FR2933730A1 - Restoration of degraded permeability of porous rock, comprises passing the rock by circulating aqueous solution containing associative polymers; and treating the rock by injecting the solution of surfactant in a determined concentration - Google Patents

Abstract

The present invention relates to a method for restoring the degraded permeability of a porous rock, in which the following steps are carried out: the rock is traversed by the circulation of at least one aqueous solution containing associative polymers; rock by injecting an aqueous solution of at least one surfactant in a concentration sufficient to remove the hydrophobic bonds between chains of associative polymers.

Description

The invention relates to the field of use of water-soluble polymers said to be hydrophobically associative in order to improve the recovery rate of hydrocarbons contained in the porous rocks of the subsoil. In particular, the present invention provides a method for restoring the permeability of porous and permeable rocks which have been traversed by at least one associative polymer solution.

Polymeric systems in aqueous solution are used in various applications, particularly related to the recovery of hydrocarbons. The field of the present invention is constituted by all the applications that involve the injection of polymeric systems in aqueous solution into wells. Among these applications, the invention relates more particularly to the enhanced hydrocarbon recovery (RAH) by polymer injection, as well as to well treatment techniques designed to prevent water inflow (PVE) or to obtain control. injection or production profiles. At the beginning of the exploitation of a deposit, the natural pressure of the hydrocarbons is often sufficient to ensure their displacement towards the wells. However, this pressure decreases during production and, when it has become too low, so-called secondary recovery techniques must be used to provide the energy necessary for the displacement of the hydrocarbons. The basic technique for secondary recovery consists in injecting an aqueous phase into the underground reservoir by so-called injection wells, located at a distance from producing wells. The injected aqueous phase first makes it possible to maintain the pressure in the reservoir, and then, when it comes into contact with the hydrocarbons, to move them to the production wells.

The quantity of hydrocarbons that can be recovered in the context of secondary recovery is notably limited by the sweeping efficiency. The sweeping efficiency is related to the vertical permeability contrasts of the reservoir as well as the difference in viscosity between the aqueous phase and the oil phase. The existence of permeability contrasts leads to poorer water propagation in layers of lower permeability. The difference in viscosity is responsible for horizontal effects of viscous digitation which cause a more or less rapid breakthrough of the aqueous phase injected into producing wells. A useful parameter for estimating the sweeping efficiency is the ratio R between the mobility of the aqueous phase and the mobility of the oil phase: R = Maq = Kaq> 7 or Moii K oii 1l ag where the mobilities Maq and Moi 'are Defined for the aqueous phase and for the oil phase respectively, as the ratio between permeability (Kaq or Koif) and viscosity (rlaq or blur). To improve the scanning efficiency, chemical RAH techniques have been proposed. Some of them consist in particular in increasing the viscosity of the injected aqueous phase by adding water-soluble polymers. The increase in viscosity results in a reduction in the mobility of this aqueous phase and, consequently, in a reduction of the mobility ratio. This therefore makes it possible at the same time to improve the vertical scanning profiles and to reduce the effects of viscous digitation. In this context, polymers are called "mobility control agents".

During the exploitation of a deposit, and more particularly at the secondary recovery stage, so-called well-treatment operations may be carried out. Some of these operations use polymeric systems in aqueous solution, with two types of objectives: to carry out PVE treatments and / or to carry out profile control treatments. 30 The goal of the POI is to reduce the ratio between the amount of water and the amount of oil produced by producing wells. Water is always produced, in greater or lesser quantities, with hydrocarbons. This water comes either from layers of porous rocks saturated with water that communicate with the well, or layers of porous rocks initially saturated with hydrocarbons through which the water injected as part of the secondary recovery pierced prematurely. The aqueous polymer systems used in PVE are generally polymer gels, and often physical gels, which are injected by the production well during a time-limited treatment operation. The aim is to obtain a significant reduction in the water permeability of the layer or layers to be treated due to the adsorption and / or gelling of the injected systems. On the other hand, the treatment must not affect the permeability to the oil. Polymer gels, deformable and soluble only in water, are systems generally adapted to meet this condition. They are then called "relative permeability modifiers" (MPR).

Profile control treatments may concern producing wells or injection wells. They are also generally based on the injection of aqueous polymer gels in order to reduce the permeability, by the same mechanisms as PVE, of the most permeable layers. The aim is to make the vertical permeability profile of the treated well more homogeneous before the injection of another compound. This other compound may be, for example, an acidic aqueous phase in the case of an acidification treatment aimed at increasing the productivity of a production well or a mobility control agent in the case of a profile control treatment. an injector well prior to a RAH operation. Products injected in the case of profile control operation are often referred to as "diversion agents".

The use of hydrophobically associative polymers (PHAs) for the two types of applications exemplified above has already been proposed. PHAs are polymers whose backbone is hydrophilic but which comprise, along the chains, small amounts (a few mole percent) of hydrophobic monomers capable of associating in water in the form of hydrophobic nano-domains, also called " hydrophobic bonds ". These act as temporary crosslinking points and give the PHAs very interesting properties, especially with respect to their rheology and adsorption on surfaces.

The viscosities of concentrated-phase PHA solutions are thus significantly higher at low shear gradients than those of non-associative polymer solutions equivalent at the same concentration. This major property makes it possible to envisage, for the RAH application, mobility ratios that are much more favorable than with conventional polymers at the same concentrations. In addition, since the cross-linking points between the PHA chains involve low energies, they can be reversibly broken by shearing. The PHA solutions thus have a rheo-fluidizing character markedly more marked than equivalent non-associative polymer solutions, which favors their injectivity in the wells. Finally, in general, significant low gradient viscosities are obtained with PHA chains much shorter than conventional polymer chains usually used in tertiary recovery. PHA solutions therefore intrinsically have better resistance to mechanical degradation than conventional polymer solutions. The use of PHAs as mobility control agents has been proposed since the early 1980s (US-4529523, US-4432881, WO-85/03510, US-4702319, US-4709759, US-4638865, US-4780517). , US-4852652, US-4861499), and several experimental studies in porous media have been performed (Taylor, KC, Nasr-el-Din, HA: "Water-soluble hydrophobically associating polymers for improved oil recovery." Journal of Physical Chemistry B, 19, 265-280 (1998)).

As regards adsorption properties on surfaces, the PHAs exhibit a behavior very different from that conventionally observed with water-soluble polymers: the adsorption isotherms of the PHAs do not tend towards a plateau when the concentration of polymers increases (Volpert, E. , Selb, J., Candau, F., Green, N., Argillier, JF, Bai, B., Audibert, A .: "Adsorption of Hydrophobically Associating Polyacrylamides on Clay." Langmuir, 14, 1870-1879 (1998) .). This particular adsorption results from the formation of hydrophobic bonds between the successive layers of adsorbed chains. Thus, a first layer of adsorbed PHA is formed in the same way as for non-associative polymers, that is to say in particular via Van der Waals type bonds with the surface. Subsequently, additional PHA chains bind to those of the first layer without being in contact with the surface, via hydrophobic bonds with the hydrophobic groups of already adsorbed chains. Multilayers can thus be formed, which explains the particular adsorption of the PHAs. This mechanism is relevant both for static adsorption on surfaces (powders, etc.) and for adsorption on the pore surface or pore thresholds in porous media. This important adsorption is of interest for well treatment type applications because it makes it possible to obtain greater reductions in permeability than those caused by the adsorption of non-associative polymers. PHAs also have an advantage over physical gels: the use of these requires indeed to control the kinetics of gelling, to ensure good injectivity and good propagation in depth. PHA solutions, on the other hand, are kinetically stable. The use of PHA for well treatment operations has been the subject of detailed work, and has industrial applications (Eoff et al., Dalrymple et al., Reddy et al., Al Taq et al.). The control of the adsorption and implicitly of the injectivity remain however sensitive points for a good efficiency of the process in PVE or in control of profile.

The present invention is in the problem of the adsorption of PHA on surfaces, which is a parameter that determines both the success of the RAH operations and the well treatment operations. In RAH, indeed, a strong adsorption is not desirable. It can, in the first place, severely penalize injectivity and the deep propagation of PHAs. In addition, it is unfavorable from an economic point of view since the adsorbed material no longer contributes to the viscosity of the aqueous phase, which increases the mobility ratio. In well treatment, strong adsorption of PHA is necessary to ensure significant reductions in water permeability, but adsorption control is essential to ensure good injectivity and good deep propagation of PHAs.

The present invention relates to cases where the injection of PHA into a well, whether for an ARD or well-treatment operation or any other operation, affects the injectivity or productivity of the well due to greater adsorption. higher than expected PHA. One of the objects of the invention is to provide a method of restoring damage to the rock formation in the vicinity of the well, which makes it possible to recover a level of permeability close to that of origin, that is to say before PHA injection.

Thus, the present invention relates to a method for restoring the degraded permeability of a porous rock, in which the following steps are carried out: - crossing said rock by the circulation of at least one aqueous solution containing associative polymers, - treats said rock by injecting an aqueous solution of at least one surfactant at a determined concentration to remove the hydrophobic bonds between chains of associative polymers. The concentration of surfactants can be determined as a function of the critical micellar concentration of the surfactant.

The concentration of surfactants may be close to the critical micellar concentration of the surfactant. The concentration of surfactants can be between 2 and 5 times the critical micellar concentration of the surfactant. The method according to the invention can be implemented following a treatment of water inflow in a reservoir by means of associative polymers, a treatment of permeability profiles by means of associative polymers, or a hydrocarbon scanning operation contained in a rock, using associative polymers. Depending on the method, the permeability of porous rocks around a well can be restored.

The interactions between the hydrophobic parts of the surfactants and the hydrophobic groups of the PHAs are responsible for significant modifications of the rheological properties of the PHA solutions. Depending on the concentration of surfactants in the PHA solution, two regimes can be distinguished: at low concentration, the surfactant molecules tend to associate with the hydrophobic groups of the chains and to reinforce the hydrophobic bonds. This results in an increase in the viscosity of the PHA solution. when the concentration of surfactants reaches and exceeds a certain value, for example of the order of magnitude of the critical micellar concentration of the surfactant measured in the absence of PHA, a maximum and then a decrease in the viscosity are observed. For even higher surfactant concentrations, the viscosity reaches a plateau value, which is lower than the initial viscosity of the PHA solution without surfactants. This behavior is interpreted by the formation of mixed micelles between hydrophobic groups of the chains and molecules of surfactants. As the concentration of surfactants increases, the surfactant molecules become more and more dominant in the micelles, until there is only one hydrophobic group per micelle. The hydrophobic bonds between polymer chains, whether they are intra or inter-chains, therefore disappear, and the viscosity of the solution approaches that of a solution of non-associative polymers, with a skeleton equivalent to that of the PHAs.

The use of PHA and surfactant mixtures has been proposed for RAH applications (Evani US Pat. mobility reports. It is clear that for a PHA / surfactant mixture to be more efficient in terms of mobility control than a PHA solution alone, the choice of optimal surfactant concentration is essential.

According to the invention, which consists in injecting a solution of surfactants into the well previously treated with PHA. The concentration of the surfactant solution must be sufficient to remove the hydrophobic bonds between chains of associative polymers. For this, laboratory tests can be performed to determine the optimal concentration of surfactant. It may, for example, be several times greater than the critical micelle concentration. In this way, in accordance with the mechanism of the interactions between PHA and surfactants, the hydrophobic bonds between hydrophobic groups of the adsorbed PHA chains disappear in favor of the integration of the hydrophobic groups in mixed micelles. As a result, throughout the formation in the vicinity of the well contacted with the concentrated surfactant solution, desorption of the PHA chains not directly adsorbed on the surface of the pores. Once the surfactant treatment is complete, the residual damage to the formation is that corresponding to a monolayer of adsorbed polymers, which results in a much lower permeability reduction than that obtained without injection of surfactants. However, the invention does not exclude that the concentration useful surfactant is less than its critical micelle concentration, or neighboring.

The reservoir conditions in which the proposed method can be applied are any. They include in particular temperatures up to 300 ° C and pressures up to 1000 bar. Rock facies are also unspecified. They include rocky type rocks and carbonate type rocks. The invention involves broad-ranging PHAs, which can be a wide variety of hydrophilic monomers and hydrophobic monomers, and which can be prepared by a variety of methods. Among the synthetic methods, mention may be made, in a non-exhaustive manner: - post-modification, which consists of grafting hydrophobic groups onto pre-existing hydrophilic polymer chains (Feng, Y., Billon, L., Grassl, B., Khoukh , A., François, J .: "Hydrophobically associating polyacrylamides and their partially hydrolyzed derivatives prepared by post-modification 1. Synthesis and characterization", Polymer, 43-7, 2055-2064 (2002)), - micellar copolymerization which consists in simultaneously polymerizing in aqueous solution the hydrophilic monomers and the hydrophobic monomers, the latter being solubilized in micelles (Bastiat, G., Grassl, B., François, J.: "Micellar copolymerization of associative polymers: Study of the effect of acrylamide on sodium dodecyl sulfate-poly (propylene oxide) methacrylate mixed micelles "Journal of Colloid and Interface Science, 289, 359-370 (2005)).

The molar mass of the PHA can range from about 20 × 10 3 to about 20 × 10 6 g / mol. The chains consist of one or more types of hydrophilic monomers and one or more types of hydrophobic monomers. The total molar fractions in hydrophobic monomers may vary between about 0.05% and about 10%, preferably between 0.1% and 3%. The hydrophilic monomers which can be used are listed in the following non-exhaustive list: acrylamide, methacrylamide, 2-acrylamido-2-methylpropanesulphonic acid (AMPS), acrylic acid, N, N-dimethylacrylamide dimethylaminoethyl methacrylate (DMAEMA), dimethylaminopropyl methacrylamide, hydroxyethyl acrylate (ethylene glycol monoacrylate), vinyl pyrrolidone and trimethylammonium methyl methacrylate chloride. In the case of post-modification synthesis, the pre-existing hydrophilic chains may be of different chemical natures, among which all of the polymers obtained by polymerizing one or more of the types of monomers which have just been mentioned, as well as non-limiting: polyethylene oxide (or polyethylene glycol), cellulose ethers (such as hydroxyethylcellulose), natural polysaccharides (such as guar gums), and polyvinyl alcohol. Examples of hydrophobic monomers that may be used include, but are not limited to: alkyl acrylates, alkyl methacrylates, alkyl acrylamides, alkyl methacrylamides, alkyl dimethyl ammonium methyl methacrylate bromides alkyl dimethylammoniumethyl methacrylate chlorides, alkyl dimethylammoniumethyl methacrylate iodides, alkyl dimethylammoniumpropyl methacrylamide bromides, alkyl dimethylammoniumpropyl methacrylamide chloride and alkyl dimethylammoniumpropyl methacrylamide iodides. The alkyl chains mentioned below may or may not be branched and may comprise between about 4 and 25 carbon atoms.

The invention is not limited to surfactants of a particular chemical nature. These are systems composed of one or more types of molecules with surfactant properties in the broad sense. They can be ionic or nonionic. Examples of families of surfactant molecules are, without limitation: polyethoxylated fatty alcohols, alkali metal salts of alkyl ethoxy sulphate and trimethyl ammonium bromides.

The example presented below demonstrates the positive effect on the permeability of the injection of surfactants in a porous medium in which PHAs were previously injected and strongly adsorbed.

The experiment reported in this example was performed with PHA prepared by micellar copolymerization. The PHAs are polyacrylamides comprising 20% by mole fraction of AMPS groups and 1% by mole fraction of 12-carbon alkyl acrylamide. Their molar mass is close to Mw = 7.105 g / mol with a polydispersity index Ip = 2.

The surfactant used is a nonionic surfactant composed of a polyethoxylated fatty alcohol mixture. This surfactant is marketed by Clariant under the name "Genapol 0 120".

All solutions were prepared in a brine composed of Milli-Q grade ultrapure water with 1 g / L NaCl and 0.4 g / L NaN3, used as a bactericide. The experiment was carried out on a model porous medium consisting of sharp grains of silicon carbide (SiC). The adsorption of water-soluble polymers on SiC has been widely studied in the past (Zitha PLJ, Chauveteau, G., Léger, L. "Unsteady-State Flow of Flexible Polymers in Porous Media", J. of Colloid and Interf. Sci., 234, 269-283 (2001).). The permeability, porosity and average hydrodynamic diameter of the pores of the porous medium used were 1100 + 100 mD, 40 + 1% and d = 11 m, respectively. The sample cell was equipped with intermediate pressure taps allowing the measurement of the losses of loads (denoted 4P) on different sections, in particular, internal of the porous medium, which made it possible to study at the same time the effects of clogging of the face of entry and the deep propagation of the injected complex fluids. The porous medium used was 10 cm long and 1.5 cm in diameter. The intermediate pressure taps made it possible to measure dP between 0 - 2 cm, 2 - 5 cm and 5 - 10 cm, of the length of the porous medium, in the direction of flow. The experiment was carried out at a temperature T = 30 ° C.

The PHA solution prepared for injection into a porous medium had a concentration of Cpo1 400 mg / L, for a relative viscosity λ1. The choice of a dilute solution is justified by the fact that the experiment was intended solely for to study the adsorption of polymers. The surfactant solution prepared for injection into a porous medium had a CTA concentration of 100 mg / L, a concentration well above the critical micelle concentration, which is of the order of 25 mg / L. Prior to the PHA and surfactant injections, the porous medium was saturated with brine to 1 g / L NaCl and 0.4 g / L NaN3, then its permeability was measured at different flow rates, using the same brine. The porous medium was then left under brine flow at a rate of 2 cm 3 / h, for a time corresponding to the injection of several pore volumes. Injections of PHA solutions, surfactants, and brine to 1 g / L NaCl and 0.4 g / L NaN3 were subsequently made at a flow rate of 2 cm3 / h.

The steps performed are as follows: injection of the PHA solution for a period corresponding to approximately 1 volume of pore, then injection of the brine for a duration corresponding to several pore volumes in order to displace all the non-adsorbed chains; injection of the surfactant solution.

The results are presented in terms of mobility reduction (Rm), also called resistance factor (RF). The mobility reductions are calculated by dividing the zPpo1 or .APTA pressure losses measured during the injection of PHA or surfactants by the APYef pressure losses measured during the reference brine injection. During a polymer injection, an increase in Rm results from both the increase in viscosity of the injected solution and the reduction in permeability related to the reduction in pore size caused by the adsorption of the chains. . During the injection of surfactants, since the viscosity of the surfactant solution is close to that of the brine, the Rm curves are representative of the variation in the permeability reduction of the porous medium, denoted Rk or residual resistance factor. (RRF).

The single figure shows the mobility reduction curves Rm obtained during the injection of the surfactant solution on the different sections of the porous medium as a function of the number of injected pore volumes (# PVI) of the surfactant solution. The Rm values at the beginning of the injection of the surfactant solution are approximately 15, 9 and 6, respectively on the 0-2 cm, 2-5 cm and 5-10 cm sections. This indicates that prior to injection of surfactants, the permeability of the porous medium is reduced from the initial permeability by a factor of about 15 over the 0-2 cm section by a factor of about 9 on section 2-5. cm and a factor of about 6 on the section 5-10 cm. This permeability reduction results from the existence of PHA layers which have been adsorbed on the pore surface during the prior injection of the PHA solution.

The transient increases in Rm observed during injection of the surfactant solution are caused by the desorption of the PHA chains which results in an increase in the viscosity of the liquid flowing in the porous medium. When all the desorbed PHAs leave the porous medium, the viscosity decreases and tends towards that of the surfactant solution, itself close to the viscosity of the reference brine. The passages during which the Rm curves on the 0-2 cm and 2-5 cm sections appear constant correspond to a transient saturation of the pressure sensors used to measure the pressure drops.

After the injection of about 10 pore volumes of surfactant solution, the final Rm values are close to unity on all sections. This indicates that the permeability on all sections is close to the initial permeability of the porous medium. The fact that the Rm values are slightly greater than 1 indicates the existence of a residual monolayer of PHA adsorbed directly on the surface of the pores. 25

Claims (8)

1) A method for restoring the degraded permeability of a porous rock, in which the following steps are carried out: - said rock is passed through the circulation of at least one aqueous solution containing associative polymers, - said rock is treated by injecting an aqueous solution of at least one surfactant at a concentration determined to remove the hydrophobic bonds between chains of associative polymers. 2) The method of claim 1, wherein said surfactant concentration is determined based on the critical micellar concentration of the surfactant. 3) Method according to claim 1 or 2, wherein said surfactant concentration is close to the critical micellar concentration of the surfactant. 4) Method according to one of claims 1 or 2, wherein said surfactant concentration is between 2 and 5 times higher than the critical micellar concentration of the surfactant. 5) Method according to one of the preceding claims, implemented as a result of a treatment of water inflow into a reservoir by means of associative polymers. 6) Method according to one of the preceding claims, implemented as a result of a treatment of permeability profiles using associative polymers. 7) Method according to one of the preceding claims, implemented as a result of a hydrocarbon scanning operation contained in a rock, using associative polymers. 15 8) Method according to one of the preceding claims, wherein restores the permeability of porous rocks near a well.