POLYMERIC COMPOSITIONS

Abstract: Stimulating fluids, especially acidising fluids, or completion or drilling fluids include as viscosifier a high molecular weight polymer that is resistant to hydrolysis and a viscosity-degradation inhibitor that is an ethylenically unsaturated material.

Description: Polymeric Compositions This invention relates to the stabilisation of viscosifying polymers in downhole fluids that are exposed to high temperature and, often, aggressive chemical conditions. These fluids include completion and drilling fluids and, in particular, stimulating fluids such as fracturing fluids and, especially, acidising fluids. These fluids are often exposed to temperatures in excess of 150°C, often above 200°C, and it is essential that they maintain adequate viscosity despite these very high temperatures and chemically aggressive environments such as acid in acidising compositions. it is well known to include viscosifying polymers in downhole compositions that are to be used for enhanced oil recovery. Because of the economic and technical limitations associated with enhanced oil recovery, it is normally only performed at relatively low temperatures, e.g. belov; 100°C, and it is well recognised that it is technically and commercially inappropriate to contemplate conducting enhanced oil recovery using synthetic polymeric viscosifiers at higher temperatures. However, these higher temperatures cannot be avoided in drilling, completion and stimulation processes and so it is essential to be able to provide synthetic polymers that will withstand these high temperatures for the duration of such processes, typically 1 to 24 hours. It is known that the polymer must be resistant to hydrolysis at these high temperature, and often chemically aggressive, conditions and it is known that, despite choosing polymers having the greatest possible degree of resistance to thermal hydrolysis, the viscosity still tends to decrease significantly during use. Some of this degradation may be due to reaction of the pendant groups despite the use of monomers that are intended to be substantially resistent to hydrolysis. Other degradation is thought to be due to cleavage of the backbone, i.e reduction of molecular weight and it can be relatively difficult to prove clearly which effect is occurring. There have been numerous proposals to incorporate various degradation inhibitors in polymers but none have solved the problem of viscosity degradation in hot drilling, completion and stimulation fluids. Examples are given in the following Chemical Abstracts, namely isobutanol, trichlorphenolate and amino acids in volume 108 189657g, phosphonates in volume 106 51187f, N-methyl-2-pyrrolidone in volume 105 229619t, maleic anhydride acylation derivatives of urea, thiourea, phenylurea or ethanolamine in volume 99 140769b, various sulphur compounds such as thiocarbonates in volume 56 52294m, thiourea and polyethylene glycol in volume 91 23783g and various compounds such as napthoquinone in volume 98 108362g. Many of the additives are described as being added to prevent oxidation of the polymer and in Chemical Abstract 88 23892y an inorganic reducing agent is used. In FR-A-2604444 the viscosity of a polyacrylamide for enchanced oil recovery is stabilised by adding at least 5% acrylamide monomer, based on the polymer. In JP-A-60/210657 it is proposed to stabilise polyacrylamide homopolymers, and copolymers of acrylamide with less than 50% of other monomers, that are to be used for purposes such as flocculation, paper-making, enhanced oil recovery, viscosifiers and soil improvers. The stabilisation is by the addition of at least 0.5% of a water soluble vinyl monomer and in the examples the monomers used are acrylamide (in an amount up to 7%) sodium acrylate, methacrylamide, acrylonitrile, dimethylaminoethyl acrylate and 2-acrylamido-2- methylpropane εulphonic acid AMPS (US Trade Mark) . The environment in which these polymers will be used are the traditional relatively non-aggressive environments listed above, and will clearly exclude the very hot and aggressive environments to which fluids such as acidising fluids may be subjected. This is confirmed by the fact that the specifically described polymers, and in particular the requirement for large amounts of acrylamide, again would contraindicate the use of such polymers in these highly aggressive environments. Accordingly, there iε nothing in the literature to suggest how one could hope to achieve improved stabilisation against viscosity degradation of synthetic polymers to be used in, for instance, acidising fluids. This iε confirmed by the fact that despite all the extensive literature on stabilising polymers, commercial reality is that a very limited range of additives have been incorporated to improve stability even in gentle environments. Urea is included for various reasons and can give some improvement. Greater improvement is achieved with thiourea, sodium nitrite or trimethoxyphenol but each of these materials are rather inconvenient to incorporate into the polymer, and the polymer is still liable to undergo substanial viscosity reduction during storage and use. As a result, it has been accepted that stimulating, completion and drilling fluids are liable to undergo substantial viscosity degradation, due to reduction in molecular weight, during use. We have now surprisingly found that it is possibe to stabilise such fluids so as to obtain a very much better retention in viscosity than has previously been obtainable. A downhole stimulating, drilling or completion fluid according to the invention is suitable for ue as at a temperature above 150°C and is viscoεified by the presence in the liquid of a polymeric composition that comprises a blend of a polymer degradation inhibitor and a water soluble cationic viscosity formed from ethylenically water soluble unsaturated monomer or monomer blend that provides groups that include cationic groups that are substantially resistent to hydrolysis during use and inhibitor is an ethylenically unsaturated material. The composition preferably retains at least 60% (preferably at least 80%) of its viscosity when (a) present as a 0.25% solution in water at 150°C for 16 hours or (b) present as a 1% solution in 15% aqueous hydrochloric acid at 200 C for 2 hours. The ethylenically unsaturated degradation inhibitor can be acrylamide, acrylonitrile or acrylic acid. Preferably, however, it is a material that has LD of above 400 that is to say more than 400mg of the inhibitor is required per kilogram body weight to achieve 50% lethality when administrated orally to rats. Polymeric compositions containing such inhibitors, and various uses of the } are also described in PCT application GB/90/„ filed today by the same applicant and inventor. Monomers that can be used include low toxicity ethylenically unsaturated carboxylic acids, for instance maleic acid (LD -.- = 708) and fumaric acid (LD - _, = DU bo 10700) and ethylenically unsaturated cationic monomers, especially dialkyl aminoalkyl (meth) acrylates such as dimethyl aminoethyl (meth) acrylate (LD 50 = 1751) and dialkyl aminoalkyl (meth) acrylamides especially those where the central alkylene group contains at least two chain carbon atoms, . for instance methyl chloride quaternary salt of dimethyl aminopropyl (meth) acrylamide (Maptac LD ςo = 715) . Diallyl dialkyl monomers can be used especially diallyl dimethyl ammonium chloride (Dadmac, LD 50 = 1700) . A wide variety of other ethylenically unsaturated monomers are of course well known and potentially uεeable, for instance, monoallyl trialkyl quaternary compounds such as allyl trimethyl ammonium chloride (atmac) , other allyl materials such as allyl sulphonate, other anionic materials such as vinyl sulphonate and sytrene phosponate, maleic anhydride, fumaric anhydride, dimethyl amino ethyl acrylate and dimethyl amino propyl acrylamide. The use of monomers containing one or more allyl groups is particularly preferred, especially Dadmac. The ethylenically unsaturated material does not have to be a monomer and it can instead be a material that contains a polymeric backbone with ethylenic unεaturation either in the backbone, in terminal groups or in pendant groups. The use of a polymer in this manner is particularly desirable since it can contribute to the viscosity or other properties of the polymer that is being stabilised or, at least, minimise the dilution effect on the polymer that might be provided otherwise by the monomer. The polymer is preferably a polymer formed from allylic monomer, optionally with other ethylenically unsaturated monomers, and the preferred allylic monomer is a dialkyl diallyl monomer. The polymers of diallyl dimethyl ammonium chloride (especially the homo polymer) are particularly preferred in this respect. Such polymers are believed to be terminated by free allyl groups. Other suitable polymers are acrylic terminated polyethylene glycolε and other unsaturated polymers including, especially, the prepolymers described in our European application 89301075 (EP-A-328321) It can be acceptable for the ethylenically unsaturated material to undergo polymerisation during the storage or use conditions that would otherwise have caused viscosity degradation, but it is generally preferred that the material merely acts as a free radial sink and does not undergo any substantial chemical change as a result of this. One disadvantage of the monomer being too reactive is that it may tend to react onto the polymer itself and this may alter the performance characteristics of the polymer. In some instances this iε undesirable. For instance, if it is important for the polymer to remain as a soluble polymer, and if the inhibitor monomer tends to react in εuch a way as to insolubilise the monomer, then such monomers should be avoided in those particular circumstances. AMPS is an example of a monomer that can have a reactivity that is too high in some circumεtances. The material may be non ionic or it may be co-ionic or counter-ionic with the polymer. If it is counter-ionic, the amount must not be such that the monomer forms an insoluble complex with the polymer. The polymer that is to be stabiliεed muεt be formed of monomers that provide groups that are substantially resistant to hydrolysis that may occur during use of the polymer in the chosen environment. Preferably therefore the polymer iε anionic or cationic. Preferably it iε formed wholly from anionic or cationic groups but it can be copolymerised with non-ionic groupε such as K,N-dimethylacrylamide; Vinyl Pyrollidone; N-Vinyl-N- Methyl Acetcimide or (meth) acrylamide. However large amounts of such groups, and especially large amounts of (meth) acrylamide are undeεirable and preferably the amount of (meth) acrylamide, and preferably the total amount of non-ionic monomer should preferably always be below 75%, and preferably below 40%, and most preferably below 20% by weight of the total weight of monomers. In particular, the amounts of acrylamide or other non-ionic monomer must be such that the polymers are stable against precipitation being cause by hydrolysis of the polymer. Anionic monomers that can be used include any of the conventional water soluble carboxylic and sulphonic ethylenically unεaturated monomers. Carboxylic monomers, εuch as (meth) acrylic acid are suitable in some instances but may tend to cause precipitation if the environment contains a significant amount of polyvalent metal ions, such as calcium or other alkaline earth ions and so if anionic monomers are requires it is generally preferred to use a sulphonate, especially AMPS. Thus one preferred type of polymer according to the inventin iε formed from AMPS and acrylamide wherein the amount of AMPS iε at least 30% by weight, preferably at leaεt 50% and most preferably at least 70% by weight. Such polymers are of particular use in fracturing fluids. Preferred cationic monomers are dialkylaminoalkyl methacrylateε and dialkylaminoalkyl (meth) acrylamideε where the central alkylene group preferably contains at leaεt 2 carbon atoms, generally 2 to 8 carbon atoms. Preferred materials are the dimethyl and diethyl aminoethyl methacrylates and the dimethyl or diethyl aminopropyl (meth) acrylamideε. The cationic monomers are generally present as acid addition of quaternary ammonium salts. Preferred monomers according to the invention are formed from 70 to 100%, preferably 80 to 100% and most preferably about 100% of such cationic monomers, with the balance being acrylamide. The cationic polymers are of particular value in acidising fluids. In order that the polymer has a viscosifying action in the downhole fluid, it generally has molecular weight above 1 million, often above 5 million. It generally has intrinsic viεcoεity (measured in 1 molar εodium chloride at 25°C by suspended level viεcometer) above 4 dl/g. When the polymer is anionic the IV is typically in the range 10 to 30. When the polymer is cationic the IV is typically in the range 6 to 18. The polymer may be made by polymerisation in conventional manner, for instance by gel or reverse phase polymerisation. The inhibitor can be present throughout the formation of the polymer if it has a sufficiently low reactivity rate that it will not participate in or interfere with the polymerisation reaction. Generally, however, it is added after the polymerisation is completed. If the polymer is present as a fluid εolution or emulεion or diεperεion (for instance a reverse phase dispersion or emulsion) the inhibitor can conveniently be incorporated into this fluid composition merely by stirring. A particularily preferred method of blending the inhibitor with a water soluble or water swellable polymer comprises providing the polymer aε a diεpersion of particles in an non-aqueous liquid (for instance by dispersing chopped gel into oil or, preferably, reverse phase polymerisation) and then mixing the inhibitor into the diεpersion, the inhibitor and the non-aqueous liquid being selected such that the inhibitor is preferently soluble in the particles. Preferably the particles in the dispersion are substantially dry before the inhibitor is added. For inεtance a diεpersion may be made by reverse phase polymerisation followed by azeotroping, and inhibitor (generally diεεolved in water) may then be mixed into the diεperεion. If the polymer is provided as a solid gel, for instance as beads or comminuted gel particles, the inhibitor or other, (generally as an aqueous solution) may be imbibed into the particles either before they are dried or after drying (in which event a further drying step maybe necessary) . The amount of inhibitor that has to be added will be found by trial and experiment. It usually at least 0.05% and generally at least 0.1%. Since the inhibitor may tend to dilute the activity of the polymer it iε generally preferred that the amount should be below 20%, preferably below 10% and most preferably below 5% by weight of the polymer. However larger amounts, e.g. up to 50%, can be used when the inhibitor does not contribute an undesirable dilution effect. Although it generally preferred to provide the inhibitor aε part of a polymeric composition that is formed previously (for instance to improve storage stability) , the invention can also include fluids where some or all of the inhibitor is introduced direct into the fluid, i.e. spearate from the polymer. The polymeric compositions can be included inconventional amounts to provide the desired viscosifying effects in εtimulating y drilling or completion fluids, the other components of which can be conventional. However, because there is less viscosity reduction, it may be possible to use less than would be required in the absence of the inhibitor. Preferably the polymeric compoεitions are used as viscosifying components in fracturing fluids or, especially, in acidising fluids having a composition that can otherwise be conventional. Preferably the fluid contains at least 10% HC1. Although the downhole fluidε can be used in environments where the temperature is relatively low, the invention is of particular value when the temperature to which the fluid iε expoεed is above 150°C, most preferably at least 200°C. Generally, the polymer is exposed to such high temperatures for periods in excesε of 1 hours, usually at least 2 hours. The period of exposure can be for several days but is generally for not more than 1 or 2 days. It is a particular advantage of the invention that, by appropriate choice of the polymer and the inhibitor (most preferably Dadmac) it is poεεible to formulate downhole fluidε that maintain most or all their initial viεcoεity deεpite prolonged exposure to the aggressive downhole conditions. The following are some examples. Example 1. To demonstrate the potential benefit of various inhibitors on polymerε made from ethylenically unsaturated monomers a laboratory test was developed observing the viscosity changes from initially making up a 1% εolution of the polymer in 15% aqueous hydrochloric acid followed by storing it for one hour at 200°C. The polymer for this laboratory test was a homopolymer of dimethyl aminoethyl (meth) acrylate (dmaema) quaterniεed with methyl chloride. The proceεε waε conducted for each inhibitor at a inhibitor doεage of lOOppm and at a inhibitor doεage of 500ppm. The polymer to which no inhibitor had been added underwent 63.5% viεcoεity degradation during the test. Thus any value higher than this is am improvement. The values in the presence of the various stabilisers are shown in table 1 below, Viεcoεity was measured as in Example 2 % Viscosity Retained Degradation Inhibitor 100 ppm 500 ppm Blank 63.5% 63.5% Dadmac 69.3% 83.3% Atmac 75.6% 79.8% Allyl Sulphonate 65.5% 71.9% Vinyl Sulphonate 78.6% Maleic Anhydride 76.3% 71.8% Amps (Sodium Salt) 43.2% 38.6% Dmaema 73.6% 78.6% Aptac 74.9% 91.4% Acrylic Acid 79.2% 75.2% Acrylamide 79.8% 86.0% Acrylic Pre-Polymer 67.0% 65.9% Thiε iε a pre-polymer according to the Example of European Application 89301075. Example 2. A 0.25% εolution of a co-polymer of 40% by weight sodium acrylate 60% by weight acrylamide was made in fresh water and the solution aged at 150 C for various times. The viscosity was measured by Brookfield RVT, εpindle 1 at 20 rpm. The reεults on the polymer alone and the polymer to which 1000 ppm Dadmac (based on the polymer solution) were added are shown below. Blank Dadmac Viscosity % Viscosity % (CP) Retained (CP) Retained Example 3 When the process of example 2 was repeated using the co-polymer of 60 parts sodium amps, 32.5 parts by weight acrylamide and 7.5 partε by weight εodium acrylate, the addition of 1000 ppm dadmac again gave a dramatic improvement in viεcoεity retention. Example 4 To demonεtrate the effect of the invention in an acidiεing polymer, a 1% solution was formed of DMAEMA homopolymer in 15% hydrochloric acid and was aged for 2 hours in a oven at 204 C, this being estimated to result in the solution being subjected to the oven temperature for about 1 hour. It was then rapidly cooled to room temperature and the viscosity was recorded as a percentage of the original viεcosity. When no degradation inhibitor was added, 20% of the viscoεity waε retained. When, respectively, 100, 500 and 2000ppm DADMAC monomer was added, the viεcoεity retention waε respectively, 46%, 73% and 76%. When the inhibitor was MAPTAC in amounts of, respectively, 100, 500 and 2000 ppm the viscoεity retained waε, reεpectively, 24%, 69% and 75%.

Claims: CLAIMS 1. A downhole stimulating, drilling or completion fluid viεcoεified by the preεence therein of a polymeric compoεition compriεing a blend of a viscosity-degradation inhibitor and a water soluble polymer formed from water soluble ethylenic monomer or monomer blend that provide groups that are subεtantially reεiεtant to hydrolyεiε during use, characterised in that the inhibitor is an ethylenically unsaturated material. 2. A fluid according to claim 1 that retains at leaεt 60% of its viscosity when (a) present aε a 0.25% solution in water at 150°C for 16 hours or (b) present aε a 1% solution in 15% aqueous hydrochloric acid at 200 C for 2 hours. 3. A fluid according to claim 1 which retains at least 80% of its viscoεity when (a) preεent as a 0.25% solution in water at 150°C for 16 hours or (b) present as a 1% εolution in 15% aqueouε hydrochloric acid at 200 C for 2 hours. 4. A fluid according to any preceding claim in which the inhibitor iε εelected from ethylenically unsaturated cationic monomers, allylic monomers, polymer containing ethylenic unsaturation, maleic acid or anhydride, fumaric acid or anhydride, vinyl sulphonate or styrene phosphonate. 5. A fluid according to any of claims 1 to 3 in which the inhibitor is an allylic material. 6. A fluid according to any of claims 1 to 3 in which the inhibitor iε diallyl dimethyl ammonium chloride. 7. A fluid according to any preceding claim in which the polymer haε intrinεic viεcoεity of at leaεt 4 dl/g and iε a copoly er of 0 to 70% non-ionic monomer with 30 to 100% ionic ethylenically unsaturated monomer εelected from carboxylic monomers, sulphonic monomers, dialkylaminoalkyl methacrylates and dialkylaminoalkyl- (meth) acryla ides in which the central alkyl group contains at 2 carbon atoms. 8. A fluid according to claim 7 in which the amount of ionic monomer is at least 60% by weight. 9. A fluid according to claim 7 in which the amount of ionic monomer is at least 80% by weight. 10. A fluid according to claim 7 in which the amount of ionic monomer iε εubstantially 100% by weight. 11. A fluid according to any of claimε 7 to 10 in which the anionic monomer iε 2-acrylamido-2-methylpropane sulphonate. 12. A fluid according to any of claims 7 to 10 in which the cationic monomer is dimethyl or diethyl aminoethyl methacrylate or dimethyl or diethyl aminopropyl (meth) acrylamide. 13. A fluid according to any preceding claim and which is an acidising fluid. 14. A fluid according to claim 1 which is an acidising fluid and in which the inhibitor is diallyl dimethyl ammonium chloride and the polymer is formed from 80 to 100% dialkylaminoalkyl methacrylate and 0 to 20% acrylamide. 15. A downhole method of drilling, completion or stimulation comprising the use of a fluid according to any preceding claim. 16. A downhole method according to claim 15 in which the fluid is exposed to a temperature of above 150°C for at least 1 hour. 17. The use of a polymeric composition as defined in any of claims 1 to 12 as the viscosifier of a stimulating, drilling or completion fluid.

Inventors: FARRAR DAVID, FARRAR, DAVID
Applicants: ALLIED COLLOIDS LTD, ALLIED COLLOIDS LIMITED
Application year: 1990
Main classification: C08J3
IPC codes: C08J3, C09K8