EA043421B1 - FLUID FOR REGULATING THE PERMEABILITY OF UNDERGROUND FORMATIONS AND ITS APPLICATION - Google Patents

Description

The invention relates to a fluid for controlling the permeability of a subterranean formation and to its use.

In particular, the present invention relates to an aqueous fluid containing a temperature-sensitive polymer. Once injected into a subterranean formation, a fluid containing a thermosensitive polymer undergoes an in situ phase transition under the influence of geothermal heat from the subterranean formation, becoming a gel with a viscosity that locally changes the permeability of the formation to fluids. The present invention can be successfully used in the petroleum industry, especially in improved oil recovery (IOR) or enhanced oil recovery (EOR) operations, to change the permeability of a subterranean formation to displacement fluids, such as water or gas, that are used in the production of oil and hydrocarbon gases.

As is known, in methods for improved recovery of hydrocarbon fluids (oil and gas) from subterranean formations, the hydrocarbon fluid initially located in the pores of the formation rock is brought to the surface by displacement by an immiscible fluid (also called a displacement fluid) that takes its place. For this purpose, a well is drilled in an oil field to inject a displacing fluid, usually water, which is placed in such a way as to create in the underlying soil layer the most uniformly advancing front of hydrocarbon fluid replacement in the direction of the production well.

Water flooding technology has long been the simplest and most cost-effective method used to maintain oil field production and increase overall hydrocarbon fluid recovery.

The amount of hydrocarbon fluid that can be displaced toward production wells by water injection depends, among other factors, on the degree of heterogeneity of the formation rock and the properties of the hydrocarbon fluid (primarily viscosity). In particular, faults, channels or levels with high permeability (the latter also called loss zones) form preferential flow paths. The natural tendency of fluids to flow through the most permeable parts of the reservoir rock means that over time, water pumped into the underlying soil continues to flow along these faults, channels or levels of high permeability, reaching production wells directly with no or only minimal penetration into the zones reservoir rocks in which hydrocarbon fluid is still present without producing a displacement effect.

In such situations, water supply may increase until water production outweighs hydrocarbon fluid production, making hydrocarbon fluid production little or no economically viable.

Moreover, associated water extraction implies the introduction of special treatment facilities to be able to safely discharge the extracted water, or systems for its re-injection into the ground layer. These activities result in high energy consumption and an increase in the overall cost of hydrocarbon production.

At the current level of technology, the problem of unwanted influx of water and insufficient efficiency of hydrocarbon extraction when displaced by water is solved by injecting into the underlying soil layer liquid compositions containing chemical compounds that are capable of changing the permeability characteristics of the underground formation. The compounds most commonly used for this purpose are usually in the form of polymers, gels or foams. These compounds are hereinafter referred to as capping agents. Plugging agents block the pores of formation zones with the highest permeability, diverting the flow of displacing fluid towards zones that are enriched in hydrocarbon fluid, thereby increasing well productivity.

Document US 2009/0264321 describes a method for changing the permeability of a subterranean formation based on the injection of a composition containing encapsulated foaming polymer microspheres into the underlying soil layer. Once injected into the soil, due to an activation event (such as a change in temperature or pH), the polymer microspheres are released from the capsules in which they are enclosed and distributed throughout the formation, where they swell to absorb the displacing fluid with which they come into contact. The swelling of the microspheres within the pores of the formation impedes the flow of the displacing fluid, which is consequently diverted towards other zones of the formation. However, the properties of the above encapsulated polymer microcapsules cannot be easily changed and therefore optimized depending on the specific formation conditions. Moreover, the microcapsules discussed above change the permeability of the formation irreversibly, so that any errors when injecting a plugging agent cannot be corrected.

Closure agents consisting of heat-sensitive polymers are also known from the state of the art. Thermosensitive polymers are polymers that exhibit abrupt and discontinuous changes in chemical and physical properties with changes in temperature. The term heat sensitive is usually used in reference to the solubility of a polymer in a given solvent. A polymer having solubility with temperature-sensitive characteristics produces an insolubility region on the temperature-composition diagram characterized by at least one phase

- 1 043421 transition in response to temperature changes. In general, two different types of phase transition are observed in a temperature-sensitive polymer solution, each characterized by a specific critical temperature:

in the first type of transition, polymers that are soluble in a given solvent become insoluble in the same solvent as its temperature increases; the temperature at which this phase transition occurs is called the lower critical solution temperature (LCST);

in the second type of transition, polymers that are insoluble in a given solvent become soluble as the temperature increases; The temperature at which this phase transition occurs is called the upper critical solution temperature (UCST).

Additional information on temperature-sensitive polymers and their solubility is given, for example, in Chem publications. Soc. Rev., 2013, 42, 7214, and Polymers 2011, 3, 1215-1242.

US 2012/0264655 describes the use of plugging agents based on temperature-sensitive polymers and hydrogels in combination with a variety of working fluids for treating near-wellbore zones in oil production. Working fluids for treating the near-wellbore zone (for example, drilling fluids, fracturing fluids, etc.) contain a gelling agent formed by a heat-sensitive graft polymer containing a water-soluble main chain (for example, polyacrylic acid), onto which many side chains having heat-sensitive functional groups are grafted . Polymer side chains give the polymer its characteristic LCST value, determining its solubility parameters as a function of temperature.

WO 1995/026455 describes a method for controlling permeability in a subterranean formation, which includes injecting an aqueous solution of at least one temperature-sensitive polymer into a wellbore up to a zone of the formation having a temperature higher than the temperature of the solution in the wellbore. The temperature-sensitive polymer has an LCST intermediate between the temperature of the solution in the well and the temperature of the zone. Due to the effect of increasing temperature due to the heat transferred from the zone to the solution, the temperature-sensitive polymer becomes insoluble, thereby changing the permeability of the formation. According to WO 1995/026455, the LCST of a temperature-sensitive polymer can be adjusted by preferentially selecting the type of monomers and/or comonomers, their weight ratios, or by adding additional compounds capable of changing the LCST. Thermosensitive polymers are produced by bulk polymerization or suspension polymerization. The effects of changes in permeability can be made reversible, for example, by injecting liquid refrigerants into the formation, which, by lowering the formation temperature to values below the LCST, cause a reverse phase transition of polymer solubilization.

Methods known in the prior art for controlling the permeability of a subterranean formation that rely on temperature-sensitive polymers have the disadvantage that they do not adequately control the permeability and modifying effects caused by phase change at temperatures above the LCST. In particular, the changes are often irreversible or require additional interventions such as injection of liquid refrigerants, as described in WO 1995/026455.

Thus, in view of the above-mentioned prior art, it is an object to provide a fluid for controlling the permeability of a subterranean formation that at least partially overcomes the disadvantages of the prior art.

Within this general objective, one object of the present invention is to provide an aqueous fluid for controlling the permeability of a subterranean formation containing a temperature-sensitive polymer, wherein the LCST temperature at which the phase transition from a soluble polymer to a viscous gel occurs is easily controlled, and therefore can be adapted to the specific conditions of a subterranean formation whose permeability is subject to change.

A second object of the present invention is to provide a fluid for controlling the permeability of a subterranean formation, which can be easily injected into the formation and whose pore-clogging effect due to the formation of a gel phase is at least partially reversible, preferably without inducing this reversibility by external influences, such as, for example , injection of liquid refrigerants.

A third object of the present invention is to provide a fluid for controlling the permeability of a subterranean formation that can be produced in a simple manner at a reasonable cost.

It has now been found that these and other objects, which will be better illustrated in the following description, can be achieved using an aqueous fluid containing a thermosensitive polymer having a block structure A-B-A', where the outer blocks A and A' are the same or different from each other are oligomeric groups with thermosensitive characteristics and therefore having a characteristic and defined LCST, while the central block B is a hydrophilic oligomeric group of relatively long length.

In fact, it has been established that by synthesizing a polymer with the above-mentioned block structure A-BA', in which the thermosensitive blocks A and A' are separated from each other by a hydrophilic oligomer

- 2 043421 segment of sufficient length, it is possible to obtain aqueous solutions of these polymers, which change state, becoming viscous gels, at a well-defined temperature (LCST). The LCST value of the polymer A-B-A' and, therefore, the temperature at which the phase transition occurs, can be easily adjusted by modifying the composition of the thermosensitive blocks A and/or A', for example, by introducing hydrophilic and hydrophobic monomers and/or by changing lengths of oligomers A and/or A'.

The above-mentioned thermosensitive A-B-A' polymers are soluble at low temperatures, ie below the LCST, while they form a gel phase with a relatively high viscosity at high temperatures, ie above the LCST. As a result, they can be easily injected into the subterranean formation in the form of low-viscosity aqueous solutions at temperatures below the LCST, while they act as plugging agents after phase transition to a viscous gel in zones where the formation temperature exceeds the LCST.

Without reference to any particular theory, it is believed that at temperatures above the LCST, the A blocks of the polymer A-B-A' disintegrate to form micellar structures which, when linked to each other through a bridge consisting of the oligomeric block B, form a network structure with relatively high viscosity (physical gel).

Preferably, the blocks A and A' of the A-B-A' polymers may also include portions that can degrade over time under the temperature conditions of the subterranean formation, for example, by a hydrolysis reaction. The decomposition of the above degradable parts leads to a structural modification of the parent blocks A and A' with a consequent change in the thermosensitive behavior of the polymer. In particular, after decomposition, the LCST of the polymer increases; When the LCST of the decomposed polymer exceeds the temperature of the underground formation, a phase transition of the viscous gel into a flowable polymer solution occurs.

Due to the A-B-A' block structure, the polymers of the present invention can be formulated to have an LCST more suitable for the specific geothermal profile of the subterranean formation, thereby increasing the efficiency of the displacement fluids (low water production) and hence hydrocarbon production. fluids (oil or gas).

In accordance with a first aspect, the present invention provides a fluid for controlling the permeability of a subterranean formation, comprising an aqueous solution of at least one thermosensitive polymer with an A-B-A' block structure, wherein:

A and A', whether the same or different from each other, each represent a thermosensitive oligomeric group;

B represents an oligomeric group containing n repeating units, identical or different from each other, where:

(i) at least one of the repeating units has a solubility of the corresponding monomer in water at 20°C equal to or greater than 120 g/l, (ii) n is an integer within the range of 30-1000.

In accordance with a second aspect, the present invention relates to a method for controlling the permeability of a subterranean formation, comprising:

A. preparing a fluid containing an aqueous solution of at least one thermosensitive polymer with a block structure A-B-A', where:

A and A', whether the same or different from each other, each represent a thermosensitive oligomeric group;

B represents an oligomeric group containing n repeating units, identical or different from each other, where:

(i) at least one of the repeating units has a solubility of the corresponding monomer in water at 20° C. equal to or greater than 120 g/l, (ii) n is an integer within the range of 30-1000;

b. placing said fluid to treat a near-wellbore zone in a subterranean formation.

In accordance with a third aspect, the present invention relates to the use of the above fluid to change the permeability of a subterranean formation.

In accordance with a further aspect, the present invention relates to the use of the above fluid to limit water infiltration into a well for extracting oil or hydrocarbon gas from a subterranean formation.

For purposes of the present description and the appended claims, the lower critical solubility temperature (LCST) of a polymer should be understood as the temperature determined by the turbidimetric method described in the examples.

As used herein and the appended claims, the degree of polymerization of a polymer, also referred to as DP, means the number of repeating units present in a polymer, as determined by 1H NMR spectroscopy methods known to one skilled in the art, in a solution of the polymer in CDCl ₃ as solvent

- 3 043421 (99.8% D-atoms) at a polymer concentration of 15 mg/ml.

As used in the present specification and the appended claims, the terms oligomer and polymer include the terms homopolymer and copolymer unless expressly stated otherwise or in any case unless otherwise indicated by the text. Oligomers and polymers can therefore be formed from the same repeating unit or from two or more repeating units that are different from each other. As used herein, the term polymer includes oligomers and polymers.

In connection with the present invention, the average molecular weight as the M _n number of the oligomer or polymer is intended to be determined by gel permeation chromatography (GPC) using polystyrene as a standard.

For purposes of the present invention, a repeating unit is meant to mean a constituent unit of a polymer or oligomer, said constituent unit may correspond to one monomer or two or more monomers from which it is formed after polymerization; said monomer or monomers from which the repeating unit is formed are also referred to herein as the corresponding repeating unit monomer.

Additional information on the structure of oligomers and polymers can be found, for example, in the publication Alfred Rudin, Phillip Choi, The Elements of Polymer Science and Engineering, 3rd edition, Elsevier, 2013.

For purposes of this specification and the appended claims, the verb contains and terms derived therefrom also include the verbs consists of and consists essentially of and terms derived from them.

The boundary values and numerical intervals expressed in the present description and in the accompanying claims also include the mentioned numerical value or numerical values. Moreover, all values and partial intervals of a boundary value or numerical interval are to be considered specifically included as if they were explicitly mentioned.

In order to better understand the essential features of the present invention, reference is made in the description to the following drawings.

Fig. 1 (degradation test): graph of percentage weight loss of polymer 1 (relative to original sample weight) as a function of observation time.

Fig. 2 (decomposition test): graph of the pH value of the liquid phase as a function of observation time.

Block A or A' of an A-B-A' polymer may be formed by an oligomer containing a single type of repeating units or two or more repeating units that are different from each other. Blocks A and A' may be the same or may be different from each other.

The oligomeric group A and/or A' preferably has a lower critical solubility temperature (LCST) within the range of 30-100°C, more preferably within the range of 40-80°C.

In order to have the thermal sensitivity characteristic, the oligomer that forms the block A and/or A' preferably contains at least one hydrophilic part and at least one hydrophobic part.

The hydrophilic part is preferably formed from polyoxyethylene chains, polylactide chains, C ₂ -C ₁₀ alkyl chains, linear or branched, containing amide groups.

The hydrophobic moiety is preferably derived from monomers containing at least one ester or amide group, such as, for example, (meth)acrylate and (meth)acrylamide based compounds.

In a preferred embodiment, the temperature-sensitive block A and/or A' is a graft polymer containing a poly(meth)acrylic or poly(meth)acrylamide linear main chain and a plurality of side chains attached to said main chain selected from: poly(ethylene glycol) -[-CH ₂ -CH ₂ -O] _t -H, poly(ethylene glycol) methyl ether -[-CH ₂ -CH ₂ -O] _t -CH ₃ , HEMA(HEMA)-polylactide -CO-O-CH2 -CH2-O-[-CO-(CH3)CH-O]t-H, alkylamide, linear or branched, and combinations thereof, where m is an integer within the range of 2-10.

In an embodiment, block A and/or block A' is an oligomer containing multiple polymer side chains.

In an embodiment, block B is a linear oligomer that does not contain polymer side chains.

In a preferred embodiment, the temperature sensitive blocks A and/or A' are preferably oligomeric groups formed from one or more repeating units derived from monomers selected from: (meth)acrylic acid, oligo(ethylene glycol) methyl ester methacrylate (OEGMA), [2-(methacryloyloxy)ethyl]-trimethylammonium chloride (MADQUAT), HEMA-polylactide, potassium 3-sulfo-propyl methacrylate, 2-acrylamido-2-methylpropane sulfonic acid, polymerizable esters containing polyoxyethylene chains -[-CH ₂ - CH ₂ -O] _X -H, where x is an integer within the range of 10-1000, N-isopropylacrylamide, N,N-dimethylaminoethyl methacrylate, dimethylacrylamide.

Preferably, the OEGMA compound used to form block A and/or A' has

- 4 043421 average molecular weight M _n within the range of 100-10000 Dalton, more preferably within the range of 100-5000 Dalton.

In a preferred embodiment, the thermosensitive block A and/or A' is preferably a copolymer of two or more OEGMA compounds having different molecular weights, for example, OEGMAs that differ from each other in side chain lengths.

In a preferred embodiment, the thermosensitive block A and/or A' is preferably a copolymer containing OEGMA as a first comonomer and at least one second hydrophobic comonomer selected from: C1- _C4 -alkyl(meth)acrylate (e.g. MMA or BA), (dimethylamino)ethyl methacrylate (DMAEMA) and combinations thereof.

In one embodiment of the present invention, the selected hydrophobic comonomer is a vinyl oligoester that is biodegradable by hydrolysis, such as, for example, HEMA-polylactide.

By properly selecting the type of hydrophilic and hydrophobic units, their molecular weight and relative mass ratio, it is possible to control the LCST value of unit A or A' and, consequently, the behavior of the final polymer A-B-A' in response to changes in temperature.

Preferably, the degree of DP polymerization of the polymer that forms the A and A' blocks, that is, the total number of repeat units present in the A and A' blocks, is within the range of 2-1000, preferably 3-500, more preferably 5-450 , even more preferably 6400.

The A-B-A' polymer block B is an oligomer formed by a single type of repeating units (homopolymer) or two or more different types of repeating units (copolymer).

Preferably, the oligomeric group which forms block B contains one or more repeating units corresponding to monomers selected from: (meth)acrylic acid, polymerizable esters containing polyoxyethylene chains -[-CH ₂ -CH ₂ -O] _g - H, where r is an integer within the range of 10-500, more preferably 20-300, oligo(ethylene glycol) methyl ether methacrylate (OEGMA), [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MADQUAT), potassium 3sulfopropyl methacrylate, 2-acrylamido -2-methylpropane-sulfonic acid, (meth)acrylamide, 2-hydroxypropyl methacrylamide, glycerol mono-methacrylate, 2-hydroxyethyl methacrylate, methacryloyloxyethylphosphorylcholine, carboxybetaine, sulfobetaine and combinations thereof.

In an alternative embodiment, the oligomeric group that forms block B is a poly(ethylene glycol) chain (PEG, HO-[CH ₂ CH ₂ -O] _p -H), where n is an integer between 30 and 300, preferably functionalized at both ends with ACA (chain transfer agent (CTA)) by reversible chain transfer polymerization (RAFT polymerization) to obtain a macromolecule of the type APC-PEG-CTA (CTA-PEG-CTA), which makes it possible to obtain a symmetrical A-B-A structure at one stage of polymerization.

Preferably, the total degree of polymerization of block B SP, that is, the total number of repeating units, identical or different from each other, present in block B, is within the range of 10-1000, preferably 20-500, more preferably 50-500.

In one embodiment, block B does not actually have a thermal response characteristic.

In another embodiment, block B does have a thermal response characteristic, but this does not significantly change the thermal response properties of the polymer A-B-A'. For example, block B may be a temperature-sensitive polymer having an LCST higher than the LCST of blocks A and A'. Therefore, this polymer does not affect the behavior of the polymer A-B-A' under the influence of heat, since the phase transition to a viscous gel occurs at an LCST of blocks A and A', which is lower than the LCST of block B.

In an embodiment in a block copolymer A-B-A':

The block contains poly(ethylene glycol) (PEG) in the main chain, having an average molecular weight M _n within the range of 500-10000 Daltons, more preferably within the range of 1000-5000 Daltons;

The A block and/or the A' block are oligomeric groups containing as the corresponding monomer oligo(ethylene glycol) methyl ether methacrylate (OEGMA), preferably having an average molecular weight M _n in the range of 100-1000.

In another embodiment, in the block copolymer A-B-A':

The B block is an oligomeric group containing [2-(methacryloyloxy)ethyl]-trimethylammonium chloride (MADQUAT) as the corresponding monomer,

The A block and/or the A' block are oligomeric groups containing as the corresponding monomer an OEGMA compound having an average molecular weight M _n within the range of 100-1000.

In another preferred embodiment, the oligomer that forms the A and/or A' block is

- 5 043421 contains one or more degradable repeat units, that is, containing weak bonds that are capable of progressively breaking down over time in the conditions of an area of underground formation in which the polymer A-B-A' is present in the form of a gel. Examples of degradable units are monomers used in the art for the synthesis of biodegradable polymers.

For example, in a preferred embodiment, one biodegradable unit suitable for use for the purposes of the present invention is a HEMA-poly(lactide) macromonomer prepared, for example, by ring opening polymerization (ROP) of a cyclic lactide ester with hydroxyethyl methacrylate (HEMA). NEMA)). This polymerization reaction is schematically represented below in the diagram:

where n is preferably between 1 and 10, and x is preferably between 10 and 500.

Polymerization of the HEMA-lactide macromonomer into block A and/or A' leads to the introduction into these blocks of side chains consisting of oligoesters, which are capable of decomposition upon hydrolysis of the ester bonds. The time required for degradation is determined primarily by the chain length of the oligoesters and can be adjusted by appropriate selection of the HEMA/lactide ratio during macromonomer synthesis.

Preferably, the HEMA/lactide molar ratio is within the range of 1-10.

When the polymer A-B-A' is in the underground formation in the form of a gel, after the decomposition of the oligoester chains, there is a progressive increase in the hydrophilic properties of the polymer A-B-A' and, consequently, its LCST. When the LCST of the polymer exceeds the temperature of the formation in which the gel is found, the latter undergoes a change of state, changing from a gel to a soluble polymer and forming a fluid aqueous phase, which can then move again through the formation.

Moreover, it should be noted that in the case of HEMA-lactide, the decomposition product of the oligoester chains consists predominantly of lactic acid molecules, which is a biodegradable substance.

In a preferred embodiment, block B contains at least one cationic functional group. This cationic functionality can be introduced by preparing polymer B from monomers containing cationic functionality. The following compounds are examples of such monomers: [2-(methacryloyloxy)ethyl]trimethyl ammonium chloride (MADQUAT), 2-(dimethylamino)ethyl methacrylate (DMAEMA), 2-aminoethyl methacrylate hydrochloride.

When block B contains cationic functional groups, the gel that is formed from the soluble polymer A-B-A' is able to interact more effectively with the pore surface of the subterranean formation, establishing electrostatic interactions with the carbonate rocks of the formation that make the plugging action of the gel more effective.

The A-B-A' triblock polymers of the present invention can be prepared by polymerization methods known in the art, such as free radical polymerization (FRP) and controlled radical polymerization (CRP). DRP methods include, for example, atom transfer radical polymerization (ATRP), nitroxide-mediated polymerization (NMP) and reversible addition-fragmentation chain transfer polymerization (RAFT).

In particular, for the production of thermally sensitive block polymers in accordance with the present invention, controlled polymerization PSA processes that produce polymer chains of uniform length and therefore having similar thermal response behavior are preferred. The uniformity of the length of the polymer chains guarantees the production of A-B-A' polymers having a clearly defined and predetermined LCST. Among the SRP methods, RAFT polymerization is a particularly preferred method. As is known, RAFT polymerization is a degenerative transfer polymerization process in which a chain transfer agent is attached to the parent monomers to extremely rapidly transfer radical function from one polymer chain to another during polymerization, thereby ensuring uniform growth of the polymer chains. The transfer of radical function occurs due to the interaction between the terminal part of the chain transfer agent

- 6 043421 (APC (STA)) and an active radical chain, which transfers the latter to a passive state, while making the chain previously associated with the APC active again.

Due to the mechanism discussed above, RAFT polymerization is particularly suitable for the production of block copolymers and is an ideal solution for the production of temperature-sensitive materials having well-defined and predetermined LCST.

Using RAFT polymerization, it is possible to obtain polymers that have polymer chains of clearly defined length and are also mutually homogeneous. The length of the polymer chain in this case is expressed by the SP marker defined earlier.

In RAFT polymerization, the DP of the polymer can be easily adjusted by choosing the ratio of the concentration of the initial monomer to the concentration of APC in the polymerization mixture.

In general, the polymers A-B-A' in accordance with the present invention can be prepared by RAFT polymerization in successive steps, as described by way of example below.

At the first stage, block A is synthesized by polymerization, for example, OEGMA300 (that is, oligo(ethylene glycol) methyl ether methacrylate, M _n =300), to obtain poly[OEGMA300 ₅₀₀ ], where the marker DP = 500 indicates the degree of polymerization, that is average number of OEGMA300 repeat units present in the A block of the oligomer. To do this, the OEGMA300 monomer is reacted with an APC (CTA), for example, 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid, and with a polymerization initiator, for example, with 4,4'-azobis(cyanovaleric) acid (ACVA )), in a polar solvent (for example, ethanol) at a temperature of approximately 50-80°C at atmospheric pressure for 24-48 hours. The relative amounts of reactants can be calculated to obtain the desired DP.

After evaporation of the solvent and isolation of the unreacted monomer (eg, extraction with diethyl ether), the oligomer that forms the A block can be isolated in the form of a viscous liquid.

The oligomer that forms block A has polymer chains terminated on either side by specific functional groups of the APC used in the first step. Thus, it is a macro-APC that can be reacted with the B segment monomer in the second step of RAFT polymerization. In this second step, the block B monomer acts essentially as a chain extender for block A. The B block, for example, can be formed by using OEGMA2000 (ie, oligo(ethylene glycol) methyl ether methacrylate, M _n =2000) as the monomer to produce poly[ OEGMA2000 ₅₀₀ ] with SP 500. RAFT polymerization in the second stage can be carried out under conditions similar to the conditions of the first stage. At the end of the second stage, therefore, a diblock polymer A-B of the poly[OEGMA300 ₅₀₀ -OEGMA2000 ₅₀₀ ] type is obtained. This polymer, since it contains chains terminated on both sides with APC functional groups, can also be used as a macro-APC in the third stage of RAFT polymerization to attach the A' block to the A-B diblock polymer and obtain an A-B- triblock polymer. A'. To this end, the poly[OEGMA300 ₅₀₀ OEGMA2000 ₅₀₀ ] polymer is reacted, for example, with the same monomer used to form segment A in the first step, in the presence of an ACVA initiator under conditions similar to those in the first step, resulting in A-B-A symmetrical polymer poly[OEGMA300 ₅₀₀ OEGMA2000 ₅₀₀ -OEGMA300 ₅₀₀ ]. However, if a monomer A' other than A is used, an unsymmetrical polymer A-B-A' is obtained.

Symmetric polymers A-B-A, that is, where A is equivalent to A', can be obtained by RAFT polymerization also through a two-step process. In the first step, the repeating unit (or repeating units) that form oligomer B (which was synthesized using the RAFT method or other polymerization method) is reacted with an APC compound, the latter being present in the reaction mixture in such an amount as to obtain oligomer B, terminated on both sides by specific functional groups of the APC used (macro-APC), for example, in a molar ratio to oligomer B greater than or equal to 2, preferably between 2.1 and 4. In the second stage, the repeat unit (or repeat units ), which should form oligomer A, is polymerized in the presence of macro-APC obtained in the first stage, obtaining the final symmetric polymer A-B-A.

According to an alternative synthesis method, symmetrical polymers A-B-A can be obtained by first-stage synthesis of oligomeric blocks A by RAFT polymerization in the presence of bifunctional APC (to produce A-APC-A). In the second stage, the A-APC-A oligomer is reacted with the repeating unit, which forms the B block, so as to extend the chain of the A-APC-A oligomer, starting from the central unit, to obtain the A-B-A polymer.

In connection with the present invention, thermosensitive A-B-A polymers are injected into a subterranean formation whose permeability to fluids in the form of an aqueous fluid must be altered. Preferably, the wellbore treatment fluid contains A-B-A' polymers in the form of an aqueous solution. The concentration of the polymer in the aqueous solution is preferably in the range of 0.5 to 40% by weight, more preferably in the range of 1 to 20% by weight, based on the weight of the aqueous solution.

- 7 043421

An aqueous fluid containing an A-B-A' polymer can be used to control the permeability of a subterranean formation during oil production.

As used herein, subterranean formation means the area below the surface of the earth, including the surface of the seabed. For example, the subterranean formation may be a formation storing a hydrocarbon fluid (oil or gas) and any well zone in fluid communication with the formation.

The method for controlling permeability to subterranean formation fluids in accordance with the present invention can be applied both before production of hydrocarbon fluid from the subterranean formation begins and while the production well is already in production.

The method in accordance with the present invention can be advantageously applied to production wells called mature wells, that is, wells that have now reached the limit of their production capacity and are characterized by the recovery of significant quantities of water in combination with oil or hydrocarbon gas.

The placement of fluid to control permeability in a subterranean formation can be accomplished using equipment and in accordance with methods that are known in the petroleum industry.

Placement, for example, can be accomplished by injecting a control fluid through both a well to extract the hydrocarbon fluid and through other wells commonly found in an oil field, such as wells for injecting steam, water, or other displacement fluids into the underlying soil layer (known as injection wells).

Based on the criteria and using the production methods discussed above, one skilled in the art can select a control fluid containing polymer A-B-A' such that said polymer has an LCST higher than the fluid injection temperature and lower than the deposit temperature at area in which permeability control is to be performed.

The method in accordance with the present invention can be used in secondary and tertiary hydrocarbon oil recovery activities, both in water plugging interventions (water sealing treatment) and in subterranean formation matching treatments (optimizing treatment).

The amounts of aqueous fluid containing temperature-sensitive polymers that must be injected into a subterranean formation may vary widely depending on the specific geological configuration. These amounts can be readily determined by one skilled in the art based on the geological characteristics of the formation and simple routine experiments.

The following exemplary embodiments are presented for illustration of the present invention only and should not be construed as limiting the scope of protection as defined by the appended claims.

Examples

1. Definition of NCTR

For the purposes of the present invention, the lower critical solubility temperature (LCST) of a polymer is determined by the turbidimetric method by measuring the optical transmittance of a 0.2 wt.% aqueous solution of the polymer under study at ambient pressure. Transmittance measurements are carried out on a sample maintained at different temperatures. By plotting the percent transmittance versus the corresponding sample temperature, a sigmoidal curve is obtained. The temperature at the inflection point of the curve is taken as the LCST of the polymer.

2. Preparation of thermosensitive polymers

2.1 Polymer 1

A temperature-sensitive polymer having the following A-B-A block structure is prepared as follows.

Hydrophilic PEG inner block

Heat Sensitive Biodegradable Block

Heat Sensitive Biodegradable Block

Stage 1 - Obtaining macro-APC APC-V-APC

Hydrophilic and bifunctional macro-APC is synthesized by Steglich esterification between an equivalent of poly(ethylene glycol) (PEG) HO-[CH ₂ -CH ₂ ] _p -OH (Mn=4000 g/mol) and two equivalents

- 8 043421

4-phenyl(carbonothioylthio)pentanoic acid (CPA (CPA)). For this purpose, 50 g of PEG and 7.7 g of CPA are solubilized in 300 ml of dichloromethane (DCM). The resulting solution is loaded into a round-bottomed flask immersed in an ice water bath. Separately, 5.7 g of dicyclohexylcarbodiimide (DCC) and 0.27 g of 4-dimethylaminopyridine (DMAP) are solubilized in 25 ml of DCM. The resulting solution is added dropwise to a solution containing PEG and CPA, while stirring with a magnetic stirrer for an hour. The resulting mixture is allowed to equilibrate at ambient temperature and then reacted for 18 hours. After concentration in vacuo to reduce the volume by half, the product is isolated by precipitation of the reaction mixture in diethyl ether and dried in vacuo at 35 °C.

The resulting macro-APC is a hydrophilic polymer that does not exhibit thermosensitive behavior.

Stage 2 - Preparation of A-B-A polymer

Macro-APC obtained in step 1 is used in the RAFT polymerization of OEGMA300 and HEMAlactide to obtain the A-B-A block structure (polymer 1). For this purpose, 2.59 g of OEGMA300, 2.41 g of HEMA-lactide, 0.065 g of macro-APC from step 1 and 1 mg of azobiscyanovaleric acid (ACVA) are dissolved in 25 ml of ethanol. Nitrogen is bubbled through the solution for 30 minutes in an ice water bath. Then RAFT polymerization is carried out at 65°C for 24 hours. After this, the polymer is isolated by precipitation from the reaction mixture in diethyl ether, then dried in an oven in a vacuum at 35°C.

The SP of external A blocks is set to 500.

The LCST temperature of the polymer was found to be 45 °C.

2.2 Polymer 2

A temperature-sensitive polymer having the following A-B-A block structure is prepared as follows.

Heat Sensitive Heat Sensitive Biodegradable Block Biodegradable Block

Stage 1 - Obtaining APC-A macro-APC

The first thermosensitive A block is synthesized by dissolving 13 g OEGMA300, 12 g HEMA-lactide, 40 mg CPA and 10 mg ACVA in 130 ml ethanol. Nitrogen is bubbled through the solution for 30 minutes and then reacted at 65°C for 24 hours. The polymer is precipitated in diethyl ether and its DP is estimated to be 500. GPC analysis shows the average molecular weight of the product to be approximately 188,000 g/mol with a polydispersity of 1.19.

Stage 2 - Obtaining APC-V-A macro-APC

Macro-APC 1 obtained in step 1 is used in the second RAFT polymerization step to bind the hydrophilic B block. That is, 10 g of macro-APC, 2.4 g of MADQUAT and 5 mg of ACQA are solubilized in 50 ml of a mixture of water and ethanol (50:50 v/v). Nitrogen is bubbled through the solution for 30 minutes, keeping the solution in an ice water bath, and then allowed to react at 65°C for 24 hours. The block polymer A-B (macro-APC 2) is then precipitated in diethyl ether and isolated by vacuum filtration. The estimated SP for block B is found to be 200.

Stage 3 - Preparation of A-B-A polymer

Finally, in the third RAFT polymerization step, the A-B-A triblock polymer is obtained by extending the macro-APC 2 obtained in the previous step 2 with OEGMA300 and HEMA-lactide. For this purpose, 10 g of macro-APC 2, 4.2 g of OEGMA300, 3.88 g of HEMA-lactide and 3 mg of ACVA are dissolved in 80 ml of a mixture of water and ethanol (50:50 v/v). Nitrogen was bubbled through the reaction mixture for 30 minutes while keeping the mixture in an ice water bath, and then reacted at 65°C for 24 hours. The A-B-A block polymer is then precipitated in diethyl ether and isolated by vacuum filtration. The SP of the third block (A) is established to be 500.

The LCST temperature of polymer 2 is 45°C.

- 9 043421

3. Thermosensitive solubility

The phase transition of thermosensitive polymers 1 and 2 from an aqueous polymer solution to a viscous gel is tested experimentally by keeping a test tube containing a sample of an aqueous polymer solution (30 wt.%, polymer relative to the weight of the solution) at a temperature of 55 ° C for 30 minutes.

In both cases, at the end of incubation, the formation of a viscous gel is observed at the bottom of the test tube. When the tube containing the gel is kept again at ambient temperature (25°C), a reverse transition from the gel to an aqueous polymer solution is observed.

4.1 Permeability tests on compacted sand

The effectiveness of the aqueous fluid containing polymer 1 according to the present invention in controlling the permeability of a compacted sand sample is experimentally tested as follows.

A solution of polymer 1 (1 wt.%) in mineralized water is prepared using artificial mineralized water having the composition indicated in the table below (density at 70°C 0.9465 kg/l; viscosity at 70°C 0.4058 cP).

Table 1

Composition of mineralized water

Compound Concentration (g/l) NaCl 22 MgCl ₂ (6H ₂ O) 9.7 Na ₂ SO ₄ (anhydrous) 37 CaCl ₂ (anhydrous) 1 KS1 0.65 _NaHCO3 0.2 H3BO4 0.023 Total 37,273

A sample (cylindrical rod) of compacted sand (passed through 20 mesh and 40 mesh screens) 14.50 cm long and 5.10 cm in diameter is measured using a solution of polymer 1 and mineralized water from table 1 (without temperature sensitive polymer) as control fluid.

A cylindrical rod housed in a sample holder is placed inside a temperature controlled oven. In addition to the mentioned furnace, the measurement system includes a pump system for pumping fluids into the rod at preset flow rates and devices for measuring fluid flow rates. A holding superpressure (nitrogen) of 50 bar is applied to the rod (over internal pressure) to eliminate effects due to sand compressibility during fluid injection. As fluids flow through the sample, electronic sensors measure the load loss between the fluid entering and leaving the sample.

Initially, the permeability of the sample is measured by flushing with a control fluid consisting of saline water. Then the treatment fluid, consisting of a solution containing polymer 1, is pumped until the rod is saturated. The rod is then held at a temperature of 70°C for the time necessary for a phase transition to occur from a polymer solution to a gel capable of resisting the flow resistance of the injected fluid. At the final stage, mineralized water is again pumped into the rod to determine the change in its permeability after treatment with polymer solution 1.

The permeability of a sample is determined by measuring the load loss in response to changes in the flow rate of the injected fluid.

The resistance factor F _r gives an idea of the relative mobility of the aqueous solution of polymer 1 inside the sample relative to the mobility of mineralized water in the same sample.

The residual resistance factor F _rr , in contrast, provides an indicator of the decrease in the relative mobility of the control fluid in the sample after treatment of the sample with the treatment fluid, relative to the mobility of the same control fluid before treatment of the sample with the treatment fluid. Higher values of the parameters F _r and F _rr indicate higher resistance of the sample to the passage of fluid.

As the polymer solution 1 flows through the rod at room temperature, the F _r value is found to be approximately 1.15. This value, being low, indicates that polymer 1 solution can be easily pumped into the sample.

The saturated sample with polymer solution 1 is kept at a temperature of 70°C for 48 hours and then washed with mineralized water to assess the degree of change in permeability. The F _rr value determined in the case of washing with mineralized water is 22.7. Treatment with polymer 1 thus provides a reduction in permeability to saline water by a factor of approximately 23.

4.2 Permeability test on Chashach sandstone

The test described in the previous section 4.1 is repeated on a cylindrical sand rod

-

Claims (6)

a Clashach-type nick, 14.50 cm long and 5.10 cm in diameter, using a solution of polymer 1 and the control mineralized water described above. During the flow of polymer solution 1 in the rod at ambient temperature, the value of F _r was found to be approximately 10.6. This value indicates an acceptable injection resistance. The F„ value determined in the case of mineralized water flow is 142.8. This Frr value confirms the effectiveness of the treatment to control the permeability of the sample using polymers in accordance with the present invention. 5. Decomposition test To test the reversibility of the sol-gel transition after hydrolysis of the ester bonds, polymer 2 inside the container is kept at 55°C for 30 min to allow the formation of a gel. Then a column of mineralized water is added to the top of the gel layer. The change in the two-phase system maintained at 55°C is observed by measuring the mass loss by thermogravimetry and the pH of the water column. Fig. 1 and 2 show experimental values for mass loss and pH of a two-phase system observed as a function of time. Fig. 1 shows that after an induction period of 7 days, during which only a slight decrease (less than 10%) in gel mass was recorded, the gel mass begins to fall linearly with time until after 20 days this results in solubilization of the gel in the water column and, consequently, to its complete disappearance. In parallel, as shown in FIG. 2, the pH value of the liquid phase decreases over time. The decrease in pH can be attributed to the release of acidic moieties, particularly lactic acid, which accompanies polymer degradation. CLAIM 1. A fluid for controlling the permeability of a subterranean formation, containing an aqueous solution of at least one thermosensitive triblock polymer of a block structure A-B-A', where A and A', the same or different from each other, represent outer blocks and each represent a thermosensitive oligomeric group, said A, A' having a certain lower critical solubility temperature (LCST); B represents an oligomeric group containing n repeating units, identical or different from each other, where (i) at least one of the repeating units has a solubility of the corresponding monomer in water at 20°C equal to or greater than 120 g/l, ( ii) n means an integer within the range of 30-1000, where block B is a non-thermosensitive or temperature-sensitive block, provided that if B is temperature sensitive, then block B has an LCST higher than the LCST of blocks A and A', and therefore it does not affect the thermal response behavior of the polymer A-B-A', since the phase transition to a viscous polymer gel occurs at the LCST of blocks A and A', where the specified block B contains poly(ethylene glycol) (PEG) in the main chain , having an average molecular weight M _n within the range of 500-10000 Daltons, or the specified block B is an oligomeric group containing as the corresponding monomer [2-(methacryloyloxy)ethyl]-trimethylammonium chloride (MADQUAT), where the specified block A and/or said block A' is selected from oligomeric groups formed by one or more corresponding monomer units selected from: oligo(ethylene glycol) methyl ether methacrylate (OEGMA) and HEMA-polylactide. 2. The fluid according to claim 1, in which the specified block A and/or the specified block A' have a lower critical solubility temperature (LCST) within the range of 30-100°C. 3. The fluid according to claim 1, wherein said A and/or said block A' are oligomeric groups formed by two or more OEGMA compounds having a molecular weight different from each other. 4. The fluid according to claim 1, wherein said block A and/or said block A' have a degree of polymerization DP (DP) within the range of 2-1000, preferably 3-500, more preferably 5-450, even more preferably 6- 400. 5. The fluid according to claim 1, wherein said OEGMA compound has an average molecular weight M _n within the range of 100-10,000 Dalton, more preferably within the range of 100-5,000 Dalton. 6. The fluid according to claim 1, wherein said B block contains poly(ethylene glycol) (PEG) in the main chain, having an average molecular weight Mn within the range of 500-10000 Dalton, more preferably within the range of 1000-5000 Dalton ; said block A and/or block A' are oligomeric groups containing as the corresponding monomer oligo(ethylene glycol) methyl ether methacrylate (OEGMA), having -