GB2420572A

GB2420572A - Inhibition of clay or shale

Info

Publication number: GB2420572A
Application number: GB0424768A
Authority: GB
Inventors: Michael Singh; Brian Dymond; Anne Frances Hooley; Kenneth Charles Symes
Original assignee: Ciba Specialty Chemicals Water Treatments Ltd
Current assignee: Ciba Specialty Chemicals Water Treatments Ltd
Priority date: 2004-11-10
Filing date: 2004-11-10
Publication date: 2006-05-31
Also published as: GB0424768D0

Abstract

A method of reducing or preventing the disintegration of clay and/or shale in a subterranean formation or a well bore, by bringing an aqueous fluid comprising a polymer into contact with the clay and/or shale, and in which the polymer is a copolymer by the reaction of an oligomeric or polymeric substrate with at least one ethylenically unsaturated monomer, wherein the reaction is conducted in the presence of a type II photo initiator and by the action of actinic radiation.

Description

Inhibition of Clay or Shale The present invention concerns methods of

reducing or preventing the disintegration of clay and/or shale in a subterranean formation or a well bore.

The invention also relates to an aqueous drilling fluid containing a polymeric shale and/or clay stabilizer.

It is known that when aqueous based fluids are used in drilling, particularly in a formation formed from the minerals known as shales that significant problems can result from the interaction of water with the shale. Water can become absorbed into the shale that swells or weakens thus disrupting its internal structure. This can lead to contraction of the weilbore and softening and disintegration of the wall of the well shaft.

The use of oil-based drilling fluids would alleviate these problems but they are expensive and are also thought of as environmentally undesirable. Therefore methods have been sought for the inhibition of shale disintegration (shale inhibition') when using aqueous drilling fluids. Similar problems occur with swelling and disintegration of clay materials within oil- and gas-bearing reservoirs on contact with aqueous reservoir fluids. Such swelling tends to lead to permeability problems with these reservoirs. Therefore it is also desirable to provide methods of inhibiting disintegration of clay (clay inhibition) in such environments.

Various polymeric materials are known for incorporation into drilling fluids as shale and clay inhibitors. High molecular weight (5 to 15 million) polyacrylam ides and acrylamide/acrylate copolymers (anionic polyacrylam ides) are known for this purpose. They are believed to work by absorbing onto the shale, coating it and preventing penetration by water. However, it is common to incorporate bentonite as a component of drilling fluids as a viscosifier.

Polyacrylamides, in particular anionic polyacrylamides, tend to absorb onto the : **. :. *.. *S.

S S * * S 5 S * S S * S 5 * S 5 ** * * S S S * * I. *S * S surface of bentonite in the drilling fluid and portions of the polymer dose are lost.

It is also known to incorporate polyglycols into drilling fluids as inhibitors of shale disintegration. These act by penetrating the shale and aiding in retaining its internal structure. They are also believed to cause some dehydration of the shale. Polyglycols are generally accepted as the industry standard shale inhibitors. The use of water-based drilling fluids containing potassium salts and polyalkylene glycols for shale inhibition is described in EP-A-495579 and Special Publication - Royal Society of Chemistry (1998), 211 (Chemicals in the Oil Industry), 58 70). This is also believed to work by absorbing onto the shale or clay, coating it and preventing penetration by water.

US-A-4,440,649 suggests the use of a vinyl amide-vinyl suiphonate terpolymer with acrylamide for the prevention of disintegration of claycontaining materials.

The terpolymer suggested is described in U.S. Pat. No. 4,309,523. All of the exemplified polymers contain 2-acrylamide-2-methyl-propane-3-sulphon ic acid (AMPS). Amounts of AMPS are often very high, for instance at least 50 wt %, often at least 65 wt %. The vinyl amide used is N-vinyl-N-methylacetamide, vinyl acetamide or vinyl formamide. These monomers are generally present in minor amounts in the exemplified polymers, in particular never more than 50 wt % of the polymer. When such monomers are present in amounts of 50% it is always in combination with significant amounts of AMPS (for instance at least wt %). US-A-4,536,297 also suggests the use of a vinyl amide-vinyl sulphonate terpolymer for prevention of disintegration of clay-containing materials. This terpolymer is described in DE-A-3,144,770. Again, the exemplified polymers contain significant amounts of anionic monomer, in this case sodium styrene sulphonate. In the terpolymers described the sulphonate is often present in an amount of at least 50 wt %. Amide monomers such as N- vinyl-N-methyl acetamide, N-vinyl formamide are also used. Generally however these are used in minor amounts, in particular in the terpolymers described, in which they are always used in amounts of less than 50 wt %. These * S * S S * *** *.* *..

*.* * * S * * S S S * 5 * * S * * S S ** * S * * * : : : predominantly anionic polymers can suffer from similar problems as those seen with anionic polyacrylamides.

It is also known to use rather low molecular weight highly cationic polymers, such as diallyl dimethyl ammonium chloride (DADMAC) as shaleinhibiting components of drilling fluids. These act to inhibit disintegration of shale by penetrating the shale and acting to increase its internal strength and reduce swelling on contact with water. Unfortunately, these cationic polymers have a tendency to absorb onto solid surfaces other than the shale with the result that a portion of the dose is lost and the use of such polymers is inefficient GB-A-2267921 describes an aqueous based drilling fluid comprising polyvinylpyrrolidone (PVP) as a shale inhibitor. This is the only material mentioned as shale inhibitor. It is stated that the PVP polymer may have a molecular weight from 5,000 upwards, but that it is preferably greater than one million. The examples show that high molecular weight is clearly preferred. It appears that the PVP is acting as a coating polymer to prevent penetration of water into the shale.

W096/04349 mentions the possibility of including dissolved molecular solutes.

These may be polymers. No specific polymers are suggested. W096/03474 suggests a particular composition which includes a specific surfactant together with a water soluble polymer such as PVP, polyvinyl alcohol, polysaccharide or partially hydrolysed (i.e. anionic) polyacrylamide US-A6,020,289 describes low molecular weight polymers of monomers selected from dialkyl (meth)acrylamides, N-vinyl formamide, N-vinyl acetamide and diacetone acrylamide in a drilling fluid for inhibiting shale disintegration during the drilling of a welibore in shale-containing rock.

It is known to employ water-soluble graft polymers for the inhibition of shale or : *. e:. *s.

S.. * * * . S * S S S * S * * S S ** * . S S * *S ** ** S * * clay.

An article by Li-Ming Zhang et al., Polymer International 48: 921-926 [1999] describes modified cellulosic polymers with amphoteric character prepared by grafting 2-dimethyl amino ethyl methacrylate and 4-vinyl benzene sulfonic acid sodium salt onto sodium carboxy methyl cellulose useful as drilling fluid additives in oilfields in order to achieve shale inhibition, filtration control and viscosity enhancement.

An article by Li-Ming Zhang et al., Journal of Applied Polymer Science, vols. 74, 3088-3093 [1999] discloses water soluble cationic graft polymers of hydroxy ethyl cellulose with vinyl monomers dimethyldiallyl ammonium chloride and acrylamide for inhibiting dehydration of water sensitive clay in oilfields.

The preparation of graft polymers in general is well-documented in the prior art.

Numerous methods are given in the literature for preparing a multitude of different types of graft polymers, covering a variety of different chemical structures and physical forms. Typically graft polymers described in the literature are used for a variety of applications. In general the preparation of a specific graft polymer, having particular properties designed for a particular application, is dependent upon the choice of starting materials and the process conditions.

In PCT/EP 04/005657 (Attorneys docket 22346), which was unpublished at the date of filing of the present application, graft polymeric surfactants formed from ethylenically unsaturated monomer and oligomeric or polymeric substrate polymer using a Type II photoinitiator are described. The polymers are of relatively low molecular weight, having a weight average molecular weight of below 100,000. No description of polymers suitable for shale or clay stabilisation is given.

S

* S * *S S.. *** *.s *.S * * * . . S * . S * * S * * * . S ** * S * * * . : :. : US 4111769 and an article entitled, "Ultraviolet cured pressure sensitive adhesives", Kenneth C Stueben, Union Carbide Corporation, Polymer Science and Technology, 1984 (29) 319-350, describes photo cure of mono- and multifunctional acrylate-poly ethers-benzophenone blends. Grafting is effected by ultraviolet radiation. The reference suggests that carbamyloxy alkyl acrylates can be grafted onto polyethylene oxides of molecular weight 1700 to 90,000 preferably 2500 to 21,000. The graft polymer thus formed would tend to have a high molecular weight in order to function as a pressure sensitive adhesive and the polymers described would be water insoluble.

To our knowledge graft polymers of oligomeric or polymeric substrates, such an polyalkylene glycol with water-soluble monomers prepared using Type II photo initiators suitable for inhibiting swelling and/or disintegration of clay and/or shale in subterranean formations or in the walls of well bores have never been disclosed.

It is an objective of the present invention to provide an improved method for preventing the swelling and/or disintegration of shale and/or clay in a subterranean formations or in a wellbore. It is a further objective to provide a method of inhibiting shale and/or clay employing an additive which is more effective than conventional clay or shale stabilizers. It is also an objective to provide methods of inhibiting shale and/or clay that utilizes additives that can be conveniently and inexpensively manufactured.

Thus according to the invention we provide a method of reducing or preventing the disintegration of clay and/or shale in a subterranean formation or a well bore, by bringing an aqueous fluid comprising a polymer into contact with the clay and/or shale, and in which the polymer is a copolymer by the reaction of an oligomeric or polymeric substrate with at least one ethylenically unsaturated monomer, : **. : *.* I.. * * * S S * * * * S * * S S ** S * * . * * S. ** S * wherein the reaction is conducted in the presence of a type II photo initiator and by the action of actinic radiation.

The action of the actinic radiation upon the photo initiator induces photo- initiation. In principle there are two types of photoinitiation mechanisms according to the process by which the initiating radicals are formed. Compounds undergoing homolytic cleavage are termed type I photoinitiators, which is not the case in the present invention, and compounds that interact with a second molecule (known as co-initiators) as in the present invention, are known as type Il photo initiators. The reaction pathways available for Type II photoinitiators are via hydrogen abstraction or electron transfer (followed by proton transfer) mechanisms, but under certain conditions both mechanisms may be involved.

These type II photoinitiations thus require co-initiators (usually hydrogen donating compounds typified by amines, alcohols, thiols or ethers) generating the radicals that trigger polymerisation and thus this type of initiation has the potential to incorporate hydrogen donating compounds into polymers (i.e. photografted polymers). However, photo initiated graft polymerisation using type II initiators has previously only been used to prepare hard resinous solids or adhesives.

Generally in the preparation of water-soluble graft polymers it can be difficult to control the water-solubility and the molecular weight, in particular the molecular weight of the pendant graft polymer chains. Furthermore, the overall structure of the graft polymer is critical in order to provide the right properties.

Surprisingly, we have been able to provide a graft polymer, which is suitable as a clay and/or shale inhibitor or flocculant, using a photo initiated graft polymerisation process employing type II photo initiators. Thus the process not only has the advantage in terms of convenience, but also can be controlled easily to provide a graft polymer with the right properties for a given application.

The graft polymers employed in the present invention have a molecular : **. I:. ** , 0s* . S * S

S S S S S

* S * S * * * :. * ** S S * * structure comprising polymer chains, formed from the chosen monomer, grafted directly onto the substrate by covalent bonding at a position on the substrate previously occupied by hydrogen atoms.

The graft polymer preferably has a molecular weight of at least 100,000, although it may have a molecular weight of several hundred thousand up to 50 million or more. Preferably, it would have a molecular weight of 120, 000 and one or two million.

Desirably the graft polymer can be water swellable or water dispersible, but preferably it will be water-soluble. By water-soluble we mean that it as a solubility in water of at least 5 g per 100cc of water and 25 C.

The graft polymer desirably is prepared from a substrate, to which grafting of ethylenically unsaturated monomers is enabled using a type II photo initiator. In this process a substrate may be regarded as a co initiator for the type II photo initiator. Therefore, although separate co initiators are not precluded, and for some systems may be beneficial, inclusion of a co initiator additional to the substrate is not essential. Typically, the substrate can be any natural polymer, sugars, vinyl addition polymers, polyethers, synthetic or semisynthetic condensation polymers, or inorganic substances and in which the substrate contains one or more abstractable hydrogen atoms. Examples of preferred substrates include substances selected from the group consisting of polyalkylene oxides, polyalkylene glycols, (meth) acrylam ide polymers, (meth) acrylic acid polymers (esters or salts thereof), polyvinyl alcohol, polyvinyl acetate, polyvinylpyrrolidone, polyethylenimine, polyesters, polyamides, sugars, polysaccharides, amino acids, proteins, natural oils such as Castor oil and inorganic substances containing abstractable hydrogen.

Particularly preferred as oligomeric or polymeric substrates are polyalkylene glycols. The polyalkylene glycol may be unsubstituted or substituted for instance : **. : **.

I.. * * * S I 5 S * * * S ** S * I * *I ** S * a dialkyl end caped polyalkylene glycol. Typically, it may be one of a number of polyethylene glycols or polypropylene glycols that preferably it is a polyethylene glycol which is desirably unsubstituted.

The polyalkylene glycol will usually have a weight average molecular weight of between 200 and 10,000. Preferably it is in the range of 1000 to 3000 and more preferably around 2000. A particularly preferred polyalkylene glycol is PEG 2000 (polyethylene glycol with a molecular weight of 2000).

The water-soluble ethylenically unsaturated monomer that is grafted onto the polyalkylene glycol substrate can be any monomer that readily undergoes direct reaction with the activated sites on the substrate. Typically such activated sites would include positions on the polyalkylene glycol where a type II photo initiator has generated a radical. In general the monomer molecule will tend to react through the double bond and become covalently bonded directly to the polyalkylene glycol. The monomer unit can then carry a radical onto which a further monomer molecule will react through the double bond and this process will continue to form a grafted polymer. It may also be possible for further polymer chains to develop from a grafted polymer chain rather than directly from the polyalkylene glycol. Such a process may be termed secondary grafting.

Desirably, the monomer comprises any one of acrylic, vinylic and allylic monomers.

The water-soluble ethylenically unsaturated monomer desirably has a solubility in water of at least 5g monomer per 100 mIs of water at 25 C. The monomer may be potentially water-soluble such that it can be modified, for instance after polymerization, to provide a monomer unit that would have been soluble in water, for instance having the above defined solubility.

Typical water-soluble monomers that may be used in the invention include water-soluble or potentially water-soluble monomers are selected from the : **. : * a I I I * * * I * I I e ** * I S * II ** 4. I group consisting of acrylamides (e.g. acrylamide and methacrylamide), N- alkylacrylamides, hydroxy alkyl (meth) acrylates (e.g. hydroxyethyl acrylate), Nvinylpyrrolidone, vinyl acetate, vinyl acetamide, acrylic acid (or salts thereof), methacrylic acid (or salts thereof), maleic acid (or salts thereof), maleic anhydride, itaconic acid (or salts thereof), crotonic acid (or salts), 2- acrylamido- 2-methyl propane sulfonic acid (or salts thereof), (moth) allyl sulfonic acid (or salts thereof), vinyl sulfonic acid (or salts thereof). dialkyl amino alkyl (moth) acrylates or quaternary ammonium or acid addition salts thereof, dialkyl amino alkyl (meth) acrylamides or quaternary ammonium and acid addition salts thereof and diallyl dialkyl ammonium halide (e.g. diallyl dimethyl ammonium chloride).

Preferred cationic monomers include the methyl chloride quaternary ammonium salts of dimethylamino ethyl acrylate and dimethyl aminoethyl methacrylate.

Other ethylenically unsaturated monomers may also be included with the water soluble monomer such as, styrenes, C1-30 alkyl (moth) acrylates, (meth) acrylonitrile and halogenated vinylic monomers, such as vinylidene chloride or vinyl chloride. If such other monomers are included, they tend to be included in small amounts based on total weight of definitely unsaturated monomer, for instance below 20% by weight and typically been a 10% by weight. Preferably the graft polymer is formed using definitely a saturated monomer or monomer blend consisting essentially of water-soluble monomer or potentially water- soluble monomer.

The graft polymer may be nonionic and in this case would be made by grafting nonionic monomers onto the polyalkylene glycol. Typically the non ionic monomers may be one or more of acrylam ides (e.g. acrylamide and methacrylamide), N-alkylacrylamides, hydroxy alkyl (moth) acrylates (e.g. hydroxyethyl acrylate), N-vinylpyrrolidone, vinyl acetate, vinyl acetamide etc. The graft polymer may be anionic and as that may be formed by grafting any : .*. *4* .. a * a

* * * * * 4. a p * a. *, * 4 anionic monomers onto the polyalkylene glycol substrate. The anionic monomers usually carry an acid group such as carboxylate, sulfonate or sulfate etc. Preferred anionic monomers include acrylic acid (or salts thereof), methacrylic acid (or salts thereof), maleic acid (or salts thereof, itaconic acid (or salts thereof), crotonic acid (or salts), 2- acrylamido-2-methyl propane sulfonic acid (or salts thereof), (meth) allyl sulfonic acid (or salts thereof), vinyl sulfonic acid (or salts thereof). The anionic monomer may be potentially anionic for example) maleic anhydride or any other anhydride monomer which can be hydrolysed after polymerisation to for instance yield the corresponding carboxylic acid. The anionic monomer may be a blend of anionic monomers or a blend of at least one anionic monomer with one or more nonionic monomers such as those given above, for instance acrylamide.

Particularly suitable polymers for the application of clay or shale stabilisation can be prepared using water-soluble ethylenically unsaturated monomers which are cationic or potentially cationic. By potentially cationic we include groups that are able to subsequently exhibit a cationic charge, for instance following a chemical reaction, such as quaternisation of a tertiary amine all acidification of a primary or secondary amine. Preferably, the water-soluble ethylenically unsaturated monomer is selected from the group consisting of dialkyl amino alkyl (meth) acrylates or quaternary ammonium or acid addition salts thereof, dialkyl amino alkyl (meth) acrylamides or quaternary ammonium and acid addition salts thereof, diallyl dialkyl ammonium halide. Dialkyl amino alkyl (meth) acrylates or acrylamides may be rendered cationic by reducing the pH to below 4 or 5 with for instance sulphuric acid or hydrochloric acid or preferably by quaternising using an alkyl halide such as methyl chloride or an alkyl sulphate such as dimethyl sulphate. A more preferred class of water-soluble monomers include (meth)acryloyloxy alkyl trialkyl ammonium compounds, normally obtained by the quaternisation of dialkyl amino alkyl acrylates or methacrylates.

Preferably the (meth)acryloyloxy alkyl trialkyl ammonium compound are as defined by formula (I) : . I * * I * I. * I * * I I - * I * * . ;. :.

R 0 R1 I ii I CH2=C-C-O--A--N_R2 (I)

I

X R

in which R is -H or CH3; R1, R2, R3 are each independently C 1 to 4 alkyl; A is an alkylene group; and X is an anion. A particularly preferred water-soluble ethylenically unsaturated monomer is acryloyloxy ethyl trimethyl ammonium chloride.

When the graft polymer is cationic it may be formed by grafting onto the polyalkylene glycol water-soluble monomer comprising at least one cationic monomer. It may be desirable to used two or more cationic monomers or a blend of at least one cationic monomer with a nonionic monomer such as acrylamide or methacrylamide or other nonionic monomers identified above.

The weight ratio of oligomeric or polymeric substrate, for instance polyalkylene glycol substrate to grafted moiety (i.e. portion formed from the ethylenically unsaturated monomer) is desirably in the range of 99:1 to 1:99. Preferably the ratio would be in the range 50:1 - 1:50. More preferably still the ratio should be between 20:1 - 1:20, especially between 10: 1 and 1: 10 and in particular around 1:1.

The type II photo initiator may be any substance which is capable of a photoreaction with a so-called co initiator or substrate to form a radical when exposed to a suitable actinic radiation. Preferably, the photo initiator may be any one of benzophenone, diaryl ketones, xanthones, thioxanthones, acridones, anthraquinones, diketones, ketocoumarines or imides. More preferably, the photo initiator is selected from the group consisting of ammoniumalkyl derivatives of benzophenones, sulfonylalkyl derivatives of benzophenones, * * *** ** * * * * I.e S S S S * : : : . :. * ** S S S S * * ammoniumalkyl derivatives of anthraquinones, sulfonylal kyl derivatives of anthraquinones and th ioxanthones.

It is further possible to use type II photoinitiators substituted by copolymerisable groups that are co-polymerised with the growing side chain, or type II photoinitiators that are bound to a polymer backbone, It is in addition possible to use blends of one or more of the previously mentioned type II photoinitiators.

Actinic radiation includes any electromagnetic radiation capable of initiating photochemical reactions. The choice of actinic radiation will depend upon the particular initiator used and will also depend to some extent on the reactants and if used the solvent. Desirably the actinic radiation includes electromagnetic radiation selected from the group consisting of ultraviolet light, infrared light, and visible light.

The graft polymer can be prepared by contacting the oligomeric or polymeric substrate with water-soluble ethylenically unsaturated monomer and a type II photo initiator to form a reaction mixture and wherein the reaction mixture is subjected to actinic radiation to induce grafting of the water-soluble ethylenically unsaturated monomer onto the polyalkylene glycol and photo polymerisation to form the graft copolymer.

The amount of type Il photo initiator will depend upon many factors, including choice of photo initiator, substrate, monomer and solvent (if used). Typically the photo initiator may be used in amount of 0.005 - 30% w/w of oligomeric or polymeric substrate. Preferably this will be within the range of 0.01 - 10 % and more preferably 0.1-5%. Particularly suitable polymers may be prepared using between 0.5 and 3.5%, especially between 1 and 3%.

It is particularly preferable to use ultraviolet light as the actinic radiation. The UV light source may include low, medium or high-pressure UV lamps, metal * S *SS S*S **I 555 * I S I S * * I.. I S S S S * : : : . :. *.

* S I S * S halide lamps, microwave-stimulated metal vapor lamps, carbon arc lamps, xenon arc lamps, excimer lamps, superactinic fluorescent tubes, fluorescent lamps or light emitting diodes, and the UV spectral output should ideally match the UV absorbance of the chosen photoinitiator.

A suitable UV light source for a typical photoinitiator such as benzophenone can be a medium pressure mercury UV lamp. Desirably, the UV light should have a wavelength in the range of 200 - 500 nm and a peak power density of at least 0.1 mW/ cm2 and normally at least 0.5 mW/ cm2. Usually the peak power density will be in the range of 1 mW/cm2 to 200W/cm2, and typically 1.5 mW/ cm2 to 100 mW/ cm2. A particularly preferred irradiation employs UV light at a wavelength between 360 and 370 nm and an intensity between 20 and 60 mW/cm2, more particularly between 30 and 50 mW/cm2.

The exposure length of the UV irradiation can be as short as a fraction of a second to several hours. Typically, the irradiation will be for a duration of up to 300 minutes. Preferably the actinic radiation is applied for a period of less than or 70 minutes. The irradiation may be from as little as a few seconds, usually one or more minutes. In some cases he may be desirable to irradiate the reaction mixture for between 45 and 70 minutes. In other cases this can be within the range of 1 to 35 or 45 minutes.

The UV light source can be applied to the reactants internally or externally provided there is no barrier to the specific wavelength of UV light that is required by the photoinitiator to function.

The reactant mixture can be introduced into the reactor vessel separately, together as a mixture, or passed through an irradiating chamber in a continuous fashion, or can be recycled with similar or different monomer/initiators mixtures.

The reactants can be degassed in order to remove unwanted oxygen from the reaction medium. Degassing is typically carried out by passing an inert gas * * *** *SS **. *e* * * * * * *** * S S S S * : : : :. :. ** * * S S S such as nitrogen to the liquid reaction medium for sufficient time and after sufficient rate to remove dissolved and entrained oxygen or by using a vacuum technique (3 or 4 freeze-pump thaw cycles) at <iOfl Torr.

Preferably, the reaction mixture is dissolved or dispersed in a liquid medium. In one aspect the reaction mixture is dispersed in the liquid medium to form a suspension or alternatively may be emulsified in the liquid medium to form an emulsion, for example according to a process defined by EP-A-150933, EP-A- 102760 or EP-A-126528. Preferably though the reaction mixture is dissolved in the liquid medium. The liquid medium can for instance be one of the range of solvents suitable for carrying out photo polymerisation reactions and desirably will be selected from the group consisting of methanol, toluene, cyclohexane, acetonitrile, dimethylformamide and water etc. Preferably, the solvents should not participate as hydrogen donors in the photo initiation step.

Therefore in one preferred form of the method of preparing the polymeric product, the oligomeric or polymeric substrate [for instance polyalkylene glycol], ethylenically unsaturated monomer and type II photo initiator are combined together in a suitable solvent to form a reaction mixture. All of the components may be dissolved in the solvent or one or more of the components may be dispersed throughout the solvent, provided that this does not adversely affect the reaction. The reaction mixture is then irradiated using UV light to initiate the reaction. The solvent used can be present in an amount of 10-90% of the reaction mixture. The photopolymerisation reaction can be carried out at a range of different temperatures, isothermally or adibatically. Typical temperature ranges include 10- 100 C.

More preferably, the liquid medium is a polar solvent, preferably water, and the type II photo initiator is soluble in said polar solvent.

* * S.. * . * S * S S S.. S S S S * * . S S * * * S S *S ** 55 5

S U S S S S If the photoreaction is performed in water or a polar solvent, the use of

water- soluble type II photoinitiators, such as the salts of ammoniumalkyl or sulfonylalkyl derivatives of benzophenones, anthraquinones or thioxanthones is preferred.

The polymerisation may be carried out in an aqueous medium as a pH of between 2 and 10. Advantageously, we find that the polymerisation may be carried out at essentially relatively neutral pH, for instance between pH 6 and 8.

In the method of reducing or preventing the disintegration of clay and/or shale in a subterranean formation or a well bore an aqueous fluid comprising the above described graft polymer is brought into contact with the clay and/or shale, We find that the use of aqueous drilling fluids containing these graft polymers gives better results than the use of conventional polymers such as nonionic polyacrylamide, anionic polyacrylamide, other anionic polymers and cationic polymers, such as p0IyDADMACs and poly acryloyl oxy ethyl trimethyl ammonium chloride. We also find that they give shale inhibition performance improved over the industry standard, polyglycol. We find also that the fluids give improved clay inhibition performance. In particular, we find that the fluids give improved clay inhibition performance when they are reservoir fluids. The invention is applicable also to workover and completion fluids as well as drilling and reservoir fluids and references to drilling and reservoir fluids should be interpreted accordingly. It is preferred in the invention that the fluid is one to be used as a drilling or reservoir fluid The polymer is dissolved in the aqueous fluid. It is usually present in amounts up to 5%, often 0.5 to 3%, preferably I to 3%, by weight of fluid. It is important that the polymer is dissolved. Highly soluble polymers may be included in the fluid in amounts at the higher end of the above ranges. Polymer of lower solubility may be included in amounts at the lower ends of these ranges to S.. *.* **. S.- : * . * * : S S S * * 5 5 5 * S S S S *S 55 55 5*

S I I S S

ensure it is dissolved.

Thus in one preferred form of the invention the method is part of a process of drilling a well bore. Typically the aqueous fluid would be a drilling fluid.

An aqueous drilling fluid of the invention may comprise any conventional drilling fluid additives found to be desirable in the circumstances. These include viscosifiers such as clay (e.g. bentonite), xanthan gum and hydroxyethyl cellulose polymer; weighting agents such as barytes and haematite; inorganic salts which aid in shale inhibition, such as sodium chloride, potassium chloride, calcium chloride, potassium carbonate, sodium acetate, and calcium sulphate; other materials added as shale inhibitors or fluid loss additives (where necessary) such as carboxylated celluloses, partially hydrolysed polyacrylamide and starch.

The aqueous drilling fluid of the invention may be used in any conventional drilling process in the same way as known aqueous drilling fluids. The invention thus provides, in a second aspect, a process of drilling a wellbore in shale- containing rock in which material is removed from the rock and flushed in an aqueous drilling fluid to the surface of the wellbore, in which case the aqueous drilling fluid contains as a clay or shale inhibitor a dissolved polymer and in which the polymer is a copolymer by the reaction of an oligomeric or polymeric substrate with at least one ethylenically unsaturated monomer, wherein the reaction is conducted in the presence of a type II photo initiator and by the action of actinic radiation. This aspect of the invention also includes any of the aforementioned more specifically defined features.

According to the invention we also provide an aqueous drilling fluid containing a dissolved polymer, and in which the polymer is a copolymer by the reaction of an oligomeric or polymeric substrate with at least one ethylenically unsaturated monomer, * * *** * : : * 0 * S * S : : : : : * * S S ** ** St * e S S S wherein the reaction is conducted in the presence of a type II photo initiator and by the action of actinic radiation. Any of the aforementioned characteristics may be incorporated into this aspect.

The aqueous fluid comprising the graft may be a reservoir fluid with clay or shale inhibiting activity. It may also be a workover or completion fluid.

In a further aspect of the invention the method is employed as part of an oil recovery process.

In an embodiment the present invention provides a composition suitable as an additive for inhibiting clay swelling or disintegration of shale in downhole locations comprising an aqueous solution of a polymer which is a copolymer by the reaction of an oligomeric or polymeric substrate with at least one ethylenically unsaturated monomer, wherein the reaction is conducted in the presence of a type II photo initiator and by the action of actinic radiation. Any of the aforementioned characteristics relating to the graft polymer are also applicable to this aspect of the invention. The graft polymer will be comprised in the aqueous fluid in an amount between 0.1 to 12% by weight, preferably between 1 and 10% by weight.

The aqueous fluid may be a stimulation fluid which is preferably prepared by admixing a quantity of the clay or shale stabilizing additive composition of the present invention and a polymeric viscosifying agent with an aqueous liquid.

Alternatively, the stimulation fluid may be prepared by blending together the various components, comprising the graft polymer and the viscosifier, in the desired proportion in the desired proportion in any combination or order.

Typically, the viscosifying agent is a soluble polysaccharide or soluble or swellable synthetic polymer. Representative examples pf soluble polysaccharides include galatomannan gums (guar), glucomannan gums, cellulose derivatives, and the like. Synthetic polymers include polyacrylamides, * * *.. *** *I.

* * * * : * **. I I S S * * * : : : :. :. * : * I * S sodium acrylate acrylamide copolymers and copolymers of acrylamide with 2acrylamido-2-methyl propane sulfonic acid. The stimulation fluid generally comprises a viscosifying agent in a concentration of about 100 to about 600 pounds per 1,000 gallons of the aqueous stimulation fluid.

The clay swelling or shale disintegration inhibitor composition, or components thereof, are admixed with an aqueous stimulation fluid in an amount sufficient to substantially stabilize the formation against permeability damage as the result of contact with the aqueous stimulation fluid. The graft polymer additive solution is preferably admixed with the stimulation fluid in an amount of at least about 0.5 pounds of the salt per 1,000 gallons, more preferably from about 1.25 to about pounds per 1, 000 gallons, and especially from about 2.5 to about 15 pounds per 1,000 gallons. The stimulation fluid obtained thereby preferably has at least about 0.1 pounds of the polyelectrolyte per 1,000 gallons of the stimulation fluid, more preferably from about 0.3 to about 10 pounds per 1,000 gallons, and especially from about 0.6 to about 5 pounds per 1,000 gallons.

The clay swelling or shale disintegration inhibitor additive is effective in treating a down hole formation when transported in a carrier fluid such as a well- stimulation fluid having either an acid, alkaline or neutral pH. The stimulation fluid of the present invention may have a pH in the range of from about 1 to about 11 without any significant negative effects upon the activity thereof, although preferably the pH of the stimulation fluid is within the more moderate range of from about a pH of 3 to about a pH of 10.

In this aspect of the invention the aqueous fluid may be introduced into the subterranean formation either through an injection well or through a production well.

* S *** S.. *S* * * S * * * S S.. * * * . . S * * S S * * * . * . . S. ** *5 * * I * * * U Particularly suitable polymers for use in the present invention are described in the UK patent application filed on the equivalent date under the attorney docket number (0S13-22367/P1).

The invention will now be illustrated with reference to the following examples.

The examples relate to inhibition of shale disintegration but similar compositions and testing methods are applicable to inhibition of clay disintegration.

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Examples

General Preparation of Graft Copolymers With (or without stirring) a mixture of the hydrogen donating substrate, monomer, photoinitiator, and cosolvent (if required) in a suitable reactor, is irradiated with UV light for a period of time until all or most of the monomer has been consumed. Cosolvent if used is removed from the crude product mixture, which affords the product.

Reduced Scale Shale Inhibition Testing Test work was carried out following a standard shale inhibition recovery test.

The test was carried out at a reduced size compared to normal due to the limited quantity of samples available for examples 1 and 2.

1) 500m1 of Base Fluid was prepared using: I) 57g of 10% wt Bentonite Wyoming Gel ii) 348g of Bradford Tap Water iii) 17.lgofKCl 2) The base fluid was mixed on the Hamilton Beach for 15 minutes and divided into 50m1 aliquots in 4oz plastic bottles.

3) The appropriate amount of dry polymer was weighed allowing for the active content, and added to the base fluid, this was then mixed on the Hamilton Beach for 30 seconds.

4) After 30 minutes tumbling, the contents were divided into 25m1 aliquots and placed in 2oz plastic bottles.

5) 3.75g(3.24g dry weight) of 2mm to 4mm of shale was added and hot rolled at 60 C for 2 hours.

6) The contents were sieved through a preweighed 500im sieve using l5ppb KCI washwater as required.

7) The sieves were dried in an oven at 105 C overnight, cooled in a desiccator

. S.. *** * * * * * : S.. * * * . . S * S S S * * * *e *5 ** ** * S S S S and weighed...DTD: 8) The % shale recoveries were calculated from the weights obtained, taking the shale dry weight into account Standard Scale Shale Inhibition Testing Test work was carried out following a standard shale inhibition recovery test.

1) A bulk solution of the base fluid was prepared: i) 400g of 10% wt Bentonite Wyoming Gel ii) 2440g of Bradford Tap Water iii) l2OgofKCl 2) The base fluid was mixed for 10 minutes on a silverson with a large holed head in a 5 litre plastic beaker and divided into 350 g aliquots into Hamilton beech cups 3) 4g (allowing for active content) of polymer product at 4ppb was added to the base fluid, this was then mixed on the Hamilton Beach for 15 minutes.

4) The contents were divided into 1 75ml aliquots and placed in small roach bombs with 0 rings 4) Add 25g of shale (dry wt. 21.87), shake for 10 seconds and lay down 5) Place the bombs in an oven and hot roll at 60 C for 2 hours.

6) The sieves were weighed and the samples sieved through 500 micron sieve using l5ppb KCI wash water as required 7) The sieves were dried in an oven at 105 C overnight and reweighed.

8) The % shale recoveries were calculated from the weights obtained, taking the shale dry weight into account.

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* S S S S S Synthesis of Graft Copolymers

Example I

Preparation of a 1:1 w/w Polyethyleneglycol - [2: (acryloyloxy)ethyl]trimethylarnrnonu chloride graft polymer.

(4-Benzoylbenzyl)trimethylammonium chloride (0.06 parts), 3.16 parts of polyethyleneglycol (Mwt 2000), 3.64 parts of a 80% aqueous solution of [2(acryloyloxy)ethyl]trimethylammonium chloride and 5.39 parts of water were placed in a glass petri dish (11 cm diameter and 2 cm deep). The petri dish containing the mixture was placed at a distance of 12 cm under a medium pressure Hg Black Ray UV lamp 360-370 nm, 40mW/cm2. After 60 minutes the sample had completely gelled to form a polymeric film.

Example 2

Preparation of a 1:1.9 w/w Polypropyleneglycol (Average M ca. 2000)Polyvinylpyrrolidone graft polymer.

Benzophenone (0.10 parts), 4.01 parts polypropyleneglycol (average M ca. 2000), and 7.55 parts of 1-vinyl-2-pyrrolidinone were placed in a glass petri dish (11 cm diameter and 2 cm deep). The petri dish containing the mixture was placed at a distance of 12 cm under a medium pressure Hg Black Ray UV lamp 360-370 nm, 40mW/cm2. After 60 minutes the sample had completely gelled to form a hard polymeric film.

Example 3

Preparation of a larger sample of a 1:1 w/w Polyethyleneglycol - [2(acryloyloxy)ethyl]trimethylammonium chloride uaternarised graft polymer.

(4-Benzoylbenzyl)trimethylammon ium chloride (0.46 parts), 25.85 parts of polyethyleneglycol (Mwt 2000), 29.67 parts of a 80% aqueous solution of [2- (acryloyloxy)ethyl]triniethylammonium chloride and 44.00 parts of water were * * .** **. .,. **.

* * * * * : *** * * * : * * : : : :. :. *e *.

* a S * * S placed in a glass petri dish (11 cm diameter and 2 cm deep). The petri dish containing the mixture was placed at a distance of 12 cm under a medium pressure Hg Black Ray UV lamp 360-370 nm, 40mW/cm2. After 60 minutes the sample had completely gelled to form a polymeric film.

Shale Inhibition Results for Examples I and 2 The polymers generated according the preparations as outlined above were then subjected to the shale inhibition testing protocol as outlined earlier

Table 1

Product Active Shale Recovery Average Improvement Solubility Dose Recovered (%) (%) (%) (ppb) (g) Series 1 Blank 0 1.71 52.8 54.3 - - 1.81 55.9 Example 2 4 2.59 79.9 80.1 47.4 Good 2.60 80.2 Series 2 Blank 0 1.68 51.9 51.5 - - 1.66 51.2 Example 1 4 3.02 93.2 91.2 76.9 Good 2.89 89.2 Comparative 4 2.87 88.6 89.0 72.8 - ymer 2.90 89.5 The shale inhibition results were run as two separate series and a comparative polymer (a low molecular weight polyamide) was used in series 2 as a comparison to demonstrate the benefits of the products of this invention. It was * * *** *.. S.. **.

* * * * * : *** * * * : . * : : : :. ** * * S I S * * clear from this set of results that the polymer produced in example 1 produced % shale recoveries that were as good as a benchmark comparative polymer when a reduced scale inhibition test was used. To confirm these initial findings the test work was repeated using a standard scale inhibition test.

Shale Inhibition Results for Examples 3

Table 2

Product Active Shale Recovery verage Improvement Solubility Dose Recovered (%) (%) (%) __________________ (ppb) (%) Blank 0 12.71 58.1 58.3 __________________ ________ 12.79 58.5 _______ ___________ ________________ comparative polymer 4 16.89 77.2 76.8 31.8 Excellent _________________ _______ 16.72 76.5 _______ __________ Solution Polymer AOETMAC 4 7.83 35.8 7.5 -18.5 Swellable __________________ _______ 12.94 59.2 _______ __________ _______________ Example 3 4 21.61 98.8 98.3 68.5 Good _________________ _______ 21.37 97.7 _______ __________ ______________ AOETMAC is Homo [2-(acryloyloxy)ethyl]trimethylammonium chloride Polymer It was clear from these results that the polymer produced in example 3 was superior in % shale recovered when compared not only to the comparative (benchmark) polymer but also against a homo[2(acryloyloxy)ethyltrimethlammonium chloride polymer. This example demonstrates the effectiveness of the graft copolymers of PEG2000 and 2(acryloyloxy)ethyltrimethlammonium chloride monomer produced by the technique described in this invention as superior shale inhibitors.

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Claims

Claims 1. A method of reducing or preventing the disintegration of clay

and/or shale in a subterranean formation or a well bore, by bringing an aqueous fluid comprising a polymer into contact with the clay and/or shale, and in which the polymer is a copolymer by the reaction of an oligomeric or polymeric substrate with at least one ethylenically unsaturated monomer, wherein the reaction is conducted in the presence of a type II photo initiator and by the action of actinic radiation.
2. A method according to claim 1 in which the polymeric material has a weight average molecular weight of above 100,000.
3. A method according to claim 1 or claim 2 in which the polymeric material is water-soluble.
4. A method according to any of claims I to 3, in which the substrate is selected from the group consisting of polyalkylene oxides, polyalkylene glycols, (meth) acrylamide polymers, N-alkyl (meth) acrylamide polymers, (meth) acrylic acid polymers (esters or salts thereof), polyvinyl alcohol, polyvinyl acetate, polyvinylpyrrolidone, polyethylenimine, polyesters, polyamides, sugars, polysaccharides, amino acids, proteins and inorganic substances containing abstractable hydrogen.
5. A method according to any of claims I to 4 in which the substrate is a polyalkylene glycol, preferably polyethyleneglycol.
6. A method according to any of claims I to 5 in which the substrate is a polyalkylene glycol that has a weight average molecular weight of between 200 and 10,000, preferably between 1,000 and 3,000, more preferably around 2,000.
7. A method according to any of claims I to 6, in which the monomer comprises any one of acrylic, vinylic and allylic monomers.
8. A method according to any of claims I to 7, in which the monomer comprises any of the group consisting of (meth) acrylamides, N-alkyl (meth) acrylamides, N-vinyl pyrrolidone, hydroxy ethyl acrylate, (meth) acrylic acid or * * *** *** *..

* * * * . * S *** * * * : * : :: . : : salts thereof, maleic acid or salts thereof, itaconic acid or salts thereof, 2- acrylamido-2-methyl propane sulfonic acid or salts thereof, vinyl sulfonic acid, allyl sulfonic acid, dialkyl amino alkyl (meth) acrylates or quaternary ammonium or acid addition salts thereof, dialkyl amino alkyl (meth) acrylamides or quaternary ammonium or acid addition salts thereof or diallyl dialkyl ammonium halide.
9. A method according to any of claims 1 to 8 in which the monomer is cationic or potentially cationic.
10. A method according to any of claims I to 9 in which the monomer is selected from the group consisting of dialkyl amino alkyl (meth) acrylates or quaternary ammonium or acid addition salts thereof, dialkyl amino alkyl (meth) acrylamides or quaternary ammonium and acid addition salts thereof, diallyl dialkyl ammonium halide.
11. A method according to any of claims I to 10 in which the watersoluble ethylenically unsaturated monomer is a (meth)acryloyloxy alkyl trialkyl ammonium compound of formula (I) R 0 R1

I II I

CH2=C-C-O-A-N-R2 (I) X R3 in which R is -H or CH3; R1, R2, R3 are each independently C I to 4 alkyl; A is an alkylene group; and X is an anion.
12. A method according to claim 11 in which the water-soluble ethylenically unsaturated monomer is acryloyloxy ethyl trimethyl ammonium chloride.
13. A method according to any of claims I to 12, in which the photoinitiator comprises one or more compounds selected from the group consisting of benzophenones, diaryl ketones, xanthones, thioxanthones, acridones, anthraquinones, diketones, 2-ketocoumarins, and imides.

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14. A method according to any of claims 1 to 13, in which the actinic radiation is selected from ultraviolet light, infrared light, and visible light.
15. A method according to any of claims I to 14 in which the method is part of process of drilling a well bore and said aqueous fluid is a drilling fluid.
16. A method according to any of claims 1 to 14 in which the method is to assist an oil recovery process.
17. A method according to claim 16 in which the aqueous fluid is introduced into through a injection well or through a production well.
18. A process of drilling a well bore in a shale or clay containing rock in which the material is removed from the rock and flushed by an aqueous drilling fluid to the surface of the well bore, in which the aqueous drilling fluid contains a dissolved polymer, and in which the polymer is a copolymer by the reaction of an oligomeric or polymeric substrate with at least one ethylenically unsaturated monomer, wherein the reaction is conducted in the presence of a type II photo initiator and by the action of actinic radiation.
19. A process according to claim 18 including the features of any of claims 2 to 14.
20. An aqueous drilling fluid containing a dissolved polymer, and in which the polymer is a copolymer by the reaction of an oligomeric or polymeric substrate with at least one ethylenically unsaturated monomer, wherein the reaction is conducted in the presence of a type II photo initiator and by the action of actinic radiation.
21. A process according to claim 20 including the features of any of claims 2 to 14.

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