(1-3) allowing the aqueous polyacrylamide premix (P) to ripen by allowing the components of the aqueous premix (P) to interact with each other for a period of time after the components have been mixed in step (1-2) and before the mixture is further diluted in step (1-4), wherein said time period is at least 1 min,

(1-4) mixing the ripened aqueous polyacrylamide premix (P) with at least an aqueous base fluid at the amounts necessary to obtain the aqueous fracturing fluid mentioned above comprising 0.002 % by weight to 0.12 % by weight of polyacrylamides. With regard to the invention, the following should be stated specifically:

For carrying out the method according to the following invention an aqueous fracturing fluid comprising at least an aqueous base fluid and a polyacrylamide friction reducer is provided. The polyacrylamide friction reducer comprises at least one polyacrylamide.

According to the invention, the aqueous fracturing fluid is made in a 2-step process. In a first step, an inverse emulsion or a liquid polymer dispersion of polyacrylamides is mixed with an aqueous liquid to obtain an aqueous premix having a concentration of 2 to 19.9 % by weight of polyacrylamides. In a second step, the aqueous premix is further diluted with additional aqueous liquid to obtain a fracturing fluid in which having a concentration of polyacrylamides from 20 ppm (0.002 % by weight) to 1200 ppm (0.12 % by weight). According to the invention, the premix is ripened for at least 1 minute, for example from 5 min to 1 day, i.e. the components of the aqueous premix are allowed to interact for said time period, before the premix is further diluted with aqueous fluid.

Polvacrylamides

The term “polyacrylamides” as used herein means water-soluble homopolymers of acrylamide, or water-soluble copolymers comprising at least 10 mole %, preferably at least 20 mole %, and more preferably at least 30 mole % of acrylamide and at least one additional water-soluble, monoethylenically unsaturated monomer different from acrylamide, wherein the amounts relate to the total amount of all monomers in the polymer. Copolymers are preferred.

The term “water-soluble monomers” in the context of this invention means that the monomers are to be soluble in the aqueous monomer solution to be used for polymerization in the desired use concentration. It is thus not absolutely necessary that the monomers to be used are miscible with water without any gap; instead, it is sufficient if they meet the minimum requirement mentioned. It is to be noted that the presence of acrylamide in the monomer solution might enhance the solubility of other monomers as compared to water only. In general, the solubility of the water-soluble monomers in water at room temperature should be at least 50 g/l, preferably at least 100 g/l.

Basically, the kind and amount of water-soluble, monoethylenically unsaturated comonomers to be used besides acrylamide is not limited and depends on the desired properties of the polyacrylamide friction reducer to be used within the contact of the present invention.

Neutral comonomers

In one embodiment of the invention, comonomers may be selected from uncharged water- soluble, monoethylenically unsaturated monomers. Examples comprise methacrylamide, N- methyl(meth)acrylamide, N,N’-dimethyl(meth)acrylamide, N-methylol(meth)acrylamide or N- vinylpyrrolidone. Further examples have been mentioned in WO 2015/158517 A1 page 7, lines 9 to 14. Anionic comonomers

In a further embodiment of the invention, comonomers may be selected from water-soluble, monoethylenically unsaturated monomers comprising at least one acidic group, or salts thereof. The acidic groups are preferably selected from the group of -COOH, -SO3H and -PO3H2 or salts thereof. Preference is given to monomers comprising COOH groups and/or -SO3H groups or salts thereof. Suitable counterions include earth alkaline metal ions such as Ca²⁺ ions, especially alkali metal ions such as Li⁺, Na⁺ or K⁺, and also ammonium ions such as Nh or ammonium ions having organic radicals. Examples of ammonium ions having organic radicals include [NH(CH₃)3]⁺, [NH₂(CH₃)2]⁺, [NH₃(CH₃)]⁺, [NH(C₂H₅)3]⁺,

[N H₂(C₂H₅ )₂]⁺, [NH₃(C₂H₅)]⁺, [NH₃(CH₂CH₂OH)]⁺, [H₃N-CH₂CH₂-NH₃]²⁺ or [H(H₃C)₂N- CH₂CH₂CH₂NH₃]²⁺.

Examples of monomers comprising -COOH groups include acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid or fumaric acid or salts thereof. Preference is given to acrylic acid or salts thereof.

Examples of monomers comprising -SO₃H groups or salts thereof include vinylsulfonic acid, allylsulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid (ATBS), 2-methacrylamido-2- methylpropanesulfonic acid, 2-acrylamidobutanesulfonic acid, 3-acrylamido-3- methylbutanesulfonic acid or 2-acrylamido-2,4,4-trimethylpentanesulfonic acid. Preference is given to 2-acrylamido-2-methylpropanesulfonic acid (ATBS) or salts thereof.

Examples of monomers comprising -P0₃H₂ groups or salts thereof include vinylphosphonic acid, allylphosphonic acid, N-(meth)acrylamidoalkylphosphonic acids or (meth)acryloyloxyalkylphosphonic acids, preferably vinylphosphonic acid.

Preferred monomers comprising acidic groups comprise acrylic acid and/or ATBS or salts thereof.

Cationic comonomers

In a further embodiment of the invention, comonomers may be selected from water-soluble, monoethylenically unsaturated monomers comprising cationic groups. Suitable cationic monomers include especially monomers having ammonium groups, especially ammonium derivatives of N-(co-aminoalkyl)(meth)acrylamides or co-aminoalkyl (meth)acrylates such as 2-trimethylammonioethyl acrylate chloride H₂C=CH-CO-CH₂CH₂N⁺(CH3)3 CI (DMA3Q). Further examples have been mentioned in WO 2015/158517 A1 page 8, lines 15 to 37. Preference is given to DMA3Q. Associative comonomers

In a further embodiment of the invention, comonomers may be selected from associative monomers.

Associative monomers impart hydrophobically associating properties to polyacrylamides. Associative monomers to be used in the context of this invention are water-soluble, monoethylenically unsaturated monomers having at least one hydrophilic group and at least one, preferably terminal, hydrophobic group. Examples of associative monomers have been described for example in WO 2010/133527, WO 2012/069478, WO 2015/086468 or WO 2015/158517.

“Hydrophobically associating copolymers” are understood by a person skilled in the art to mean water-soluble copolymers which, as well as hydrophilic units (in a sufficient amount to assure water solubility), have hydrophobic groups in lateral or terminal positions. In aqueous solution, the hydrophobic groups can associate with one another. Because of this associative interaction, there is an increase in the viscosity of the aqueous polymer solution compared to a polymer of the same kind that merely does not have any associative groups.

Examples of suitable associative monomers comprise monomers having the general formula H2C=C(R¹)-R²-R³ (I) wherein R¹ is H or methyl, R² is a linking hydrophilic group and R³ is a terminal hydrophobic group. Further examples comprise having the general formula H2C=C(R¹)-R²-R³-R⁴ (II) wherein R¹, R² and R³ are each as defined above, and R⁴ is a hydrophilic group.

The linking hydrophilic R² group may be a group comprising ethylene oxide units, for example a group comprising 5 to 80 ethylene oxide units, which is joined to the H2C=C(R¹)- group in a suitable manner, for example by means of a single bond or of a suitable linking group. In another embodiment, the hydrophilic linking group R² may be a group comprising quaternary ammonium groups.

In one embodiment, the associative monomers are monomers of the general formula H₂C=C(R¹)-0-(CH₂CH₂0)_k-R^3a (III) or H₂C=C(R⁵)-(C=0)-0-(CH₂CH₂0)_k-R^3a (IV), wherein R¹ has the meaning defined above and k is a number from 10 to 80, for example, 20 to 40. R^3a is an aliphatic and/or aromatic, straight-chain or branched hydrocarbyl radical having 8 to 40 carbon atoms, preferably 12 to 32 carbon atoms. Examples of such groups include n-octyl, n- decyl, n-dodecyl, n-tetradecyl, n-hexadecyl or n-octadecyl groups. In a further embodiment, the groups are aromatic groups, especially substituted phenyl radicals, especially distyrylphenyl groups and/or tristyrylphenyl groups.

In another embodiment, the associative monomers are monomers of the general formula H₂C=C(R¹)-0-(CH₂)n-0-(CH₂CH₂0)x-(CH₂-CH(R⁵)0)_y-(CH₂CH₂0)zH (V), wherein R¹ is defined as above and the R⁵ radicals are each independently selected from hydrocarbyl radicals comprising at least 2 carbon atoms, preferably from ethyl or propyl groups. In formula (V) n is a natural number from 2 to 6, for example 4, x is a number from 10 to 50, preferably from 12 to 40, and for example, from 20 to 30 and y is a number from 5 to 30, preferably 8 to 25. In formula (V), z is a number from 0 to 5, for example 1 to 4, i.e. the terminal block of ethylene oxide units is thus merely optionally present. In an embodiment of the invention, it is possible to use at least two monomers (V), wherein the R¹ and R⁶ radicals and indices n, x and y are each the same, but in one of the monomers z = 0 while z > 0 in the other, preferably 1 to 4.

In another embodiment, the associative monomers are cationic monomers. Examples of cationic associative monomers have been disclosed in WO 2015/158517 A1 , page 11 , line 20 to page 12, lines 14 to 42. In one embodiment, the cationic monomers having the general formula H₂C=C(R¹)-C(=0)0-(CH2)k-N⁺(CH3)(CH₃)(R⁶) X (VI) or H₂C=C(R¹)-C(=0)N(R¹)- (CH₂)_k-N⁺(CH3)(CH3)(R⁶) X (VII) may be used, wherein R¹ has the meaning as defined above, k is 2 or 3, R⁶ is a hydrocarbyl group, preferably an aliphatic hydrocarbyl group, having 8 to 18 carbon atoms, and X is a negatively charged counterion, preferably Cl and/or Br.

Further comonomers

Besides water-soluble monoethylenically unsaturated monomers, also water-soluble, ethylenically unsaturated monomers having more than one ethylenic group may be used. Monomers of this kind can be used in special cases in order to achieve easy crosslinking of the acrylamide polymers. The amount thereof should generally not exceed 2 mole %, preferably 1 mole % and especially 0.5 mole %, based on the sum total of all the monomers. More preferably, the monomers to be used in the present invention are only monoethylenically unsaturated monomers.

Composition of polyacrylamides

The specific composition of the polyacrylamides to be used in the process of slickwater fracturing according to the present invention may be selected by the skilled artisan according to his/her needs. The following compositions preferred:

Preferred polyacrylamides comprise, besides at least 10 mole % of acrylamide, at least one comonomer, preferably at least one comonomer selected from the group anionic comonomers, cationic comonomers or associative comonomers as described above. In one embodiment, such preferred polyacrylamides comprise at least one comonomer selected from the group of acrylic acid or salts thereof, ATBS or salts thereof, associative monomers, in particular those of formula (V) or DMA3Q, more preferably at least one comonomer selected from acrylic acid or salts thereof, ATBS or salts thereof, associative monomers, in particular those of formula (V).

In one embodiment, the polyacrylamides comprise 20 mole % to 95 mole % of acrylamide and 5 mole % to 80 mole % of acrylic acid and/or salts thereof, wherein the amounts of the monomers relate to the total of all monomers in the polymer.

In one embodiment, the polyacrylamides comprise 70 mole % to 95 mole % of acrylamide and 5 mole % to 30 mole % of acrylic acid and/or salts thereof.

In one embodiment, the polyacrylamides comprise 40 mole % to 90 mole % of acrylamide, 5 mole % to 30 mole % of acrylic acid and/or salts thereof, and 5 mole % to 30 mole % of ATBS and/or salts thereof.

In one embodiment, the polyacrylamides comprise 69 mole % to 94.995 mole % of acrylamide, 0.005 mole % to 1 mole % of at least one associative monomer of the general formula (V) mentioned above, including the preferred embodiments, and 5 mole % to 30 mole % of acrylic acid or salts thereof.

In one embodiment, the polyacrylamides comprise 70 mole % to 99 mole% of acrylamide and 1 mole % to 30 mole % of a cationic comonomer, preferably DMA3Q.

In one embodiment, the polyacrylamides comprise 69 mole % to 99.995 mole % of acrylamide, 0.005 mole % to 1 mole % of at least one associative monomer, and 0 mole % to 30 mole % of an anionic monomer, for example ATBS or a cationic monomer, for example DM3AQ. Preferably, the associative monomer(s) have the general formula (V) including the preferred embodiments mentioned above.

In all embodiments mentioned above, the amount of the monomers relates to the total of all monomers in the polyacrylamide. Further water-soluble, monoethylenically unsaturated monomers may be present besides those specifically mentioned, however, the embodiments each include also one embodiment in which besides the monomers specifically mentioned no further monomers are present, i.e. in these embodiments the total amount of the monomers specifically mentioned is 100 mole %.

The intrinsic viscosity of the polyacrylamides to be used in the process of slickwater fracturing may be selected by the skilled artisan according to his/her needs. In general, for use as friction reducer, higher intrinsic viscosities are advantageous. In one embodiment of the invention, the intrinsic viscosity may be at least 17 deciliter/gram (dL/g), in particular 19 dl_/g to 28 dL/g. Method of fracturina

For carrying out the method of fracturing according to the present invention, the formation is penetrated by at least one wellbore. The wellbore may be a “fresh” wellbore drilled into the formation which needs to become prepared for oil and/or gas production. In another embodiment the wellbore may be a production well which already has been used for producing oil and/or gas but the production rate decreased and it is necessary to fracture the formation (again) in order to increase production.

The method of fracturing according to the present invention is a method of slickwater fracturing. The term “slickwater fracturing” is known to the skilled artisan. Slickwater fracturing is carried out by injecting the fracturing fluid into the subterranean formation at a high flow sufficiently to transport the proppants with the fluid into the formation without settling of the proppants. Due to the very high flow, it is not necessary to use thickeners in the fracturing fluid in order to avoid settling of the proppants. As outlined already above, excessive pressure caused by turbulent flow of the fracturing fluid may be a big problem in slickwater fracturing. The polyacrylamides added to the fracturing fluid in the present invention serve as friction reducers that dampen the turbulent eddies, thereby avoiding excessive pressures. For that purpose, the polyacrylamides are used in low amounts so that they usually have no pronounced effect on the viscosity of the fracturing fluid.

The process of fracturing according to the present invention comprises at least the two steps (1), and (2).

Step (1)

In course of step (1) an aqueous fracturing fluid comprising at least an aqueous base fluid and a polyacrylamide is provided. At least a part of the aqueous fracturing fluid additionally comprises a proppant. The polyacrylamide serves as friction reducer and is used in an amount of 20 ppm (0.002 % by weight) to 1200 ppm (0.12 % by weight), in particular 20 ppm (0.002 % by weight) to 300 ppm (0.03 % by weight), preferably from 20 (0.002 % by weight) to 100 ppm (0.01 % by weight), wherein the amount of the polyacrylamide friction reducer relates to the total of all components of the aqueous fracturing fluid except the proppants.

Examples of aqueous base fluids comprise fresh water, brines, sea water, formation water treated water or mixtures thereof. The salinity of the water may be -for example- from 500 ppm to 300,000 ppm total dissolved solids (TDS), for example from 1,000 ppm to 100,000 ppm.

Proppants are small hard particles which cause that fractures formed in course of the process do not close after removing the pressure. Suitable proppants and suitable amounts thereof are known to the skilled artisan. Examples of proppants include naturally-occurring sand grains, resin-coated sand, sintered bauxite, glass beads, or ultra-lightweight polymer beads.

At least a part of the aqueous fracturing fluid provided in course of step (1) comprises a proppant. Of course, the entire amount of fracturing fluid may comprise a proppant. Also, the amounts of proppants in the fluid may be varied in course of the process. As will be detailed below, the process comprises embodiments in which the process starts with injecting an aqueous fracturing fluid comprising no proppants followed by the injection of an aqueous fracturing fluid comprising proppants.

Besides the aqueous base fluid, optionally the proppants and the polyacrylamides, the aqueous fracturing fluid may optionally comprise further components. Examples of such additional components comprise biocides, corrosion inhibitors, scale inhibitors, iron control agents and clay control agents. The skilled artisan may select such further depending on the needs of the frac job.

According to the present invention, step (1) comprises at least the sub-steps (1-1), (1-2), (1-3), and (1-4). It may of course comprise further sub-steps.

Step (1-1)

Step (1-1) comprises providing a composition (C) comprising at least 20 % by weight, preferably at least 25 % by wt. of polyacrylamides, relating to the total of all components of the composition, wherein the composition (C) is selected from

• a liquid dispersion polymer composition (C-l I) comprising an organic, hydrophobic liquid and particles of polyacrylamides dispersed therein, wherein the amount of water in the composition is less than 10 % by weight, relating to the total of all components of the composition (C-l I).

Suitable polyacrylamides and preferred compositions have already been described above.

For carrying out the process, preferably either compositions (C-l) or compositions (C-l I) are used, however, in exceptional cases also both compositions (C-1) and (C-l I) may be used together. Preferably, a composition (C-l) is used.

Composition (C-l)

The composition (C-l) is a water-in-oil-emulsion comprising an oil phase and an aqueous phase comprising at least one polyacrylamide. Water-in-oil emulsions are also known as inverse emulsions. The oil phase of the water-in-oil-emulsion basically may be any kind of mineral oil. In one embodiment of the invention aliphatic mineral oils with a content of aromatic hydrocarbons of less than 5 % by wt., preferably less than 1 % by wt. may be used. Preferably, the flash point of the mineral oil should be at least 70°C. The aqueous phase comprises at least one polyacrylamide. The emulsion is stabilized in known manner by suitable surfactants. The manufacture of water-in-oil emulsions comprising polyacrylamides basically is known in the art and such emulsions are commercially available.

The water-in-oil-emulsion may additionally comprise additives for accelerating the inversion of the polyacrylamides into the aqueous solution after mixing the water-in-oil emulsion with water. Such additives, also known as activating surfactants or boosters are basically is known in the art. Examples comprise nonionic surfactants, in particular polyalkoxylated alcohols, in particular polyalkoxylates of Cs to C22 alkohols. The polyalkoxy groups preferably may be selected from ethoxy, propopxy or butoxy groups. In one embodiment, the polyalkoxy groups comprise at least ethoxy groups and optionally propoxy and/or butoxy groups.

Specific examples include surfactants based on oxo alcohols, in particular based on C13/15 oxo alcohols and comprising 5 to 15 EO units and optionally PO and/or BuO with the proviso that the number of PO and/or BuO units is less than the number of EO units. The amount of such additives may for example be from 1 % to 5 % by weight relating to total weight of the water-in-oil emulsion.

The composition (C-l) comprises at least 20 % by weight of polyacrylamides relating to the total of all components of the composition (C-l), preferably at least 25 % by weight. In one embodiment of the invention, the composition (C-l) comprises from 20 to 45 % by weight of polyacrylamides, in particular from 25 to 45 % by weight, preferably from 25 to 40 % by weight and for example from 25 to 35 % by weight.

Composition (C-ll)

The composition (C-ll) is a liquid dispersion polymer composition comprising at least an organic, hydrophobic liquid and particles of polyacrylamides dispersed therein. The composition may also comprise additionally small amounts of water, however, the amount of water in the composition is less than 10 % by weight, relating to the total of all components of the composition (C-ll), preferably less than 5% by weight.

In one embodiment of the invention, the particles of polyacrylamides may an average particle size of 0.4 mGh to 5 mhi, preferably 0.5 mhi to 2 mhi. Average particle size here means the d50 value of the particle size distribution (number average) which may be measured by the skilled artisan using known techniques for determining the particle size distribution.

Compositions (C-ll) may preferably be manufactured by inverse emulsion polymerization followed by removing water from the inverse emulsion until the amount of water is at least less than 10 % by weight. The removal of water preferably is carried out at reduced pressure, for example at a pressure of 30 hPa to 500 hPa. A suitable manufacturing procedure is for example disclosed in US 2014/0131039 A1. Liquid dispersion polymer compositions are commercially available.

In other embodiment, compositions (C-ll) may be manufactured by dispersing dry powders of polyacrylamides in organic, hydrophobic liquids. The dry powders should be ground to the desired particle size before use. In such products, typically no water is added additionally. Nevertheless, the obtained compositions (C-ll) may comprise small amounts of water because even “dry” polyacrylamide powders, especially if they are fine particles, typically comprise up to a few percent of water.

Also, the composition (C-ll) may comprise additives for accelerating inversion. Examples have already been mentioned above.

The composition (C-ll) comprises at least 20 % by weight of polyacrylamides relating to the total of all components of the composition (C-ll), preferably at least 25 % by weight. In one embodiment of the invention, the composition (C-l) comprises from 20 to 70 % by weight of polyacrylamides, in particular from 30 to 60 % by weight, preferably from 40 to 60 % by weight and for example from 45 to 55 % by weight.

Step (1-2)

In course of step (1-2) at least one composition (C) is mixed with an aqueous liquid, thereby obtaining an aqueous polyacrylamide premix (P). The premix (P) is a concentrate, in which the polyacrylamides not yet have the final concentration for use but further dilution in additional steps is necessary.

Aqueous liquids comprise water. Besides water also small amounts of organic liquid miscible with water may be used, however, their extent should not exceed 20 wt. %, preferably not 10 wt. %. Preferably, no organic liquids are present. Water may be fresh water but also water comprising salts such as sea water or formation water or mixtures thereof may be used. In one embodiment of the invention, fresh water is used.

Mixing may be carried out by means of customary mixing means such as static mixers or stirred vessels. Preferably, high-shear mixing units should be avoided. The aqueous polyacrylamide premix (P) is a homogeneous mixture.

The concentration of polyacrylamides in the aqueous premix (P) is from 2 % to 19.9 % by weight, relating to the total of all components of the aqueous premix (P), in particular from 2 to 10 % by weight, preferably from 2 % to 8 % by weight and for example from 3 to 6 % by weight. Step (1-3)

In course of step (1-3), the aqueous polyacrylamide premix (P) obtained in course of step (1- 2) is allowed to ripen for at least 1 min. In one embodiment, the ripening time is at least 5 min, in other embodiments, it is at least ½ h. As will be shown in the examples and comparative examples, the ripening step (1-3) increases the performance of the polyacrylamide as friction reducer in the method of fracturing.

In one embodiment of the invention the ripening time is from 1 min to 1 day, in particular from 5 min to 1 day, and for example from 5 min to 2 h. In other embodiments, the ripening time is from ½ to 1 day, for example from ½ h to 2 h.

The term “ripening” simply means, that the components of the aqueous premix (P) obtained in course of step (1-2) by mixing an aqueous liquid and the composition (C) are allowed to interact for an additional period of time after the components have been mixed with each other and before the mixture is further processed in step (1-4) With other words, the steps of preparing the premix (P) (1-2) and of further diluting the premix (P) with additional aqueous fluid (1-4) don’t follow directly after each other, but there is a certain period of time, the so- called ripening time, in between.

In one embodiment, step (1-3) is carried out by allowing the aqueous premix (P) to rest in a vessel. In other embodiments, the contents of the vessel is mixed, for example by an internal stirrer and/or the contents of the vessel may be circulated through an external mixing circuit by means of a pump. The external loop may comprise a static mixer. In one embodiment, the vessel for ripening the aqueous premix (P) may be the same vessel as used for mixing. In other embodiments ripening may be carried by pumping the aqueous premix (P) through a pipe which comprises static mixers.

In yet another embodiment of the invention, the aqueous premix (P) is filled into a transport unit after step (1-2) at a location A and the transport unit filled with the premix (P) is transported from said location A to a different location B, so that ripening of the premix (P) (step 1-3) happens in course of the transport.

A may be a central plant for making an aqueous polyacrylamide premix (P) as described above which serves a number of oil wells on an oilfield with aqueous polyacrylamide premix (P) for fracturing. Such a plant may be erected at a central location on an oilfield. Location (B) is a location at an oilwell at which step (2) is carried out. Locations A and B may be -by the way of example- from 1 to 500 km, or from 10 to 100 km apart from each other. In such a case, the ripening times correspond to the transport time and may also be a few days.

The transport may be carried out by any transport means suitable for transporting the transport unit, for example by trucks, railcars or ships. In one embodiment, the transport is carried out by trucks. In another embodiment, tanks fixed on a truck may be used. In one embodiment, the tank comprises an outlet opening at the rear end of the truck and for supporting removal of the contents the tank may be tilted. In another embodiment, the tank comprises an outlet opening at the bottom side of the tank. Additionally, the tank may comprise a conus at the bottom side of the tank and the outlet opening in located at the lower end of the conus. The tank may also be rotatable. For example, a concrete mixer may be used for transporting the concentrate.

(Step 1-4)

In course of step (1-4), the ripened aqueous premix (P) is mixed with at least an aqueous base fluid at the amounts necessary to obtain the aqueous fracturing fluid mentioned above. It is known in the art, to use so-called “blenders” for mixing the components of the fracturing fluid. Such blenders may be fixed on a truck, mounted on a trailer or mounted in a skid and are commercially available. It is known in the art to meter inverse emulsion or aqueous solution of polyacrylamides into such blenders for making aqueous fracturing fluids. Advantageously, the aqueous premix (P) to be used according to the present invention may be simply metered into such blenders in the same manner as inverse emulsions or aqueous solutions.

Further embodiments comprise adding the aqueous premix (P) into the pipe which transports the aqueous fracturing fluid to the wellbore. The pipes may comprise static mixers additionally. Alternatively, only static mixers may be used.

Step (21

In course of step (2), the aqueous fracturing fluid provided in step (1) is injected into the wellbore at a rate and pressure sufficient to penetrate into the formation, and to initiate or extend fractures in the formation. For those parts of the aqueous fracturing fluid which comprise proppants, the aqueous fracturing fluid also serves to transporting the proppants into thus generated fractures.

The bottomhole pressure is determined by the surface pressure produced by the surface pumping equipment and the hydrostatic pressure of the fluid column in the wellbore, less any pressure loss caused by friction. The minimum bottomhole pressure required to initiate and/or to extend fractures is determined by formation properties and therefore will vary from application to application.

The amounts of proppants in the aqueous fracturing fluid may vary in course of carrying out the process.

In one embodiment of the process, the process starts with the injection of an aqueous fracturing fluid not comprising a proppant, followed by the injection of an aqueous fracturing fluid additionally comprising a proppant. The amounts of proppants may be increased stepwise or continuously.

In another embodiment of the process, an aqueous fracturing fluid comprising a proppant is injected during the entire process.

As outlined above, the injections rates are high for slickwater fracturing thereby causing turbulent flow of the aqueous fracturing fluid and -as a consequence thereof- high pressure losses. Such high pressure losses may significantly limit the amount of fracturing fluid to become injected. Friction reducers dampen the turbulent eddies and may create pressure drops approaching those of laminar flow. In course of slickwater fracturing, the aqueous fracturing fluid typically is injected at such a rate that is typically takes -depending on the depth of the well- about 3 to 7 min to flow through the pipe into the subterranean formation.

Suitable concentrations for the polyacrylamides in aqueous fracturing fluids for slickwater fracturing have already been mentioned above.

Further steps

After carrying out the process steps (1) and (2) further process steps might follow.

Typically, the applied pressure is reduced thereby allowing at least a portion of the injected fracturing fluid to flow back from the formation into the wellbore. Reducing the pressure allows the fractures to close. At least a part of the proppants injected with the fracturing fluid remains in the initiated or extended fractures generated in course of step (2), thereby holding opens such fractures. The aqueous fracturing fluid flown back from the formation into the wellbore may be removed from the wellbore. It goes without saying for the skilled artisan that the aqueous fracturing fluid recovered may no longer have exactly the same composition as he injected fluid but may be mixed with formation fluids such as oil and/or formation water. Furthermore, at least a portion of the proppants remains in the formation.

Advantages of the present invention

The steps of preparing an aqueous premix (P) followed by ripening the aqueous premix (P) yields an improved performance as friction reducer as compared to a process without such a preparation of an aqueous premix (P) followed by ripening. Consequently, the amount of inverse emulsion or liquid dispersion polymer needed may be reduced without loss of performance, thereby yielding a more economic process. The invention is illustrated in detail by the examples which follow:

Step 1: Preparation of a polyacrylamide inverse emulsion (IE)

Inverse emulsion of a copolymer comprising 69.4 wt.% (75.0 mol%) of acrylamide and 30.6 wt.% (25 mol%) of sodium acrylate stabilized with 0.25 wt.% Na-MBT relating to polymer (solids content 23 % by weight relating to the total of the inverse emulsion).

A 600 ml_ beaker with magnetic stirrer, pH meter and thermometer was charged with 150.44 g of sodium acrylate (35% by weight in water), 128.97 g of distilled water, 229.65 g of acrylamide (52% by weight in water), 0.5 g of diethylenetriaminepentaacetic acid pentasodium salt (Trilon C; 5% by weight in water), and 0.86 g of the stabilizer sodium 2- mercaptobenzothiazole (Na-MBT; 50% by weight in water).

After adjustment to pH 6.4 with sulfuric acid (20% by weight in water), the rest of the water to attain the desired monomer concentration of 23% by weight (total amount of water 138.61 g minus the amount of water already added, minus the amount of acid required) was added.

A high 1 L beaker was charged with 12.2 g sorbitan monooleate (Span® 80) and 189.9 g of a high-boiling dearomatized hydrocarbon mixture (Exxsol^® D100) was added and stirred with a spatula. The beaker with the oil solution was fixed in a Silverson high shear mixer. While mixing the oil solution at 4000 rpm, the aqueous solution was poured in quickly. Then, the Silverson high shear mixer is turned up to 8000 rpm for 2 min 48 sec. The emulsion was transferred to a double jacketed reactor, stirred at 200 rpm and adjusted to the initiation temperature of 10 °C. During this time the emulsion was purged with nitrogen (for 60 minutes). The polymerization was drop-wise initiated with 9 g of a 0.1% sodium bisulfite solution and 5 g of 0.1% t -butyl hydroperoxide solution.

The initiators were added with a squeezing pump, controlled by hand. When the respective 0.1% solutions were empty, the initiators were changed to 9 g of a 1% sodium bisulfite solution and 5 g of a 1% t -butyl hydroperoxide solution. Thereby, the temperature rose 1 °C per minute up to 40 °C, from there the temperature was maintained at 40 °C. When the second initiator was added completely, the emulsion was stirred for additional 60 minutes at 40 °C. The emulsion was then filtered through a 190 pm filter prior to use.

Step 2:

Preparation of an aqueous polyacrylamide premix made by mixing (pre-hydrating) an inverse emulsion polyacrylamide with an aqueous liquid (2.3 wt.%)

Example 1:

Ripening time 60 min

An amount of 270 ml of water was added into a 600 ml beaker while mixing using an overhead mixer with a 75 mm diameter half-moon propeller. The mixing rate was initially set at 300 rpm. Thereafter 30 ml of the polyacrylamide inverse emulsion obtained in course of step 1 was slowly added to the vortex over a few seconds to avoid the formation of lumps. Within 5 s, the inverse emulsion was mixed with the water.

The obtained mixture was allowed to ripen for 60 min. Ripening was carried out by continuing mixing at 300 rpm for 1 min. After 1 min, the mixing rate was lowered to 50 rpms for an additional 59 minutes, thereby obtaining an aqueous polyacrylamide premix comprising 2.3 % by weight of the polyacrylamide relating to the total of all components of the aqueous polyacrylamide premix.

The aqueous polyacrylamide premix was used in subsequent Friction Flow Loop testing immediately following the 60 minutes total ripening time.

Example 2:

Ripening time 1 min

An amount of 270ml of water was added into a 600ml beaker while mixing using an overhead mixer with a 75mm half-moon propeller. The mixing rate was set at 300 rpm. Thereafter 30 ml of the polyacrylamide inverse emulsion obtained in course of step 1 was slowly added to the vortex over a few seconds to avoid the formation of lumps. Within 5 s, the inverse emulsion was mixed with the water.

The obtained mixture was allowed to ripen for 1.0 minute. Ripening was carried out by continuous mixing for 1.0 min at 300 rpms.

The aqueous polyacrylamide premix was used in subsequent Friction Flow Loop testing immediately following the 1.0 minute total ripening time.

Friction Loop Apparatus

The friction reduction performance of the friction reducing agent was assessed using a Chandler model M5600 friction loop, which circulates fluid through a section of known diameter pipe to determine the effectiveness and longevity of a friction reducing agent added to a test fluid. Fluid in the loop flows from a -37.8 I (~ 10 gallon) reservoir through a pump, mass flow meter and then two - 250 cm (10 feet) long sections of pipe before returning to the reservoir to be recirculated. Pressure drop is measured over the two sections of pipe. One is 1.27 cm outer diameter (1/2 inch), the other is 1.91 cm outer diameter (3/4” inch), giving different ranges of Reynolds number.

The friction loop was loaded with 37.85 I (10 gallons) of aqueous test fluid (fresh water or brines). The flow rate was set to 37.85 I per minute (10 gallons per minute) and once a stable, initial pressure was recorded. Thereafter, the friction reducing composition to be tested was injected, using a plastic syringe, into the vortex of the fluid reservoir, mixing at 600 rpms using an overhead mixer with a 2 inch 3-blade propeller.

The injection time was taken as the start of the test (time = 0 seconds). The subsequent drop in pressure measured the performance of the friction reducing composition. The pressure data from the 1.27 cm pipe is reported, because it reflected a higher Reynolds number than the 1.91 cm pipe.

Pressure data was converted to friction reduction using the formula:

Initial Pressure with no FR — Pressure with FR

% Friction Reduction (% FR) = Initial Pressure with no FR

Friction loop tests

Comparative example (C1):

Use of inverse emulsion without pre-hydrating

For comparative purposes, the friction loop apparatus was charged with 10 gal (37.8 I) of fresh tap water. 3.78 ml of polyacrylamide inverse emulsion (equating to 25 ppm of polyacrylamide polymer once fully dispersed in the 10 gal (37.8 I) of aqueous fluid) was directly added to the reservoir undergoing agitation to disperse the inverse emulsion. At the same time the aqueous fluid containing the polyacrylamide friction reducer was circulated through the instrument while measuring differential pressure. The change in differential pressure was then recorded over time by the instrument’s software and reported as Friction Reduction (%). The results are shown in figure 1.

Example 3:

Use of the 2.3 wt. % premix of example 2 (1 min ripening)

Under the same conditions, 37.80 g of the aqueous polyacrylamide premix (2.5 wt. %) obtained in course of step 2 example 2 was injected into the reservoir via a syringe (equating to 25 ppm of polyacrylamide polymer once fully dispersed in the 10 gal (37.8 I) of aqueous fluid). The results are shown in figure 1.

Example 4:

Use of the 2.3 wt. % premix of example 1 (60 min ripening)

Under the same conditions, 37.80 g of the aqueous polyacrylamide premix (2.3 wt. %) obtained in course of step 2 example 1 was injected into the reservoir via a syringe (equating to 25 ppm of polyacrylamide polymer once fully dispersed in the 10 gal (37.8 I) of aqueous fluid). The results are shown in figure 1.

Comments: Figure 1 shows the results of the three tests. It represents the Friction Reduction (%) as a function of time.

In the comparative example C1, an inverse emulsion was injected directly into the friction loop. The friction reduction effect was only 58 % and it took about 3 min to get at the number.

Example 3 represents the procedure according to the present invention. At first an aqueous premix (P) comprising 2.3 wt. % of polyacrylamides was prepared and thereafter allowed to ripen for 1 min. After 1 min of ripening, the aqueous premix (P) was injected into the friction loop in the same manner as described above. The friction reduction effect was about 77 % and it took less than one minute to reach at the number.

Example 4 also represents the procedure according to the present invention. The test was carried out as example 3, except that the ripening time was increased from 1 min to 30 min. After 30 min of ripening, the aqueous premix (P) was injected into the friction loop in the same manner as described above. The friction reduction effect increased again to about 80 % and it took less than one minute to reach at the number.

Claims

Claims:

1. Method of fracturing subterranean, oil- and/or gas-bearing formations penetrated by at least a wellbore comprising at least the steps of

(1-3) allowing the aqueous polyacrylamide premix (P) to ripen by allowing the components of the aqueous premix (P) to interact with each other for a period of time after the components have been mixed in step (1-2) and before the mixture is further diluted in step (1-4), wherein said period of time is at least 1 min,

(1-4) mixing the ripened aqueous polyacrylamide premix (P) with at least an aqueous base fluid at the amounts necessary to obtain the aqueous fracturing fluid mentioned above comprising 0.002 % by weight to 0.12 % by weight of polyacrylamides.

2. Method according to claim 1, wherein the ripening time in course of step (1-3) is from 1 min to 1 day.

3. Method according to claim 1, wherein the ripening time in course of step (1-3) is from ½ h to 2 h.

4. Method according to any of claims 1 to 3, wherein step (1-3) is carried out by allowing the aqueous polyacrylamide premix (P) to rest in a vessel.

5. Method according to claim 4, wherein the contents of the vessel is mixed by means of an internal stirrer and/or by circulating through an external circuit mixing circuit by means of a pump.

6. Method according to any of claims 1 to 3, wherein step (1-3) is carried out by filling the premix (P) into a transport unit after step (1-2) at a location A and the transport unit filled with the premix (P) is transported from said location A to a different location B, so that ripening of the premix (P) (step 1-3) happens in course of the transport.

7. Method according to claim 6, wherein the transport unit filled with the premix (P) is transported by means of a truck from location A to location B.

8. Method according to claims 6 or 7, wherein locations A and B are 1 to 500 km apart from each other.

9. Method according to any of claims 1 to 8, wherein the concentration of polyacrylamides in the premix (P) is from 2 % to 8 % by weight, relating to the total of all components of the premix (P).

10. Method according to any of claims 1 to 9, wherein the concentration of polyacrylamide friction reducer is from 20 to 300 ppm, relating to the total of all components of the aqueous fracturing fluid except the proppants.

11. Method according to any of claims 1 to 10, wherein a composition (C-l) is used.

12. Method according to any of claims 1 to 11 , wherein the components of the aqueous fracturing fluid are mixed in a blender.

13. Method according to claim 12, wherein the blender is selected from blenders fixed on a truck, mounted on a trailer or mounted in a skid.

14. Method according to any of claims 1 to 13, wherein the process starts with the injection of an aqueous fracturing fluid not comprising a proppant, followed by the injection of an aqueous fracturing fluid additionally comprising a proppant.

15. Method according to any of claims 1 to 14, wherein during the entire process an aqueous fracturing fluid comprising a proppant is injected.