WO2020079123A1

WO2020079123A1 - Method of fracturing subterranean formations using aqueous solutions comprising hydrophobically associating copolymers

Info

Publication number: WO2020079123A1
Application number: PCT/EP2019/078164
Authority: WO
Inventors: Tobias Joachim Zimmermann; Alexander KRONAST; Anna-Corina SCHMIDT
Original assignee: Basf Se
Priority date: 2018-10-18
Filing date: 2019-10-17
Publication date: 2020-04-23

Abstract

Method of fracturing subterranean, oil and/or gas-bearing formations by injecting a viscosified aqueous fracturing fluid comprising at least an aqueous base fluid, hydrophobically associating polyacrylamides and proppants into a subterranean formation at a pressure sufficient to create fractures wherein the hydrophobically associating polyacrylamides are manufactured by adiabatic gel polymerization of an aqueous monomer solution, wherein the concentration of the monomers in the aqueous solution is from 1mole / kg to 3.3 mole / kg, relating to the total of all components of the aqueous monomer solution and wherein after fracturing the aqueous fracturing fluid is treated with at least one viscosity breaker comprising at least one non-ionic surfactant having an HLB-value of at least 8.

Description

Method of fracturing subterranean formations using aqueous solutions comprising hydrophobically associating copolymers

The present invention relates to a method of fracturing subterranean, oil and/or gas-bearing formations by injecting a viscosified aqueous fracturing fluid comprising at least an aqueous base fluid, hydrophobically associating polyacrylamides and proppants into a subterranean formation at a pressure sufficient to create fractures wherein the hydrophobically associating polyacrylamides are manufactured by adiabatic gel polymerization of an aqueous monomer solution, wherein the concentration of the monomers in the aqueous solution is from 1 mole / kg to 3.3 mole / kg, relating to the total of all components of the aqueous monomer solution and wherein after fracturing the aqueous fracturing fluid is treated with at least one viscosity breaker comprising at least one non-ionic surfactant having an HLB-value of at least 8.

Aqueous solutions of water-soluble, high molecular weight homo- and copolymers of acrylamide may be used for various applications such as mining and oilfield applications, water treatment, sewage treatment, papermaking, and agriculture. Examples include its use in the exploration and production of mineral oil, in particular as thickener in aqueous injection fluids for fracturing, enhanced oil recovery or as rheology modifier for aqueous drilling fluids.

Hydrophobically associating copolymers are basically known in the art.“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.

It is also known in the art to enhance the thickening effect of polyacrylamides by using associative monomers besides acrylamide and optionally further comonomers thereby obtaining hydrophobically associating polyacrylamides. Such associative monomers are water-soluble, monoethylenically unsaturated monomers having at least one hydrophilic group and at least one hydrophobic group. Examples of polyacrylamides comprising associative monomers have been described for example in EP 705 854 B1 , DE 100 37 629 A1 , DE 10 2004 032 304 A1 , WO 2010/133527 A2, WO 2012/069477 A1 , WO 2012/069478 A1 , WO 2012/069438 A1 , WO 2014/095621 A1 , WO 2014/095621 A1 , WO 2015/086468 A1 or WO 2017/121669 A1.

WO 2012/069942 A1 discloses the use of hydrophobically associating polyacrylamides for hydraulic fracturing. It is often necessary, to alter the viscosity of aqueous polymer solutions in course of using them. For example, in hydraulic fracturing, an aqueous solution comprising at least a thickening polymer and proppants is injected into an oil- and/or gas-bearing subterranean formation at a pressure sufficient to extend existing fractures and/or to create new fractures in the formation. While before and in course of fracturing a high viscosity is desirable in order to properly transport the proppants into the formation, after fracturing a high viscosity is undesirable because it is necessary to remove the polymer solution as complete as possible from the formation in order to ensure production of oil and/or gas from the formation without hindrance. For that purpose, it is known in the art to add so-called breakers to the polymer solution which reduce its viscosity. Examples of breakers comprise oxidants or enzymes capable of cleaving the polymer chain.

It is known in the art, that surfactants may alter the viscosity of aqueous solutions of hydrophobically associating polymers.

WO 2012/069438 A1 discloses a method of tertiary oil recovery in which an aqueous solution of hydrophobically associating polymers and surfactants is used. The surfactants are selected from anionic surfactants, non-ionic surfactants and star-like, non-ionic surfactants. Adding said surfactants to an aqueous solution of hydrophobically associating polymers increases its viscosity. Adding a cationic surfactant decreases the viscosity. The publication also discloses a manufacturing method for the polymers by gel polymerization wherein the temperature of the polymer gel after polymerization is about 80°C.

WO 2016/030341 A1 discloses a method of tertiary oil recovery in which an aqueous solution of hydrophobically associating polymers and 0.5 to 50 ppm surfactants is used. The surfactants are selected from non-ionic surfactants having an HLB value of more than 1 1 , anionic surfactants, cationic surfactants, and zwitterionic surfactants. The viscosity of the aqueous solutions is reduced by adding the surfactants. The publication also discloses a manufacturing method for the polymers by gel polymerization wherein the polymerization is started by means of UV light at 20°C.

Our older patent application WO 2019/081328 A1 discloses a process for producing hydrophobically associating polyacrylamides by adiabatic gel polymerization thereby obtaining an aqueous polyacrylamide gel, wherein the concentration of the monomers is from 1 mole / kg to 3.3 mole / kg, the aqueous monomer solution has a temperature not exceeding 30°C before the onset of polymerization, and the temperature of the aqueous

polyacrylamide gel after polymerization is from 45°C to 80°C, preferably 50°C to 70°C. Said polymerization method yields aqueous polyacrylamide solutions having a significantly higher viscosity compared to other polymerization methods.

WO 2017/186697 A1 , WO 2017/186685 A1 , and WO 2017/186698 A1 disclose methods of manufacturing polyacrylamides on-site in containerized plants. Our older applications WO 2019/081318 A1 , WO 2019/081319 A1 , WO 2019/081320 A1 ,

WO 2019/081321 A1 , WO 2019/081323 A1 , WO 2019/081327 A1 , and WO 2019/081330 A1 disclose the manufacture of aqueous polyacrylamide solution on-site in modular plants.

It was therefore an object of the invention to provide an improved process of hydraulic fracturing.

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 a viscosified aqueous fracturing fluid comprising at least an aqueous base fluid, a viscosifier comprising hydrophobically associating polyacrylamides and proppants, wherein the concentration of hydrophobically associating polyacrylamides is from 0.05 % by weight to 2 % by weight, relating to the total of all components of the fracturing fluid except the proppants,

(2) injecting the aqueous fracturing fluid into a wellbore at least at a rate and pressure sufficient to penetrate into the formation, and to initiate or extend fractures in the formation and to transport proppants into thus generated fractures,

(3) treating the injected fracturing fluid with at least one viscosity breaker, thereby

reducing its viscosity,

(4) reducing the applied pressure thereby allowing at least a portion of the injected

fracturing fluid to flow back from the formation into the wellbore, wherein 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 open such fractures, and

(5) removing such flowed back fracturing fluid from the wellbore,

wherein the polyacrylamides for making the aqueous fracturing fluid are provided as aqueous polyacrylamide gel, and

wherein the aqueous polyacrylamide gel is manufactured in an additional process step

(0) by radically polymerizing an aqueous solution comprising water-soluble,

monoethylenically unsaturated monomers comprising at least

• water,

• 40 mole % to 99.995 mole % of at least one monomer (A) selected from the group of (meth)acrylamide, N-methyl(meth)acrylamide, N,N'-dimethyl(meth)acrylamide or N- methylol(meth)acrylamide, wherein the amount relates to the total of all ethylenically unsaturated monomers in the aqueous solution, and

• 0.005 mole % to 1 mole % of at least one monoethylenically unsaturated monomer (B) selected from the group of

H₂C=C(R¹)-0-(-CH₂-CH(R²)-0-)_k-R³ (I), H₂C=C(R¹)-(C=0)-0-(-CH₂-CH(R²)-0-)_k-R³ (II),

H₂C=C(R¹)-R⁴-0-(-CH₂-CH(R⁵)-0-)x-(-CH₂-CH(R⁶)-0-)y-(-CH₂-CH₂0-)z-R⁷ (III), wherein the radicals and indices are defined as follows:

R¹: H or methyl;

R²: independently H, methyl or ethyl, with the proviso that at least

70 mol% of the R² radicals are H,

R³: aliphatic and/or aromatic, linear or branched hydrocarbyl

radicals having 8 to 40 carbon atoms,

R⁴: a single bond or a divalent linking group selected from the

group consisting of -(C_nH_2n)-, -0-(C_n'H_2n')- and

-C(0)-0-(C_n"H_2n")-, wherein n is a natural number from 1 to 6, and n' and n" are natural numbers from 2 to 6,

R⁵: independently H, methyl or ethyl, with the proviso that at least

70 mol% of the R⁵ moieties are H,

R⁶: independently hydrocarbyl radicals of at least 2 carbon

atoms,

R⁷: H or a hydrocarbyl radical having 1 to 30 carbon atoms,

k a number from 10 to 80,

x a number from 10 to 50,

y a number from 5 to 30,

z a number from 0 to 10, and

• 0.5 to 3 % by wt. of a non-polymerizable surfactant (P), wherein the amounts relate to the total of monomers in the aqueous solution,

under adiabatic conditions in the presence of suitable initiators for radical polymerization thereby obtaining an aqueous polyacrylamide gel, wherein

^■ the concentration of the monomers is from 1 mole / kg to 3.3 mole / kg, relating to the total of all components of the aqueous monomer solution,

■ the aqueous monomer solution has a temperature Ti not exceeding 30°C before the onset of polymerization, and

^■ the peak temperature of the aqueous polyacrylamide gel T₂ in course of

polymerization is from 45°C to 70°C,

and wherein the viscosity breaker comprises at least a non-ionic surfactant (S) having an HLB-value of at least 8, and wherein the concentration of the non-ionic surfactant (S) is at least 5 % by weight relating to the concentration of the hydrophobically associating polyacrylamides in the aqueous fracturing fluid. With regard to the invention, the following should be stated specifically:

The method of fracturing subterranean, oil- and/or gas-bearing formations according to the present invention comprises injecting a viscosified aqueous fracturing fluid into at least one wellbore penetrating the formation. Hydrophobically associating polyacrylamides are used as viscosifier which are provided as aqueous polyacrylamide gels. Such aqueous

polyacrylamide gels are not dried but used as such for making the aqueous fracturing fluid.

In a first step [0] of the method of fracturing subterranean formations according to the present invention, an aqueous polyacrylamide gel comprising hydrophobically associating

polyacrylamides is synthesized. In course of step (0), an aqueous solution of water-soluble, ethylenically unsaturated monomers is polymerized in the presence of suitable initiators for radical polymerization under adiabatic conditions thereby obtaining an aqueous

polyacrylamide gel.

Step (0): Synthesis of an aqueous polyacrylamide gel comprising hydrophobically

associating polyacrylamides

Aqueous monomer solution

For polymerization, an aqueous solution comprising at least water and water-soluble, ethylenically unsaturated monomers is provided. Besides the monomers, further additives and auxiliaries may be added to the aqueous monomer solution. As will be detailed below, before polymerization also suitable initiators for radical polymerization are added.

Besides water, the aqueous monomer solution may also comprise additionally water-miscible organic solvents. However, as a rule the amount of water should be at least 70 % by wt. relating to the total of all solvents used, preferably at least 85 % by wt. and more preferably at least 95 % by weight. In one embodiment, only water is used as solvent.

The term“water-soluble monomers” in the context of this invention means that the monomers are 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 monomers (A) 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.

According to the invention, the aqueous solution comprises at least the monoethylenically unsaturated monomers (A) and (B). In other embodiments of the invention further water- soluble, monoethylenically unsaturated monomers (C) different from monomers (A) and (B) may be present. Optionally, further comonomers (D) may be present.

Monomers (A)

Monomers (A) selected from the group of (meth)acrylamide, N-methyl(meth)acryl-amide, N,N'-dimethyl(meth)acrylamide or N-methylol(meth)acrylamide. Monomer (A) preferably is (meth)acrylamide, especially acrylamide. If mixtures of different monomers (A) are used, at least 50 mole % of the monomers (A) should be (meth)acrylamide, preferably acrylamide. In one embodiment of the invention, the monomer (A) is acrylamide.

According to the invention, the amount of the monomers (A) is from 40 mole % to 99.995 mole %, preferably from 45 mole % to 99.995 mole %, for example from 65 mole % to 79.995 mole %, wherein the amount relates to the total of all ethylenically unsaturated monomers in the aqueous solution.

Monomers (B)

Besides monomers (A) the aqueous solution comprises at least one monomer (B). The monomers (B) are selected from monomers having the general formula

H₂C=C(R¹)-0-(-CH₂-CH(R²)-0-)_k-R³ (I),

H₂C=C(R¹)-(C=0)-0-(-CH₂-CH(R²)-0-)_k-R³ (II), or

H₂C=C(R¹)-R⁴-0-(-CH₂-CH(R⁵)-0-)x-(-CH₂-CH(R⁶)-0-)y-(-CH₂-CH₂0-)z-R⁷ (III).

In the formulae (I), (II) and (III), R¹ is H or methyl, preferably H.

The R² moieties are each independently H, methyl or ethyl, preferably H or methyl, with the proviso that at least 70 mole % of the R² radicals are H. Preferably at least 80 mole % of the R² radicals are H, more preferably at least 90 mole %, and they are most preferably exclusively H. This block is thus a polyoxyethylene block which may optionally include certain proportions of propylene oxide and/or butylene oxide units, preferably a pure

polyoxyethylene block.

The number of alkylene oxide units k is a number from 10 to 80, preferably 12 to 60, more preferably 15 to 50 and, for example, 20 to 40. It will be apparent to the person skilled in the art in the field of alkylene oxides that the values mentioned are mean values.

R³ is an aliphatic and/or aromatic, straight-chain or branched hydrocarbyl radical having 8 to 40 carbon atoms, preferably 12 to 32 carbon atoms. In one embodiment, the aliphatic hydrocarbyl groups are those having 8 to 22 and preferably 12 to 18 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 the monomers (B) of the formula (III), an ethylenic H2C=C(R²)- group is bonded via a divalent linking -R⁴-0- group to a polyoxyalkylene radical having block structure, where the - (-CH₂-CH(R⁵)-0-)_X-, -(-CH₂-CH(R⁶)-0-)I- and optionally -(-CH₂-CH₂0-)_z-R⁷ blocks are arranged in the sequence shown in formula (III). The transition between the two blocks may be abrupt or else continuous.

In formula (III), R¹ has the definition already defined, i.e. R¹ is H or a methyl group, preferably H.

R⁴ is a single bond or a divalent linking group selected from the group consisting of

-(CnH2n)-, -0-(Cn'H2n')- and -C(0)-0-(C_n"H2n')-· In the formulae mentioned, n in each case is a natural number from 1 to 6; n' and n" are each a natural number from 2 to 6. In other words, the linking group comprises straight-chain or branched aliphatic hydrocarbyl groups which have 1 to 6 carbon atoms and may be joined directly, via an ether group -O- or via an ester group -C(0)-0- to the ethylenic H2C=C(R²)- group. The -(Cnl-hn)-, -(Cn l- n )- and -(C_n Thn )- groups are preferably linear aliphatic hydrocarbyl groups.

Preferably, the -(Cnl-hn)- group is a group selected from -CH2-, -CH2-CH2- and -CH2-CH2- CH2-, more preferably a methylene group -CH2-.

Preferably, the -0-(O_hΉ2_h )- group is a group selected from -O-CH2-CH2-, -O-CH2-CH2-CH2- and -O-CH2-CH2-CH2-CH2-, more preferably -O-CH2-CH2-CH2-CH2-.

Preferably, the -C(0)-0-(C_n H2n”)- group is a group selected from -C(0)-0-CH2-CH2-, - C(0)0-CH(CH₃)-CH₂-, -C(0)0-CH₂-CH(CH₃)-, -C(0)0-CH2-CH₂-CH2-CH₂- and -C(0)0-CH₂- CH2-CH2-CH2-CH2-CH2-, more preferably -C(0)-0-CH₂-CH₂- and -C(0)0-CH₂-CH₂-CH₂- CH2-, and most preferably is -C(0)-0-CH2-CH2-.

More preferably, the R⁴ group is a -0-(C_n'H2n')- group, most preferably a group

-O-CH2-CH2-CH2-CH2-.

In the -(-CH₂-CH(R⁵)-0-)_X- block, the R⁵ radicals are independently H, methyl or ethyl, preferably H or methyl, with the proviso that at least 70 mole % of the R⁵ radicals are H. Preferably at least 80 mole % of the R⁵ radicals are H, more preferably at least 90 mole %, and they are most preferably exclusively H. This block is thus a polyoxyethylene block which may optionally include certain proportions of propylene oxide and/or butylene oxide units, preferably a pure polyoxyethylene block. The number of alkylene oxide units x is a number from 10 to 50, preferably 12 to 40, more preferably 15 to 35, even more preferably 20 to 30 and, for example, 23 to 26. It will be apparent to the person skilled in the art in the field of polyalkylene oxides that the numbers mentioned are mean values of distributions.

In the second -(CH₂-CH(R⁶)-0)_y- block, the R⁶ radicals are independently hydrocarbyl radicals of at least 2 carbon atoms, for example 2 to 10 carbon atoms, preferably 2 or 3 carbon atoms. This may be an aliphatic and/or aromatic, linear or branched carbon radical. Preference is given to aliphatic radicals.

Examples of suitable R⁶ radicals include ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl or n-decyl and phenyl. Examples of preferred radicals include ethyl, n-propyl, n-butyl, n-pentyl, especially ethyl and/or n-propyl radicals, and more preferably ethyl radicals. The -(-CH2-CH(R⁶)-0-)y- block is thus a block consisting of alkylene oxide units having at least 4 carbon atoms.

The number of alkylene oxide units y is a number from 5 to 30, preferably 8 to 25.

In one embodiment of the invention, R⁶ is ethyl, i.e. the alkylene oxy groups are butylene oxy groups, and y is a number from 10 to 25, in particular from 12 to 25, preferably from 12 to 20 and more preferably from 14 to 18.

In formula (III), z is a number from 0 to 10, preferably 0 to 5, i.e. the terminal block of ethylene oxide units is thus only optionally present. In one embodiment of the invention, z is a number > 0 to 10, especially > 0 to 10, preferably >0 to 5, for example from 1 to 5.

The R⁷ radical is H or a preferably aliphatic hydrocarbyl radical having 1 to 30 carbon atoms, preferably 1 to 10 and more preferably 1 to 5 carbon atoms. R⁷ is preferably H, methyl or ethyl, more preferably H or methyl and most preferably H.

In a preferred embodiment of the invention, at least one of the monomers (B) is a monomer of the formula (III). Preferably, all of the monomers (B) are monomers of the formula (III).

In one embodiment, the monomers (III) are monomers (Ilia), having the general formula

H2C=CH-0-(Cn_'H2n_')-0-(-CH2-CH2-0-)x-(-CH2-CH(C₂H₅)-0-)y-(-CH2-CH2-0-)z-H (Ilia), wherein n is a natural number from 2 to 6, x is from 15 to 35, y is from 10 to 25, and z is from >0 to 5. Preferably, in formula (Ilia), n is 4, x is from 20 to 30, y is from 12 to 20, and z is from 1 to 5. For example, in formula (Ilia), n is 4, x is from 23 to 36, y is from 14 to 28, and z is from 1 to 5 In another embodiment of the invention, a mixture of at least two different monomers (B) of the formula (III) is used, where the radicals R¹, R⁴, R⁵, R⁶, and R⁷ and the indices x and y are the same in each case. In addition, z = 0 in one of the monomers, while z is a number > 0 to 10, preferably 1 to 4, in the other. Said preferred embodiment is thus a mixture of the following composition:

H₂C=C(R¹)-R⁴-0-(-CH₂-CH(R⁵)-0-)_x-(-CH₂-CH(R⁶)-0-)_y-H (Nib) and

H₂C=C(R¹)-R⁴-0-(-CH₂-CH(R⁵)-0-)_x-(-CH₂-CH(R⁶)-0-)_y-(-CH₂-CH₂0-)_z-H (lllc), where the radicals and indices have the definition outlined above, including the preferred embodiments thereof, with the proviso that, in the formula (lllc), z is a number > 0 to 10.

Preferably, in the formulae (Nib) and (lllc), R¹ is H, R⁴ is -O-CH2CH2CH2CH2-, R⁵ is H, R⁶ is ethyl, x is 20 to 30, preferably 23 to 26, y is 12 to 25, preferably 14 to 18, and z is 3 to 5.

The monomers (B) of the formulae (I), (II) and (III), the preparation thereof and acrylamide copolymers comprising these monomers and the preparation thereof are known in principle to those skilled in the art, for example from WO 85/03510 A1 , WO 2010/133527 A1 , WO 2012/069478 A1 , WO 2014/095608 A1 , WO 2014/095621 A1 and WO 2015/086486 A1 and in the literature cited therein.

According to the invention, the amount of the monomers (B) is 0.005 mole % to 1 mole % based on the sum total of all the monomers, preferably 0.005 mole % to 0.2 mole %, and more preferably 0.005 mole % to 0.1 mole %.

Monomers (C)

In other embodiments of the invention in the monomer aqueous solution further water- soluble, monoethylenically unsaturated monomers (C) different from monomers (A) and (B) may be present. Preferably, the hydrophobically associating polyacrylamides according to the present invention comprise at least the monomers (A), (B), and (C).

Basically, the kind of water-soluble monomers (C) is not limited and depends on the desired properties and the desired use of the hydrophobically associating polyacrylamides to be manufactured. The amount of monomers (C) may be up to 59.995 mole % relating to the total of all monomers, for example from 1 mol % to 59.995 mole % or from 10 mole % to 59.995 mole %, for example from 20 mole % to 34.995 mole %.

Neutral monomers (C)

Examples of monomers (C) include neutral monomers comprising hydroxyl and/or ether groups, for example hydroxyethyl(meth)acrylate, hydroxypropyl(meth)acrylate, allyl alcohol, hydroxyvinylethylether, hydroxyvinylpropylether, hydroxyvinylbutylether, polyethylene glycol (meth)acrylate, N-vinylformamide, N-vinylacetamide, N-vinylpyrrolidone or N- vinylcaprolactam, and vinyl esters, for example vinylformate or vinyl acetate.

Anionic monomers (C)

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 especially alkali metal ions such as Li⁺, Na⁺ or K⁺, and also ammonium ions such as NH₄ ⁺ 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]⁺, [NH₂(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 Monomers (C)

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. Further comonomers (D)

Besides the monomers (A), (B), and optionally (C), the aqueous monomer solution may comprise further ethylenically unsaturated monomers different from (A), (B), and (C).

Examples comprise water-soluble, ethylenically unsaturated monomers having more than one ethylenic group. Monomers of this kind can be used in special cases in order to achieve easy crosslinking of the acrylamide polymers. The amount of such monomers comprising more than one ethylenically unsaturated group should generally not exceed 1 mole %, preferably 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, in particular only monoethylenically unsaturated monomers (A), (B), and (C) are used.

Concentration of the monomers

According to the present invention, the concentration of the monomers is from 1 mole / kg to 3.3 mole / kg, relating to the total of all components of the aqueous monomer solution.

Preferably, the concentration is from 1.5 mole / kg to 3.3 mole / kg. As will be detailed below, the choice of said concentration range yields hydrophobically associating polyacrylamides with improved viscosity efficiency.

Further components

Besides the monomers, further additives and auxiliaries may be added to the aqueous monomer solution. As will be detailed below, before polymerization also suitable initiators for radical polymerization are added. Examples of such further additives and auxiliaries comprise complexing agents, defoamers, surfactants, stabilizers, and bases or acids for adjusting the pH value. In certain embodiments of the invention, the pH-value of the aqueous monomer solution is adjusted to values from pH 4 to pH 7, for example to pH 6 to pH 7.

In one embodiment, the aqueous monomer solution comprises at least one stabilizer for the prevention of polymer degradation. Such stabilizers for the prevention of polymer degradation are what are called“free-radical scavengers”, i.e. compounds which can react with free radicals (for example free radicals formed by heat, light, redox processes), such that said radicals can no longer attack and hence degrade the polymer. Using such kind of stabilizers for the stabilization of aqueous solutions of polyacrylamides basically is known in the art, as disclosed for example in WO 2015/158517 A1 , WO 2016/131940 A1 , or WO 2016/131941 A1.

The stabilizers may be selected from the group of non-polymerizable stabilizers and polymerizable stabilizers. Polymerizable stabilizers comprise a monoethylenically unsaturated group and become incorporated into the polymer chain in course of polymerization. Non-polymerizable stabilizers don’t comprise such monoethylenically unsaturated groups and are not incorporated into the polymer chain.

In one embodiment of the invention, stabilizers are non-polymerizable stabilizers selected from the group of sulfur compounds, sterically hindered amines, N-oxides, nitroso

compounds, aromatic hydroxyl compounds or ketones.

Examples of sulfur compounds include thiourea, substituted thioureas such as N,N‘- dimethylthiourea, N,N‘-diethylthiourea, N,N‘-diphenylthiourea, thiocyanates, for example ammonium thiocyanate or potassium thiocyanate, tetramethylthiuram disulfide, and mercaptans such as 2-mercaptobenzothiazole or 2-mercaptobenzimidazole or salts thereof, for example the sodium salts, sodium dimethyldithiocarbamate, 2,2‘-dithiobis(benzothiazole), 4,4‘-thiobis(6-t-butyl-m-cresol).

Further examples include dicyandiamide, guanidine, cyanamide, paramethoxyphenol, 2,6-di- t-butyl-4-methylphenol, butylhydroxyanisole, 8-hydroxyquinoline, 2,5-di(t-amyl)- hydroquinone, 5-hydroxy-1 ,4-naphthoquinone, 2,5-di(t-amyl)hydroquinone, dimedone, propyl 3,4,5-trihydroxybenzoate, ammonium N-nitrosophenylhydroxylamine, 4-hydroxy-2, 2,6,6- tetramethyoxylpiperidine, (N-(1 ,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine and

1 ,2,2,6,6-pentamethyl-4-piperidinol.

Preference is given to sterically hindered amines such as 1 ,2,2,6, 6-pentamethyl-4-piperidinol and sulfur compounds, preferably mercapto compounds, especially 2-mercaptobenzothiazole or 2-mercaptobenzimidazole or the respective salts thereof, for example the sodium salts, and particular preference is given to 2-mercaptobenzothiazole or salts thereof, for example the sodium salts.

The amount of such non-polymerizable stabilizers -if present- may be from 0.1 % to 2.0 % by weight, relating to the total of all monomers in the aqueous monomer solution, preferably from 0.15 % to 1.0 % by weight and more preferably from 0.2 % to 0.75 % by weight.

In another embodiment of the invention, the stabilizers are polymerizable stabilizers substituted by a monoethylenically unsaturated group. With other words, such stabilizers are also monomers (C). Examples of stabilizers comprising monoethylenically unsaturated groups comprise (meth)acrylic acid esters of 1 ,2,2,6,-pentamethyl-4-piperidinol or other monoethylenically unsaturated groups comprising 1 ,2,2,6,6-pentamethyl-piperidin-4-yl groups. Specific examples of suitable polymerizable stabilizers are disclosed in WO

2015/024865 A1 , page 22, lines 9 to 19. In one embodiment of the invention, the stabilizer is a (meth)acrylic acid ester of 1 ,2,2,6,6-pentamethyl-4-piperidinol. The amount of polymerizable stabilizers -if present at all- may be from 0.01 to 2% by weight, based on the sum total of all the monomers in the aqueous monomer solution, preferably from 0.02 % to 1 % by weight, more preferably from 0.05 % to 0.5 % by weight.

In one embodiment, the aqueous monomer solution comprises at least one non- polymerizable surfactant (P). Examples of suitable surfactants including preferred amounts have been disclosed in WO 2015/158517 A1 , page 19, line, 23 to page 20, line 27.

The non-polymerizable surfactant (P) preferably is a nonionic surfactant, although anionic and cationic surfactants are also suitable, provided, that they do not take part in the polymerization reaction.

These compounds may especially be surfactants, preferably nonionic surfactants of the general formula R¹²-Y wherein R¹² is a hydrocarbyl radical having 8 to 32, preferably 10 to 20 and more preferably 12 to 18 carbon atoms and Y is a hydrophilic group, preferably a nonionic hydrophilic group, and in particular a polyalkoxy group, especially a polyethoxy group.

The nonionic surfactant is preferably an ethoxylated long-chain aliphatic alcohol having the general formula R¹²-0-(CH₂CH₂0)_mH, wherein R¹² has the meaning as defined above and m is a number from 5 to 20, preferably 8 to 18. Examples include Ci Ci -fatty alcohol ethoxylates, Ci₆Cis-fatty alcohol ethoxylates, C13-OXO alcohol ethoxylates, C- -oxo alcohol ethoxylates, C13C15-OXO alcohol ethoxylates, Cio-Guerbet alcohol ethoxylates and alkylphenol ethoxylates, wherein the number of ethoxy groups is m. Optionally, a small amount of ethoxy groups may be substituted by propyleneoxy and/or butyleneoxy units, although the amount as ethyleneoxy units should generally be at least 80 mol% based on all the alkyleneoxy units.

In one embodiment of the invention, the non-polymerizable surfactant (P) may be selected from the group of ethoxylated alkylphenols, ethoxylated saturated iso-C13 alcohols and/or ethoxylated C10 Guerbet alcohols, where 5 to 20 ethyleneoxy units, preferably 8 to 18 ethyleneoxy units, are present in each of the alkyleneoxy radicals. For example, the non- polymerizable surfactant (P) may have the formula iso-Ci₃-0(CH₂CH₂0)_mH, wherein m is from 8 to 18, preferably from 10 to 14.

In the manufacture of hydrophobically associating polyacrylamides, the surfactants lead to a distinct improvement of the product properties.

If present, such a non-polymerizable surfactant (P) may be used in an amount from 0.1 to 5% by weight, for example from 0.5 to 3 % by weight based on the amount of all the monomers used.

Preferred compositions In one embodiment, the aqueous solution comprises 40 mole % to 99.995 mole % of acrylamide and 0.005 mole % to 0.2 mole % of monomers (B), preferably those of formula (III), more preferably those of formula (Ilia) wherein the amounts relate to the total amount of all monomers in the aqueous monomer solution. Besides acrylamide and monomers (B) further monomers (C) and optionally (D) may be present.

In another embodiment, the aqueous solution comprises 40 mole % to 98.995 mole % of acrylamide and 0.005 mole % to 0.2 mole % of monomers (B), preferably those of formula (III), more preferably those of formula (Ilia), and 1 mole % to 59.995 mole % of at least one monomer (C), preferably an anionic monomer (C), more preferably acrylic acid and/or ATBS or salts thereof.

In another embodiment, the aqueous solution comprises 65 mole % to 79.995 mole % of acrylamide and 0.005 mole % to 0.2 mole % of monomers (B), preferably those of formula (III), more preferably those of formula (Ilia), and 20 mole % to 34.995 mole % of at least one monomer (C), preferably an anionic monomer (C), more preferably acrylic acid and/or ATBS or salts thereof.

In all embodiments, the amounts relate to the total amount of all monomers in the aqueous monomer solution.

Polymerization

According to the present invention, the aqueous monomer solution is polymerized in the presence of suitable initiators for radical polymerization under adiabatic conditions thereby obtaining an aqueous polyacrylamide gel.

Such a polymerization technique is also briefly denominated by the skilled artisan

as“adiabatic gel polymerization”. Reactors for adiabatic gel polymerization are unstirred.

Due to the relatively high monomer concentration the aqueous monomer solution used solidifies in course of polymerization thereby yielding an aqueous polymer gel. The term “polymer gel” has been defined for instance by L. Z. Rogovina et al., Polymer Science, Ser.

C, 2008, Vol. 50, No. 1 , pp. 85-92.

“Adiabatic” is understood by the person skilled in the art to mean that there is no exchange of heat with the environment. This ideal is naturally difficult to achieve in practical chemical engineering. In the context of this invention,“adiabatic” shall consequently be understood to mean“essentially adiabatic”, meaning that the reactor is not supplied with any heat from the outside during the polymerization, i.e. is not heated, and the reactor is not cooled during the polymerization. However, it will be clear to the person skilled in the art that - according to the internal temperature of the reactor and the ambient temperature - certain amounts of heat can be released or absorbed via the reactor wall because of temperature gradients.

Naturally, this effect plays an ever lesser role with increasing reactor size.

The polymerization of the aqueous monomer solution generates polymerization heat. Due to the adiabatic reaction conditions the temperature of the polymerization mixture increases in course of polymerization.

Suitable reactors for performing adiabatic gel polymerizations are known in the art.

Particularly advantageously, the polymerization can be conducted using conical reactors, as described, for example, by US 5,633,329 or US 7,619,046 B2. In one embodiment of the invention, the reactor comprises a cylindrical upper part and a conical part at its lower end. At the lower end, there is a bottom opening which may be opened and closed. After

polymerization, the aqueous polyacrylamide gel formed is removed through the opening.

The volume of suitable polymerization units typically is more than 1 m³, for example from 1 m³ to 200 m³, in particular from 5 m³ to 200 m³. In one embodiment, the polymerization unit has a volume from 5 m³ to 40 m³, and more preferably 20 m³ to 30 m³. In other

embodiments, larger polymerization units may be used, for example polymerization units having a volume from 100 m³ to 200 m³, or from 120 m³ to 160 m³.

The polymerization is performed in the presence of suitable initiators for radical

polymerization. Suitable initiators for radical polymerization, in particular for adiabatic gel polymerization are known to the skilled artisan.

In a preferred embodiment, redox initiators are used for initiating. Redox initiators can initiate a free-radical polymerization even at temperatures of less than +5 °C. Examples of redox initiators are known to the skilled artisan and include systems based on Fe²⁺/Fe³⁺ - H O , Fe²⁺/Fe³⁺ - alkyl hydroperoxides, alkyl hydroperoxides - sulfite, for example t-butyl hydroperoxide - sodium sulfite, peroxides - thiosulfate or alkyl hydroperoxides - sulfinates, for example alkyl hydroperoxides/ hydroxymethane-sulfinates, for example t-butyl hydroperoxide - sodium hydroxymethanesulfinate.

Furthermore, water-soluble azo initiators may be used. The azo initiators are preferably fully water-soluble, but it is sufficient that they are soluble in the monomer solution in the desired amount. Preferably, azo initiators having a 10 h t- in water of 40 °C to 70 °C may be used. The 10-hour half-life temperature of azo initiators is a parameter known in the art. It describes the temperature at which, after 10 h in each case, half of the amount of initiator originally present has decomposed.

Examples of suitable azo initiators having a 10 h t- temperature between 40 and 70 °C include 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (10 h t- (water): 44 °C), 2,2'-azobis(2-methylpropionamidine) dihydrochloride (10 h t- (water): 56 °C), 2,2'-azobis[N- (2-carboxyethyl)-2-methylpropionamidine hydrate (10 h ti_/2 (water): 57 °C), 2,2'-azobis{2-[1- (2-hydroxyethyl)-2-imidazolin-2-yl]propane} dihydrochloride (10 h ti_/2 (water): 60 °C), 2,2'- azobis(1-imino-1-pyrrolidino-2-ethylpropane) dihydrochloride (10 h ti_/2 (water): 67 °C) or azobis(isobutyronitrile) (10 h ti_/2 (toluene): 67 °C).

In one embodiment of the invention a combination of at least one redox initiator and at least one azo initiator is used. The redox initiator efficiently starts polymerization already at temperatures below +5 °C. When the reaction mixture heats up, also the azo initiators decompose and also start polymerization.

In the following, the temperature of the aqueous monomer solution before the onset of polymerization shall be denominated as Ti and peak temperature of the aqueous polymer in course of polymerization shall be denominated as T₂. It goes without saying that T₂ > Ti.

“Peak temperature” shall mean the highest temperature of the polymerization mixture in course of polymerization. Under ideal adiabatic conditions, this temperature is equivalent to the temperature of the aqueous polyacrylamide gel after polymerization (and the temperature would stay at the peak temperature until removal of the gel from the reactor). In the real world, small heat losses through the reactor wall cannot be avoided completely, and therefore, the peak temperature of the gel may be slightly higher than the temperature at the end of polymerization. It is self-evident, that this effect is more pronounced in those portions of the gel close to the reactor walls as compared to gel portions in the center of the reactor, and generally heat losses become less important with increasing reactor size.

Within the context of the present invention, the temperature Ti should not exceed 30°C. In particular, Ti should not exceed 25 °C, preferably not 20 °C.

As the polymerization is carried out under adiabatic conditions, the temperature T₂ reached in course of polymerization is not influenced by external heating or cooling but only depends on the polymerization parameters chosen. By suitable choice of the polymerization parameters, the skilled artisan can adjust T₂. Because the reaction is adiabatic, the temperature increase in course of polymerization basically depends on the heat of polymerization generated in course of polymerization, the heat capacity of contents of the polymerization unit and the temperature Ti of the monomer solution, i.e. the temperature before the onset of polymerization. Due to high water contents of the mixture for

polymerization the heat capacity of the mixture for polymerization is dominated by the heat capacity of water and it may of course be measured. The polymerization heat per mole for common monoethylenically unsaturated monomers is known in the art and may therefore be gathered from the scientific literature. Of course, it may also be measured. So, it is possible for the skilled artisan to calculate at least roughly the heat of polymerization for specific monomer compositions and specific monomer concentrations. The higher the concentration of the monoethylenically unsaturated monomers in the aqueous solution the more heat of polymerization is generated. T₂ may be roughly calculated from the parameter mentioned above by the formula T₂ = Ti + [(polymerization heat) / (heat capacity)].

According to the invention, the starting temperature Ti and the concentration of the monomers in the aqueous monomer solution is selected such, that the temperature T₂ from 45 °C to 70 °C, preferably from 50 °C to 70 °C, for example from 55 °C to 70 °C.

So, in certain embodiments of the invention, Ti may be in the range from -5 °C to +5 °C, in other embodiments of the invention Ti may be in the range from +5 °C and +25 °C.

In one embodiment, Ti is from +5 °C to +15 °C and T₂ is from 45 °C to 70 °C, preferably from 50 °C to 70 °C and for example from 55 °C to 70 °C.

As will be detailed in the examples and comparative examples, limiting T₂ to not more than 70 °C by a suitable choice of the concentration of the monomers and Ti yields

hydrophobically associating polyacrylamides having improved viscosity at the same polymer concentration. Furthermore, the viscosity of aqueous solutions of such polymers is far more sensitive to added surfactants than of polymers obtained at higher temperatures T₂.

Before polymerization oxygen from the reactor and the aqueous monomer solution to be polymerized is removed in basically known manner. Deoxygenation is also known as inertization. By the way of example, inert gases such as nitrogen or argon may be injected into the reactor filled with the aqueous monomer solution.

The polymerization yields an aqueous polyacrylamide gel hold in the polymerization reactor. For further processing, the aqueous polyacrylamide gel is removed from the polymerization reactor. Preferably, the aqueous polyacrylamide gel may be removed by applying pressure onto the gel and pressing it through an opening in the polymerization reactor. By the way of example, pressure may be generated by mechanical means such as a piston, by means of gases such as compressed air, nitrogen, argon or by means of aqueous fluids, in particular water.

In the method of fracturing subterranean formations according to the present invention the aqueous polyacrylamide gel is used as such for making the aqueous fracturing fluid, i.e. there is no step of drying the aqueous polyacrylamide gels and re-dissolving the resulting dry polyacrylamide powders. The direct use of gels saves significant costs for drying and re- dissolving polyacrylamides

As will be shown below, for use the aqueous polyacrylamide gels are dissolved in an aqueous fluid. Said step provides the opportunity to simultaneously modify the polymers obtained. For that purpose, suitable agents for modifying the polymers may be added to the aqueous fluid used for dissolving the aqueous polyacrylamide gel. By the way of example, the polyacrylamides may be partially hydrolyzed thereby obtaining polyacrylamides comprising also -COOH groups or salts thereof. In certain embodiments, about 30 mol % of the amide groups may be hydrolyzed to carboxylic groups. Partially hydrolyzed

polyacrylamides are known in the art. For that purpose, bases such as NaOH are added to the aqueous liquid. In another embodiment, hydroxylamine and a base may be added to the aqueous liquid thereby obtaining polyacrylamides in which a part of the amide groups are converted to hydroxamic acid groups.

The weight average molecular weight M_w of the hydrophobically associating polyacrylamides to be used is at least 100,000 g/mol, in particular from 1 ,000,000 g/mol to 30,000,000 g/mol.

Provision of the aqueous polyacrylamide gel to the site-of-use

In one embodiment of the invention, the aqueous polyacrylamide gel is manufactured on-site.

Όh-site” shall mean, that the aqueous polyacrylamide gel is manufactured at the location at which the polyacrylamide solutions are used or at least at a location close to such a location of use. In one embodiment, the manufacture may be carried out at an oil and/or gas well to be treated with aqueous polyacrylamide solutions for example in a hydraulic fracturing operation, or close to such an oil and/or gas well. In another embodiment, the location of manufacture may be on an oilfield in between a plurality of such oil and/or gas wells or at one of them. Aqueous polyacrylamide solutions may be distributed from such a location to all injection wells, for example by means of pipelines. Suitable plants for on-site manufacturing have been disclosed for example in WO 2017/186697 A1 , WO 2017/186685 A1 , or WO 2017/186698 A1 .

Όh-site” manufacture is advantageous because it is not necessary to dry the aqueous polymer gel obtained in course of gel polymerization and to re-dissolve the powder later but rather the aqueous polyacrylamide gel can be dissolved directly. It should be kept in mind that aqueous polyacrylamide gels obtained from gel polymerization typically comprise from 65 % to 80 % of water. So,“drying” such polyacrylamide gels does not mean to remove only some residual moisture in course of drying but rather about 0.55 to 0.75 kg of water need to be removed per kg of polymer gel. It goes without saying that such a procedure is very energy-intensive. Furthermore, dryers and equipment for post-processing dried powders are needed.

In a preferred embodiment of the invention, the aqueous polyacrylamide gel is manufactured in a modular plant.

The modular plant comprises relocatable units. Each relocatable unit bundles certain functions of the plant.“Relocatable unit” means that the unit is transportable basically as a whole and that is it not necessary to disassemble the entire unit into individual parts for transport. Transport may happen on trucks, railcars or ships.

In one embodiment, such modular, relocatable units are containerized units which may be transported in the same manner as closed intermodal containers for example on trucks, railcars or ships. In another embodiment, the relocatable units may be fixed on trucks or on trailers. With other words, for such relocatable units not a container or something similar is deployed, but the entire truck or the trailer including the unit in its loading spaces is deployed. Such a modular construction using relocatable units provides the advantage, that the plant may be easily relocated if aqueous polyacrylamide solutions are no longer needed at one location but at another location.

Examples of such relocatable units comprise units for storing the monomers and other raw materials, mixing monomers, polymerization and dissolution of the aqueous polymer gels obtained. For performing the process according to the present invention individual units are connected with each other in a suitable manner thereby obtaining a production line.

In another embodiment of the invention, the plant for manufacturing polyacrylamide gels, in particular, a modular plant for manufacturing the aqueous polyacrylamide gels is not located on-site, but at a different manufacturing location apart from the site-of-use. The aqueous polyacrylamide gel is then transported to the site-of-use for manufacturing the aqueous fracturing fluid and use in the method of fracturing subterranean, oil- and/or gas-bearing formations. By the way of example, a relocatable polymerization unit may be used for polymerization and the polymerization unit filled with aqueous polyacrylamide gel is transported to the site of use wherein it can be removed from the polymerization unit.

As will be outlined below, it may be helpful to pre-dilute the aqueous polyacrylamide gel with some aqueous liquid before preparing the aqueous fracturing fluid. In another embodiment of the invention, such a pre-dilution step may also be carried out in a plant apart from the site- of-use. In such case, a pre-diluted aqueous fracturing fluid is transported from the site-of- manufacture to the site-of-use.

Method of fracturing

For applying the method of fracturing subterranean oil- and/or gas-bearing formation according to the present invention to the formation, 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 comprises at least six steps (0), (1 ), (2), (3), (4), and (5). Step (0), i.e. the manufacture of aqueous gels comprising hydrophobically associating polyacrylamides has already been detailed above.

Step (1 )

In course of step (1 ), an aqueous fracturing fluid comprising at least an aqueous base fluid, a viscosifier comprising hydrophobically associating polyacrylamides and proppants is provided. The hydrophobically associating polyacrylamides increase the viscosity of the aqueous fluid significantly, thereby ensuring that the proppants are properly transported by the fracturing fluid into the subterranean formation.

The aqueous base fluid comprises water. Examples of aqueous base fluids comprise fresh water, brines, sea water, formation water or mixtures of such fluids.

In one embodiment, the aqueous base fluid may have a salinity of 30,000 ppm to 350,000 ppm. The salts may especially be alkali metal salts and alkaline earth metal salts. Examples of typical cations comprise Na⁺, K⁺, Mg²⁺ or Ca²⁺, and examples of typical anions comprise chloride, bromide, hydrogen carbonate, sulfate or borate. Furthermore, alkaline earth metal ions may be present, and the weight ratio of alkali metal ions/alkaline earth metal ions is generally > 2, preferably > 3. The anions present are generally at least one or more than one halide ion, especially at least Ch In general, the amount of Ch is at least 50% by weight, preferably at least 80% by weight, based on the sum of all anions.

Proppants are small hard particles which cause that fractures formed in course of the process do not close after removing the pressure. Suitable proppants 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. The amount of proppants in the aqueous fracturing fluid may be from 50 kg/m³ to 3500 m³/kg of the fracturing fluid, preferably from 50 kg/m³ to 1200 kg/m³ of the fracturing fluid.

Hydrophobically associating polyacrylamides to be used in the present application and their manufacture have been described above.

The aqueous fracturing fluid may comprise optionally further components. Examples of such additional components comprise acids, biocides, buffers, clay stabilizers, corrosion inhibitors, defoamers, non-emulsifying agents, scale inhibitors, oxygen scavengers, or flowback aids. The skilled artisan may select such further depending on the needs of the frac job.

For making the fracturing fluid, the hydrophobically associating polyacrylamides are provided as aqueous polyacrylamide gel as already outlined above. The aqueous fracturing fluid may be manufactured by mixing at least said aqueous polyacrylamide gel comprising hydrophobically associating polyacrylamides, the aqueous base fluid and the proppants.

It is advisable, to comminute the gel obtained from polymerization before mixing it with the aqueous base fluid, the proppants and optionally further components.

The mixing step may be a one-step process or mixing may be carried out in two or more consecutive steps. In one embodiment of the invention, the aqueous polyacrylamide gel is pre-diluted with a part of aqueous base fluid thereby obtaining a more diluted polyacrylamide gel or already a polyacrylamide solution. Said more diluted gel or solution is then mixed in a second step with additional aqueous base fluid, the proppants and optionally further components thereby obtaining the final aqueous fracturing fluid. In one embodiment, the aqueous polyacrylamide gel may be diluted in a first step to a concentration of 1 % to 8 % by weight, preferably 3 % to 7 % by weight, relating to the total of all components of the diluted gel.

Methods of mixing the components are known in the art. For example, so-called blenders (often mounted on trucks) may be used. In one embodiment of the present invention, an aqueous base fluid, proppants, and a pre-diluted aqueous polyacrylamide gel, preferably having a polyacrylamide concentration from 3 % to 7 % by weight and optionally further components are mixed with each other by means of a customary blender thereby obtaining an aqueous fracturing fluid.

The amount of the hydrophobically associating polyacrylamides in the aqueous fracturing fluid is from 0.05 % to 2 % by weight, relating to the aqueous fracturing fluid except the proppants, in particular from 0.05 % to 0.5 % by weight or from 0.05 % by wt. to 0.3 % by weight, for example from 0.1 % by wt. to 0.3 % by weight.

Step (2)

In course of process step (2) the aqueous fracturing fluid is injected into a wellbore at a rate and pressure sufficient to flow into the formation and to initiate or extend a fracture in the formation. In order to initiate or to extend fractures in the formation a bottomhole pressure sufficient to open a fracture in the formation is necessary. 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. Methods and equipment for fracturing procedures are known to the skilled artisan. The aqueous fracturing fluid simultaneously transports suspended proppants and the proppants become deposited into the fractures and hold fractures open after the pressure exerted on the fracturing fluid has been released Step (3)

In course of step (3), the injected fracturing fluid is treated with at least one viscosity breaker, thereby reducing its viscosity, and wherein the viscosity breaker comprises at least a non- ionic surfactant (S) having an HLB-value of at least 8. Step (3) is necessarily carried out after step (2).

According to the invention, the viscosity breaker comprises at least a non-ionic surfactant (S) having an HLB > 8. Of course, a mixture of two or more different surfactants (S) may be used.

HLB stands for hydrophilic-hydrophobic balance and is a number for characterizing the hydrophilic or hydrophobic nature of surfactants, in particular non-ionic surfactants.

Within the scope of the present invention, we refer to the following definition of the HLB-value which was suggested by Griffin:

HLB = 20 (1 - Mi/ M),

wherein Mi is the molar mass of the lipophilic parts of the molecule and M is the total molar mass. Details may be found in H.-D. Dorfler,„Grenzflachen und kolloid-disperse System ', Springer Verlag 2002, Kapitel 9.3„Physikalische Eigenschaften und Wirkungen der Tenside".

Preferably the HLB is ³ 9, more preferably > 10. In particular, the HLB-value may be a number from 8 to 18, preferably from 9 to 18, more preferably from 10 to 18.

Basically, any kind of non-ionic surfactant may be used which has an HLB of at least 8.

In one embodiment of the invention, the non-ionic surfactant (S) may be selected from non- ionic surfactants having the general formula (IV)

R⁸-0-(CH₂CH₂0)_IH (IV),

wherein R⁸ is an aliphatic, saturated or unsaturated, linear or branched hydrocarbon moiety having 8 to 22 carbon atoms, preferably 10 to 20 carbon atoms and more preferably 12 to 18 carbon atoms. I is a number from 5 to 20, preferably from 8 to 15.

In one embodiment, R⁸ is a branched saturated hydrocarbon moiety derived from an oxo- alcohol, preferably a moiety having 13 and/or 15 carbon atoms.

Examples of surfactants of formula (IV) comprise surfactants iCi₃-0-(CH₂CH₂0)iH, wherein I is from 10 to 15 and 1C13 is a branched saturated hydrocarbon moiety derived from an oxo- alcohol. In one embodiment of the invention, in the method according to the present invention hydrophobically associating polyacrylamides comprising at least one monomer (B) of the general formula (III), preferably of the general formula (Ilia) are used and the non-ionic surfactant (S) is at least one of the general formula (IV). Monomers of the general formula (Ilia) including preferred embodiments and surfactants of the general formula (IV) including preferred embodiments have already been described above.

In another embodiment of the invention, the non-ionic surfactant (S) may be selected from non-ionic surfactants having the general formula (V)

R⁹-O-(CH₂CH₂O)_o(CH₂CH(R¹⁰)O)_p(CH₂CH₂O)_qH (V),

wherein the moieties R¹⁰ are - independently from each other - aliphatic hydrocarbon moieties having 2 to 10 carbon atoms, preferably 2 or 3 carbons atoms o is a number from 10 to 80, preferably 15 to 40, more preferably 20 to 30, p is a number from 8 to 30, preferably 10 to 25 and for example from 12 to 20, and q is a number of 0 to 15, preferably 0 to 10, more preferably 0 to 5 and for example from 1 to 5. The polyalkoxy groups are arranged blockwise in the order as indicated in formula (V). So, the head group R⁹ is first alkoxylated with a polyoxyethylene block, then with a polyoxylakylene block, preferably a

polyoxybutylene or a polyoxypropylene block and optionally (because q may be 0) a second polyoxyethylene block.

The head group R⁹ is a group selected from the group of R^9a and R^9b.

R^9a is a saturated or unsaturated hydrocarbon group having from 1 to 6 carbon atoms, preferably from 1 to 4 carbon atoms. Examples comprise methyl, ethyl, n-propyl and n-butyl groups. Examples of surfactants (V) with a group R^9a comprise surfactants of the general formula n-C4H9-(CH2CH2O)_o(CH₂CH(R¹⁰)O)_pH, wherein R¹⁰ is ethyl, o is from 20 to 30, preferably from 22 to 28, and p is from 12 to 20, preferably 14 to 18.

R^9b is a group R^9a-0-(R¹¹0)_s- in which R^9a is as defined above and s is a number from 1 to 4. R¹¹ is an alkylene group with 1 to 4 carbon atoms. In one preferred embodiment R¹¹ is an 1 ,4-butylene group -CH2CH2CH2CH2- and s is 1.

Examples of surfactants (V) with a group R^9b comprise surfactants of the general formula H₂C=CH-O-(CH₂CH₂CH₂CH₂O)-(CH₂CH₂O)_o(CH₂CH(R¹⁰)O)_p(CH₂CH₂O)_qH, wherein R¹⁰ is an ethyl group, o is from 20 to 30, preferably from 22 to 28, and p is from 12 to 20, preferably 14 to 18, and q is from 0 to 5, preferably from 1 to 5.

In one embodiment of the invention, in the method according to the present invention hydrophobically associating polyacrylamides comprising at least one monomer (B) of the general formula (III), preferably of the general formula (Ilia) are used and the non-ionic surfactant (S) is at least one of the general formula (V). Monomers of the general formula (Ilia) including preferred embodiments and surfactants of the general formula (IV) including preferred embodiments have already been described above.

Step (3) may be carried out in at least two different ways.

In one embodiment of the invention, an aqueous solution comprising at least one non-ionic surfactant (S) is injected into the subterranean formation after step (2). When the aqueous surfactant solution contacts the aqueous fracturing fluid comprising the hydrophobically associating polyacrylamides in the formation, the viscosity of the aqueous fracturing fluid is reduced thereby easing its removal from the subterranean formation in course of step (4).

Ideally, the solution comprising the breaking surfactant (S) should mix completely with the fracturing fluid in the formation. It goes without saying that this ideal conception is difficult - if not impossible - to achieve in daily routine. For that reason, it is also difficult to adjust the concentration of the surfactant in the formation properly. As will be shown in the experimental part, a certain minimum concentration of the surfactant in the formation is necessary to reduce the viscosity to sufficiently low values. Such minimum can be determined in the laboratory. For daily routine, it is advisable to use an excess of surfactant in the injection fluid in order to achieve that the concentration of the non-ionic surfactant (S) in the formation is sufficient to drop the viscosity.

In another embodiment of the invention, the non-ionic surfactant (S) may be a so-called “delayed surfactant” which can be added to the fracturing fluid.“Delaying” means that particles comprising the non-ionic surfactant (S) are added to the fracturing fluid, which release the non-ionic surfactants (S) not immediately but after injecting the particles into the formation under the conditions prevailing in the formation. For example, the non-ionic surfactants (S) may become released when the temperature exceeds a certain limit.

Techniques of delaying the effect of chemical compounds are basically known to the skilled artisan.

For example, the surfactants (S) or a suitable formulation thereof may be encapsulated. The encapsulating material is designed such that the capsules open with delay in the formation, for example a caused by exceeding a specific temperature and/or simply after some time. Techniques of encapsulating materials are known in the art.

In another example, the surfactants (S) may be embedded in particles which release the surfactants (S) after some time and/or according to the certain formation conditions. In such an embodiment, the surfactants (S) are mixed with a matrix material, such as a wax or a polymer and particles made thereof, which can become added to the fracturing fluid. For example, the surfactant may be mixed with a wax and fine particles made thereof. After injection into the formation, the wax melts after exceeding a certain temperature thereby releasing the surfactants (S). In other embodiments, a polymer matrix may be used which releases the surfactant (S) in course of time. In one embodiment, a polymer having a glass transition temperature above ambient temperatures but below the formation temperature is used. At ambient temperatures, the surfactants (S) are fixed in the polymer, while they may become released at temperatures above the glass transition temperature.

Using delayed surfactants (S) is advantageous, because the surfactants are a component to the fracturing fluid and therefore, the abovementioned problem of mixing does not occur. Furthermore, it is easier to adjust the concentration of the surfactants. As above, the minimum amount necessary can be determined in the laboratory.

The concentration of the non-ionic surfactant (S) is at least 5 % by weight relating to the concentration of the hydrophobically associating polyacrylamides in the aqueous fracturing fluid.

Depending on the method of applying the breaker, said concentration relates either to the concentration of the non-ionic surfactant(s) (S) in an aqueous solution which is injected into the formation after injecting the aqueous fracturing fluid or to the concentration of the non- ionic surfactants(s) (S) in the fracturing fluid, if the non-ionic surfactant(s) (S) are added as delayed surfactants, i.e. as particles which release the non-ionic surfactants not immediately but in course of time and/or under certain conditions.

In one embodiment, the concentration of the non-ionic surfactant (S) is from 5 % to 100 % by weight relating to the concentration of the hydrophobically associating polyacrylamides in the aqueous fracturing fluid, preferably from 5 % to 50 % by weight. In other embodiments, the concentration is from 10 % to 100 % by weight or from 20 % to 100 % by weight.

Step (4)

In course of process step (4) the applied pressure is reduced thereby allowing at least a portion of the injected aqueous 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.

Step (5)

In course of step (5) the aqueous fracturing fluid flown back from the formation into the wellbore is 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. Further steps

The method according to the present invention may of course comprise further steps. By the way of example, before the injection of the fracturing fluid comprising proppants, a fluid comprising only the aqueous base fluids and hydrophobically associating polyacrylamides may be injected.

Method of reducing the viscosity

In another embodiment, the present invention relates to a method of reducing the viscosity of aqueous solutions comprising hydrophobically associating polyacrylamides, comprising at least the steps of

(i) providing a viscosified aqueous solution comprising at least hydrophobically

associating polyacrylamides, wherein the concentration of hydrophobically associating polyacrylamides is from 0.05 % by weight to 2 % by weight, relating to the total of all components of the aqueous solution,

wherein the polyacrylamides are manufactured in a preceding step (0) by radically polymerizing an aqueous solution comprising water-soluble, monoethylenically unsaturated monomers comprising at least

• water,

• 40 mole % to 99.995 mole % of at least one monomer (A) selected from the group of (meth)acrylamide, N-methyl(meth)acrylamide, N,N'- dimethyl(meth)acrylamide or N-methylol(meth)acrylamide, wherein the amount relates to the total of all ethylenically unsaturated monomers in the aqueous solution, and

• 0.005 mole % to 1 mole % of at least one monoethylenically unsaturated

monomer (B) selected from the group of

H₂C=C(R¹)-0-(-CH₂-CH(R²)-0-)_k-R³ (I),

H₂C=C(R¹)-(C=0)-0-(-CH₂-CH(R²)-0-)_k-R³ (II),

H₂C=C(R¹)-R⁴-0-(-CH₂-CH(R⁵)-0-)x-(-CH₂-CH(R⁶)-0-)y-(-CH₂-CH₂0-)z-R⁷ (III), wherein the radicals and indices are defined as follows: R¹: H or methyl;

R²: independently H, methyl or ethyl, with the proviso that at least 70 mole % of the R² radicals are H,

R³: aliphatic and/or aromatic, linear or branched hydrocarbyl radicals having 8 to 40 carbon atoms,

R⁴: a single bond or a divalent linking group selected from the group consisting of -(Cnl-hn)-, -0-(C_n'H2n')- and -C(0)-0- (C_n"H2_r/)-, where n is a natural number from 1 to 6, and n' and n" are a natural number from 2 to 6,

R⁵: independently H, methyl or ethyl, with the proviso that at least 70 mole % of the R⁵ moieties are H,

R⁶: independently hydrocarbyl radicals of at least 2 carbon

atoms,

R⁷: H or a hydrocarbyl radical having 1 to 30 carbon atoms, k a number from 10 to 80,

x a number from 10 to 50,

y a number from 5 to 30,

z a number from 0 to 10, and

^■ the aqueous monomer solution has a temperature Ti not exceeding 30 °C before the onset of polymerization, and

■ the temperature of the aqueous polyacrylamide gel T2 after polymerization is from

45 °C to 70 °C, and

(ii) adding at least a non-ionic surfactant (S) having an HLB of at least 8 to the aqueous solution, thereby reducing its viscosity, wherein the concentration of the non-ionic surfactant (S) is at least 5 % by weight relating to the concentration of the hydrophobically associating polyacrylamides in the aqueous solution. Step (i)

In the method of reducing the viscosity of aqueous solutions of hydrophobically associating polyacrylamides in the first step (i) an aqueous solution comprising at least hydrophobically associating polyacrylamides is provided. The aqueous solution may comprise optionally further components depending on the intended use of the aqueous solution.

The hydrophobically associating polyacrylamides are manufactured in a preceding step (0) as already described above and we explicitly refer to said passages.

In one embodiment of the invention, the aqueous solution comprising at least

hydrophobically associating polyacrylamides is provided by dissolving the aqueous polymer gel comprising hydrophobically associating polyacrylamides obtained in course of step (0) in an aqueous liquid.

In another embodiment of the inventions, the aqueous solution comprising at least hydrophobically associating polyacrylamides is provided by dissolving solid hydrophobically associating polyacrylamides in an aqueous liquid. The solid hydrophobically associating polyacrylamides are obtained by drying the aqueous polymer gel obtained in course of step (0).

The aqueous liquid comprises water. Examples of aqueous liquids comprise fresh water, brines, sea water, formation water or mixtures of such fluids.

Upon dissolving the hydrophobically associating polyacrylamides in the aqueous liquid its viscosity increases.

The amount of the hydrophobically associating polyacrylamides in the aqueous solution may be adjusted by skilled artisan according to the intended use of the aqueous solution. In one embodiment, the amount of the hydrophobically associating polyacrylamides may be from 500 ppm to 3000 ppm, relating to the total of all components of the aqueous solution.

Step (ii)

In course of step (ii) at least a surfactant (S) is added to the aqueous solution, thereby reducing its viscosity. Of course, a mixture of two or more different surfactants (S) may be used.

Suitable surfactants and preferred surfactants have already been disclosed above and we explicitly refer to said passages. The surfactants are preferably added as aqueous solutions to the aqueous solution comprising the hydrophobically associating polyacrylamides. Advantages of the methods according to the present invention

In the method according to the present invention, polyacrylamides are used which have two advantageous properties, namely an improved viscosity efficiency and improved sensitivity of the polymers upon the addition of surfactants. Viscosity efficiency is the viscosity / mass. If polymer has a higher viscosity efficiency than another polymer, either the viscosity achieved is higher for the same amount of polymer or less polymer may be used for achieving the same viscosity. Said advantageous property is achieved by limiting T_max to not more than 70 °C in course of polymerization by limiting the concentration of the monomers in the aqueous monomer solution for polymerization to not more 3.3 mole/kg.

The viscosity of the polymer solutions may be decreased by adding non-ionic surfactants having an HLB > 8. Surprisingly, the viscosity drop upon adding surfactants is larger for polymer synthesized with limited T_max.

Consequently, the products are very efficient in a process of fracturing. In course of injecting the fracturing fluid a higher viscosity efficiency yields a more economic process. After fracking, injecting a surfactant (S) as viscosity breaker yields an efficient viscosity reduction.

The invention is illustrated in detail by the examples which follow.

Synthesis of hydrophobically associating polyacrylamides:

Associative monomer:

For the examples, the following macromonomer was used (synthesis according to the procedure disclosed in WO 2017/121669 A1 , pages 23 - 24):

H2C = CH-0-(CH2)4-0-(CH2CH20)24.5-(CH2CH(C2H₅)0)I6-(CH2CH₂0)3.5H

Comparative example 1 :

Synthesis of a copolymer comprising 47.6 wt. % (75.1 mole %) of acrylamide, 50.5 wt. % (24.8 mole %) of sodium ATBS and 1.9 wt. % (0.0854 mol%) of the macromonomer;

Monomer concentration: 3.49 mole/kg (40 % by weight)

A 5 I beaker with magnetic stirrer, pH meter, and thermometer was initially charged with 1385.6 g of a 50% aqueous solution of Na-ATBS, and then the following components were added successively: 730 g of distilled water, 1254.5 g of acrylamide (52 % by weight in water), 3.5 g of a commercially available silicone defoamer (Xiameter^® AFE-0400), 10.5 g of a 5 % aqueous solution of the pentasodium salt of diethylenetriamine-pentaacetic acid, 33,9 g of a 85 % aqueous solution of the surfactant iCi₃0(CH₂CH₂0)i₂H (Lutensol^® T0129), 7 g of a 0.1 wt. % aqueous solution of sodium hypophosphite hydrate. After adjustment to pH 6.0 with a 10 % by weight solution of sulfuric acid, 30 g of an 87% aqueous solution of the macromonomer were added, the pH adjusted back to pH 6.0 and the rest of the water was added to attain the desired monomer concentration of 40 % by weight (total amount of water 755.3 g minus the amount of water already added, minus the amount of acid required). 21 g of a 10% aqueous solution of the water-soluble azo initiator 2,2‘- azobis(2-methylpropionamidine) dihydrochloride (Wako V-50; 10h ti_/2 in water 56°C) was added and the monomer solution was adjusted to the initiation temperature of 0°C. The solution was transferred to a Dewar vessel, the temperature sensor for the temperature recording was inserted, and the flask was purged with nitrogen for 45 minutes. The polymerization was initiated with 1.05 g of a 1 % t-BHPO solution and 2.1 g of a 1 % sodium sulphite solution. With the onset of the polymerization, the temperature rose to 84 °C within about 25 min. A solid polymer gel was obtained.

After the polymerization, the gel was incubated for 4 hours at T_max and the gel block was comminuted with the aid of a meat grinder. The comminuted aqueous polyacrylamide gel was kept for further testing without drying.

Comparative example 2:

Synthesis of a copolymer comprising 47.6 wt. % (75.1 mole %) of acrylamide, 50.5 wt. % (24.8 mole % ) of sodium ATBS and 1.9 wt. % (0.0854 mol%) of the macromonomer;

Monomer concentration: 3.36 mole/kg (38.5 % by weight)

The copolymer was synthesized according to the same procedure as in comparative example 1 , except that the concentration of the monomers was reduced from 40 % to 38.5 %.

Example 1 :

Monomer concentration: 3.1 mole/kg (35.5 % by weight)

The copolymer was synthesized according to the same procedure as in comparative example 1 , except that the concentration of the monomers was reduced from 40 % to 35.5 %.

Example 2:

Synthesis of a copolymer comprising 47.6 wt. % (75.1 mole %) of acrylamide, 50.5 wt. % (24.8 mole % ) of sodium ATBS and 1.9 wt. % (0.0854 mole %) of the macromonomer; Monomer concentration: 2.83 mole/kg (32.5 % by weight)

The copolymer was synthesized according to the same procedure as in comparative example 1 , except that the concentration of the monomers was reduced from 40 % to 32.5 %.

The polymerization parameters for the polymers C1 , C2, 1 , and 2 are summarized in table 1.

Table 1 : Summary of important polymerization parameters

DB: double-bond number (moles reactive monomers per kg monomer mixture)

Viscosity reduction tests

For the tests, the non-ionic surfactants S1 , S2, and S3 were tested. For comparative purposes, the anionic surfactant C1 was tested. The surfactants are listed in the following table 2.

Table 2: Tested surfactants (EO = ethyleneoxy unit, PO = propyleneoxy unit,

BuO = butylenoxy unit, 1C₁₃: C-₁₃-oxoalcohol) Test procedure:

In order to illustrate the effects of the present invention, the viscosity of aqueous solutions comprising 2000 ppm of the polymers synthesized as above and the surfactants S1 , S2, S3, and C1 in various amounts from 25 ppm to 1600 ppm were measured in synthetic sea water.

Synthetic sea water

For the tests, synthetic sea water having a total salt concentration of 35 g/l was used.

The salt mixture used had the following composition:

Salt amount [% by wt.]

NaHCOs 0.5

MgSC x 7 H2O 16.3

MgCI₂ x 6 H2O 12.1

CaCI₂ x 6 H₂0 5.4

KCI 1.9

NaCI 63.8

Sample preparation

For the tests the respective polymer gel and the respective surfactant to be tested were mixed with synthetic sea water in such amounts to achieve a final concentration of the polymer of 2000 ppm and surfactant concentrations of 25, 50, 100, 200, 400, 800, and 1600 ppm by weight relating to the total of the aqueous solution. The amounts relate to the polymer as such and to the surfactant as such.

Viscosity measurements

The viscosity measurements were carried out at 30 °C using an Anton Paar viscosimeter at 7 s ¹. The viscosity data are represented in the following tables 3 to 6 and the also in figures

1 to 4.

Table 3: Test results with surfactants S1 as breaker

Table 4: Test results with surfactants S2 as breaker

Table 5: Test results with surfactants S3 as breaker

Table 6: Test results with surfactants S4 as breaker

Table 3 and Figure 1 show the viscosity of solutions comprising various amounts of surfactant S1 (which is a surfactant within the general formula (IV)) having an HLB of 15. The samples without any surfactant demonstrate the effect of T_max, i.e. the effect of limiting the polymerization temperature by limiting the concentration in course of polymerization. For the sample obtained at a T_{max O}f 84 °C (comparative polymer C1 ), the viscosity is ~ 52 mPas and for the sample obtained at a T_{max O}f 81 °C (comparative polymer C2), the viscosity is ~ 63 mPas. When further reducing T_max, the viscosity increases to 315 mPas at 67°C (polymer 1 ) and to 285 mPas at 61 mPas (polymer 2).

Adding only 100 ppm of the surfactant S1 reduces the viscosities of the two samples having a high initial viscosity very significantly (polymer 1 and polymer 2). The viscosity of the two other samples comparative polymer 1 and comparative polymer 2) is also reduced a bit, however not that much.

So, lowering T_max in course of gel polymerization yields polymers which have a higher viscosity efficiency (viscosity efficiency = viscosity / mass) and whose viscosity is more sensitive to the addition of non-ionic surfactants. Both properties are important for the process of fracturing according to the present invention: In course of fracking, the viscosity should be high. The improved viscosity efficiency allows to use less polymer for achieving a certain viscosity. After fracking, the viscosity should drop in order to enable a quick and complete removal of the fracking fluid from the formation.

Using Surfactant S1 is a preferred embodiment, because already an amount of 5% of the surfactant relating to the polymer yields a very significant drop in the viscosities.

Figure 2 and table 4 show that with surfactant S2 having an HLB of 10.8 similar effects are obtained except that a higher amount of surfactant is necessary in order to drop the viscosity to low values.

Figure 3 and table 5 show that with surfactant S3 having an HLB of 10.4 again similar effects are obtained, however also here higher amounts of surfactants are necessary than with surfactant S1.

Figure 4 and table 6 show the results with the comparative surfactant C1 which is not a non- ionic but which is an anionic surfactant. Adding small amounts of C1 yields a decrease in viscosity, however when further increasing the amount of surfactant, the viscosity begins to rise again. Because it is extremely difficult if not impossible to properly adjust a certain concentration of surfactants injected as breakers in the formation, such a behavior is disadvantageous in daily routine.

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 ) providing a viscosified aqueous fracturing fluid comprising at least an aqueous base fluid, hydrophobically associating polyacrylamides and proppants, wherein the concentration of hydrophobically associating polyacrylamides is from 0.05 % by weight to 2 % by weight, relating to the total of all components of the fracturing fluid except the proppants,

(2) injecting the aqueous fracturing fluid into at least one wellbore at a rate and pressure sufficient to penetrate into the formation, and to initiate or extend fractures in the formation and to transport proppants into thus generated fractures,

reducing its viscosity,

(5) removing such flowed back fracturing fluid from the wellbore,

wherein the aqueous polyacrylamide gel is manufactured in an additional process step (0) by radically polymerizing an aqueous solution comprising water-soluble,

monoethylenically unsaturated monomers comprising at least

• water,

H₂C=C(R¹)-0-(-CH₂-CH(R²)-0-)_k-R³ (I),

H₂C=C(R¹)-(C=0)-0-(-CH₂-CH(R²)-0-)_k-R³ (II),

R²: independently H, methyl or ethyl, with the proviso that at least

70 mole % of the R² radicals are H,

R³: aliphatic and/or aromatic, linear or branched hydrocarbyl

radicals having 8 to 40 carbon atoms,

R⁴: a single bond or a divalent linking group selected from the

group consisting of -(Cnl-hn)-, -0-(C_n'H2n')- and -C(0)-0- (C_n"H2_r/)-, where n is a natural number from 1 to 6, and n' and

n" are a natural number from 2 to 6,

R⁵: independently H, methyl or ethyl, with the proviso that at least

70 mol% of the R⁵ moieties are H,

R⁶: independently hydrocarbyl radicals of at least 2 carbon

atoms,

R⁷: H or a hydrocarbyl radical having 1 to 30 carbon atoms,

k a number from 10 to 80,

x a number from 10 to 50,

y a number from 5 to 30,

z a number from 0 to 10, and

^■ the aqueous monomer solution has a temperature Ti not exceeding 30°C before the onset of polymerization, and

■ the peak temperature of the aqueous polyacrylamide gel T2 in course of

polymerization is from 45°C to 70°C,

and wherein the viscosity breaker comprises at least a non-ionic surfactant (S) having an HLB-value of at least 8, and wherein the concentration of the non-ionic surfactant (S) is at least 5 % by weight relating to the concentration of the hydrophobically associating polyacrylamides in the aqueous fracturing fluid.

2. Method according to claim 1 , wherein the concentration of the monomers is from 1.5 mole / kg to 3.3 mole / kg.

3. Method according to claims 1 or 2, wherein Ti is from -5 °C to + 5 °C and T₂ is from 50 °C to 70 °C.

4. Method according to claims 1 to 3, wherein the monomer (B) is at least one monomer of the general formula (III).

5. Method according to claim 4, wherein the monomer (B) has the general formula

H₂C=CH-0-(C_n'H_2n')-0-(-CH₂-CH₂-0-)_x-(-CH₂-CH(C₂H₅)-0-)_y-(-CH₂-CH₂-0-)_z-H (Ilia), wherein n is a natural number from 2 to 6, x is from 15 to 35, y is from 10 to 25, and z is from >0 to 5.

6. Method according to claim 5, wherein n is 4, x is from 20 to 30, y is from 12 to 20, and z is from 1 to 5.

7. Method according to claim 4, wherein the monomers (B) are a mixture comprising at least the following monomers:

H₂C=C(R¹)-R⁴-0-(-CH₂-CH(R⁵)-0-)_x-(-CH₂-CH(R⁶)-0-)_y-H (Nib) and

H₂C=C(R¹)-R⁴-0-(-CH₂-CH(R⁵)-0-)_x-(-CH₂-CH(R⁶)-0-)_y-(-CH₂-CH₂0-)_z-H (lllc), where the radicals and indices have the definition outlined above, with the proviso that, in the formula (lllc), z is a number > 0 to 10.

8. Method according to claim 7, wherein, in the formulae (Nib) and (lllc), R¹ is H, R⁴ is a -O- (C_n'H_2n')- group, R⁵ is H, R⁶ is ethyl, x is 20 to 30, y is 12 to 25, and z is 1 to 6.

9. Method according to claim 7, wherein, in the formulae (Nib) and (lllc), R¹ is H, R⁴ is -O- CH₂CH₂CH₂CH₂-, R⁵ is H, R⁶ is ethyl, x is 23 to 26, y is 14 to 18, and z is 3 to 5.

10. Method according to any of claims 1 to 9, wherein the aqueous solution comprises

additionally up to 59.995 mole % of at least one water-soluble, monoethylenically unsaturated monomer (C) different from monomers (A) and (B).

1 1. Method according to claim 10, wherein monomer (C) comprises at least one acidic group selected from the group of -COOH, -SO3H and -P0₃H₂ or salts thereof.

12. Method according to claim 1 1 , wherein monomers (C) are selected from acrylic acid and/or ATBS or salts thereof.

13. Method according to any of claims 1 to 9, wherein the aqueous solution comprises 65 mole % to 79.995 mole % of acrylamide, 0.005 mole % to 0.2 mole % of monomers (B) of formula (III), and 20 mole % to 34.995 mole % of at least one monomer (C), selected from the group of acrylic acid and/or ATBS or salts thereof.

14. Method according to any of claims 1 to 13, wherein the non-ionic surfactant (S) is at least one having the general formula (IV)

R⁸-0-(CH₂CH₂0)_nH (IV),

wherein R⁸ is an aliphatic, saturated or unsaturated, linear or branched hydrocarbon moiety having 8 to 22 carbon atoms, and n is a number from 5 to 20.

15. Method according to claim 14, wherein the monomer (B) has the general formula

16. Method according to any of claims 1 to 13, wherein the non-ionic surfactant (S) is at least one having the general formula (V)

R⁹-O-(CH₂CH₂O)_o(CH₂CH(R¹⁰)O)_p(CH₂CH₂O)_q-H (V),

wherein R⁹ is a group selected from the group of R^9a and R^9b, wherein

R^9a is a saturated or unsaturated hydrocarbon group having from 1 to 6 carbon atoms, and

R^9b is a group R^9a-0-(R¹¹0)_s- in which R^9a is as defined above, s is a number from 1 to 4, and R¹¹ is an alkylene group with 1 to 4 carbon atoms,

the moieties R¹⁰ are -independently from each other- aliphatic hydrocarbon moieties having 2 to 10 carbon atoms, o is a number from 10 to 80, p is a number from 8 to 30, and q is a number of 0 to 15.

17. Method according to any of claims 1 to 16, wherein step (1 ) is carried out by pre-diluting the aqueous polyacrylamide gel with aqueous base fluid, thereby obtaining a diluted polyacrylamide gel, and mixing said diluted gel at least with further aqueous base fluid and proppants.

18. Method according to claim 17, wherein the pre-diluted polyacrylamide gel has a

concentration from 3 % to 7 % by weight, relating to the total of all components of the diluted gel.

19. Method according to any of claims 1 to 18, wherein step (3) is carried out by injecting an aqueous solution comprising at least one non-ionic surfactant (S) into the subterranean formation.

20. Method according to any of claims 1 to 18, wherein step (3) is carried out by adding particles comprising non-ionic surfactants (S) to the aqueous fracturing fluid, wherein the particles release the non-ionic surfactants (S) not immediately but after injection into the formation under formation conditions.

21. Method according to claim 20, wherein the non-ionic surfactants (S) are encapsulated surfactants.

22. Method according to any of claims 1 to 21 , wherein the concentration of the

hydrophobically associating polyacrylamides in the aqueous fracturing fluid is from 0.05 % to 0.3 % by weight, relating to the aqueous fracturing fluid except the proppants.

23. Method of reducing the viscosity of aqueous solutions of hydrophobically associating polyacrylamides, comprising at least the steps of

(i) providing a viscosified aqueous solution comprising at least hydrophobically associating polyacrylamides, wherein the concentration of hydrophobically associating polyacrylamides is from 0.05 % by weight to 2 % by weight, relating to the total of all components of the aqueous solution,

• water,

• 0.005 mole % to 1 mole % of at least one monoethylenically unsaturated

monomer (B) selected from the group of

H₂C=C(R¹)-0-(-CH₂-CH(R²)-0-)_k-R³ (I),

H₂C=C(R¹)-(C=0)-0-(-CH₂-CH(R²)-0-)_k-R³ (II),

R¹: H or methyl;

R⁶: independently hydrocarbyl radicals of at least 2 carbon

atoms,

x a number from 10 to 50,

y a number from 5 to 30,

z a number from 0 to 10, and

■ the temperature of the aqueous polyacrylamide gel T2 after polymerization is from 45 °C to 70 °C, and

(ii) adding at least a non-ionic surfactant (S) having an HLB of at least 8 to the aqueous solution, thereby reducing its viscosity, wherein the concentration of the non-ionic surfactant (S) is at least 5 % by weight relating to the concentration of the hydrophobically associating polyacrylamides in the aqueous solution.