OA20732A - Fluid for Controlling the Permeability of a Subterranean Formation, and Use Thereof. - Google Patents

Abstract

The present invention relates to a fluid for controlling the permeability of a subterranean formation comprising an aqueous solution of at least one thermo-responsive polymer with block structure A-B-A', wherein: - A and A' , the same as or different from one another, each represent a thermo-responsive oligomer group; - B is an oligomer group comprising n repeat units, the same as or different from one another, wherein: (i) at least one of the repeat units has a solubility of the corresponding monomer, in water at 20°C, equal to or greater than 120 g/1, (ii) n is a whole number within the range 30 1000. The present invention also relates to a method for controlling the permeability of a subterranean formation in which the aforesaid fluid is used.

Description

FLUID FOR CONTROLLING THE PERMEABILITY OF A SUBTERRANEAN FORMATION, AND USE THEREOF

The présent invention relates to a fluid for 5 controlling the permeability of a subterranean formation, and use thereof.

In particular, the présent invention relates to an aqueous fluid comprising a thermo-responsive polymer. Once injected into a subterranean formation, through an 10 effect of the geothermal heat of the subterranean formation the fluid comprising the thermo-responsive polymer undergoes in situ a phase transition, becoming a gel of viscosity such as to locally modify the permeability of the formation to fluids. The présent 15 invention may be used advantageously in the oil industry, particularly in improved oil recovery operations (IOR) or enhanced oil recovery (EOR) , to modify the permeability of a subterranean formation to displacement fluids, for example water or gas, that are used in the 20 production of oils and hydrocarbon gases.

As is known, in processes of improved recovery of hydrocarbon fluids (oil and gas) from a subterranean formation, the hydrocarbon fluid initially présent in the pores of the réservoir rock is brought to the surface 25 by means of displacement by an immiscible fluid (also called a displacement fluid) that takes its place. To this end, wells are drilled within the oil field for the injection of the displacement fluid, generally water, that are arranged in such a way as to create in the 30 subsoil the most uniform possible advancing front to displace the hydrocarbon fluid towards the production well.

-2The technique of displacement by injections of water (water flooding) has long been the simplest and most economical method used to sustain production in an oil field and increase the overall recovery factor of 5 the hydrocarbon fluid.

The quantity of hydrocarbon fluid that can be displaced towards the production wells by water injection dépends, among other factors, on the degree of heterogeneity of the réservoir rock and the properties 10 of the hydrocarbon fluid (above ail the viscosity). In particular, fractures, channels or levels of high permeability (the latter also called thief zones) constitute preferential flow pathways. The natural tendency of fluids to flow through the most permeable 15 portions of the réservoir rock means that, with the passage of time, the water injected into the subsoil continues to flow away along these fractures, channels or levels of high permeability, reaching the producer wells directly without or only minimally infiltrating 20 the zones of the réservoir rock in which the hydrocarbon fluid is still présent, thus producing no displacement effect.

In these situations, the production of water can increase until it dominâtes that of the hydrocarbon 25 fluid, thus rendering the production of the latter barely or not at ail advantageous from an économie standpoint.

Furthermore, the co-production of water implicates the adoption of spécifie treatment plants so as to be able to safely dispose of the water produced, or of 30 Systems for its re-injection into the subsoil. These measures involve high consumption of energy and materials, and increase the overall cost of hydrocarbon extraction.

In the State of the art, the problem of undesirable production of water and of the inadéquate efficiency of the recovery of the hydrocarbons by displacement with 5 water is dealt with by injecting into the subsoil liquid compositions containing Chemical compounds that are capable of modifying the permeability characteristics of the subterranean formation. The compounds most in use for this purpose are generally in the form of polymers, 10 gels or foams. These compounds are here also called ''blocking agents. The blocking agents block the pores of the formation zones with the highest permeability, diverting the flow of the displacement fluid towards the zones that are still rich in hydrocarbon fluid, thus 15 increasing the production capacity of the well.

US 2009/0264321 describes a method for modifying the permeability of a subterranean formation based on injecting into the subsoil a composition comprising encapsulated expandable polymeric microspheres. Once 20 injected into the subsoil, following an activation event (for exemple a variation in température or pH) , the polymeric microspheres escape from the capsules in which they are enclosed and disperse into the formation, where they swell up by absorbing the displacement fluid with 25 which they are in contact. Swelling of the microspheres within the pores of the formation prevents the flow of the displacement fluid, which is thus diverted towards other zones of the formation. However, the properties of the aforesaid encapsulated polymeric microspheres cannot 30 be easily modifiable and therefore optimizable as a function of the spécifie conditions of the formation. Furthermore, the aforesaid microspheres alter the

-4permeability of the formation irreversibly, so that any errors of injecting the blocking agent are not remediable.

Also known in the state of the art are blocking agents composed of thermo-responsive polymers. Thermoresponsive polymers are polymers that show a drastic and discontinuons change in chemicophysical properties with température. The term thermo-responsive is commonly used with reference to the solubility of the polymer in a given solvent. A polymer having a solubility with thermoresponsive characteristics présents a région of immiscibility in the temperature-composition diagram, characterised by at least one phase transition that is réversible in response to a variation in température.

In general, two different types of phase transition are seen in a solution of a thermo-responsive polymer, each of which is characterised by a spécifie critical température :

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

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

Further information on thermo-responsive polymers 30 and their solubility properties are given, for example, in Chem. Soc. Rev., 2013, 42, 7214, and Polymers 2011, 3, 1215-1242.

-5US 2012/0264655 describes the use of blocking agents based on thermo-responsive polymers and hydrogels in combination with varions treatment fluids for oil extraction operations. The treatment fluids (e.g. 5 drilling fluids, fracturing fluids, etc.) contain a gelling agent formed of a thermo-responsive graft polymer comprising a water-soluble main chain (e.g. Polyacrylic acid) onto which are grafted a plurality of side chains having thermo-responsive functional groups.

The polymeric side chains confer on the polymer a characteristic LCST value, determining the course of its solubility as a function of température.

WO 1995026455 describes a method for controlling the permeability in a subterranean formation, which 15 comprises injecting an aqueous solution of at least one thermo-responsive polymer into a well up to a zone of the formation having a température higher than the température of the solution in the well. The thermoresponsive polymer has an LCST température intermediate 2 0 between the température of the solution in the well and the température of the zone. Through an effect of the température increase caused by the heat transmitted from the zone to the solution, the thermo-responsive polymer becomes insoluble, thus altering the permeability of the formation. According to WO1995026455, the LCST température of the thermo-responsive polymer may be regulated by advantageous sélection of the type of monomers and/or co-monomers, their weight ratios, or by adding further compounds capable of modifying the LCST 30 température, The thermo-responsive polymers are prepared by bulk polymérisation or suspension polymérisation. The effects of permeability modification may be rendered

-6réversible, for example by injecting réfrigérant liquida into the formation which, by lowering the température of the formation to values below the LCST température, cause the inverse transition phase of solubilisation of the 5 polymer.

The methods known in the prior art for controlling the permeability of a subterranean formation, that are based on the use of thermo-responsive polymers, hâve the disadvantage of not allowing adéquate controlling of the 10 permeability-modifying effects induced by the transition phase of the polymer at températures above the LCST. In particular, the modifications are often irréversible or necessitate further interventions, such as the injection of réfrigérant fluids as described in WO 1995026455.

In considération of the aforesaid State of the art, the Applicant therefore set the primary objective of providing a fluid for controlling the permeability of a subterranean formation, which at least partially overcomes the drawbacks of the prior art.

Within the scope of this primary objective, one object of the présent invention is to provide an aqueous fluid for controlling the permeability of a subterranean formation comprising a thermo-responsive polymer, wherein the LCST température at which occurs the 25 transition phase from soluble polymer to viscous gel is easily regulable, and therefore adaptable to the spécifie conditions of the subterranean formation the permeability of which is to be modified.

A second object of the présent invention is to 30 provide a fluid for controlling the permeability of a subterranean formation that is easily injectable into the formation and whose effect of blocking of the pores due to formation of the gel phase is at least partially réversible, preferably without this reversibility being induced by external actions, such as for exemple the injection of réfrigérant fluids.

ή third object of the présent invention is to provide a fluid for controlling the permeability of a subterranean formation that can be prepared by a simple method at moderate cost.

The Applicant has now found that these and other objects, which will be better illustrated in the description that follows, can be achieved by means of an aqueous fluid comprising a thermo-responsive polymer having a block structure A-B-A', wherein the outer blocks A and A', the same as or different from one another, are oligomer groups with thermo-responsive characteristics, and therefore having a characteristic and defined LCST, whereas the central block B is a hydrophilic oligomer group of relatively high length.

It has in fact been observed that by synthesising a polymer with the af orementioned A-B-A' block structure, in which the thermo-responsive blocks A and A' are distanced one from the other by a hydrophilic oligomer segment of sufficient length, it is possible to préparé aqueous solutions of said polymers which change state, becoming viscous gels, at a well-defined température (LCST). The LCST value of the polymer A-BA' , and therefore the température at which the phase transition occurs, may be easily regulated by modifying the composition of the thermo-responsive blocks A and/or

A' , for example by incorporating hydrophilic and hydrophobie monomers and/or by varying the length of the oligomers A and/or A' .

-8The aforementioned thermo-responsive A-B-A' polymers are soluble at low températures, that is, at a température below the LCST, whereas they form a gel phase of relatively high viscosity at high températures, that 5 is, at a température above the LCST. As a resuit of this, they can be easily injected into the subterranean formation in the form of aqueous solutions of low viscosity at températures below the LCST, whereas they act as blocking agents following phase transition to 10 viscous gel, in the zones in which the température of the formation exceeds the LCST.

Without making reference to any particular theory, it is considered that, at températures above the LCST, the blocks A of the polymer A-B-A' collapse, forming 15 micellar structures which, being linked to one another via a bridge composed of the oligomer block B, give rise to a reticulated structure of relatively high viscosity (physical gel).

Advantageously, the blocks A and A' of the A-B-A' 20 polymers can also include portions that are degradable over time in the température conditions of the subterranean formation, for example by effect of a hydrolysis reaction. Dégradation of the aforesaid degradable portions leads to a structural modification 25 of the original blocks A and A', with conséquent variation of the thermo-responsive behaviour of the polymer. In particular, following the dégradation an increase in the LCST of the polymer occurs; when the LCST of the degraded polymer exceeds the température of 30 the subterranean formation, the phase transition of the viscous gel into fluid polymeric solution occurs.

Due to the A-B-A' block structure, the polymers according to the présent invention may be prepared so as to hâve the LCST more suitable for the spécifie geothermal profile of the subterranean formation, so increasing the efficacy of the displacement fluids 5 (lower production of water) and consequently the productivity of the fluid hydrocarbons (oil or gas).

In accordance with a first aspect, the présent invention therefore relates to a fluid for controlling the permeability of a subterranean formation comprising 10 an aqueous solution of at least one thermo-responsive polymer with an Ά-Β-Ά' block structure, wherein:

- A and A' , the same as or different from one another, each represent a thermo-responsive oligomer group;

- B is an oligomer group comprising n repeat units, the same as or different from one another, wherein:

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

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

a. providing a fluid comprising an aqueous solution 25 of at least one thermo-responsive polymer with block structure A-B-A', wherein:

- A and A' , the same as or different from one another, each represent a thermo-responsive oligomer group;

- B is an oligomer group comprising n repeat units, the same as or different from one another, wherein:

(i) at least one of the repeat units has a

- 10solubility of the corresponding monomer, in water at 20°C, equal to or greater than 120 g/1, (ii) n is a whole number within the range 30 - 1000;

b. positioning said treatment fluid in a 5 subterranean formation.

In accordance with a third aspect, the présent invention relates to the use of the aforesaid fluid to modify the permeability of a subterranean formation.

In accordance with a further aspect, the présent 10 invention relates to the use of the aforesaid fluid to limit the infiltration of water into a well for extracting an oil or hydrocarbon gas from a subterranean formation.

For the purposes of the présent description and of 15 the attached daims, the lower critical solubility température (LCST) of a polymer is to be understood as determined by the turbidimetric method described in the examples.

For the purposes of the présent description and of 20 the attached daims, the degree of polymérisation of a polymer, also denoted as DP (degree of polymérisation) , is the mean number of repeat units présent in a polymer, determined by ¹H NMR spectroscopy with methods known to the person skilled in the art, on a solution of the polymer in CDCI3 as the solvent (99.8% D atoms) at a polymer concentration of 15 mg/mL.

For the purposes of the présent description and of the attached daims, the terms oligomer and polymer include the terms homopolymer and co-polymer, unless 30 explicitly indicated otherwise or in any event unless otherwise deducible from the text. The oligomers and the polymers, that is, can be formed from the sanie repeat

unit or from two or more repeat units different from each other. As used herein, the term polymer includes oligomers and polymers.

For the purposes of the présent invention, the mean 5 mdecular weight in number M_n of an oligomer or polymer is understood to be determined by gel perméation chromatography (GPC), using polystyrène as the standard.

For the purposes of the présent invention, repeat unit is understood to mean the constitutive unit of a 10 polymer or an oligomer; said constitutive unit may correspond to one monomer or to two or more monomers, from which it dérivés following polymérisation; said monomer or monomers from which the repeat unit dérivés are here also denoted as corresponding monomer of the 15 repeat unit.

Further information about the structure of oligomers and polymers can be found, for example, in Alfred Rudin, Phillip Choi, The Eléments of Polymer Science and Engineering, 3rd édition, Elsevier, 2013.

0 For the purposes of the présent description and of the attached daims, the verb comprise and the terms derived therefrom also include the verb consist and consist essentially in, and the terms derived therefrom.

The limits and the numerical intervals expressed in the présent description and in the attached daims also include the numerical value or numerical values mentioned. Furthermore, ail the values and sub-intervals of a limit or numerical interval must be understood as 30 specifically included as if they were explicitly mentioned.

For a better understanding of the features of the

-12present invention, in the description reference will be made to the following drawings:

Fig. 1 (dégradation test): graph of the percentage loss of mass of Polymer 1 (with respect to the initial weight of the sample) as a function of the observation time ;

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

The block A or A' of the polymer A-B-A' may be formed of an oligomer comprising a single type of repeat units or two or more repeat units different from each other. The blocks A and A' may be the same as or different from one another.

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

To hâve thermo-responsive characteristics, the oligomer that forms the block A and/or A' preferably comprises at least one hydrophilic portion and at least one hydrophobie portion.

The hydrophilic portion is preferably formed of polyoxyethylene chains, polylactide chains, C2-C10 alkyl chains, linear or branched, containing amide groups.

The hydrophobie portion is preferably obtained from monomers comprising at least one ester or amide group, such as, for example, the compounds (meth)acrylate and (meth)acrylamide.

In a preferred embodiment, the thermo-responsive block A and/or A' is a graft polymer comprising a poly(meth)acrylic or poly(meth)acrylamide linear main

-13 Chain and a plurality of aide chains, connected to said main chain, selected from: poly(ethylene glycoi) -[-CH2CH2-0]_m-H, poly(ethylene glycoi) methyl ether -[-CH2-CH20] m~CH3, HEMA-polylactide --CO-O-CH2-CH2-O- [ -CO- (CH3) CH5 0]_ra-H, alkylamide, linear or branched, and combinations thereof, where m is an integer number within the range 2 - 10.

In an embodiment, the A block and/or A' block is an oligomer containing a plurality of polymer side chains. 10 In an embodiment, the B block is a linear oligomer that contains no polymer side chains.

In a preferred embodiment, the thermo-responsive blocks A and/or A' are preferably oligomer groups formed of one or more repeat units deriving from monomers 15 selected from: (meth)acrylic acid, oligo(ethylene glycoi) methyl ether méthacrylate (OEGMA), [2(methacryloyloxy)ethyl]- trimethylammonium chloride (MADQUAT), HEMA-polylactide, potassium 3-sulfopropyl méthacrylate, 2-acrylamido-2-methylpropanesulfonic acid, polymerisable esters comprising polyoxyethylene chains - [-CH2-CH2-O] _X-H where x is an integer number within the range 10 - 1000, N-isopropylacrylamide, N,Ndimethylaminoethyl méthacrylate, dimethylacrylamide.

Preferably, the OEGMA compound used to form the 25 block A and/or A' has a mean molecular weight M_n within the range 100 - 10,000 Dalton, more preferably within the range 100 - 5,000 Dalton.

In a preferred embodiment, the thermo-responsive block A and/or A' is preferably a co-polymer of two or 30 more OEGMAs having a different molecular weight from one another, for example OEGMAs that differ from one another in the length of the side chains.

- 14In a preferred embodiment, the thermo-responsive block A and/or A' is preferably a co-polymer comprising OEGMA as a first co-polymer and at least one second hydrophobie co-monomer selected from: C1-C4 alkyl5 (meth)acrylate (e.g. MMA or BA) , (dimethylamino)ethyl méthacrylate (DMAEMA) and combinations thereof.

In one embodiment of the présent invention, the hydrophobie co-monomer selected is a vinyl oligoester that is biodégradable using hydrolysis reactions, such 10 as for example HEMA-polylactide.

By appropriately selecting the type of hydrophilic and hydrophobie units, their molecular weight and the relative molar ratio, it is possible to regulate the LCST value of the A or A' unit and therefore also the 15 behaviour of the final polymer A-B-A' in response to variations of température.

Preferably the degree of polymérisation DP of the polymer that forms the blocks A and A', that is, the total number of repeat units présent in the blocks A and 20 A' , is within the range 2 - 1000, preferably 3 - 500, more preferably 5 - 450, even more preferably 6 - 400.

The block B of the polymer A-B-A' is an oligomer formed of a single type of repeat units or (homopolymer) or of two or more different types of repeat units 25 (copolymer) ,

Preferably, the oligomer group that forms the block B comprises one or more repeat units corresponding to monomers selected from: (meth)acrylic acid, polymerisable esters comprising polyoxyethylene chains 30 -[-CH2-CH2-O] r-H where r is an integer number within the range 10 - 500, more preferably 20 - 300, oligo(ethylene glycol)methyl ether méthacrylate (OEGMA), [220732

-15 (methacryloyloxy)ethyl]- trimethylammonium chloride (MADQUAT), potassium 3-sulfopropyl méthacrylate, 2acrylamido-2-methylpropanesulfonic acid, (meth)acrylamide 2-hydroxypropyl methacrylamide, 5 glycerol monomethacrylate, 2-hydroxyethyl méthacrylate, methacryloyloxyethyl phosphorylcholine, carboxybetaine, sulfobetaine and combinations thereof.

In an alternative embodiment, the oligomer group that forms the block B is a poly(ethylene glycol) chain (PEG, HO- [CH2CH2-O] n-H) , where n is a whole number comprised between 30 and 300, preferably functionalised at both ends with the CTA of RAFT polymérisation to obtain a macromoleouïe of the type CTA-PEG-CTA which enables the symmetrical structure A-B-A to be obtained in a single polymérisation stage.

Preferably the overall degree of polymérisation DP of the block B, that is, the total number of repeat units, the same as or different from one another, présent in the block B, is within the range 10 - 1000, preferably 20 20 - 500, more preferably 50 - 500.

In one embodiment, the block B possesses no thermal response characteristic.

In another embodiment, the block B does possess thermal response characteristics, but these do not 25 substantially alter the thermal response properties of the polymer A-B-A'. For example, the block B may be a thermo-responsive polymer having an LC ST température higher than the LCST température of the blocks A and A' . Therefore this polymer has no influence on the thermal 30 response behaviour of the polymer A-B-A', in that the phase transition to vis cous gel occurs at the LCST of the blocks A and A' , which is lower than that of the

- 16block B.

In an embodiment, in the block copolymer A-B-A^r:

- the B block comprises poly(ethylene glycol) (PEG) in the main Chain having a mean molecular weight Mn 5 within the range 500 - 10,000 Dalton, more preferably within the range 1,000 - 5,000 Dalton;

- the A block and/or A' block are oligomer groups comprising, as corresponding monomer, oligo(ethylene glycol)methyl ether méthacrylate (OEGMA), preferably 10 having a mean molecular weight M_n within the range 100 1,000.

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

- the B block is an oligomer group comprising, as 15 corresponding monomer, [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MADQUAT),

- the A block and/or A' block are oligomer groups comprising, as corresponding monomer, OEGMA having a mean molecular weight M_n within the range 100 - 1,000.

In a further preferred embodiment, the oligomer that forms the block A and/or A' comprises one or more degradable repeat units, that is, containing weak bonds capable of progressively breaking over time in the conditions of the région of the subterranean formation 25 in which the polymer A-B-A' is présent in the form of a gel. Examples of these degradable units are the monomers used in the state of the art for the synthesis of biodégradable polymers.

For example, in a preferred embodiment, one biodégradable unit utilisable for the purposes of the présent invention is the macromonomer HEMApoly(lactide), obtainable for example by ring opening

- 17polymerisation (ROP) of the lactide cyclic ester with hydroxyethyl méthacrylate (HEMA). This polymérisation reaction is illustrated schematically in the figure below;

with n preferably comprised between 1 and 10 and x 15 preferably comprised between 10 and 500.

The polymérisation of the HEMA-lactide macromonomer in the block A and/or A' leads to the introduction into these blocks of side chains compose! of oligoesters, that are degradable by effect of hydrolysis of the ester 20 bonds. The time necessary for the dégradation is determined principally by the length of the chain of oligoesters and can be regulated by appropriate sélection of the HEMA/lactide ratio during the synthesis of the macromonomer.

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

When the polymer A-B-A' is présent in the subterranean formation in the form of a gel, following dégradation of the oligoester chains, a progressive 30 increase in the hydrophilic properties of the polymer AB-A' and, consequently, of its LCST is observed. When the LCST of the polymer exceeds the température of the

-18formation in which the gel is found, the latter undergoes a change in State, transitioning from gel to soluble polymer and forming a fluid aqueous phase, which can then once again move through the formation.

Moreover, it will be noted that, in the case of the

HEMA-lactide, the product of dégradation of the oligoester chains is composed principally of molécules of lactic acid, a biocompatible substance.

In a preferred embodiment, block B comprises at 10 least one cation functionality. This cation functionality may be introduced by preparing the polymer B from monomers comprising cation functionalities. The following are examples of these monomers: [2(methacryloyloxy)ethyl]- trimethylammonium chloride 15 (MADQUAT), 2-(dimethylamino)ethyl méthacrylate (DMAEMA), 2-aminoethyl méthacrylate hydrochloride.

When block B comprises cation functionalities, the gel that is formed from the soluble polymer A-B-A' is capable of interacting more effectively with the surface 20 of the pores of the subterranean formation, establishing electrostatic interactions with the carbonate rocks of the formation, which render the blocking action of the gel more effective.

The triblock A-B-A' polymers according to the 25 présent invention may be prepared with the polymérisation techniques known in the State of the art, such as free-radical polymérisation (ERP) and controlled radical polymerization (CRP). CRP techniques comprise, for example: atom transfer radical polymerization 30 (ATRP), nitroxide mediated polymerization (NMP) and réversible addition-fragmentation transfer polymerization (RAFT).

- 19In particular, for the préparation of thermoresponsive block polymers according to the présent invention, CRP techniques of controlled polymérisation are preferred, which yield polymer chains of homogeneous 5 length and therefore having a similar thermal response behaviour. The homogeneity of the length of the polymer chains guarantees the obtaining of A-B-A' polymers having a well-defined and predeterminable LCST. Among the CRP techniques, the one especially preferred is the

RAFT polymérisation technique. As is known, RAFT polymérisation is a procèss of dégénérative transfer polymérisation, in which a chain-transfer agent is added to the starting monomers with the aim of transferring, during polymérisation, the radical function from one 15 polymer chain to another extremely rapidly, thus enabling uniform growth of the polymer chains. Transfer of the radical function occurs by interaction between the terminal portion of the chain-transfer agent (CTA) and the active radical chain, which brings the latter to a dormant State while rendering the chain previously bound to the CTA agent active again.

On account of the above-mentioned mechanism, RAFT polymérisation is especially suitable for the préparation of block copolymers and represents the idéal 25 solution in the production of thermo-responsive materiels having well-defined and predeterminable LCSTs.

With RAFT polymérisation it is also possible to préparé polymers having polymer chains of well-defined length, as well as being mutually homogeneous. The length 30 of a polymer chain is here expressed by the marker DP, previously defined.

In RAFT polymérisation, the DP of the polymer can be

-20regulated simply, selecting the ratio of the initial monomer concentration to that of the CTA in the polymérisation mixture.

In general, the A-B-A^f polymers according to the 5 présent invention may be prepared by RAFT polymérisation in successive steps, as described by way of example hereinafter.

In a first step, the synthesis of block A is performed, by polymerising for example OEGMA300 (that 10 is, oligo (ethylene glycol) methyl ether méthacrylate, Mn = 300) to obtain poly [OEGMASOOsoo] , where the marker DP = 500 indicates the degree of polymérisation, that is, the average number of OEGMA300 repeat units présent in the block A oligomer. To this end, the OEGMA300 monomer 15 is set to react with a CTA, for example 4-cyano-4(phenylcarbonothioylthio) pentanoic acid, and a polymérisation initiator, for example 4,4'— azobis(cyanovaleric) acid (ACVA), in a polar solvent (e. g. éthanol), at a température of approximately 5020 8 0°C, at atmospheric pressure, for 24-48 hours. The relative quantifies of reagents may be calculated in such a way as to détermine the desired DP.

Following évaporation of the solvent and recovery of the unreacted monomer (for example, by extraction 25 with diethyl ether), the oligomer that forms block A can be recovered in the form of a viscous liquid.

The oligomer that forms block A has polymer chains terminating on both sides by the spécifie functionalities of the CTA used in the first step. This 30 therefore représenta a macro-CTA that may be set to react with the monomer of segment B, in a second step of RAFT polymérisation. In this second step, the monomer of block

-21 B acts essentially as a block A chain extender. Block B, for example, may be formed by using OEGMA2000 (that is, oligo(ethylene glycol) methyl ether méthacrylate, Mn = 2,000) as the monomer, to obtain poly[OEGMA20005oo] , with

DP = 500. The RAFT polymérisation of the second step may be performed under conditions similar to those of the f irst step. At the end of the second step, a diblock polymer A-B of the type poly [OEGMASOOsoo ~ OEGMAlOOOsoo] is therefore obtained. This polymer too, because it 10 contains chains terminated on both sides by the CTA functionalities, may be used as a macro-CTA in a third step of RAFT polymérisation to add the block A' to the diblock polymer A-B and obtain the triblock polymer A-BA' . To this end, the polymer poly [OEGMASOOsoq

OEGMA2000500] is set to react, for example, with the same monomer used to form the segment A in the first step in the presence of the initiator ACVA in conditions analogous to those of the first step, thus obtaining the A-B-A symmetrical polymer poly [OEGMASOOsoo - 0EGMA2 OOOsoo

0 - OEGMA30Û5oo] . However, if a monomer A' different from

A is used, an asymmetrical polymer A-B-A' is obtained.

The symmetrical polymers A-B-A, that is where A is egual to A', may be prepared by RAFT polymérisation also via a two-step process. In the first step, the repeat 25 unit (or repeat units) that forms the oligomer B (which may hâve been synthesised via RAFT or another polymérisation technique) is reacted with a CTA compound, the latter being présent in the reaction mixture in a quantity such as to obtain the oligomer B 30 terminating on both sides by the spécifie functionalities of the CTA used (macro-CTA), for example in a molar ratio with respect to the oligomer B gréater

-22than or equal to 2, preferably comprised between 2.1 and 4. In the second step, the repeat unit (or repeat units) that is to form the oligomer A is made to polymerise in the presence of the macro-CTA produced in the first step 5 to obtain the final symmetrical polymer A-B-A.

According to an alternative synthesis route, the symmetrical polymers A-B-A may be prepared by synthesising, in a first step, the oligomer blocks A by RAFT polymérisation in the presence of a bifunctional 10 CTA (to obtain A-CTA-A). In a second step, the oligomer A-CTA-A is reacted with the repeat unit that forms the block B, so as to extend the chain of the oligomer ACTA-A starting from the central unit until the polymer A-B-A is obtained.

For the purposes of the présent invention, the thermo-responsive A-B-A polymers are introduced into the subterranean formation, the permeability of which to fluids in the form of aqueous fluid is to be modified. Preferably, the treatment fluid comprises the A-B-A' polymers in the form of aqueous solution. The concentration of the polymer in the aqueous solution is preferably within the range 0.5-40% by weight, more preferably within the range from 1 to 20% by weight, with respect to the weight of the aqueous solution,

The aqueous fluid comprising the A-B-A' polymer may be used for controlling the permeability of a subterranean formation during oil extraction activity.

For the purposes of the présent invention, subterranean formation is intended to mean a zone beneath 30 the land surface, including the surfaces of the seabed. For example, a subterranean formation may be any zone of a formation containing a rock storing a hydrocarbon fluid

-23(oil or gas) and any zone of a well in fluid communication with said formation.

The method for controlling the permeability to fluids of a subterranean formation according to the 5 présent invention may be applied both before starting extraction of the hydrocarbon fluid from the subterranean formation and when the extraction well is already in production.

The method according to the présent invention may 10 be advantageously applied to the extraction wells termed mature, that is, to wells that hâve now reached the limit of their production capacity and are characterised by the extraction of significant quantifies of water in association with the oil or hydrocarbon gas.

The positioning of the fluid for controlling permeability in a subterranean formation may be carried out with the equipment and according to the techniques known in the oil extraction industry.

The positioning may, for example, be done by 20 injecting the control fluid both via the well for extracting the hydrocarbon fluid and through the other wells generally présent in an oil field, such as the wells for injecting steam, water or other displacement fluids into the subsoil (known as injection wells).

On the basis of the criteria and using the préparation methods disclosed above, the person skilled in the art can select the control fluid comprising the polymer A-B-A' in such a way that said polymer has an LCST higher than the injection température of the fluid 30 and lower than the température of the deposits within the zone in which the permeability control is to be carried out.

-24The method according to the présent invention may be used within the scope of secondary and tertiary activities for recovery of a hydrocarbon oil, both in water-blocking interventions (water shut-off treatment) 5 and in treatments conforming the subterranean formation (conformation treatment).

The quantifies of aqueous fluid comprising the thermo-responsive polymers to be inj ected into the subterranean formation can vary widely as a function of 10 its spécifie geological conformation. The quantifies may be easily determined by the person skilled in the art on the basis of the geological characteristics of the formation and of simple routine expérimenta,

The following example embodiments are provided 15 merely in order to illustrate the présent invention and are not to be understood in a sense limiting the scope of protection defined by the attached daims.

EXAMPLES

1. Détermination of the LCST

For the purposes of the présent invention, the Tower critical solubility température (LCST) of a polymer is intended to be determined by the curbidimetry method, by measuring the optical transmittance of an aqueous solution 0.2% by weight of the polymer under investigation, at ambient pressure. The transmittance measurements are conducted on the sample maintained at different températures. By plotting the percentage transmittance recorded against the corresponding température of the sample, a sigmoidal curve is obtained.

The température at the inflection point of the curve is considered the LCST of the polymer.

2. Préparation of the thermo-responsive polymers

2.1 Polymer 1

A thermo-responsive polymer having the following AB-A block structure was prepared as follows.

*ΠΓ thermo-responsive biodégradable block thermo-responsive biodégradable block

Step 1 - Préparation of the macro-CTA CTA-B-CTA

A hydrophilic and bifunctional macro-CTA was synthesised by Steglich estérification between an 15 équivalent of poly(ethylene glycol) (PEG) HO-[CH2-CH2] nOH (Mn = 4000 g 'mol'¹) and two équivalents of 4phenyl(carbonothioylthio) pentanoic acid (CPA). To this end, 50 g PEG and 7.7 g CPA were solubilised in 300 mL dichloromethane (DCM). The solution obtained was loaded 20 into a round-bottomed flask immersed in an ice and water bath. Separately, 5.7 g dicyclohexylcarbodiimide (DCC) and 0.2 7 g 4-dimethylamino pyridine (DMAP) were solubilised in 25 mL DCM. The solution obtained was added dropwise to the one containing PEG and CPA, maintained 25 under magnetic stirring, within an hour. The mixture thus obtained was left to equilibrate at ambrent température, and then reacted for 18 hours. After concentrating under vacuum to reduce the volume to about half, the product was recovered by précipitation of the 30 reaction mixture in diethyl ether and left to dry in an oven under vacuum at 35°C.

The resulting macro-CTA is a hydrophilic polymer

-26that exhibits no thermal response behaviour.

Step 2 - Préparation of the A-B-A polymer

The macro-CTA obtained in step 1 was used in the RAFT polymérisation of OEGMA300 and of HEMA-lactide to 5 obtain the A-B-A block structure (Polymer 1). To this end, 2.59 g OEGMA300, 2.41 g HEMA-lactide, 0.065 g of the macro-CTA in step 1 and 1 mg azobiscyanovaleric acid (ACVA) were dissolved in 25 mL éthanol. The solution was bubbled with nitrogen for 30 minutes, keeping it in an ice and water bath. The RAFT polymérisation was then conducted at 65°C for 24 hours. The polymer was subsequently recovered by précipitation from the reaction mixture in diethyl ether, followed by drying in an oven under vacuum at 35°C.

The DP of the external A blocks was found to be

500 .

The LCST température of the Polymer 1 was found to be 45°C.

2.2 Polymer 2

A thermo-responsive polymer having the following AB-A block structure was prepared as follows.

thermo-responsive biodégradable block. thermo-responsive biodégradable block.

-27 Step 1 - Préparation of the CTA-A macro-CTA

The first thermo-responsive A block was synthesised by dissolving 13 g OEGMA300, 12 g HEMA-lactide, 40 mg CPA and 10 mg ACVA in 130 mL éthanol. The solution was 5 bubbled with nitrogen for 30 minutes and then reacted at 65°C for 24 hours. The polymer was précipitated in diethyl ether and its DP was evaluated as 500. By GPC analysis it was determined that the mean molecular weight of the product was approximately 188000 g-mol·¹, with 10 polydispersity of 1.19.

Step 2 - Préparation of the CTA-B-A macro-CTA

The macro-CTA 1 obtained in step 1 was used in a second RAFT stage to bind the hydrophilic B block. Specifically, 10 g of macro CTA, 2.4 g MADQUAT and 5 mg 15 ACVA were solubilised in 50 mL of a 50:50 v/v water:éthanol mixture. The solution was bubbled with nitrogen for 30 minutes while keeping it in an ice and water bath, and then left to react at 65°C for 24 hours. The A-B block polymer (macro-CTA 2) was then precipitated 20 in diethyl ether and recovered by filtration under vacuum. The DP calculated for the B block B was found to be 200.

Step 3 - Préparation of the A-B-A polymer

Finally, in the third RAFT stage, the A-B-A triblock 25 copolymer was obtained by extending the macro-CTA 2 obtained in the previous step 2 with OEGMA300 and HEMAlactide. To this end, 10 g of macro-CTA 2, 4.2g OEMA300, 3.88 g HEMA-lactide and 3 mg ACVA were dissolved in 80 mL of a 50:50 v/v water: éthanol mixture. The reactive 30 mixture was bubbled with nitrogen for 30 minutes, keeping it in an ice and water bath, and then reacted at 65 °C for 24 hours. The A-B-A block polymer was then

-28precipitated in diethyl ether and recovered by filtration under vacuum. The DP of the third block (A) was found to be 500.

The LCST température of the Polymer 2 is 45°C.

3. Thermo-responsive solubility

The phase transition of the thermo-responsive polymers 1 and 2 from aqueous polymer solution to viscous gel was verified experimental!y, conditioning a test tube containing a sample of aqueous solution of the 10 polymer (30% by weight of polymer with respect to the weight of the solution) at a température of 55°C for 30 minutes.

In both cases, at the end of the conditioning, the formation of a viscous gel at the bottom of the test 15 tube was observed. By conditioning the test tube containing the gel again at ambient température (25°C), the inverse transition from gel to aqueous polymer solution was observed.

4.1 Permeability modification tests on packed sand 20 The effectiveness of an aqueous fluid containing the Polymer 1 according to the présent invention in controlling the permeability of a sample of packed sand was verified experimentally in the following way.

A solution of Polymer 1 (1% by weight) in saltwater 25 was prepared using a synthetic saltwater having the composition given in Table 1 below (density at 70°C = 0.9465 kg/L; viscosity at 70°C = 0.4058 cP).

-29Table 1 - saltwater composition

Compound	Concentration (g/L)
NaCl	22
MgCl2(6H2O)	9.7
Na2SO4 (anhydrous)	37
CaCl2 (anhydrous)	1
KC1	0.65
NaHCO3	0.2
H3BO4	0.023
Total	37.273

On a sample (cylindrical core) of packed sand (passed through a 20-mesh and a 40-mesh sieve) 14.50 cm 5 in length and 5.10 cm in diameter, measurements of permeability to fluids were made using the solution of the Polymer 1 and the saltwater of Table 1 (without thermo-responsive polymer), as the reference fluid.

The cylindrical core, housed in a sample holder, 10 was placed inside a temperature-controlled oven. The measûrement System included, in addition to the aforesaid oven, a System of pumps for inj ecting the fluids into the core at preset flow rate values, and devices for measuring the flow rates of the fluids. A 15 containment overpressure (nitrogen) of 50 bar was applied to the core (on top of the internai pressure) to avoid the effects linked to the compressibility of the sand during injection of the fluids. During fluxing of

-30the fluids in the sample, the load losses between inflow and outflow of the fluid from the sample were measured by means of electronic transducers.

Initially, the permeability of the sample was 5 measured by flushing the reference fluid consisting of the saltwater. The treatment fluid consisting of the solution containing Polymer 1 was then injected, up to saturation of the core. The core was then maintained at a température of 70°C for the time necessary for the 10 phase transition to occur, from soluble polymer to gel capable of opposing résistance to the flow of the injected fluid. In a final step, saltwater was again injected into the core to détermine the variation in its permeability following the treatment with the solution 15 of Polymer 1.

The permeability of the sample was determined by measuring the load losses in response to variation in the flow rate of the injected fluid.

The résistance factor F_r gives an indication of the 20 relative mobility of the aqueous solution of Polymer 1 within the sample with respect to the mobility of the saltwater alone in the same sample.

The residual résistance factor F_rr instead provides a measure of the réduction in relative mobility of the 25 reference fluid in the sample following treatment of the sample with the treatment fluid, with respect to the mobility of the same reference fluid prior to the treatment of the sample with the treatment fluid. Higher values of the parameters F_r and. F_rr indicate a higher 30 résistance of the sample to passage of a fluid.

During fluxing of the solution of Polymer 1 in the core at ambient température, the value of F_r was found

-31 to be equal to approximately 1.15. This value, being low, indicates that the solution of Polymer 1 is easily injectable into the sample.

The saturated sample of solution of Polymer 1 was 5 maintained at a température of 70°C for 48 hours and then flushed with saitwater to verify the extent of modification of permeability. The value of F_rr determined for the saltwater flushing was 22.7. The treatment with Polymer 1 thus determined a réduction in the permeability 10 of the saltwater by a factor of approximately 23.

4.2 Tests of modification of the permeability on sandstone - Chashach

The test described in the preceding point 4.1 was repeated on a cylindrical core of sandstone of Clashach 15 type, length 14.50 cm and diameoer 5.10 cm, using the solution of Polymer 1 and the reference saltwater described above.

During fluxing of the solution of Polymer 1 in the core at ambient température, the value of F_r was found 20 to be approximately 10.6. This value indicates a résistance to the injection of acceptable magnitude.

The value of F_rr determined for the fluxing of saltwater was 142.8. This value of F_rr confirms the efficacy of the treatment for controlling the permeability of the sample with the polymers according to the présent invention.

5. TEST OF DETERIORATION

To verify the reversibility of the sol-gel transition following hydrolysis of the ester bonds, 30 Polymer 2 inside a container was conditioned at a température of 55°C for 30 minutes, so as to enable formation of the gel. A head of saltwater was then added

-32on top of the gel layer. Change in the biphasic System, maintained at 55°C, was monitored by measuring the loss of mass by thermogravimetry and the pH of the head of water.

Figs. 1 and 2 give the experimental values for loss of mass and pH of the biphasic System observed as a function of time.

Fig. 1 shows that, after an induction period of around 7 days in duration, in which only a slight décliné 10 (less than 10%) in the mass of the gel was recorded, the mass of the gel began to fall off linearly over time, until, after 20 days, it resulted in the solubilisation of the gel in the head of water, and therefore in its total disappearanoe. In parallel, as shown by Fig. 2, 15 the pH of the liquid phase declined over time. The réduction in the pH can be attributed to the release of acid species, in particular lactic acid, which accompanies dégradation of the polymer.

Claims (20)

1. Fluid for controlling the permeability of a subterranean formation comprising an aqueous solution of at least one thermo-responsive polymer of block

5 structure A-B-A', wherein:

- A and A' , the same as or different from one another, each represent a thermo-responsive oligomer group;

- B is an oligomer group comprising n repeat units, 10 the same as or different from one another, wherein:

(i) at least one of the repeat units has a solubility of the correspond!ng monomer, in water at 20°C, egual to or greater than 120 g/1, (ii> n is a whole number within the range 30 - 1000.

15

2. Fluid according to daim 1, wherein said A block and/or A' block is an oligomer ccntaining a plurality of polymer side chains.

3. Fluid according to anyone of the preceding claims, wherein said B block is a linear oligomer that 20 contains no polymer side chains.

4. Fluid according to claim 1, wherein said A block and/or said A' block has a lower critical solubility température (LCST) within the range 30°C - 100’C.

5. Fluid according to claim 1, wherein said A block

25 and/or said A' block are a graft oligomer group comprising a poly(meth)acrylic or poly(meth)acrylamide linear main Chain and a plurality of side chains, connected to said main chain, selected from : poly(ethylene glycol) -[-CH2-CH2-O] _m-H, poly(ethylene

30 glycol ) methyl ether - [ -CH2-CH2-O] m-CHa, HEMA-polylactide —CO-O-CH2-CH2-O- [-CO- (CH₃) CH-Od-H, alkylamide, linear or branched, and combinations thereof, where m is a whole

-34number within the range 2 - 10.

6, Fluid according to claim 1, wherein said A block and/or said A' block are oligomer groupa formed by one or more corresponding monomeric units selected from:

5 (meth)acrylic acid, oligo(ethylene glycol)methyl ether méthacrylate (OEGMA), [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MADQDAT), HEMA-polylactide, potassium 3-sulfopropyl méthacrylate, 2-acrylamido-2methylpropanesulfonic acid, polymerisable esters

10 comprising polyoxyethylene chains - [-CH2-CH2-O]_x- where x is an integer number within the range 10 - 1000, Nisopropylacrylamide, N,N-dimethylaminoethyl méthacrylate, dimethylacrylamide.

7. Fluid according to claim 1, wherein said A block 15 and/or said A' block are oligomer groups formed by two or more OEGMA compounds having molecular weight different from one another.

8. Fluid according to claim 1, wherein said A block and/or said A' block hâve a degree of polymérisation DP 20 within the range 2 - 1,000, preferably 3 - 500, more preferably 5 - 450, even more preferably 6 - 400.

9. Fluid according to claim 6, wherein said OEGMA compound has a mean molecular weight M_n within the range 100 - 10,000 Dalton, more preferably within the range 25 100 - 5,000 Dalton.

10. Fluid according to claim 1, wherein said A block and/or said A' block is a co-polymer comprising OEGMA as a first co-monomer and at least one hydrophobie second co-monomer selected from: C1-C4 alkyl -(meth)acrylate, 30 (dimethylamino)ethyl méthacrylate (DMAEMA) and combinations thereof.

11. Fluid according to claim 1, wherein the oligomer

-35group of said B block is an oligomer group comprising one or more repeat units selected from: (meth)acrylic acid, polymerisable esters comprising polyoxyethylene chains - [-CH2-CH2-O] r~H where r is an integer number 5 within the range 10 - 500, more preferably 20 - 300, oligo(ethylene glycol)methyl ether méthacrylate (OEGMA), [2-(methacryloyloxy)ethyl]- trimethylammonium chloride (MADQUAT), [2-(methacryloyloxy)ethyl]trimethylammonium chloride, potassium 3-sulfopropyl 10 méthacrylate, 2-acrylamido-2-methylpropanesulfonic acid, (meth)acrylamide 2-hydroxypropyl methacrylamide, glycerol monomethacrylate, 2-hydroxyethyl méthacrylate, methacryloyloxyethyl phosphorylcholine, carboxybetaine, sulfobetaine and combinations thereof.

15

12. Fluid according to claim 1, wherein:

said B block comprises poly(ethylene glycol) (PEG) in the main chain having a mean molecular weight M_n within the range 500 - 10,000 Dalton, more preferably within the range 1,000 - 5,000 Dalton;

20 - said A block and/or A' block are oligomer groups comprising, as corresponding monomer, oligo(ethylene glycol)methyl ether méthacrylate (OEGMA), preferably having a mean molecular weight M_n within the range 100 1,000.

25

13. Fluid according to claim 1, wherein:

- said B block is an oligomer group comprising, as corresponding monomer, [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MADQUAT),

- said A block and/or A' block are oligomer groups 30 comprising, as corresponding monomer, OEGMA having a mean molecular weight M_n within the range 100 - 1,000.

14. Fluid according to claim 1, wherein said B block

-36comprises at least one cationic pendant group.

15. Fluid according to the preceding claim, wherein said B block comprising at least one cationic pendant group comprises one or more monomers selected from: [25 (methacryloyloxy)ethyl]- trimethylammonium chloride (MADQUAT), [2-(acryloyloxy)ethyl]trimethylammonium chloride, (dimethylamino)ethyl méthacrylate DMAEMA and 2-aminoethyl méthacrylate hydrochloride.

16. Fluid according to claim 1, wherein said 10 oligomer group of the A block and/or said A' block comprises one or more degradable repeat units.

17. Fluid according to the preceding claim, wherein said degradable repeat unit is HEMA-poly(lactide).

18. Method for controlling the permeability of a 15 subterranean formation comprising:

a. providing a fluid comprising an aqueous solution of at least one thermo-responsive polymer with block structure A-B-A', wherein:

- A and A' , the same as or different from one 20 another, each represent a thermo-responsive oligomer group;

- B is an oligomer group comprising n repeat units, the same as or different from one another, wherein:

( i) at least one of the repeat units has a 25 solubility of the correspond!ng monomer, in water at 20 ° C, equal to or greater than 12 0 g/1, ( ii ) n is a whole number within the range 30 - 1000;

b. positioning said treatment fluid in a 30 subterranean formation.

19. Method according to the preceding claim, wherein said polymer A-B-A' is présent in said aqueous

-37 solution in a concentration within the range 0.5-40% by weight, more preferably within the range 1-20% by weight, with respect to the weight of the aqueous solution.

20. Method according to claim 18 or 19, wherein 5 said step b comprises injecting said fluid via at least one extraction well and/or at least one injection well.

21. Use of a fluid according to any one of claims 1-17 to modify the permeability of a subterranean formation.

10 22. Use of a fluid according to any one of claims

1-17 to limit the infiltration of water in a well for extracting a hydrocarbon oil or gas from a subterranean formation.