WO2017211563A1

WO2017211563A1 - Method for inhibiting metal corrosion in oil and natural gas production

Info

Publication number: WO2017211563A1
Application number: PCT/EP2017/061932
Authority: WO
Inventors: Benjamin Gerlach; Fatima DUGONJIC-BILIC; Jasmin MEURER; Marita Neuber
Original assignee: Tougas Oilfield Solutions Gmbh
Priority date: 2016-06-07
Filing date: 2017-05-18
Publication date: 2017-12-14

Abstract

Method for inhibiting metal corrosion in oil and natural gas production The instant invention relates to the use of synthetic polymer as corrosion inhibitor reducing or inhibiting corrosion of metal equipment being present during acidizing treatment in gas- or oilfield reservoirs with one or more acids. A further aspect of the invention is a method for reducing or inhibiting corrosion of steel equipment being present during an acid treatment of a gas- or oilfield reservoir with one or more acids by using a specific synthetic polymer as corrosion inhibitor.

Description

Method for inhibiting metal corrosion in oil and natural gas production

The resources for fossil fuels are highly exploited and also limited. With new and improved technologies these resources for oil and natural gas can be further exploited and unconventional reservoirs can be accessed. With the increasingly challenging conditions for the oil and gas production the requirements for the equipment and the chemicals also become more and more demanding.

Several techniques are used to increase oil and natural gas production from formations with low permeability or from exploited oil and gas field. In sandstone and especially in carbonate reservoirs acid treatment - that means that acid is pumped down into the borehole - is a widely used technique either to stimulate a well to improve flow or to remove damage. Formation fines, mud or cement filtrate, scale from well operations that is accumulated in the tubing, in the perforations and the area immediate to the well bore may be removed by purging the well with acid. Typically, this is done by means of coiled tubing. Acid is pumped down the well and the spent acid is returned through the annulus of the coiled tubing.

There are two types of acid treatment of the formation that are related to injection rates and pressures. Injection rates resulting in pressures below fracture pressure are termed "matrix acidizing", while those above fracture pressure are termed "fracture acidizing" or "acid fracturing".

During "matrix acidizing" the acids dissolve the sediments of the reservoir and of mud solids within the pores that are inhibiting the permeability of the rock. This process enlarges the natural pores of the reservoir, which stimulates the flow of hydrocarbons. Removal of severe plugging of the pores can result in very large increases in well productivity. The acid also dissolves rock matrix leading to the formation of highly conductive flow channels, the so-called wormholes.

In "acid fracturing" highly conductive fractures and long wormholes are created by pumping acid at pressures exceeding the minimum stress of the formation. When fractures in the formation are opened due to the high pressure the acid enters these newly formed fractures and etches channels on the fracture surface. The acid is also pressed further into the formation forming a network of wormholes. When the pressure is released and the fractures close again the etched channels and wormholes stay open. No proppants are necessary to keep the paths for oil and gas opened. Acid fracturing is typically used in formations with low

permeability and in carbonates.

The temperatures for acidizing treatments is normally in the range from slightly above ambient temperature for low depth wells up to about 100 °C, in special cases even up to 150 °C or higher.

Hydrochloric acid (HCI) is mostly used for acid treatments in carbonate reservoirs. HCI is highly reactive with carbonates and the salts from its reaction with the rock are typically water soluble and thus easy to remove from the borehole. HCI is not expensive and easily available. However, HCI can react so fast that large wormholes are created through which the acid flows with ease etching even larger channels and increasing its leak off but leaving most parts of the formation unstimulated. Methods were developed to control the placement of acid and its reactivity, for example pumping viscous fluid pads intermittently throughout the acid treatment. The viscous fluid forms a filter cake that is a temporary barrier against the acid leak-off. Another method is to make the acid more viscous by either applying emulsified acid or gelled acid. Typically, the reactivity of the acid is also influenced and the activity is retarded. Often HCI is used together with other acids, e.g. with hydrofluoric acid (HF) or organic acids like acetic acid or formic acid.

Organic acids, without HCI, are less commonly applied in acid treatments mostly because of their high costs. They are much less reactive against carbonates compared to HCI, therefore they are used preferably for high temperature acid treatments above 90 to 100 °C. However due to their lower activity they are also much less corrosive against steel tubular than HCI.

Since HCI or its mixtures with other acids are highly corrosive against steel equipment, especially at higher temperatures, corrosion inhibitors must be used to protect the tubulars from corrosive attack. Typically, corrosion inhibitors consist of a mixture of different substances to make use of synergistic effects. Often they are specific for a distinct steel quality under defined conditions. The requirements for corrosion inhibitors for acidizing treatment are challenging. They must inhibit corrosion for a variety of steels, from commonly used low alloy steel to high ranking steel for high temperature and high pressure (HTHP) applications. Furthermore, they must prevent corrosion over a broad temperature range. Especially at high temperature, protection must be effective as corrosion rate increases exponentially with temperature. And last but not least, corrosion inhibitors must not be expensive.

A broad variety of chemicals were tested for their capability to prevent corrosion of steel. Amongst them, propargylic alcohol, substituted benzimidazoles, benzyl alkyl pyridine quat, benzalkonium chloride and cinnamaldehyde were the most effective substances, see for example "Application of corrosion inhibitors for steels in acidic media for the oil and gas industry: A review", M. Finsgar, J. Jackson, Corrosion Science 86 (2014) 17-41 .

However, several substances that are effective as corrosion inhibitor are toxic and cause problems when released to the environment. Great effort is done to develop corrosion inhibitors with improved environmental profiles. C. Sitz et al, SPE 155966, 2012 describe the development of corrosion inhibitor which specifically exclude propargyl alcohol, pyridine and quinoline quats, nonylphenol ethoxylates, BTEX, methanol, ethylene glycol, and ethylene glycol monobutyl ether.

Beside the aforementioned substances, also polymeric corrosion inhibitors have been considered. Due to their less toxic properties, polymers were tested for their capability to prevent or reduce corrosion. A broad variety of different metals is used for the examination and often aqueous system and not acidic systems are investigated. Thus, Umoren et al. al reported that polyvinylpyrrolidone in 1 molar H2SO4 reduces corrosion on mild steel more effectively than polyacrylamide (Umoren S.A. et al., Surf. Rev. Lett. 2008, 25 (3), 277). Selvaraji et al. examined the influence of

polyvinylpyrrolidone on carbon steel in aqueous system in the presence and absence of 60 ppm chloride ion and zinc ion (Selvaraji S.K. et al., Corrosion Rev., 2004, 22(3), 219).

In U.S. Patent 8,372,336 the use of product obtainable by the reaction of an alkoxylated fatty amine with a dicarboxyic acid derivative, followed by partial or total quaternization of the reaction product obtained, is described. The product consists of >50% w/w of oligomers/polymers based on alkoxylated amine and dicarboxylic acids and is used as a corrosion inhibitor for metal surfaces. In U.S. Patent 4,650,591 a method of inhibition corrosion and scale formation in aqueous solution using at least 0.1 mg/l of a polymer consisting of 35 to 65 % by weight of acrylic acid or methacrylic acid, 15 to 45 % by weight of 2-acrylamido-2- methylpropylsulfonic acid and 15 to 25 % by weight of 2-acrylamido-2- methylpropylphosphonic acid is described. In the method, the polymer is only applied in aqueous system and not in acidic media.

Chinese Patent Application CN 105001366 discloses a copolymer of acrylamide and acrylic acid as corrosion inhibitor for waste water from industry, steel plant, electroplating plant, metallurgy, and sewage plant

In summary, often polymers are used to protect water treatment equipment against corrosion caused by the water or to prevent corrosion from diluted acid. For acidizing treatment in oil or gas field operations the use of acids with high concentration is necessary.

However, polymers are well known in oil or gas field operations and can act as thickener for aqueous acids (acid gallant). The polymers can be natural based polymers or synthetic polymers. Typically, as natural based polymers, polysaccharides or modified polysaccharides are used. For instance, suitable hydratable polysaccharides include starch or its derivatives, galactomannan gums, glucomannan gums, cellulosic derivatives, preferably carboxymethyl cellulose; cellulose ether, preferably hydroxyethyl cellulose; guar gums or its derivatives, preferably hydroxyalkyl guar, carboxyalkyl guar, and carboxyalkyl hydroxyalkyl guar or hydrophobically modified guar alginates,

carrageenans, tragacanth gums, glucan gums and xanthan gums. All the natural based polymers have in common that due to their glycosidic bonds they are not stable at elevated temperatures and under highly acidic conditions. Thus, the ability to gel acids may last only for a very short/limited time and such short/limited time frames are problematic in such operations. Furthermore the availability of natural based polymers may depend on weather conditions and crop yield. This dependence impacts the availability and the price of natural based polymers. In response to such problems of natural based polymers, synthetic polymers based on acrylamide are used for acidizing applications in the oil and gas production as substitutes for natural based polymers. Synthetic polymers are independent from bad weather conditions and distinguish themselves with marked better temperature and chemical stability. For example, acrylamide can be copolymerized with a broad variety of monomers to adjust the properties of the resulting water soluble polymer. Amongst others, ethylenically unsaturated carboxylic, sulfonic or phosphonic acids, their esters, unsubstituted or N- and Ν,Ν-substituted derivatives of amides of ethylenically unsaturated carboxylic acids, N-substituted (cyclic) derivatives of ethylenically unsaturated amides can be used.

The viscosity of acids containing polymers as thickener can be further increased by crosslinking the polymer chains to form a hydrogel, that is a three dimensional network of extremely high molecular weight. Typically, polyvalent cations of group IIIA, IVB, VB, VIB, VI IB and/or VI 11 B of the periodic table of the elements are used as crosslinking compound in acids, preferred are compounds of zirconium, titanium or iron, The viscosity of the viscosified acids or of the crosslinked acidic hydrogels may range from almost as thin as water (1 mPas) up to 5000 mPas. The application of polyacrylamide based polymers as thickener for aqueous acids is widely described in the literature. For example, U.S. Patent 4,244,826 discloses the use of polymers as partially hydrolyzed polyacrylamide as gellant for aqueous acids to be used for acidizing a subterranean formation; U.S. Patent 5,975,206 discloses the use of a polymer emulsion containing a polymer consisting of acrylamide and 2-acrylamido-2- methylpropane sulfonic acid; the polymer is applied in acid together with an external activator and crosslinked with zirconium compound to form the gelled acid; EP-A- 0,1 12,520 discloses the use of metal chelates of water soluble copolymers consisting of monomers carrying at least a carboxylic acid amide group, a sulfonic acid group and a phosphonic acid group as gellant for aqueous acids; U.S. Patent Publication 2003-0104948 describes a gellant for acid consisting of acrylamide and/or acrylic acid that is copolymerized with the dimethylaminoalkyl derivatives of acrylamide and/or acrylic acid.

Therefore, there is an existing need for environmental friendly corrosion inhibitors that are effective over a broad range of conditions like temperature and acid

concentrations to protect a broad variety of steel qualities in the severe conditions existing during such oil or gas field operations.

Surprisingly it was found that synthetic polymers used as gellant to viscosity acids for acidizing treatments also exhibit corrosion inhibition properties and protect metal equipment during acidizing treatment in gas- or oilfield reservoirs.

Therefore the present invention relates to the use of synthetic polymer comprising (I) at least structural units of formula (I)

(I)

wherein

R1 , R2 and R3 independently are hydrogen or d-Ce-alkyl,

from 0 to 95 % by weight structural units of formula (II)

R4

A

I

o=s=o

I

0-R5 wherein R4 is hydrogen or Ci-Ce-alkyI,

R5 is hydrogen, a cation of an alkaline metal, of an earth alkaline metal, of ammonia and/or of an organic amine,

A is a covalent C-S bond or a two-valent organic bridging group,

from 0 to 95 % by weight structural units of formula (III)

R6 R7

0-R8

(III)

wherein

R6 and R7 are independently of one another hydrogen, Ci-Ce-alkyI, -COORg or -CH2-COORg, with Rg being hydrogen, a cation of an alkaline metal, of an earth alkaline metal, of ammonia and/or of an organic amine,

R8 is hydrogen, a cation of an alkaline metal, of an earth alkaline metal, of ammonia and/or of an organic amine, or is Ci-Ce-alkyl, a group -CnH2n-OH with n being an integer between 2 and 6, preferably 2, or is a group -C0H20-NRI 0R1 1 , with 0 being an integer between 2 and 6, preferably 2, and R10 and R1 1 are independently of one another hydrogen or Ci-Ce-alkyl, preferably hydrogen,

(IV) from 0 to 95 % by weight structural units of formula (IV)

(IV)

wherein

R12 and R13 are independently of one another hydrogen, Ci-Ce-alkyl,

-COOR16 or -CH2-COOR16, with R16 being hydrogen, a cation of an alkaline metal, of an earth alkaline metal, of ammonia and/or of an organic amine,

R14 is hydrogen or, Ci-Ce-alkyl, and R15 is -COH, -CO-Ci-Ce-alkyl or

R14 and R15 together with the nitrogen atom to which they are attached form a heterocyclic group with 4 to 6 ring atoms, preferably a pyridine ring, a pyrrolidone ring or a caprolactame ring, (V) from 0 to 20 % by weight structural units of formula (V)

R17

CH^

B I

R19— 0-P=0

I

0-R18

(V)

wherein

R17 is hydrogen or, d-Ce-alkyl, and

R18 and R19 are independently of one another hydrogen, a cation of an alkaline metal, of an earth alkaline metal, of ammonia and/or of an organic amine,

B is a covalent C-P bond or a two-valent organic bridging group, with the proviso that the percentage of the structural units of formulae (I) to (V) refer to the total mass of the copolymer and the percentage of the structural units of formulae (I) to (V) amounts to 100%,

as corrosion inhibitor reducing or inhibiting corrosion of metal equipment being present during acidizing treatment in gas- or oilfield reservoirs with one or more acids. In the synthetic polymer the radicals have the following meanings:

The d-Ce-alkyl groups being present may be straight-chain or branched.

Examples of alkyl groups are methyl, ethyl, n-propyl, iso-propyl, n-butyl, sec-butyl, tert.-butyl, n-pentyl or n-hexyl. Ethyl and especially methyl are preferred.

The group A may be a C-S-covalent bond or a two-valent organic group.

Examples thereof are d-Ce-alkylene groups or -CO-d-Ce-alkylene groups. The alkylene groups may be straight-chain or branched. Examples of A groups are - CpH2_P- groups or -CO-NH-C_PH2_P- groups, with p being an integer between 1 and 6. -CO-NH-C(CH3)2-CH2- or a C-S-covalent bond is a preferred group A. The group B may be a C-P-covalent bond or a two-valent organic group.

Examples thereof are d-Ce-alkylene groups. These groups may be straight-chain or branched. Examples of alkylene groups are -Cqh q- groups, with q being an integer between 1 and 6. Methylene or a C-P-covalent bond is a preferred group A.

The structural units of formula (I) are derived from an ethylenically unsaturated carboxylic acid amide selected from the group of acrylamide, methacrylamide and/or their N-d-Ce-alkyl derivatives or N,N-Ci-C6-dialkyl derivatives.

Preferred polymers used in the instant invention further contain structural units of formula (II) to (V) which are derived from an ethylenically unsaturated sulfonic acid and/or its alkaline metal salts and /or their ammonium salts, from ethylenically unsaturated carboxylic acid and/or its alkaline metal salts and /or their ammonium salts, from N-vinylamides, and/or an ethylenically unsaturated phosphonic acid and/or its alkaline metal salts and /or their ammonium salts, optionally together with further copolymerisable monomers.

Other preferred copolymers used in the instant invention are those, wherein B is a C-P covalent bond or a -Cqhbq- group with q being an integer between 1 and 6, preferably 1 , and/or wherein A is a C-S covalent bond or a -CO— NH-Cphbp- group with p being an integer between 1 and 6, preferably between 2 and 4, B being most preferably a group -CO-NH-C(CH3)2-CH2-.

Also preferably applied are copolymers with structural units of the formula (II) derived from vinylsulfonic acid, 2-acrylamido-2-methylpropane sulfonic acid, 2- methacrylamido-2-methylpropane sulfonic acid, styrene sulfonic acid and/or their alkaline metal salts and/or their ammonium salts. Especially preferred are structural units of the formula (II) derived from 2-acrylamido-2-methylpropane sulfonic acid and/or from 2-methacrylamido-2-methylpropane sulfonic acid and/or from their alkaline metal salts and/or from their ammonium salts.

Further preferably applied monomers which are optionally used in the manufacture of the copolymers are chosen from ethylenically unsaturated carboxylic acid and/or their derivatives of the formula (III), preferably chosen from the group of alkylesters from ethylenically unsaturated carboxylic acid, oxyalkyl esters of ethylenically unsaturated carboxylic acid and/or esters of ethylenically unsaturated carboxylic acids with N-dialkylalkanolamines.

The ethylenically unsaturated carboxylic acids of the formula (III) are preferably acrylic acid, methacrylic acid, fumaric acid, maleic acid, itaconic acid and/or crotonic acid as well as their alkaline metal salts and/or their ammonium salts. The alkylesters of ethylenically unsaturated carboxylic acids are preferably alkylesters of acrylic acid, methacrylic acid, fumaric acid, maleic acid, itaconic acid and/or crotonic acid. Especially preferred are alkylesters with 1 to 6 carbon atoms.

The oxyalkyl esters of an ethylenically unsaturated carboxylic acids of the formula (III) are preferably 2-hydroxyethylester of acrylic acid, methacrylic acid, fumaric acid, maleic acid, itaconic acid and/or crotonic acid.

The ester of ethylenically unsaturated carboxylic acid of the formula (III) with N- dialkylalkanolamine is preferably Ν,Ν-dimethylethanolamine methacrylate, its salt or quaternary ammonium product.

Further preferably applied copolymers with structural units of the formula (IV) are derived from N-vinylamides.

The N-vinylamide is preferably N-vinylformamide, N-vinylacetamide, N-vinyl-N- methylacetamide, or N-vinylamide comprising cyclic N-vinylamide groups, preferably derived from N-vinylpyrrolidone, N-vinylcaprolactame or N-vinylpyridine.

Preferably applied are copolymers with structural units of the formula (V) are derived from vinylphosphonic acid and/or its alkaline metal salts and/or its ammonium salts, and/or allylphosphonic acid and/or its alkaline metal salts and/or its ammonium salts.

Preferred copolymers used in the instant invention are those, wherein Ri , P»2, Fb,

F , Rio, Ri i , Ri4, and R17 are independently of one another hydrogen or methyl or wherein R5, Rg, Ri e, Ris and R19 are independently of one another hydrogen or a cation of an alkali metal, of an earth alkaline metal, of ammonia or of an organic amine. Still other preferred copolymers used in the instant invention are those, wherein and Ri2 is hydrogen and R₇ and R13IS hydrogen or methyl, or wherein Re is - COORg and R₇ is hydrogen or wherein Re is hydrogen and R₇ is -CH2-COOR9 or wherein R12 is hydrogen and R13 is hydrogen or methyl, or wherein R12 is - COORie and Ri3 is hydrogen or wherein R12 is hydrogen and R13 is -CH2- COORie.

Preferred copolymers used in the instant invention are selected from the group consisting of polymers containing:

(i) 10 to 90 % by weight of structural formula I, preferred from 20 to 70 % by weight,

(ii) 0 to 80 % by weight of structural formula II, preferred from 10 to 60 % by weight,

(iii) 0 to 50 % by weight of structural formula III, preferred from 0 to 40 % by weight,

(iv) 0 to 50 % by weight of structural formula IV, preferred from 0 to 40 % by weight,

(v) 0 to 20 % by weight of structural formula V, preferred from 0 to 5 % by

weight,

referred to the total mass of the polymer.

The copolymer used in the instant invention may be linear or branched or crosslinked either by covalent or ionic crosslinking.

The average molecular weight of the copolymers used according to the invention is higher than 500,000 Dalton, preferably higher than 2,000,000 Dalton.

The average molecular weight can be determined via gel permeation

chromatography (GPC). Commercially available polymers, e.g. from acrylamide with molecular weight of 1 ,140,000 Dalton and 5,550,000 Dalton, can be used as standards. For separation of the sample a column consisting of a

polyhydroxymethacrylate copolymer network with a pore volume of 30,000 Angstrom (A) can be used.

The K value according to Fikentscher serves as indicator for the average molecular weight of the copolymers according to the invention. The K value of the copolymers used according to the invention is higher than 300 determined as 0.1 weight% copolymer concentration in solvent solution consisting of 0.5 % by weight of isotridecanethoxylate (6 EO) surfactant in deionized water, preferably is higher than 350.

The polymers can be synthesized by various technologies, e.g. by inverse

emulsion polymeriziation, gel polymerizaiton or precipitation polymerization.

A viscosified treatment fluid is prepared by dissolving a solid polymer or by diluting a polymer solution or by inverting a water-in-oil polymer emulsion using water or an acidic aqueous solution.

Typically, the acid used in the acidizing treatment of the instant invention consist of Bronsted acids, such as

(i) inorganic, not oxidizing acids, for example hydrochloric acid or hydrofluoric acid,

(ii) organic mono- and/or dicarboxylic acids, hydroxyl carboxylic acids, for

example acetic acid , formic acid, lactic acid, maleic acid,

(iii) alkyl sulfonic acids, for example methansulfonic acid,

and mixtures of the aforementioned acids.

The total concentration of the one or more acid(s), such as Bronsted acids, is typically from 0.1 to 40 % by weight, preferred from 1 to 25 % by weight and most preferred from 3 to 20 % by weight, referred to the mass of treatment fluid.

Typically, hydrofluoric acid is used only in combination with other acids, in particular inorganic acids. The amount of hydrofluoric acid in such acid mixture varies from 0 to 5 % by weight. The amount of the other acids typically ranges from 1 to 40 % by weight.

The acid(s) may further contain additives that are necessary for the treatment.

Typically, those additives may include surfactants and/or biocides. The concentration of the synthetic polymer is typically from 0.01 to 10 % by weight, preferred from 0.05 to 5 % by weight and most preferred from 0.2 to 2 % by weight, referred to the mass of treatment fluid. To increase the viscosity of the treatment fluid, the polymers may also be ionically crosslinked by multivalent metal ions or metal complexes selected from group IIIA, IVB, VB, VIB, IIVB and/or VIIIB of the periodic table of elements, preferably selected form the ions and/or complexes of zirconium, aluminium, titanium, boron, chromium and/or iron. Especially preferred are the ions and/or complexes of zirconium and titanium.

Typically water soluble salts of the multivalent metal ions are used. Suitable anions are e.g. halides, especially chloride, sulfate, lactate, citrate or gluconate. Also suitable are complexes of the multivalent metal ions with organic N- and O- compound, e.g. alcohols, di- and triols, mono-, di- and tri- carboxylic acids, mono-, di- and triamines and/or hydroxyalkylamines.

The quantity of transition metal compound for crosslinking the polymers ranges 0.1 to 50 % by weight, preferred from 0.5 to 30 %, more preferred from 1 to 20 % by weight, referred to the total mass of polymer.

The transition metal compounds, e.g. the salts and/or complexes of transition metal cation, are dissolved and/or diluted in water or in a water miscible solvent, and then added to the polymer solution under stirring to ensure a homogenous distribution of transition metal cation in the solution. The crosslinking of the polymer chains can be retarded or speeded up by adaptation of the stirring speed and/or adjusting the temperature.

The viscosity of the viscosified acids or of the crosslinked hydrogels typically may range from about 3 mPas to 5000 mPas, preferred from 10 to 500 mPas.

Another aspect of the instant invention is a method for reducing or inhibiting corrosion of steel equipment being present during an acid treatment of a gas- or oilfield reservoir with one or more acids comprising the measures: (i) providing an aqueous viscosified treatment fluid containing at least an acid and a water soluble synthetic polymer

(ii) pumping the treatment fluid into the formation using steel equipment,

characterised in that the synthetic polymer comprising

(I) at least structural units of formula (I)

(I)

wherein

R1 , R2 and R3 independently are hydrogen or Ci-Ce-alkyI,

(II) from 0 to 95 % by weight structural units of formula (II)

R4

— CH—

A I

O=S=O

I

O-R5

(I I)

wherein

R4 is hydrogen or Ci-Ce-alkyI,

A is a covalent C-S bond or a two-valent organic bridging group,

from 0 to 95 % by weight structural units of formula

(H i)

wherein

R6 and R7 are independently of one another hydrogen, Ci-Ce-alkyl, -COORg or -CH2-COORg, with Rg being hydrogen, a cation of an alkaline metal, of an earth alkaline metal, of ammonia and/or of an organic amine,

R8 is hydrogen, a cation of an alkaline metal, of an earth alkaline metal, of ammonia and/or of an organic amine, or is d-Ce-alkyl, a group -CnH2n-OH with n being an integer between 2 and 6, preferably 2, or is a group -C0H20-NRI 0R1 1 , with 0 being an integer between 2 and 6, preferably 2, and R10 and R1 1 are independently of one another hydrogen or d-Ce-alkyl, preferably hydrogen,

(IV) from 0 to 95 % by weight structural units of formula (IV)

R12 R13

r I 1

CH R14 Λ R15

(IV)

wherein

R12 and R13 are independently of one another hydrogen, d-Ce-alkyl,

R14 is hydrogen or, d-Ce-alkyl, and

R15 is -COH, -CO-Ci-C-6-alkyl or

R14 and R15 together with the nitrogen atom to which they are attached form a heterocyclic group with 4 to 6 ring atoms, preferably a pyridine ring, a pyrrolidone ring or a caprolactame ring,

(V) from 0 to 20 % by weight structural units of formula (V)

R17

CH^

B I

R19— 0-P=0

I

0-R18

(V)

wherein

R17 is hydrogen or, Ci-Ce-alkyl, and R18 and R19 are independently of one another hydrogen, a cation of an alkaline metal, of an earth alkaline metal, of ammonia and/or of an organic amine,

B is a covalent C-P bond or a two-valent organic bridging group, with the proviso that the percentage of the structural units of formulae (I) to (V) refer to the total mass of the copolymer and the percentage of the structural units of formulae (I) to (V) amounts to 100%

The presence of the aforementioned synthetic polymer comprising the structural units (I) to (V) reduces the corrosion of steel equipment during an acidizing treatment significantly when such synthetic polymers are used as thickener for the acid(s).

Test Methods

The following testing methods are used:

Viscosity:

The viscosity was determined using a Fann 35 rheometer or Ubbelohde capillary viscosimeter.

The Fann 35 rheometer is a Couette type coaxial cylinder rotational viscometer, equipped with R1 rotor sleeve, B1 bob and F1 torsion spring. 120 ml of the sample were poured into the viscometer cup and characterized at 100 rpm and room temperature.

For the Ubbelohde capillary viscosimeter the capillary of appropriate width was choosen, about 30 ml of the sample were filled into the capillary. The capillary was then allowed to adjust temperature to 30 °C for 10 min in a water bath. The time of the defined sample volume for passing through the capillary was taken and then multiplied with the capillary constant to give the viscosity in cP.

K value

The K-value is a method to determine the molecular mass of polymers relative to a sample of similar chemical composition.

To determine the K value, the copolymer is dissolved in a 0.5 % by weight solution of isotridecanethoxylate (6 EO) surfactant in distilled water. The quantity of copolymer in the solution is adjusted to 0.1 % by weight and added to the solvent solution under stirring.

The viscosities of the solvent solution η₀ as well as of the copolymer solution η_ε are determined by means of an Ubbelohde capillary viscometer at 25°C. This value gives the absolute viscosity of the solution (η_ε). The absolute viscosity of the solvent is η₀. The ratio of the two absolute viscosities gives the relative viscosity

From the relative viscositiy, the K value can be determined as a function of the concentration c by means of the following equations:

Log nrei = [(75k²/(1 +1 .5kc) + k]c

k = K/1000

Flow Spread:

The flow spread was used to characterize crosslinked gels. The gel is poured onto a glass plate, the diameter at 3 different places is determined and the average of these three values is calculated.

Corrosion Test:

The corrosion tests were performed at room temperature (23°C). Small metal specimen - about 12 g each - of different steel quality were purchased. To remove any coating and to create a clean and well defined surface, the specimens were pre etched for 2 h using HCI 15 % by weight. All steel specimens were cleaned carefully with alkaline surfactant solution, then with distilled water followed by acetone. The specimens were allowed to dry at the air. They were not touched with the hand, only by using forceps.

Directly before the start of a test the weight of the specimen was determined. The specimen was placed into the test solution that was kept in a glass beaker or glass bottle. After a defined time the specimen was removed and purged with alkaline surfactant solution, distilled water and acetone. After drying at the air the weight of the specimen was determined and the weight loss relative to the initial weight was calculated. Abbreviations

HLB HLB-value means the hydrophilic-lipophilic balance of a surfactant and is a measure of the degree to which it is hydrophilic or lipophilic, determined by calculating values for the different regions of the molecule. There are different methods to calculate the HLB-value. The most common results in a ranking of the surfactants between 0 and 20 with 0 corresponds to a completely lipophilic/hydrophobic molecule, and a value of 20 corresponds to a completely hydrophilic/lipophobic molecule. Typically, the suppliers specifies the HLB-value of the surfactant.

St37 St37 is the designation for an unalloyed carbon steel for construction purposes.

1 .4301 1 .4301 is the designation for an alloy steel containing 18 % by weight

Cr and 5 % by weight Ni.

1 .4401 1 .4401 is the designation for an alloy steel containing 1 6 % by

weight, Cr 10 % by weight Ni and 2 % by weight Mo.

η₀ Viscosity of solvent solution for K value determination

He Viscosity of copolymer solution for K value determination

n,rei Relation of He relative to η₀

c Concentration of polymer in solution, determination of K value

The following examples illustrate the invention without limiting it. Examples:

Example 1 : Preparation of a polymer via inverse emulsion polymerization

37 g sorbitan monooleate were dissolved in 1 60 g Cn-C-ie isoparaffin. 100 g water in a beaker were cooled to 5 °C, then 50 g 2-acrylamido-2-methylpropane sulfonic acid and 10 g vinylphosphonic acid were added. The pH was adjusted to 7.1 with aqueous ammonia solution. Subsequently 223 g acryl amide solution (60 weight% in water) were added.

Under vigorous stirring the aqueous monomer solution was added to the isoparaffin mixture. The emulsion was then purged for 45 min with nitrogen.

The polymerization was started by addition of 0.5 g azoisobutyronitrile in 12 g isoparaffin and heated to 50 °C. To complete the reaction the temperature was increased to 80 °C and maintained at this temperature for 2 h. The polymer emulsion was cooled to room temperature. As product, a viscous fluid was obtained.

The K-value of the copolymer of ex. was 390.

Example 2: Preparation of a polymer via inverse emulsion polymerization

A polymer emulsion was prepared according to example 1 but using 80 g 2- acrylamido-2-methylpropane sulfonic acid, no vinylphosphonic acid and 187.5 g acryl amide solution (60 weight% in water).

The K-value of the copolymer of ex. 2 was 441 .

Example 3: Preparation of a polymer via gel polymerization

400 ml deionized water and 9.2 ml 25 weight-% aqueous ammonia solution were placed in a reaction vessel. 70 g acryl amide and 30 g acrylic acid were added under stirring. The solution was purged with nitrogen and heated to 50°C. The polymerization was started by addition of 5 ml of a 20 % by weight aqueous solution of ammonium persulfate. To complete the reaction the temperature was increased to 80 °C and maintained at this temperature for 2 h. After cooling to room temperature a highly viscous gel was obtained. The gel was dried at 90 °C in a vacuum drying oven and carefully chopped from time to time. The dried polymer was crushed to obtain small particles.

The K-value of the copolymer of ex. 3 was 418.

Example 4 to 6:

200 ml of 20 % by weight HCI and 2 g of of isotridecanethoxylate (6 EO) surfactant were mixed in a Waring blender. Polymer emulsion was added and mixed for 4 min. The viscosity of the polymer solution was determined. The results are shown in Table 1

Table 1

Example Polymer Quantity, Viscosity, Measured with

emulsion [gram] [mPas]

Fann 35 at 100

4 Ex. 1 4.0 25.5

rpm Fann 35 at 100

5 Ex. 2 8.0 90

rpm

Ubbelohde

6 Ex. 1 1 .89 1 .9

capillary

Examples 4 to 6 clearly show that the polymers significantly thicken the acid. Example 7:

To 200 g HCI (15 % by weight) 2 g isotridecanethoxylate (6 EO) surfactant were added and mixed in a Waring Blender. 4.5 g of the polymer emulsion of example 1 were added and mixed for 10 min.

To the so prepared polymer solution 2 g of a 50 % by weight solution of a Zr-lactat complex were added and stirred for 60 s. A gel is formed that can be poured onto a glass plate. The flow spread is 15 cm.

Examples 4 to 7 clearly demonstrate that the polymers can be used as linear gel as well as crosslinked gel to thicken acid.

Examples 8 to 11 :

In a Waring Blender 0.45 g of the polymer of example 3 were added to 299.5 g

HCI 15 % by weight. The mixture was stirred for 10 min, then it was transferred to a beaker and stirred overnight using a magnetic stirrer to ensure complete dissolution of the polymer. Then the acidic polymer solution was poured into two beakers.

In example 8, a specimen St37 was mounted into one beaker.

In example 9, a specimen 1 .4301 was mounted into the other beaker.

In examples 10 and 1 1 , St37 and 1 .4301 were mounted into a beaker each containing only HCI 15 % by weight. After 6 h the specimen were removed, cleaned and the weight loss was determined. The results are shown in table 2.

Examples 12 to 21 :

Acidic solutions containing polymer or polymer emulsion were prepared according to the description in examples 8 and 4 with the concentrations given in table 2. Corrosion tests were done according to the procedure described in example 8. Steel specimen and test duration is also given in table 2. In examples 20 and 21 the test solution was stirred with a magnetic stirrer during the test.

Table 2:

Concentrations of HCI, polymer powder and polymer emulsion are given in % by weight relative to the total mass of the acidic test solution.

The weight loss of steel specimen is given in % by weight relative to the initial weight of the specimen before testing.

The examples clearly show that steel of different quality is significantly less attacked by hydrochloric acid in various concentrations when polymers according to the invention are present compared to reference tests where hydrochloric without polymers were used.

The corrosion inhibition effect is obvious even at very low concentration of polymer. Natural based polymer like guar does not show corrosion inhibition properties as the synthetic polymers describe in the present invention.

Thus, the instant invention can also prohibit corrosion in acidizing treatments which include natural based polymers, polysaccharides or modified polysaccharides being used in treatment of oil or natural gas reservoirs and required equipment. Suitable natural based polymers are for instance, suitable hydratable polysaccharides include starch or its derivatives, galactomannan gums, glucomannan gums, cellulosic derivatives, preferably carboxymethyl cellulose; cellulose ether, preferably

hydroxyethyl cellulose; guar gums or its derivatives, preferably hydroxyalkyl guar, carboxyalkyl guar, and carboxyalkyl hydroxyalkyl guar or hydrophobically modified guar alginates, carrageenans, tragacanth gums, glucan gums and xanthan gums. All the natural based polymers have in common that due to their glycosidic bonds they are not stable at elevated temperatures and under highly acidic conditions for a longer period. However, in combination with the synthetic polymers defined by the structural units of the formula (I) to (V), such period can be extended.

Claims

1 . Use of synthetic polymer comprising

(I) at least structural units of formula (I)

(I)

wherein

R1 , R2 and R3 independently are hydrogen or d -Ce-alkyl,

(II) from 0 to 95 % by weight structural units of formula (II)

R4

CH^

A

I

o=s=o

I

0-R5

(II)

wherein

R4 is hydrogen or d -Ce-alkyl,

A is a covalent C-S bond or a two-valent organic bridging group, from 0 to 95 % by weight structural units of formula (III)

R6 R7

I

^■CH-

Cr O-R8

(Hi)

wherein R6 and R7 are independently of one another hydrogen, d -Ce-alkyl, -COOR9 or -CH₂-COOR9, with R₉ being hydrogen, a cation of an alkaline metal, of an earth alkaline metal, of ammonia and/or of an organic amine,

R8 is hydrogen, a cation of an alkaline metal, of an earth

alkaline metal, of ammonia and/or of an organic amine, or is C-i -Ce-alkyl, a group -C_nH2n-OH with n being an integer between 2 and 6, preferably 2, or is a group -C0H20-

NR10R1 1 , with 0 being an integer between 2 and 6, preferably 2, and R10 and R1 1 are independently of one another hydrogen or d -Ce-alkyl, preferably hydrogen,

(IV) from 0 to 95 % by weight structural units of formula (IV)

R12 R13

I R14 Λ R15

(IV)

wherein

R12 and R13 are independently of one another hydrogen, C-i -Ce-alkyl,

-COOR16 or -CH2-COOR16, with R 16 being hydrogen, a cation of an alkaline metal, of an earth alkaline metal, of ammonia and/or of an organic amine,

R14 is hydrogen or, Ci -Ce-alkyl, and

R15 is -COH, -CO-C1 -Ce-alkyl or

from 0 to 20 % by weight structural units of formula (V)

(V)

wherein

R17 is hydrogen or, d -Ce-alkyl, and

B is a covalent C-P bond or a two-valent organic bridging group,

with the proviso that the percentage of the structural units of formulae (I) to (V) refer to the total mass of the copolymer and the percentage of the structural units of formulae (I) to (V) amounts to 100%,

as corrosion inhibitor reducing or inhibiting corrosion of metal equipment being present during acidizing treatment in gas- or oilfield reservoirs with one or more acids.

2. The use as claimed in claim 1 , wherein the water soluble synthetic polymer material is selected from the group consisting of polymers containing:

(V) 0 to 20 % by weight of structural formula V, preferred from 0 to 5 % by weight,

referred to the total mass of the polymer.

3. The use as claimed in claim 1 or 2, wherein the structural units of formula (I) are obtained from an ethylenically unsaturated carboxylic acid amide selected from the group of acrylamide, methacrylamide and/or their N-d -Ce-alkyl derivatives or N,N-Ci -C6-dialkyl derivatives.

4. The use as claimed in one or more of claims 1 to 3, wherein the polymer contain structural units of formula (II) to (V) which are obtained from an ethylenically unsaturated sulfonic acid and/or its alkaline metal salts and /or their ammonium salts, from ethylenically unsaturated carboxylic acid and/or its alkaline metal salts and /or their ammonium salts, from N-vinylamides, and/or an ethylenically unsaturated phosphonic acid and/or its alkaline metal salts and /or their ammonium salts, optionally together with further

copolymerisable monomers.

5. The use as claimed in one or more of claims 1 to 4, wherein the polymer is a copolymer with structural units of the formula (II) obtained from vinylsulfonic acid, 2-acrylamido-2-methylpropane sulfonic acid, 2-methacrylamido-2- methylpropane sulfonic acid, styrene sulfonic acid and/or their alkaline metal salts and/or their ammonium salts.

6. The use as claimed in one or more of claims 1 to 5, wherein the polymer is a copolymer with structural units of the formula (III) obtained from ethylenically unsaturated carboxylic acid and/or their derivatives, preferably chosen from the group of alkylesters from ethylenically unsaturated carboxylic acid, oxyalky I esters of ethylenically unsaturated carboxylic acid and/or esters of ethylenically unsaturated carboxylic acids with N-dialkylalkanolamines.

7. The use as claimed in one or more of claims 1 to 6, wherein the polymer is a copolymer with structural units of the formula (IV) obtained from N- vinylamides, preferably N-vinylformamide, N-vinylacetamide, N-vinyl-N- methylacetamide, or N-vinylamide comprising cyclic N-vinylamide groups.

8. The use as claimed in one or more of claims 1 to 7, wherein the polymer is a copolymer with structural units of the formula (V) being vinylphosphonic acid and/or its alkaline metal salts and/or its ammonium salts, and/or allylphosphonic acid and/or its alkaline metal salts and/or its ammonium salts.

9. The use as claimed in one or more of claims 1 to 8, wherein the acid consist of one or more Bronsted acids.

10. The use as claimed in one or more of claims 1 to 9, wherein the total

concentration of the one or more acid(s) is from 0.1 to 40 % by weight, preferably from 1 to 25 % by weight and more preferred from 3 to 20 % by weight, referred to the mass of treatment fluid.

1 1 . The use as claimed in one or more of claims 1 to 10, wherein the total

concentration of the synthetic polymer is from 0.01 to 10 % by weight, preferably from 0.05 to 5 % by weight and more preferred from 0.2 to 2 % by weight, referred to the mass of treatment fluid.

12. The use as claimed in one or more of claims 1 to 1 1 , wherein the viscosity of the treatment fluid is increased by ionically crosslinking the polymer by multivalent metal ions or metal complexes selected from group IIIA, IVB, VB, VI B, I IVB and/or VI MB of the periodic table of elements, preferably selected form the ions and/or complexes of zirconium, aluminium, titanium, boron, chromium and/or iron.

13. The use as claimed in one or more of claims 1 to 12, wherein the viscosity of the treatment fluid is from 3 mPas to 5000 mPas, preferred from 10 to 500 mPas.

14. The use as claimed in one or more of claims 1 to 13, wherein the treatment fluid further contains natural based polymers, polysaccharides or modified polysaccharides.

15. Method for reducing or inhibiting corrosion of steel equipment being present during an acid treatment of a gas- or oilfield reservoir with one or more acids comprising the measures: (i) providing an aqueous viscosified treatment fluid containing at least an acid and a water soluble synthetic polymer

(ii) pumping the treatment fluid into the formation using steel equipment, characterised in that the synthetic polymer comprising

(I) at least structural units of formula (I)

(I)

wherein

R1 , R2 and R3 independently are hydrogen or Ci-Ce-alkyI,

(II) from 0 to 95 % by weight structural units of formula (II)

R4

— CH—

A

I

o=s=o

I

0-R5

(I I)

wherein

R4 is hydrogen or Ci-Ce-alkyI,

A is a covalent C-S bond or a two-valent organic bridging group, from 0 to 95 % by weight structural units of formula

(H i)

wherein

R6 and R7 are independently of one another hydrogen, Ci-Ce-alkyl,

-COORg or -CH2-COORg, with Rg being hydrogen, a cation of an alkaline metal, of an earth alkaline metal, of ammonia and/or of an organic amine,

R8 is hydrogen, a cation of an alkaline metal, of an earth

alkaline metal, of ammonia and/or of an organic amine, or is

C-i -Ce-alkyl, a group -Cnh OH with n being an integer between 2 and 6, preferably 2, or is a group -C0H20-

NR10R1 1 , with 0 being an integer between 2 and 6, preferably 2, and R10 and R1 1 are independently of one another hydrogen or C-i -Ce-alkyl, preferably hydrogen,

(IV) from 0 to 95 % by weight structural units of formula (IV)

R12 R13

I R14 Λ R15

(IV)

wherein

R12 and R13 are independently of one another hydrogen, C-i -Ce-alkyl,

R14 is hydrogen or, Ci -Ce-alkyl, and

R15 is -COH, -CO-C1 -Ce-alkyl or

(V) from 0 to 20 % by weight structural units of formula (V)

R17

— CH-

B I

R19— 0-P=0

I

0-R18

(V)

wherein R17 is hydrogen or, d -Ce-alkyl, and

B is a covalent C-P bond or a two-valent organic bridging group,

with the proviso that the percentage of the structural units of formulae (I) to (V) refer to the total mass of the copolymer and the percentage of the structural units of formulae (I) to (V) amounts to 100%

16. The method as claimed in claim 15, wherein the water soluble synthetic

polymer material is defined in one or more of claims 2 to 8, the acid is defined in one or more of claims 9 or 10 the treatment fluid is defined in one or more of claims 1 1 to 14.