The present invention concerns a method for producing a viscosified water containing dissolved polymer for injecting in a well comprising :

- feeding solid polymer particles and water to a dynamic mixer having a rotor ;

- wetting and at least partially dissolving the solid polymer particles in water in the dynamic mixer to produce a mother solution ;

- agitating the mother solution in at least two successive tanks to obtain a matured mother solution; diluting the matured mother solution with water to obtain the viscosified water.

The method is carried out in particular to inject the viscosified water into a subterranean oil bearing reservoir.

To enhance and speed up the oil recovery from a reservoir, it is known to inject into the reservoir a polymer solution having a controlled viscosity, in order to decrease the mobility ratio between the displacing fluid (water) and the displaced fluid (oil) in the formation. The polymer solution injection allows an improved microscopic and macroscopic displacement of the oil present in the formation leading to enhanced oil recovery.

The preparation of the injected polymer solution is a two-step process. A polymer mother solution of a water-based viscosified polymer is generally prepared first from a polymer powder. Then, the mother solution is diluted with process water, prepared for example from treated sea water or from produced water. The resulting mixture is injected under pressure in the formation.

Injection in the wells of a production installation generally requires large volumes of viscosified water, e.g. in the order of several thousands of cubic meters per day for many years.

The polymers which are used to prepare the viscosified solution, e.g. polyacrylamides and/or derivatives of polyacrylamides, are very sensitive to mechanical degradation. Mechanical degradation may lead to a cleavage of some polymer chains and a consequent viscosity decrease. The viscosity decrease can be compensated by adding more polymer in the solution, with a significant increase in cost.

In subsea oil and gas fields, the polymer solution is prepared on surface facility close to the injection well in order to minimize riser lengths, for example on topsides of a FPSO or of a platform.

Dissolution of solid polymer particles in water to prepare the mother solution and further dilution of the mother solution with process water to obtain the viscosified water is a process that takes time. In particular, the solid polymer particles first have to be wet with water, which creates swelling and reptation. Then, a maturation takes place to disengage the polymer chains from the particles and allow their full diffusion in the solution. During all these steps, the viscosity increases until reaching a plateau. The time at which plateau is reached is called the maturation time.

This maturation time depends on physicochemical parameters related to the polymer, in particular polymer type, polymer composition, polymer concentration, molecular weight, polymer powder grain size. It also depends on physicochemical parameters related to the process including water salinity, water ionic composition, temperature, and on the stirring efficiency

In prior art processes such as in CA 2 602 039, the first step is carried out by mechanical agitation in a dispersing unit, using a rotor, which produces the mother solution. Then, the mother solution is matured in at least one tank under shear.

The average residence time t _res of a maturation process is equal to the volume of the tank V divided by the outlet flow rate Q.

In a batch process, the volume of the tanks must correspond to a minimum of twice the average residence time, since while solution is maturating in one tank, the other tank, in which maturation is complete, is emptied.

In order to decrease the volume of the maturation tanks, continuous maturation process is preferred. The process is fed at the same flow rate that the discharge flow rate. If the mixing is very efficient, the flow at the inlet is considered to be completely and instantaneously mixed with the bulk of the solution in the tank.

The residence time distribution of such an ideal continuous stirred tank is exponential. Therefore, having one single maturation tank would result in a wide distribution of residence time. In order to homogenize the mean residence time, several tanks have are put in series to tend towards the plug flow reactor which residence time distribution approaches a Dirac function.

Having a continuous process with several tanks of total volume V = Q x t is the best way to have a distribution of residence times close to the required maturation time with the minimum volume of tanks. However, because of the very large volume of viscosified solution injected on oil fields, the footprint of the tanks even with a continuous process can be very large, in some cases more than 200 m ³ . In addition to the footprint, it represents a big load.

The bulkiness of the equipment is nevertheless a strong disadvantage in the constrained space available on topsides of a surface facility. One aim of the invention is therefore to provide a method which allows a quick and reliable preparation of a viscosified water close to a well, with a minimal footprint and weight.

To this aim, the subject matter of the invention is a method of the above-mentioned type, characterized in that water fed to the dynamic mixer is a low salinity water having a salinity smaller than 3 g/l, the mother solution produced in the dynamic mixer having a temperature greater than 40 °C, and a polymer concentration greater than 15 g/L, in particular greater than 19 g/L.

The method according to the invention may comprise one or more of the following feature(s), taken solely or according to any technical combination:

- the at least two successive tanks comprise an upstream tank with an upstream stirring impeller and a downstream tank with a downstream stirring impeller, the average shear rate generated in the upstream tank with the upstream stirring impeller being advantageously greater than the average shear rate generated in the downstream tank with the downstream stirring impeller;

- the tanks have volumes smaller than 10 m ³ , the downstream tank having a volume greater than the volume of the upstream tank;

-the upstream stirring impeller is a ribbon impeller;

- the at least two successive tanks comprise an intermediate tank located between the upstream tank and the downstream tank, the intermediate tank having an intermediate impeller being preferentially a ribbon impeller;

- diluting the mother solution comprises introducing the diluted water in the downstream tank;

- diluting the mother solution comprises injecting the matured mother solution produced in the downstream tank in a flow of field water through a transverse perforated diffuser;

- the solid polymer particles have a number average particle size comprised between 300 micrometers and 500 micrometers, in particular comprised between 420 micrometers and 480 micrometers;

- water fed to the dynamic mixer is a low salinity water having a temperature greater than 40°C;

- the residence time of the mother solution in each tank is shorter than 10 minutes;

- the viscosified water has a polymer content smaller than 12 g/L, in particular comprised between 0.5 g/L and 11 g/L;

-the volume of each tank of the at least two successive tanks is smaller than 10m ³ , in particular smaller than 5 m ³ .

The invention also concerns an injection process in a well, comprising : - producing a viscosified water with the method as defined above ; injecting the viscosified water in a well.

In addition, the invention relates to an equipment for producing a viscosified water containing a dissolved polymer to be injected in a well, comprising : a mixing stage comprising a dynamic mixer having a rotor, the dynamic mixer having a solid polymer particles feeding inlet, a water feeding inlet and an outlet for recovering a mother solution resulting from wetting and at least partially dissolving the solid polymer particles in water in the dynamic mixer ;

- a maturing stage comprising at least two successive tanks to agitate the mother solution to produce a matured mother solution ;

- a diluting stage comprising a water pipe to dilute the matured mother solution with water to obtain the viscosified water ; characterized in that the water feeding inlet in the dynamic mixer is connected to a source of low salinity water having a salinity smaller than 3 g/l, the mixing stage being configured to produce a mother solution in the dynamic mixer at a temperature greater than 40 °C and at a polymer concentration geater than 15 g/L, in particular greater than 19 g/L.

The equipment according to the invention may comprise one or more of the following feature(s), taken solely or according to any technical combination:

- the diluting stage comprises a water pipe, a mature mother solution feed pipe connected to a downstream tank of the at least one tank and a perforated diffuser connected to the mother solution feed pipe, the perforated diffuser protruding in the water pipe to diffuse the matured mother solution in the water flowing in the water pipe.

The invention will be better understood, upon reading of the following description, taken solely as an example, and made in reference to the appended drawings, in which:

- figure 1 is a schematic view of an oil and gas production installation comprising an equipment for producing a viscosified water according to the invention ;

- figure 2 is a schematic view of the viscosified water production equipment ;

- figure 3 is a schematic view of a dynamic mixer used in the production equipment of figure 2 ;

- figure 4 is a view of an injection head of a mother solution in the equipment for producing the viscosified water ;

- figure 5 is a view similar to figure 2 of a variant of equipment for producing the viscosified water; - figure 6 is a curve illustrating the effect of temperature of the mother solution at the outlet of the dynamic mixer on the maturation of the mother solution;

- figure 7 is a curve illustrating the effect of salinity of the water fed at the inlet of the dynamic mixer on the maturation of the mother solution.

A first oil and gas production installation 10 according to the invention is schematically shown in figure 1.

The installation 10 is intended for recovering a fluid, in particular a hydrocarbonaceous fluid such as oil and/or natural gas for example, in an underwater subterranean formation 12 located in under a body of water 14. The installation 10 is also intended for conveying the recovered oil and gas towards to surface of the body of water 14.

The body of water 14 is for example a sea, an ocean, a lake and/or a river. The depth of the body of water is generally greater than 10 m and is notably comprised between 100 m and 5000 m.

The installation 10 comprises a bottom recovery facility 16, a surface facility 18 and a fluid transport assembly 20 connecting the bottom recovery facility 16 with the surface facility 18. According to the invention, the installation 10 further comprises a viscosified water producing equipment 22, which is here borne by the surface facility 18.

The bottom recovery facility 16 comprises several producing wells 24 bored in the subterranean formation 12, located at the bottom of the body of water to reach a first region of an oil-bearing reservoir (not shown).

The bottom recovery facility 16 comprises, for each well 24, a wellhead 26 selectively closing the well 24. It advantageously comprises a manifold 28 connected to one or several wells 24 to recover the oil and gas collected in each well 24.

The bottom recovery facility 16 further comprises at least an injection well 30 bored in the subterranean formation 12 from the bottom of the body of water 14 to reach a second region of the oil-bearing reservoir fluidly connected to the first region.

The injection well 30 is closed by a wellhead 32. The wellhead 32 is designed for the injection in the injection well 30 of a viscosified water prepared in the production equipment 22.

The transporting assembly 20 comprises at least a production line 34 connecting a manifold 28 to the surface facility 18 and at least a viscosified water injection line 36 connecting the viscosified water production equipment 22 to the injection well 30.

The viscosified water preferentially has a viscosity ranging from 1 to 200 times the viscosity of water, in particular from 2 to 50 times the viscosity of water, notably from 2 to 25 times the viscosity of water. On the field, viscosity is measured at a fixed temperature in the range between ambient temperature and 25 °C using a viscometer witi a Couette geometry. For example, the viscosity is measured at different shear rates in the range 7 s ^-1 - 122 s ^-1 .

The viscosified water contains at least one viscosifying dissolved polymer, preferentially a water soluble polymer. The viscosifying polymer is for example an acrylamide based polymer. In particular, the polymer is a hydrolyzed polyacrylamide (HPAM) or acrylamide tertiobutyl sulfonate polymer or copolymer of acrylamide and acrylamide tertiobutyl sulfonate or a terpolymer of acrylamide, acrylate and acrylamide tertiobutyl sulfonate. In a variant, the polymer comprises a gum, such as xanthan gum, or a polymer designed so as to withstand high temperatures (for example up to 120°C), and high salinity (up to 300 g/l).

The polymer mass content is adjusted to obtain the required viscosity. Advantageously, the mass content of polymer into the viscosified water is comprised between 100 ppm and 5000 ppm, in particular between 200 ppm and 4000 ppm, notably between 500 ppm and 3000 ppm.

The molecular weight of the polymer is preferably greater than 2x10 ⁶ g/mol and is advantageously comprised between 2x10 ⁶ g/mol and 30x10 ⁶ g/mol.

Advantageously, the viscosified water has a controlled content in dissolved ions, in particular a controlled content in divalent ions.

The total mass content in dissolved ions in the viscosified water is generally less than 15% and is for example comprised between 0.02% and 30%.

The divalent ions are for example cationic, e.g. alkaline earth metals ions such as calcium or magnesium or/and strontium ions. In a variation, the divalent ions are anionic, for example sulfates and chlorides.

In a variation, the viscosified water further comprises one or several surfactants, such as anionic, cationic and/or nonionic surfactants. It may comprise one or more alkalis, such as a carbonate, or ammonia.

The viscosified water production equipment 22 is designed to produce the viscosified water from solid polymer particles, in the form of polymer powder.

The solid polymer particles are supplied as a powder. Preferentially, the solid polymer particles have a number average particle size smaller than 1000 micrometer, and preferentially comprised between 200 and 1000 micrometers, in particular comprised between 200 micrometers and 800 micrometers.

As will be seen below, this range of particle size allows a quick wetting of the polymer particles when mixed with water. The number average particle size is measured for example by sieve analysis e.g. the powder is allowed to pass through a series of sieves of progressively smaller mesh size and weighing the amount of powder that is stopped at each sieve as a fraction of the whole mass.

In reference to figure 2, the viscosified water production equipment 22 comprises a mixing stage 40, for mixing the solid polymer particles 42 with a low salinity heated water 44 to form a mother solution 46.

The equipment 22 further comprises a maturation stage 48 for maturing the mother solution 46 and allowing the full dissolution of the polymer and a diluting stage 50 for diluting the matured mother solution to a concentration appropriate to form the viscosified water injected in the well 30.

The mixing stage 40 comprises a solid polymer storage 52 containing the solid polymer particles 42, a source 54 of low salinity water 55 and a dynamic mixer 56 connected to the polymer storage 52 and to the source 54 of low salinity water 55.

The mixing stage 40 further comprises a heater 59 for heating the low salinity water 55 to form the heated low salinity water 44 and advantageously, a metering unit 58 for dosing the solid polymer particles 42 conveyed to the dynamic mixer 56.

An example of dynamic mixer 56 is depicted schematically in figure 3. The dynamic mixer shown in figure 3 is for example supplied by YTRON, IKA or SILVERSON. Another type of dynamic mixer 56 is a polymer slicing unit (PSU) supplied by SNF which enables to wet powder particles and to grind them so as to reduce their particle size and to speed up the subsequent maturation.

The dynamic mixer 56 comprises a mixing chamber 60 containing a rotor 64 and a stator 66, a hopper 68 for conveying the solid polymer particles 42 to the mixing chamber 60 through a polymer particle inlet and an injection pipe 70 for conveying the heated low salinity water 44 to the hopper 68 or/and the mixing chamber 60 through at least a water inlet.

The dynamic mixer 56 further comprises a mother solution outlet pipe 72 connected to an outlet of the mixing chamber 60 to feed the maturing stage 48 with the mother solution 46 formed in the dynamic mixer 56.

The salinity of the low salinity water 55 fed in the dynamic mixer 56 is smaller than 3 g/L, preferentially is smaller than 2g/L and is for example comprised between 0.2 g/L and 1 g/L.

A low salinity water greatly accelerates the wetting and particularly, the subsequent dissolution of the polymer particles. An improved dissolution corresponds to a decrease of the maturation time. The salinity of the low salinity water 55 is measured at a temperature of 25°C by conductivity measurement.

The heater 59 is configured to heat the low salinity water 55 to a temperature greater than 40 °C, in particular comprised between 40 °C and 80 °C, notably between 65 °C and 75 °C. Such a water temperature also accelerates dissolution of the polymer particles 42.

The mixing chamber 60 for example has a cylindrical shape.

The rotor 64 comprises knives. It is mounted in the mixing chamber 60 to rotate along a preferentially vertical axis.

The speed of rotation of the rotor is generally between 1500 rpm and 6000 rpm, preferably between 2000 rpm and 5000 rpm. It is on average about 3000 rpm for a cutting diameter of 200 mm. It is between 3000 and 6000 rpm for a cutting diameter of 100 mm and between 1500 and 3000 rpm for a cutting diameter of 400 mm.

The stator 66 is located around the rotor 64. It comprises static blades in register with the blades of the rotor 64.

The hopper 68 opens in the mixing chamber 60 above the rotor 64. In the example of figure 3, the hopper 68 opens vertically above and advantageously coaxially with the rotor 64.

The water injection pipe 70 has a first water outlet located above the rotor 64. It advantageously opens transversally with regards to the axis of rotation of the rotor 64. Advantageously, the first water outlet of the water injection pipe 70 opens in one of the mixing chamber 60 and of the hopper 68. A second water outlet may then be provided in the other one of the mixing chamber 60 and of the hopper 68

The mother solution outlet pipe 72 opens in the mixing chamber 60 below the rotor 64 or in register with the rotor 64 and the stator 66. It advantageously opens transversally to the axis of rotation of the rotor 64.

In the particular embodiment of figure 3, the water injection pipe 70 opens in the hopper 68, preferentially at the bottom of the hopper 68. The mother solution outlet pipe 72 opens in the mixing chamber 60 in register with the rotor 64 and the stator 66 at the periphery of the stator 66.

The concentrated mother solution 46 obtained from the dynamic mixer 56 at the mother solution outlet pipe 72 has a polymer concentration greater than 10 g/L and in particular greater than 15 g/L. The polymer concentration in the concentrated mother solution is for example comprised between 15 g/L and 35 g/L. The concentrated mother solution 46 obtained from the dynamic mixer 56 at the mother solution outlet pipe 72 has a temperature greater than 40°C, in particular comprised between 40 °C and 80 °C, notably between 65 C and 75 °C.

The maturation stage 48 comprises an upstream tank 80 and a downstream tank 82. In the example of figure 2, it further comprises at least an intermediary tank 84, located between the upstream tank 80 and the downstream tank 82.

The upstream tank 80 is equipped with a rotating upstream impeller 86, the downstream tank 82 is equipped with a rotating downstream impeller 88 and the intermediate tank 84, when present, is equipped with a rotating intermediary impeller 90.

Thanks to the low salinity and the high temperature, the residence time in each tank is smaller than 5 minutes, particularly smaller than 4 minutes. The volume of each tank is smaller than 10 m ³ , in particular smaller than 4m ³ . This volume is for example comprised between 2 m ³ and 4 m ³ . The volumes of the tanks are defined in consideration of the maturation time and of the prepared viscosified water flow rate.

The volume of the downstream tank 82 is preferentially larger than the volume of the upstream tank 80 and also larger than the volume of the intermediate tank 84.

The upstream impeller 86 available in the upstream tank 80 is for example a ribbon impeller. It is configured and operated to develop an average shear rate which is greater than the average shear rate developed by the downstream impeller 88 in the downstream tank 82.

A cylindrical tank stirred by a ribbon or double ribbon impeller is the most efficient way to maximize the mixing of a viscous, non-Newtonian and elastic solution. The combination of such impeller with a cylindrical tank eliminates dead zones which, in traditional processes, contribute to enlarge the residence time distribution and/or decrease the average residence time because they do not contribute to the mixing of the solution while having non negligible footprint and weight.

Moreover, the temperature in the upstream tank 80 and in the intermediate tank 84 is controlled to be greater than the temperature in the downstream tank 82.

The temperature in the upstream tank 80 and in the intermediate tank 84 is for example comprised between 40 °C and 80 °C and the tenperature in the downstream tank 82 is for example comprised between 25 °C and 40 °C.

The temperature in the downstream tank 82 is generally adjusted as a function of the dilution ratio between the tank 82 and the field water and as a function of the field water temperature. The tanks 80, 84, 82 are mounted in series, such that the overflow of the upstream tank 80 flows downstream in the intermediate tank 84 and the overflow of the intermediate tank 84 flows downstream in the downstream tank 82.

In the example of figure 2, the dilution stage 50 comprises a matured mother solution feed pipe 100, a source of field water 102, and a field water pipe 104 connected to the source of field water 102. The dilution stage 50 further comprises a static mixing zone 106 connected to the matured mother solution feed pipe 100 and to the field water pipe 104 to mix the matured mother solution with the field water to the appropriate dilution.

The diluting stage 50 further comprises a viscosified water pipe 108 connected to an outlet of the mixer 106.

The mother solution feed pipe 100 is connected upstream to the downstream tank 82 to receive the overflow of concentrated matured mother solution produced in the maturing stage 48.

The field water source 102 contains field water at a salinity greater than 0.2 g/L and generally comprised between 0.2 g/L and 300 g/L. In the example of figure 4, the mixer 106 comprises a transverse diffuser 120 for diffusing the matured mother solution arising from the mother solution feed pipe 100 into the field water pipe 104.

The transverse diffuser 120 is formed of a perforated tubular wall 122, which is closed at its free end.

The perforated tubular wall 122 transversally protrudes in the field water pipe 104. It comprises a plurality of through openings 124 spread along the tubular wall 122 to allow a spread diffusion of the concentrated matured mother solution in the field water located in the field water pipe 104.

A method for producing a viscosified water for injecting in an injection well 30, carried out with the equipment 22 according to the invention, will now be described.

Initially, a powder comprising solid polymer particles 42 of a viscosifying polymer as described above is supplied to the solid polymer storage 52.

Preferentially, the solid polymer particles have a number average particle size smaller than 1000 micrometer, and preferentially comprised between 200 and 1000 micrometers, in particular comprised between 200 micrometers and 800 micrometers.

Similarly, low salinity water 55 is provided to the source 54 of low salinity water either by producing the low salinity water 55 on site or by supplying it from an external source.

The salinity of the low salinity water 55 fed in the dynamic mixer 56 is smaller than 3 g/L, preferentially is smaller than 2 g/L and is for example comprised between 0.2 g/L and 1 g/L. The solid polymer particles are then fed to the dynamic mixer 56 through the metering unit 58.

At the same time, low salinity water 55 is heated in the heater 59 to reach a temperature greater than 40 °C, in particular comprsed between 40 °C and 80 °C, notably between 65 °C and 75 °C.

The heated low salinity water 44 is also fed to the dynamic mixer 56 and is mixed with the solid polymer particles 42 in the dynamic mixer 56 to wet and start dissolving the solid polymer particles 42.

In the example shown in figure 3, the solid polymer particles 42 and the heated low salinity water 44 are fed upstream of the rotor 64 and stator 66 in the mixing chamber 60.The rotor 64 is rotated.

The wetted and partially dissolved solid polymer particles then form a concentrated mother solution 46 which is recovered at the outlet pipe 72.

The polymer concentration of the concentrated mother solution 46 is greater than 10 g/L, in particular greater than 15 g/L and is for example equal to 25 g/L. The temperature of the concentrated mother solution 46 is greater than 40 °C, in particular comprised between 40 °C and 80 °C, notably between 65 C and 75 °C.

The concentrated mother solution 46 then flows in succession in the upstream tank 80, in the or each intermediate tank 84, when present, and then, in the downstream tank 82.

In the upstream tank 80 and in the or each intermediate tank 84, the respective impellers 86, 88 provide a strong agitation to generate an average shear rate greater than the average shear rate in the downstream tank 82.

The temperature of the mother solution in the upstream tank 80 and in the or each intermediate tank 84 is maintained above 40 °C, inparticular between 60 °C and 75°C.

Then, in the downstream tank 82, the average shear rate is reduced as the viscosity of the mother solution increases.

The viscosity of the mother solution in the upstream tank 80 is for example smaller than 100000 mPa.s and is notably comprised between 15000 mPa.s and 100000 mPa.s. The viscosity in the downstream tank 82 is for example greater than 1000 mPa.s and notably comprised between 1000 mPa.s and 10000 mPa.s. The viscosity is for example measured at ambient temperature at a shear rate of 7.3 s ^_1 .

Thanks to the low salinity and high temperature of the water used for wetting the solid polymer particles in the dynamic mixer 56, the dissolution of the solid polymer particles is surprisingly very fast, which helps creating a homogenous concentrated mother solution 46. The maturation of the concentrated mother solution 46 is much faster than in state of the art methods. The residence time in each of the upstream tank 80 and of the intermediate tank 84 when present is smaller than 5 minutes, and is for example comprised between 3 and 4 minutes.

Then, the temperature of the mother solution is decreased in the downstream tank 82. The residence time in the downstream tank 82 is smaller than 5 minutes and is comprised between 1 minute and 3 minutes. This tank enables to dilute the concentrated mother solution prepared in tank 80 and 84.

Then, the matured mother solution is fed to the mother solution feed pipe 100 and to the transverse diffuser 120.

It is then mixed with field water in the field water pipe 104 to allow dilution to a polymer concentration of less than 15 g/L, in particular less than 11 g/L, for example comprised between 2g/L and 0.5g/L and notably equal to 1 g/L to produce the viscosified water in the pipe 108.

The viscosified water is then fed in the injection line 36 and injected in the injection well 30. Then, the viscosified water is supplied to the downhole reservoirs to enhance oil and gas production through the production wells 24, the manifold 28, and the transportation line 34.

In a variant shown in figure 5, the diluting stage 50 is located directly in the downstream tank 82 which forms a mixing zone.

The field water pipe 104 emerges in the downstream tank 84 to allow the dilution to take place in the downstream tank 82.

The viscosified water is produced at the outlet of the downstream tank 82.

As an illustration, in a state of the art method, not using the present invention, a viscosified water solution having a polymer concentration of Cinj = 1000 ppm with a flow rate of Qinj = 1125 m ³ /h is prepared.

This method requires a mother solution with a concentration of CMS = 10000 ppm and a residence time in the tanks of t res = 1 hour. The flow rate of mother solution is QMS = Qinj x Cinj / CMS = 1125 x 1000 / 10000 = 112.5 m ³ /h. The required volume for the tanks is therefore Vtot = QMS x t res = 112.5 x 1 = 112.5 m ³ . If this volume is split among four tanks, the volume per tank is 112.5/4 = 28.1 m ³ , with a liquid weight of 112.5 tons.

As illustrated in figure 6 and 7, thanks to the method according to the invention, an increase of the temperature of the concentrated mother solution above 40 °C, in particular comprised between 40 °C and 80 °C, notably between 65 C and 75 °C, combined with a decrease of the salinity of the low salinity water 55 fed in the dynamic mixer 56 below 3 g/L, preferentially below 2 g/L and is for example comprised between 0.2 g/L and 1 g/L, leads to a significant unexpected decrease of the polymer maturation time, shown by a faster increase of viscosity.

Hence, with a method according to the invention, residence times of t res(tank 80) = tres (tank 84 ) = 3.5 minutes are obtained from a mother solution of concentration CMS = 20000 ppm in these tanks 80, 84. A residence time t res(tank 82) = 2 minutes and a dilution to CMS = 10000 ppm are obtained in the last tank 82.

Thus, the flow rates in tanks 80, 84 are :

QMS (tank 80) = QMS(tank 84) = Qinj x Cinj / CMS = 1125 x 1000 / 20000 = 56.25 m ³ /h.

The volumes of the tanks 80, 84 are therefore :

V(tank 80) = V(tank 84) = QMS (tank 80) x t res(tank 80) = QMS (tank 84) x t res (tank 84)= 56,25 x 3.5 / 60 = 3.3 m ³ .

The field water flow rate is Q H20 (tank 82) = QMS (tank 84) x CMS (tank 84) / CMS (tank 82) - QMS (tank 84) = 56.25 x 20000 / 10000 - 56.25 = 56.25 m ³ /h

The tank volume for tank 82 is V(tank 82) = (QMS (tank 82) + QH20 (tank 82)) x t res(tank 82) = 112.25 x 2/60 = 3.75 m ³ .

The method according to the invention uses and optimizes the physical and chemical parameters such as temperature, salinity and overconcentration of the concentrated mother solution to significantly reduce the polymer maturation time. Very fast maturation times can be obtained, advantageously of less than 30 minutes, in particular ranging between 7 minutes and 30 minutes.

The fast maturation of the concentrated mother solution allows a great volume and weight reduction of the tanks 80, 84, 82 that are used in the maturation stage 48. Each tank 80, 84, 82 can be dimensioned to have a smaller volume, for example less than 5 m ³ for each tank 80, 84, 82, with an overall volume of less than 15 m ³ . This greatly reduces the weight and footprint on the surface facility 18, for example to less than 30 m ³ , in particular to around 20 m ³ with no dead volumes in the tanks 80 to 84, while keeping a very stable and efficient process.

Thanks to the high concentration of the mother solution and to the adapted stirring in the successive tanks 80 to 84, polymer degradation is minimized, which lowers the overall exploitation cost.

In variant, the recovered fluid is in an onshore formation 12. The installation 10 is also onshore, including the recovery facility 16 and the well(s) 24.