NO340788B1 - Improved method and chemical agent for reducing water production from oil and gas-containing wells - Google Patents

Description

IMPROVED METHOD AND CHEMICAL AGENT FOR REDUCING WATER PRODUCTION FROM OUE AND GAS CONTAINING WELLS

The invention relates to a chemical substance for reducing water production from underground zones surrounding oil and gas wells after being introduced into the formation. The invention further relates to a method for reducing water production from underground zones surrounding oil and gas wells, where the method comprises the step of introducing such a chemical substance. The invention also relates to a method for producing such a chemical substance.

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

Some of the chemicals are naturally dissolved in the water, 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 pressure. This also involves costs. The best technical and economic solution would be to shut off the water selectively in the reservoir, which is the purpose of the present means and method.

The prior art technology covers a wide range of proposals for using 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 in particulate form during injection, but stretch into linear polymers when swollen in water. Some of these may again have functional groups such as PO4, which are intended to form a form of bond (hydrogen bond or ion bond) or adhesion to the surface of the formation. In general, it is positive for the polymer's ability to take up water that it is not highly cross-linked, but capable of being converted into an essentially linear polymer when swollen in water. In this way, many water molecules can be attached 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 in terms of remaining in place inside the formation and in terms of maintaining the desired properties, such as viscosity and general chemical stability in relation to chemical influences from the environment.

Regarding polymers of negatively charged ions, these have a significant disadvantage in the form of insufficient swelling ability in environments where salts or acids are present. The same can be said, albeit to a somewhat lesser extent, about polymers based on positively charged monomers. Neutral systems are less affected 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 an underground formation is inhibited or controlled by pumping a fluid containing coated compact particles through a wellbore into the formation. The particles have previously been coated with a relative permeability modifier (RPM). Upon contact with water, the RPM coating expands or swells, inhibiting and controlling 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-hydrolyzable polymer with a weight-average molecular weight greater than 100,000. The particles may be conventional plug material or gravel. The general idea is that the RPM has the additional property that swelling can reverse when oil returns after the water has caused the particles to swell (and block the formation). The problem, however, is that in this scenario both oil and water can be produced at the same time. The water present in the oil will prevent the swelling of the coating from going back, and the system will consequently continue to block oil production. Another problem is that the particles are hard and have dimensions between 75 pm and 2000 pm, which makes the particles unsuitable for introduction into the formation, which typically has much smaller pores, often less than 50 pm.

US 2010/0132944 Al discloses a method for removing a portion of water in a well, comprising deploying a differential filter down the hole; and performing a downhole operation. The differential filter comprises a swellable polymer which swells after contact with water, so that the permeability of the differential filter is reduced by removal of said partial amount of water. The swellable polymer is insoluble in the water and/or the hydrocarbon, and where the swellable polymer comprises at least one selected from the group consisting of polyacrylic acid, polyacrylate, polymethacrylic acid, polymaleic anhydride, polyacrylamide, polyvinyl alcohol, latex, polyamide, polyester and a copolymer thereof. A disadvantage of the technology described in this document is that the water content is reduced, but not completely removed. Furthermore, the described method can reduce water production in the production well drilling, but is not applicable for blocking the water inside the pore structure outside the reservoir/formation.

US 6884760 B1 discloses a technology which uses a well drilling fluid comprising an anti-filtration loss additive and a bridging material which is hydrophobic in nature, hydrophobically modified or oil wettable. The well drilling fluid produces an active filter cake which, after it is formed, is impermeable to an aqueous phase, thus reducing fluid loss and ensuring reduced damage to the formation, yet at the same time being permeable to the return flow of hydrocarbons during a hydrocarbon extraction process. The problem with using filter cakes is that their flow resistance gets worse over time as the filter gets thicker over time, ie the effectiveness of this technology decreases over time. A further problem with this technology is that its applicability is limited to well drilling, and therefore not very efficient. During clean-up processes and restarting of wells, an enormous amount of well chemicals are specifically used for the removal of all kinds of filter cakes, and consequently there are great dangers of simultaneous damage to the filter cake disclosed in this document.

The purpose of the invention is to remedy or to reduce at least one of the disadvantages of known technology, or at least to provide a useful alternative to known technology. It is a further object of the invention to provide a technology which effectively combines the advantages of the various technologies according to known techniques which have been previously discussed here.

The purpose is achieved by features that are specified in the description below and in the following requirements.

The invention is set out in the independent patent claims. The dependent claims set forth advantageous embodiments of the invention.

In a first aspect, the invention relates to a chemical substance for reducing water production from underground zones surrounding oil and gas wells after being introduced into the formation. The chemical substance comprises a plurality of particles, each particle having a water-swellable core. Each core has been coated with a flexible coating layer which is hydrophobic under all conditions in the well, and which does not swell in the presence of water, nor does it react with water. Furthermore, the flexible coating layers of different particles in contact with each other have the property of forming an interconnected hydrophobic network (or path) of "channels", at least in a swollen state of the core, to allow hydrocarbons to pass therethrough hydrophobic the network/path while preventing water (hydrophilic) from passing through the "channels".

The effects of the combination of features according to the invention are as follows. The cores which swell in the presence of water effectively block the formation after being introduced therein. The particle concentration (weight-percentage) can be tailored so that the swollen particles form a "gel" mass of swollen deformed particles in contact with each other, while ensuring complete filling of pore cavities. The provision of such described particles having a stable and flexible (covalently chained) hydrophobic coating, however, leads to an in-situ generation of a hydrophobically connected network/path (consisting of the coating layer) which ensures only the flow of oil, but prevents the flow of water. 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 by "swelling" of particles is meant any increase in the volume of the particle, regardless of whether this is only by absorption of water, by chemical reaction with water, or a combination of these two. The invention also covers both reversible as well as irreversible swelling processes, as long as there is a coating layer which, at least in the swollen state, lets the oil through through an interconnected hydrophobic network of channels formed in this coating layer.

In one embodiment of the chemical substance according to the invention, the water-swellable core comprises a material which is initially hydrophobic and which is converted to hydrophilic by means of hydrolysis. This embodiment has the advantage that introduction into the formation is made much easier, particularly when water has already been produced or is present in the formation. The chemical substance in this embodiment comprises a plurality of particles, where each particle initially has a water-non-swellable core (hydrophobic), and where the property of the core is changed to a hydrophilic swellable core. This embodiment of the chemical substance thus makes it possible to place the system in the formation without early blocking of the pores during placement due to the delayed swelling property, i.e. swelling does not occur until the system is placed and hydrolysis takes place due to the swelling temperature. In this way, it is possible to prevent a filter cake from building up.

In one embodiment, the cores of the chemical substance comprise polymer, cross-linked particles which are stable in anhydrous solvents, where a) the particles are prepared in an oil-in-oil emulsion and are cross-linked with at least one water-stable cross-linking agent and at least one water- unstable cross-linking agent, the water-unstable cross-linking agent is selected from cross-linking agents capable, when opened in a reaction with water, of forming a hydrophilic site which increases the water swelling ability of the particles, and that b) the particles contain a controlled amount of at least one immobilized chemical compound which, by means of the aforementioned reaction between water and reactive groups in the polymer shell, is able to form a new chemical bond to another particle, to another reactive site in the same particle, to other immobilized compounds or to other connections. The cores in this embodiment have the advantageous property of changing from hydrophobic to hydrophilic during hydrolysis. These particles (cores) are hydrophobic when produced and pressed into the formation. When water meets the particles in the pores, hydrolysis occurs. This hydrolysis is faster when the temperature is increased in the well. This means there is time to push the system into the formation before hydrolysis starts. The hydrophobic cores are then converted into hydrophilic cores due to the formation of hydrophilic groups such as carboxylic acids from anhydrides (hydrophobic) during the hydrolysis. The initial particles are cross-linked with both a stable and an unstable cross-linking agent. The water splits the unstable cross-linking agent (anhydride) into two carboxylic acid groups. The stable cross-linking agent retains the "shape" and integrity of the particle. Low amount of stable cross-linking agent leads to higher swelling capacity. The material described here and many examples of this are well explained in WO 2007/126318 Al.

In an embodiment of the chemical substance according to the invention, the coating layer comprises oligomeric or polymeric materials such as homopolymers or copolymers based on monomers selected from a group comprising: styrene and styrene derivatives, acrylics and acrylic derivatives, methacryls and methacrylic derivatives, amides and imides, carbonates, dienes , esters, ethers, vinyl acetals, vinyl esters, vinyl ethers and ketones, vinyl pyridine and vinyl pyrrolidone, fluorocarbons, hydrophobic siloxanes such as fluorinated siloxanes, or materials such as condensation polymers based on epoxy or urethane resins.

In a preferred embodiment of the chemical substance, the coating layer comprises styrene and styrene derivatives, for example p-tert butylstyrene, 4-(trifluoromethyl)styrene, 2,4-dimethylstyrene, or acrylic and methacrylic derivatives. These materials are readily available and provide a tailored hydrophobic effect.

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

In an embodiment of the chemical substance according to the invention, in a non-swollen state of the core, the core has a size in the range from 0.1 pm to 75 pm, preferably in the range from 0.5 pm to 60 pm, and most preferably in the range from 1 pm to 40 pm. The range of core diameters for this embodiment has proven to be a practical size for introduction into the formation. It must be emphasized that the cores do not have to be spherical in the invention, i.e. that they can have other shapes. What is important is that the (initially hydrophobic) cores have the property of swelling (after being hydrolyzed) in the presence of water at an appropriate well temperature, so that water production is blocked as the cores completely block formation. In the case of non-spherical shapes, the diameter refers to an average diameter. In a swollen state, the nuclei may not be spherical, due to deformation and swelling to completely fill the pore cavities.

In an embodiment of the chemical substance according to the invention, in an unswollen state of the core, the flexible coating layer has a thickness in the range from 10 nm to 600 pm, preferably in the range from 50 nm to 200 pm, and most preferably in the range from 200 nm to 50 pm. The range for the thickness of the coating layer according to this embodiment has proven to be a practical size for introduction into the formation.

In a second aspect, the invention relates more specifically to a method for reducing water production from underground zones surrounding oil and gas wells, where the method includes the step of introducing the chemical substance according to the invention into the formation of the well.

In a third aspect, the invention relates more specifically to a method comprising the step of removing water-swellable cores, or after said water-swellable cores have been formed as a separate finishing step. An oil-in-oil emulsion process is a very practical process for making the chemical substance according to the invention.

In the following, an example of a preferred embodiment illustrated in the accompanying drawings is described, where: Fig. la shows a chemical substance as known from prior art when it is in a

unswollen state;

Fig. 1b shows the chemical substance in Fig. let when it is in a swollen

state;

Fig. 2a shows a chemical substance in accordance with the invention when it is in a

unswollen state, and

Fig. 2b shows the chemical substance in fig. 2a when it is in a swollen

state.

It should be pointed out that the above 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 brackets shall not be interpreted as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those set forth in a claim. The article "a" or "an" before an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several separate elements, and by means of a suitably programmed computer. In the device requirement that lists several means, several of these means can be given concrete form by one and the same item of hardware. The mere fact that certain measures are set forth in mutually different dependent claims does not indicate that a combination of these measures cannot be advantageously used. Throughout the figures, similar or corresponding features are indicated with the same reference numbers or symbols.

Fig. 1a shows a chemical substance 100 as known from the prior art when it is in an unswollen state. This figure shows a previously 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 causing hydrolysis of the hydrophobic core to yield 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 they are in contact with water in the cores of the formation. Fig. 1b shows the chemical substance 100 in Fig. la when in the swollen state, where it is schematically illustrated that the pore cavities (not shown) are hermetically sealed by the swollen particles 110'. The unfortunate consequence of this is that also any oil 10, which may be produced at the same time as the water, cannot pass 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 it is in an unswollen state. The improved chemical substance 200 comprises a plurality of particles 210, each of which comprises 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 fig. 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 conditions in the well. This layer further has the property of forming an interconnected hydrophobic network (not shown) of channels at least in the swollen state of the core 220, which network of channels facilitates the passage of hydrocarbons while blocking water (due to hydrophobic nature of the material). Fig. 2b shows the improved chemical substance 200 of Fig. 2a when in the swollen state. The cores 220' have swelled, while the coating layers 230' are now stretched and in contact with each other to block the formation for water production. At the same time, the hydrophobic network in the coating layer 230' allows hydrocarbons to pass through, promoting hydrocarbon production. For reasons of definition of certain features in the claims, the particle core diameter D and the coating layer thickness T in the unswollen state of the cores 220 have been illustrated in fig. 2a, as well as the particle core diameter D' in a swollen state of the cores 220'.

When you get down to the particles' cores 220, the invention can be applied to different types of water-swellable cores. It must be taken ad notam 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. Nevertheless, patent application publication WO 2007/126318 Al is incorporated as a reference in its entirety in this patent document. The invention is nevertheless particularly suitable for water-swellable cores, which are produced by the applicant himself. The applicant has developed a special chemical agent, which is described in the following points. Nevertheless, it must be emphasized that, in accordance with the present invention, that the previously described flexible hydrophobic coating layer 230 must still be added to the particles in the special chemical agent that is hereinafter described. 1. Chemical agent for reducing water production from underground zones surrounding oil and gas wells, comprising polymeric, cross-linked particles which are stable in anhydrous solvents, where a) the particles are prepared in an oil-in-oil emulsion and are cross-linked with at least one water-stable cross-linking agent and at least one water-unstable cross-linking agent, the water-unstable cross-linking agent being selected from cross-linking agents capable, when opened in a reaction with water, of forming a hydrophilic site which increases the water swelling ability of the particles, and that b) the particles contain a controlled amount of at least one immobilized chemical compound which, by means of said reaction between water and reactive groups in the polymer skeleton, is able to form a new chemical bond to another particle, to another reactive site in the same particle , to other immobilized compounds or to other compounds. 2. Chemical agent as described in point 1, where the polymer-crosslinked particles have a skeleton selected from functionalized homo- and copolymers of methacrylates, acrylates, acrylamides, vinyl alcohols, alginates, chitosan, xanthan, dextrans, gelatin, cellulose, amylose, biopolymers generally, vinyl pyrrolidone, vinyl sulfonates, derivatized polyethylene glycols, siloxanes, isocyanates, lactones and epoxides, which are functionalized with hydroxyl, vinyl, primary, secondary, tertiary or quaternary amines, carboxylic acids, acrylic, methacrylic, sulfonic acids, hydroxy succinimides, anhydrides, esters, lactones , azalactones, epoxides or thiols. 3. Chemical agent as described in point 1 or 2, where 2-aminoethyl methacrylic 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-linking agent methylene bisacrylamide and a water-unstable cross-linking agent dimethacrylic anhydride. 4. Chemical agent as described in point 1, where the water-unstable cross-linking agent is selected from compounds with functional groups comprising anhydrides, diesters, diimido-esters, dihydroxy-succinimides and difunctional disulfides. 5. Chemical agent as described in one of points 1-4, where the least one water-unstable cross-linking agent comprises at least one of dimethacrylic anhydride, diacrylic hydride and methylene dimethacrylate, where the carboxylic acid groups are formed by reaction with water. 6. Chemical agent as described in one of items 1-5, where the water-unstable cross-linking agent is present in an amount in the range between 0.05 and 10% by weight of the total amount of monomer, more preferably in the range between 0.05 and 3%. 7. Chemical agent as described in point 1, where the at least one immobilized compound is selected from water-soluble compounds comprising functional reactive groups selected from epoxy, amine, thiol, OH, ester, lactone, azalactone, maleimide, aldehyde and anhydride. 8. Chemical agent as described in point 1, where the at least one immobilized compound is selected from derivatives of phosphonates, borates, reactive polyvinyl alcohols, reactive polysaccharides based on derivatives of gelatin, chitosan, alginate, cellulose, amylose, dextran and xanthan; functionalized reactive synthetic polymers of polyethylene glycols, polyacrylamides, polyacrylates, polymethacrylates, polyzwitter ions, peroxides, azoinitiators, persulfates, siloxanes or sodium silicates. 9. Chemical agent as described in point 8, where 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 point 8, where the at least one immobilized compound comprises at least one biopolymer with an epoxy group, amino group, thiol group or free radical polymerisable group such as, for example, vinyl, allyl. 11. Chemical agent as described in point 1, where the at least one immobilized compound is chosen so that it contributes to increased swelling, such as, for example, polymers of 2-acrylamido propanesulfonic acid, chitosan, poly n-acrylyltris(hydroxymethyl) aminomethane (NAT) and amino-functionalized gelatin. 12. Chemical agent as described in point 1, where the at least one immobilized compound is chosen so that it contributes to an increased degree of crosslinking, such as, for example, amino-functionalized polyethylene glycols, such as Jeffamine 1000, Jeffamine 2000 and chitosan. 13. Chemical agent as described in point 1, where the at least one immobilized compound comprises alkoxysilanes which, by means of hydrolysis, are converted into silanols which are capable of binding to compounds such as quartz. 14. Chemical agent as described in point 1, where said other compounds are compounds in an underground formation where the polymer system is used. 15. Chemical agent as described in point 1, where the at least one immobilized compound is able to initiate reaction and cause interparticle bonds and/or bonds between particles and the formation, such as sodium silicate, tetraethyl orthosilicate and tetramethyl siloxane. 16. Chemical agent as described in point 1, wherein the polymer particles are mainly hydrophobic until the reaction between water and water-unstable cross-linking agents has taken place. 17. Chemical agent as described in point 1, where it takes the form of a masterbatch or a concentrate of said particles in a suitable organic solvent. 18. Chemical agent as described in point 1, where it takes the form of said particles in dry states.

The material which is preferably used for the core therefore relates to a chemical agent in the form of a polymer particle system which is prepared in an oil-in-oil emulsion. The polymer particles can be produced using a free-radical polymerization process or a step polymerization (often also referred to as condensation polymerization). When using an oil-in-oil emulsion, water-reactive monomers (monofunctional, polyfunctional) can be used, while avoiding contact between particles and water. The desired reactions should only occur after placement in the underground 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 that only provide swelling is that the particles have a high swelling capacity without being water-soluble. The latter property means that the agent maintains its structural integrity and remains in place in the formation after swelling. At the same time, the agent is chemically and thermally stable, and the rate at which it initially swells when it comes into contact with water can be adjusted by means of a selected balance between stable and unstable 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 initial swelling of the agent in water leads to the opening of cross-linking bonds, so that hydrophilic sites in the particles become available, which in turn leads to a gradually increased rate of swelling of the particles, after initiation. With the desired amount and combination of immobilized compounds, miririp|p1-<; pnpnskanpr vli-prlinprp nntimalisprpt;.pk«;pmnplvi<; fnr to riannp interparticle bonds, i.e. bonds between different particles, which also contributes to the stability of the agent in relation to influence imposed in the formation after swelling. Said features, characteristics and effects contribute to increased stability with respect to temperature, pressure, re-production, leaching, as well as chemical (salinity, pH) and mechanical impact. Such immobilized compounds with properties as mentioned can only be contained in an anhydrous environment, which is why the production in an oil-in-oil emulsion is absolutely necessary.

The chemical agent is produced in an oil-in-oil emulsion polymerization, i.e. an emulsion polymerization free of water. This ensures 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 brought into contact with water for the first time. The polymerization can be a free radical polymerization, but also a condensation polymerization.

The agent can thus be produced in a water-free environment in a way that allows various chemical compounds to be immobilized on it, which at a given time, in contact with water, can contribute to the formation of chemical intra-particle bonds, inter-particle bonds and bonds between particles and the reservoir formation. Said bonds will contribute to a more stable swollen particle system that maintains the reduced permeability over a long period of time, while the system, as mentioned, becomes more robust in relation to external influences.

The invention also relates to a method for injecting the chemical substance into an underground zone which reduces water production from a water-permeable zone. This can be done with the help of a suitable carrier fluid, preferably an organic carrier fluid that lacks reactive groups such as aliphatic or aromatic hydrocarbons or combinations of such together with other hydrophobic, organic solvents that do not include thiols, amines, or hydroxyl groups.

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

Alternatively, the coating layer 230 can be added in a separate finishing step, for example in a grafting step or a coating step. Various techniques for applying the coating layer 230 exist, and are considered to be well known to the person skilled in the art.

Claims (8)

1. Chemical substance (200) for reducing water production from underground zones surrounding oil and gas wells after being introduced into the formation, the chemical substance (200) comprising a plurality of particles (210), each particle (210) having a water-swellable core (220), characterized in that each core (220) has been coated with a flexible coating layer (230) which is hydrophobic under all conditions in the well, and which does not swell in the presence of water nor reacts with water, and where the flexible coating layer (230) has the property of forming an interconnected hydrophobic network of channels, at least in a swollen state of the core (220), to allow hydrocarbons to pass through these channels while preventing water from passing through the channels. 2. Chemical substance (200) according to claim 1, characterized in that the water-swellable core (220) comprises a material which is initially hydrophobic and which can be converted to hydrophilic by means of hydrolysis. 3. Chemical substance (200) according to claim 1 or 2, characterized in that the coating layer (230) comprises oligomeric or polymeric materials such as homopolymers or copolymers based on monomers selected from a group comprising: styrene and styrene derivatives, acrylics and acrylic derivatives, methacryls and methacrylic derivatives, amides and imides, carbonates, dienes, esters, ethers, vinyl acetals, vinyl esters, vinyl ethers and ketones, vinyl pyridine and vinyl pyrrolidone, fluorocarbons, hydrophobic siloxanes such as fluorinated siloxanes, or materials such as condensation polymers based on epoxy or urethane resins. 4. Chemical substance (200) according to claim 1, 2 or 3, characterized in that the coating layer comprises styrene and styrene derivatives, for example p-tert-butylstyrene, 4-(trifluoromethyl)styrene, 2,4-dimethylstyrene, or acrylic and methacrylic derivatives. 5. Chemical substance (200) according to any one of the preceding claims, characterized in that, in an unswollen state of the core (220), the core (220) has a size in the range from 0.1 µm to 75 µm, preferably in the range from 0.5 to 60 µm, and most preferably in the range from 1 µm to 40 µm. 6. Chemical substance (200) according to any one of the preceding claims, characterized in that, in an unswollen state of the core (220), the flexible coating layer (230) has a thickness in the range from 10 nm to 600 µm, preferably in the range from 50 nm to 200 µm, and most preferably in the range from 200 nm to 50 µm. 7. Method for reducing water production from underground zones surrounding oil and gas wells, where the method comprises the step of introducing the chemical substance (200) according to any of the preceding requirements in the formation of the well. 8. Method for producing the chemical substance (200) according to any one of claims 1 to 6, characterized in that the method comprises the step of forming the coating layer (230) on the water-swellable cores (220) during the formation of said water-swellable cores (220) or after that said water-swellable cores (220) have been formed as a separate finishing step.