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Title:
IMPROVED METHOD AND CHEMICAL AGENT FOR REDUCTION OF WATER PRODUCTION FROM OIL AND GAS CONTAINING WELLS
Document Type and Number:
WIPO Patent Application WO/2016/043598
Kind Code:
A1
Abstract:
The invention relates to a chemical substance (200) for reducing water production from subterranean zones surrounding oil and gas wells after being inserted into the formation, the chemical substance (200) comprising a plurality of particles (210), wherein each particle (210) has a water-swellable core (220). Each core (220) has been coated with a flexible coating layer (230) which is hydrophobic under all circumstances in the well and which does not swell in the presence of water nor react with water. Furthermore, the flexible coating layers from different particles contacting each other, have the property of forming an interconnected hydrophobic network (or pathway) of "channels", at least in a swollen condition of the core, for allowing hydrocarbons to go through this hydrophobic network/pathway while simultaneously preventing water (hydrophilic) to go through the "channels".

Inventors:
KILAAS, Lars (Skansegata 17C, Trondheim, N-7014, NO)
JOHNSEN, Heidi (Hoeggvegen 44, Trondheim, N-7036, NO)
Application Number:
NO2015/050160
Publication Date:
March 24, 2016
Filing Date:
September 15, 2015
Export Citation:
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Assignee:
WELLCEM INNOVATION AS (Postboks 1215 Sluppen, Trondheim, N-7469, NO)
International Classes:
C09K8/588; C09K8/88; C08F2/22; C08F291/00; C09K8/92
Domestic Patent References:
WO2010132851A12010-11-18
WO2007126318A12007-11-08
WO2013158306A12013-10-24
WO2002040828A12002-05-23
WO2010105070A12010-09-16
WO2002014453A12002-02-21
Foreign References:
US20050244641A12005-11-03
US20140187451A12014-07-03
US20120220504A12012-08-30
US20100307744A12010-12-09
US4670166A1987-06-02
US5786420A1998-07-28
Attorney, Agent or Firm:
HÅMSØ PATENTBYRÅ ANS (P.O.Box 171, Sandnes, N-4302, NO)
Download PDF:
Claims:
C l a i m s

1. A chemical substance (200) for reducing water production from subterranean zones surrounding oil and gas wells after being inserted into the formation, the chemical substance (200) comprising a plurality of particles (210), wherein each particle (210) has a water-swellable core (220), c h a r a c t e r i s e d i n that each core (220) has been coated with a flexible coating layer (230) which is hydrophobic under all circumstances in the well and which does not swell in the presence of water nor react with water, and wherein the flexible coating layer (230) has the property of forming an interconnected hydrophobic network of channels at least in a swollen condition of the core (220) for allowing hydrocarbons to go through these channels while simultaneously preventing water to go through the channels.

2. The chemical substance (200) according to claim 1, c h a r a c t e r - i s e d i n that the water-swellable core (220) comprises a material, which initially is hydrophobic and convertible to hydrophilic by means of hydrolysis.

3. The chemical substance (200) according to claim 1 or 2, c h a r a c t e r i s e d i n that the coating layer (230) comprises oligomeric or polymeric materials as homopolymers or copolymers based on monomers selected from a group comprising: styrene and styrene derivatives, acrylics and acrylic derivatives, meth- acrylics and methacrylic derivatives, amides and imides, carbonates, dienes, esters, ethers, vinyl acetals, vinyl esters, vinyl ethers and ketones, vinylpyridine and vinylpyr- rolidone, fluorocarbons, hydrophobic siloxanes like fluorinated siloxanes, or materials like condensation polymers based on epoxy or urethane resins.

4. The chemical substance (200) according to claim 1, 2, or 3, c h a r a c - t e r i s e d i n that the coating layer comprises styrene and styrene derivatives, e.g. p-tert butylstyrene, 4-(trifluoromethyl)styrene, 2,4-dimethylstyrene, or acrylic and methacrylic derivatives.

5. The chemical substance (200) according to any one of the preceding claims, c h a r a c t e r i s e d i n that, in an unswollen condition of the core (220), the core (220) has a size in the range from Ο,ΐμηη to 75μηΊ, preferably in the range from 0,5μηΊ to δθμηη, and most preferably in the range from ΙμΓΠ to 40μΓΠ.

6. The chemical substance (200) according to any one of the preceding claims, c h a r a c t e r i s e d i n that, in an unswollen condition of the core (220), the flexible coating layer (230) has a thickness in the range from 10 nm to 600 μιη, preferably in the range from 50nm to 200μηΊ, and most preferably in the range from 200nm to 50μΓη.

7. A method of reducing water production from subterranean zones surrounding oil and gas wells, wherein the method comprises the step of inserting the chemical substance (200) according to any one of the preceding claims into the formation of the well.

8. A method of manufacturing the chemical substance (200) of any one of claims 1 to 6, c h a r a c t e r i s e d i n that the method comprises the step of forming the coating layer (230) on the water-swellable cores (220) during the for- mation of said water-swellable cores (220) or after that said water-swellable cores (220) have been formed as a separate post process step.

Description:
IMPROVED METHOD AND CHEMICAL AGENT FOR REDUCTION OF WATER PRODUCTION FROM OIL AND GAS CONTAINING WELLS

The invention relates to a chemical substance for reducing water production from subterranean zones surrounding oil and gas wells after being inserted into the formation. The invention further relates to a method of reducing water production from subterranean zones surrounding oil and gas wells, wherein the method comprises the step of inserting such chemical substance. The invention also relates to a method of manufacturing such chemical substance.

In oil and gas wells, over time, there is usually also a certain production of water. After a while, the water production typically increases with time and may become so large that further production of hydrocarbons is no longer remunerative. The water can be naturally present in the reservoir. It can also be water injected from another well to maintain the production and the pressure in the reservoir. The produced water includes chemical compounds, some of which may be harmful for the environment and must be removed before the water eventually may be discharged to the sea.

Some of the chemicals are dissolved in the water from nature's side while others are added and dissolved as production chemicals. Production chemicals are thus included in the produced water and constitute an environmental problem.

Produced water is thus not only an economic disadvantage for the oil companies but also a significant environmental challenge. In many cases produced water is reinjected to maintain the pressure. This also involves costs. The best technical and economical solution would be to shut off the water selectively in the reservoir, which is the object of the present agent and method.

The prior art technology in this area covers a broad range of suggestions of use of water swellable polymers to block the flow of water through permeable zones.

There are water swellable polymers based on positively or negatively charged (ion active) monomers and polymers based on non-ion active monomers. Some polymers are particulate during injection but are stretched out to linear polymers when swelled in water. Some of these can again have functional groups like P0 4 , which are intended to form some kind of bond (hydrogen bond or ion bond) or adhesion to the surface of the formation. In general it is positive for the ability of the polymer to take up water that it is not strongly cross-linked but able to be converted to a substantially linear polymer when swelled in water. This way many water molecules may be associated to the polymer chain. On the other hand the same property (low degree of cross-linking) is negative for the stability of the polymer both with respect to remaining in position within the formation and with respect to maintaining the desired properties, such as viscosity and general chemical stability with respect to chemical influences from the environment.

With regards to polymers of negatively charged ions these have a significant disadvantage in the form of insufficient swellability in environments where salts or acids are present. The same may be said, though to a somewhat less extent, about polymers based on positively charged monomers. Neutral systems are influenced least by salt concentrations.

The known water block technologies that have been reported so far all have their limitations and problems.

US 2011/0098377 Al discloses a technology where water production from a subterranean formation is inhibited or controlled by pumping a fluid containing coated compact particles through a wellbore into the formation. The particles have been previously coated with a relative permeability modifier (RPM). Upon contact with water, the RPM coating expands or swells and inhibits and controls the production of water. The "core" particles themselves do not swell. It is only the coating that swells in contact with water. The RPM may be a water hydrolysable polymer having a weight average molecular weight greater than 100,000. The particles may be conventional proppants or gravel. The general idea is that the RPM has the further property that it may deswell when oil returns after that the water has made the particles swell (and block the formation). However, the problem is that in that scenario both oil and water may be produced simultaneously. The water present in the oil will prevent de-swelling of the coating, and hence the system will continue to block oil production. Another problem is that the particles are hard and sized between 75μΓΠ and 2000μΓη, which renders the particles unsuitable for being inserted into the formation, which typically has much smaller pores, often less than 50μηι. US 2010/0132944 Al discloses a method of removing a partial amount of water in a well, comprising deploying a differential filter downhole; and performing a downhole operation. The differential filter comprises a swellable polymer that swells after contacting water such that permeability of the differential filter is reduced by removing said partial amount of water. The swellable polymer is non-soluble in the water and/or hydrocarbon and wherein the swellable polymer comprises at least one selected from the group consisting of poiyacryiic acid, poiyacryiate, poiymethacryiic acid, poly maleic anhydride, polyacrylamide, polyvinyl alcohol, latex, polyamide, polyester, and a copolymer thereof. A disadvantage of the technology described in this document is that water content is reduced, but not completely removed. Furthermore, the described method can reduce the water production in the production wellbore, but is not applicable for blocking the water within the pore structure out in the reservoir/formation.

US 6,884,760 Bl discloses a technology, which uses a wellbore fluid comprising a fluid loss additive and a bridging material that are hydrophobic in nature, hydrophobically modified or oil wettable. The wellbore fluid generates an active filter cake that once formed, is impermeable to an aqueous phase, thus reducing fluid loss and ensuring reduced damage to the formation, yet simultaneously is permeable to the back flow of hydrocarbons during a hydrocarbon recovery process. The problem with using filter cakes is that their flow resistance gets worse over time as the filter gets thicker over time, i.e. the efficiency of this technology over time reduces. A further problem with this technology is that its applicability is limited to the wellbore and therefore not very effective. During clean-up processes and restart of wells, a huge amount of well chemicals are specifically used for removing all sorts of filter cakes, and hence there are large risks for simultaneously damage the filter cake disclosed in this document.

The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art. It is a further object of the invention to provide a technology, which effectively combines the advantages of the various technologies of the prior art discussed before.

The object is achieved through features, which are specified in the description below and in the claims that follow.

The invention is defined by the independent patent claims. The dependent claims define advantageous embodiments of the invention.

In a first aspect the invention relates to a chemical substance for reducing water production from subterranean zones surrounding oil and gas wells after being inserted into the formation. The chemical substance comprises a plurality of particles, wherein each particle has a water-swellable core. Each core has been coated with a flexible coating layer which is hydrophobic under all circumstances in the well and which does not swell in the presence of water nor react with water. Furthermore, the flexible coat ing layers from different particles contacting each other, have the property of forming an interconnected hydrophobic network (or pathway) of "channels", at least in a swollen condition of the core, for allowing hydrocarbons to go through this hydrophobic network/pathway while simultaneously preventing water (hydrophilic) to go through the "channels".

The effects of the combination of the features of the invention are as follows. The cores which swell in the presence of water effectively block the formation after being inserted there into. The particle concentration (weight percentage) may be tailored such that the swollen particles form a "jelly" mass of swollen deformed particles contacting each other while securing completely filling of pore voids. However, the provision of such described particles having a stable and flexible (covalently linked) hydrophobic coating, leads to an in-situ generation of a hydrophobic connected

network/pathway (made up by the coating layer) ensuring only flow of oil but prevent ing water flow. The swollen particle "core" will prevent a constant flow of water through the core. The invention is particularly advantageous, because water and hydrocarbons are often produced simultaneously.

In the context of this invention it is important to note that with "swelling" of particles it is meant any increase in the volume of the particle, no matter whether this is by mere absorption of water, by chemical reaction with water, or a combination of those two. Also, the invention covers both reversible as well as irreversible swelling processes, as long as there is a coating layer, which in at least the swollen state, lets through the oil through an interconnected hydrophobic network of channels formed in this coating layer.

In an embodiment of the chemical substance in accordance with the invention the water-swellable core comprises a material, which initially is hydrophobic and convertible to hydrophilic by means of hydrolysis. This embodiment has the advantage that insertion into the formation is rendered much easier, particularly when there is already water produced or present in the formation. The chemical substance in this embodiment comprises a plurality of particles, wherein each particle initially has a water-non- swellable core (hydrophobic), and wherein the property of the core changes into a hydrophilic swellable core. Thus, this embodiment of the chemical substance enables placing the system in the formation without early blocking the pores during placement due to the delayed swelling property, i.e. no swelling occurs before the system is set in place and hydrolysis due to well temperature occurs. In this way it is feasible to prevent a filter cake to be built up.

In an embodiment the cores in the chemical substance comprises polymer, cross- linked particles which are stable in water-free solvents, wherein a) the particles are manufactured in an oil-in-oil emulsion and are cross-linked with at least one water- stable cross-linker and at least one water-labile cross- linker, the water-labile cross- linker being chosen among cross-linkers able to, when opened in a reaction with water, form a hydrophilic seat which increases the particles' water-swelling ability, and that b) the particles contains a controlled amount of at least one immobilized chemical compound that by means of said reaction between water and reactive groups in the polymer skeleton is able to form a new chemical bonding to another particle, to a nother reactive seat in the same particle, to other immobilized compounds or to other compounds. The cores in this embodiment have the advantageous property of changing from hydrophobic to hydrophilic under hydrolysis. These particles (cores) are hydrophobic when produced and squeezed into the formation. When water meets the particles in the pores, a hydrolysis takes place. This hydrolysis is faster when the temperature is increased in the well. That means there is time for squeezing the system into formation before hydrolysis starts. The hydrophobic cores then convert to hydrophilic cores due to the generation of hydrophilic groups like carboxylic acids from anhydrides (hydrophobic) during the hydrolysis. The initial particles are cross-linked with both a stable and labile cross-linker. The water splits the labile cross-linker (anhydride) into two carboxylic acid groups. The stable cross-linker retains the particle "shape" and integrity. Low amount of stable cross-linker leads to higher swelling capacity. The material here described and many examples thereof are well explained in WO 2007/126318 Al .

In an embodiment of the chemical substance in accordance with the invention the coating layer comprises oligomeric or polymeric materials as homopolymers or copolymers based on monomers selected from a group comprising : styrene and styrene derivatives, acrylics and acrylic derivatives, methacrylics and methacrylic derivatives, amides and imides, carbonates, dienes, esters, ethers, vinyl acetals, vinyl esters, vinyl ethers and ketones, vinylpyridine and vinylpyrrolidone, fluorocarbons, hydrophobic siloxanes like fluorinated siloxanes, or materials like condensation polymers based on epoxy or urethane resins. In a preferred embodiment of the chemical substance, the coating layer comprises styrene and styrene derivatives, e.g. p-tert butylstyrene, 4-(trifluoromethyl)styrene, 2,4-dimethylstyrene, or acrylic and methacrylic derivatives. These materials are easily available and give a tailored hydrophobic effect.

The materials and groups of materials in this embodiment of the invention have been shown to provide, after swelling (by hydrolysis) of the core, the effect of forming an interconnected network/pathway from the hydrophobic coating and thus to facilitate the production of hydrocarbons while blocking water production.

In an embodiment of the chemical substance in accordance with the invention, in an unswollen condition of the core, the core has a size in the range from Ο,ΐμηι to 75μηΊ, preferably in the range from Ο,δμηη to δθμηη, and most preferably in the range from ΙμΓη to 40μΓη. The range of core diameters of this embodiment has been proven being a convenient size for insertion into the formation. It must be stressed that the cores do not need to be spherical in the invention, i.e. they may have other shapes. What is important is that the (initial hydrophobic) cores have the property of swelling (after being hydrolysed) in the presence of water at a suitable well temperature, such that water production is blocked as the cores completely block the formation. In case of non-spherical shapes the diameter refers to an average diameter. In a swollen state, the cores may not be spherical, due to deformation and swelling to fill completely the pore voids.

In an embodiment of the chemical substance in accordance with the invention, in an unswollen condition of the core, the flexible coating layer has a thickness in the range from 10 nm to 600 μηη, preferably in the range from 50nm to 200μηΊ, and most preferably in the range from 200nm to 50μΓΠ. The range of coating layer thickness of this embodiment has been proven being a convenient size for insertion into the formation.

In a second aspect the invention relates more particularly to a method of reducing water production from subterranean zones surrounding oil and gas wells, wherein the method comprises the step of inserting the chemical substance according to the invention into the formation of the well.

In a third aspect the invention relates more particularly to the method comprising the step of forming the coating layer on the cores during the formation of said water- swellable cores, or after that said water-swellable cores have been formed as a separate post process step. An oil-in-oil emulsion process is a very convenient process to make the chemical substance of the invention. In the following is described an example of a preferred embodiment illustrated in the accompanying drawings, wherein :

Fig. la shows a chemical substance as known from the prior art when in an un- swollen condition;

Fig. lb shows the chemical substance of Fig . la when in a swollen condition;

Fig. 2a shows a chemical substance in accordance with the invention when in an unswollen condition, and

Fig. 2b shows the chemical substance of Fig. 2a when in the swollen condition.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Throughout the Figures, similar or corresponding features are indicated by same reference numerals or labels.

Fig. la shows a chemical substance 100 as known from the prior art when in an unswollen condition. This figure shows an earlier developed particle-based system (or substance) as known from the prior art (see patent application publication WO

2007/126318 Al) . The substance 100 comprises a plurality of particles 110, which have the property of swelling in the presence of water, the water induce hydrolysis of the hydrophobic core to give a hydrophilic swellable core. Such initially unswollen particles 110 are to be injected into the formation of a well where they swell strongly over time when in contact with water in the pores of the formation. Fig. lb shows the chemical substance 100 of Fig. la when in the swollen condition, where it is schematically illustrated that the pore voids (not shown) are hermetically closed by the swollen particles 110'. The unfortunate consequence of this is that also any oil 10, which may be produced simultaneously with the water, cannot go through this swollen particle gel, i.e. is blocked (as illustrated by the cross) .

Fig. 2a shows an improved chemical substance 200 in accordance with the invention when in an unswollen condition. The improved chemical substance 200 comprises a plurality of particles 210, each comprising a water-swellable core 220, which is coated with a flexible coating layer 230. The water-swellable cores 220 may be of the same composition as the particles 110 in Figs, la, but the invention is not limited to such water-swellable cores. The flexible coating layer 230 comprises a material, which is hydrophobic under all circumstances in the well. Furthermore, this layer has the property of forming an interconnected hydrophobic network (not shown) of channels at least in the swollen condition of the core 220, which network of channels facilitates hydrocarbons to go through, while water is blocked (due to the hydrophobic nature of the material). Fig. 2b shows the improved chemical substance 200 of Fig. 2a when in the swollen condition. The cores 220' have swollen, while the coating layers 230' are now stretched and contact each other to block the formation for water production. Simultaneously, the hydrophobic network in the coating layer 230' allows for hydrocarbons to go through, thus facilitating hydrocarbon production. For the matter of definition of certain features in the claims the particle core diameter D and the coating layer thickness T in unswollen condition of the cores 220 has been illustrated in Fig. 2a, as well as the particle core diameter D' in a swollen condition of the cores 220'.

As it comes down to the cores 220 of the particles the invention may be applied to various types of water swellable cores. It must be noted that the use of water swellable substances for reducing water production in a well, as such, is considered to be known to the person skilled in the art. Yet patent application publication WO

2007/126318 Al is incorporated by reference in its entirety into this specification. Nevertheless, the invention is particularly suitable for the water swellable cores, which are manufactured by the applicant himself. The applicant has developed a special chemical agent, which is described by the following clauses. Yet it must be stressed that, in accordance with the current invention, the earlier-described flexible hydrophobic coating layer 230 still has to be added to the particles of the special chemical agent described hereinafter. 1. Chemical agent for reducing water production from subterranean zones surrounding oil and gas wells, comprising polymer, cross-linked particles which are stable in water- free solvents, wherein a) the particles are manufactured in an oil-in-oil emulsion and are cross-linked with at least one water-stable cross-linker and at least one water- labile cross-linker, the water-labile cross-linker being chosen among cross-linkers able to, when opened in a reaction with water, form a hydrophilic seat which increases the particles' water-swelling ability, and that b) the particles contains a controlled amount of at least one immobilized chemical compound that by means of said reaction between water and reactive groups in the polymer skeleton is able to form a new chemical bonding to another particle, to another reactive seat in the same particle, to other immobilized compounds or to other compounds.

2. Chemical agent as described in clause 1, wherein the polymer cross-linked particles have a skeleton chosen among functionalized homo and copolymers of methacrylates, acrylates, acrylamides, vinyl alcohols, alginates, chitosan, xanthan, dextrans, gelatine, cellulose, amylose, biopolymers in general, vinyl pyrrolidone, vinyl sulfonates, derivat- ized polyethylene glycols, siloxanes, isocyanates, lactones and epoxides, being functionalized with hydroxyl, vinyl, primary, secondary, tertiary or quaternary amines, carboxylic acids, acryl, methacryl, sulfonic acids, hydroxy succinimides, anhydrides, esters, lactones, azalactones, epoxides or thiols.

3. Chemical agent as described in clause 1 or 2, wherein 2-aminoethyl methacryl hydrochloride is used as a copolymer in a preferred amount of 1-5% by weight of the total amount of polymer, together with a water stable cross-linker methylene bis- acrylamide and a water-labile cross-linker dimethacrylic anhydride.

4. Chemical agent as described in clause 1, wherein the water-labile cross-linker is chosen among compounds having functional groups comprising anhydrides, diesters, diimodo-esters, dihydroxy succinimides and difunctional disulfides.

5. Chemical agent as described in one of the clauses 1 - 4, wherein the at least one water-labile cross-linker comprises at least one of dimethacrylic anhydride, diacrylic anhydride and methylene dimethacrylate, in which the carboxylic acid groups are formed by reaction with water.

6. Chemical agent as described in one of the clauses 1 - 5, wherein the water-labile cross-linker is present in an amount in the range between 0.05 and 10% by weight of the total amount of monomer, more preferred in the range between 0.05 and 3%.

7. Chemical agent as described in clause 1, wherein the at least one immobilized compound is chosen among water soluble compounds comprising functional reactive groups chosen among epoxy, amine, thiol, OH, ester, lactone, azalactone, maleimide, aldehyde, and anhydride.

8. Chemical agent as described in clause 1, wherein the at least one immobilized compound is chosen among derivatives of phosphonates, borates, reactive polyvinyl alcohols, reactive polysaccarides based on derivatives of gelatine, chitosan, alginat, cell ulose, amylose, dextran, and xanthan; functionalized reactive synthetic polymers of polyethylene glycols, polyacryl amides, polyacrylates, polymethacrylates, polyzwitter ions, peroxides, azo initiators, persulfates, siloxanes or sodium silicates.

9. Chemical agent as described in clause 8, wherein the functionalized, reactive synthetic polymers of polyethylene glycols, comprise diepoxy functionalized polyethylene glycols, diamino functionalized polyethylene glycols, dithio functionalized polyethylene glycols or divinyl functionalized polyethylene glycols.

10. Chemical agent as described in clause 8, wherein the at least one immobilized compound comprises at least one biopolymer with an epoxy group, amino group, thiol group, or free radical polymerizable group such as e.g. vinyl, allyl.

11. Chemical agent as described in clause 1, wherein the at least one immobilized compound is chosen such that it contributes to increased swelling, such as e.g. polymers of 2-acrylamido propanesulfonic acid, chitosan, poly n- acryloyltris(hydroxymethyl) aminomethane (NAT) and amino functionalized gelatine.

12. Chemical agent as described in clause 1, wherein the at least one immobilized compound is chosen such that it contributes to increased degree of cross-linking such as for example amino functionalized polyethylene glycols, like Jeffamine 1000, Jeffamine 2000 and Chitosan.

13. Chemical agent as described in clause 1, wherein the at least one immobilized compound comprise alkoxy silanes which by means of hydrolysis are converted to si- lanols that are able to bond to compounds like quartz. 14. Chemical agent as described in clause 1, wherein said other compounds are compounds in a subterranean formation in which the polymer system is used.

15. Chemical agent as described in clause 1, wherein the at least one immobilized compound is able to initiate reaction and cause interparticulate bondings and/or bondings between particles and the formation, such as sodium silicate, tetraethyl orthosili- cate and tetramethyl siloxane.

16. Chemical agent as described in clause 1, wherein the polymer particles are mainly hydrophobic until the reaction between water and water-labile cross-linkers has occurred.

17. Chemical agent as described in clause 1, wherein it has form of a masterbatch or a concentrate of said particles in a suitable organic solvent.

18. Chemical agent as described in clause 1, wherein it has the form of said particles in dry conditions.

Thus material that is preferably used for the core relates to a chemical agent in the form of a polymer particle system which is manufactured in an oil-in-oil emulsion. The polymer particles can be manufactured by means of a free radical polymerization process or a step polymerization (often also denoted condensation polymerization). By using an oil-in-oil emulsion water reactive monomers (mono functional, poly functional) can be used while avoiding contact between particles and water. The desired reactions should only take place subsequent the placement in the subterranean formation and such systems can only be produced in oil-in-oil emulsion systems.

The advantage of this type of water swellable cores compared to other patented methods which only provide swelling, is i.a. that the particles have a high swelling ability without being water soluble. The latter property implies that the agent mai ntains its structural integrity and remains in position in the formation after swelling. At the same time the agent is chemically and thermally stable and the rate with which it initially swells when contacted by water can be adjusted by means of a chosen balance between stable and labile cross-linking bonds. It is thus avoided that the agent swells prematurely, i.e. before it has reached its desired position in the formation. The first swelling of the agent in water leads to opening of cross-linking bonds so that hydro- philic seats in the particles become available which again leads to gradually increased rate of swelling of the particles, once initiated. By desired amount and combination of immobilized compounds the agent's properties can be further optimized, e.g . to form interparticulate bonds, i.e. bonds between different particles, which also contributes to the stability of the agent in relation to influence imposed in the formation subsequent to the swelling. Said features, characteristics and effects contribute to increased stability with respect to temperature, pressure, back-production, washing-out, as well as chemical (salinity, pH) and mechanical influence. Such immobilized compounds with properties as mentioned can only be included in a water-free environment which is why the manufacture in an oil-in-oil emulsion is imperative.

The chemical agent is manufactured in an oil-in-oil emulsion polymerization, i.e an emulsion polymerization free from water. Thereby it is ensured that the agent does not swell prematurely or lead to an early reaction involving solid immobilized compounds, but remains stable until introduced into the formation and contacted by water for the first time. The polymerization can be a free radical polymerization but also a condensation polymerization.

The agent can thus be manufactured in a water-free environment in a way with which it allows different chemical compounds to be immobilized thereon, which at a given point in time, in contact with water can contribute to the formation of chemical intra- particulate bonds, interparticulate bonds and bonds between particles and the reservoir formation. Said bonds will contribute to a more stable swollen particle system which maintains the reduced permeability for a long period of time while the system as mentioned becomes more robust in relation to external influence.

The invention also relates to a method of injection of the chemical substance into a subterranean zone which reduces the water production from a water permeable zone. This may be done by means of a suitable carrier fluid, preferable an organic carrier fluid lacking reactive groups like aliphatic or aromatic hydrocarbons or combinations of such together with other hydrophobic, organic solvents which do not comprise thiols, amines or hydroxyl groups.

The hydrophobic flexible coating layer 230 of the invention may be manufactured in different ways. A convenient way is to make such layer as a further step in the oil-in- oil emulsion process for making the cores, after that the cores have been formed. In the oil-in-oil process this means that a further monomer may be added to the emulsion, which then reacts with the surface of the particles (e.g. grafting) to form the coating layer.

Alternatively, the coating layer 230 may be added in a separate post treatment step, for instance in a grafting step or a coating step. Various techniques to apply the coating layer 230 exist and are considered to be well-known to the person skilled in the art.