Abstract:
Hydrosoluble copolymers made of hydrophilic monomers, e.g., acrylamide, and of silane or siloxane derivative-based hydrophobic monomers, e.g., a silane or or siloxane acrylate, is obtained by micellar or heterogeneous radical copolymerization, and such copolymers can be used as rheology control agents in, e.g., a well bore.

Description:
FIELD OF THE INVENTION 
     The present invention relates to new associating hydrosoluble polymers intended for various applications, for example well fluids, paints, the cosmetics industry, the paper industry. Associating polymers consist of a hydrosoluble skeleton comprising a small amount of hydrophobic groups. They are mainly used as thickening agents for rheology control of aqueous suspensions. The polymers according to the present invention have shown very interesting properties as viscosifiers and, furthermore, an adsorption power that permits the contemplation of other applications, for example as filtrate reducer, swelling inhibitor or argillaceous rock cleavage inhibitor, or for cuttings dispersion control. 
     It is well-known that hydrophobic monomers generally consist of a hydrocarbon or perfluorocarbon chain. The copolymers of the present invention are made of a hydrophilic monomer of acrylamide type for example and of a silane or siloxane derivative-based hydrophobic monomer. The temperature stability of the siloxane bonds should favour the thermal stability in solution of the copolymer of the present invention. 
     SUMMARY OF THE INVENTION 
     The present invention thus relates to a hydrosoluble copolymer obtained from hydrophilic monomers and hydrophobic monomers having at least one of the following general formula structures a, b or c:                                                     
     The proportion of hydrophobic monomer is less than 10 % by mole. 
     In the copolymer, the hydrophobic monomer can have the following general formula:                           
     According to the invention, the hydrophobic monomers can be acrylamide. 
     The copolymer according to the invention can be obtained by means of a radical, for example micellar copolymerization process. 
     In the micellar copolymerization process, dodecyl sodium sulfate can be used as surfactant. 
     The copolymer according to the invention can also be obtained by heterogeneous copolymerization. 
     The copolymer can have a molecular weight substantially ranging between 10 4  and 5·10 6 . 
     In a variant, the copolymer can have a molecular weight substantially ranging between 5·10 5  and 5·10 6 , and have at most 2% by mole of hydrophobic monomers. 
     In another variant, the copolymer can have a molecular weight substantially ranging between 10 4  and 10 6 , and have from 2% to 10% by mole of hydrophobic monomers. 
     The copolymer can be dissolved in aqueous fluids in a proportion ranging between 0.1 and 15% by weight, said fluids being used in circulation in a wellbore, for example as drilling fluids, completion or workover fluids, fracturing, sweep fluids, or cement slurry. 
     The invention also relates to an application of the copolymer for rheology control of an aqueous fluid. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the present invention will be clear from reading the description hereafter of processes allowing to obtain the copolymer, characterization tests and characteristics measurements, with reference to the accompanying drawings wherein: 
     FIG. 1 gives the compared rheological characteristics of various copolymers as a function of the concentration thereof in solution, 
     FIG. 2 gives the adsorption results of a copolymer according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The polyacrylamide is generally prepared by radical polymerization in aqueous solution. The hydrophobic monomers are insoluble in such a reaction medium. In radical copolymerization, the polymer is prepared in a single stage by copolymerization of the hydrophilic monomer with the hydrophobic monomer. In order to overcome the insolubility problem of the hydrophobic monomer, various synthesis methods can be used: 
     emulsification of the hydrophobic monomers in the presence of small surfactant amounts; 
     solubilization of the hydrophobic monomer in surfactant micelles dispersed in water (micellar copolymerization), as taught in documents U.S. Pat. Nos. 4,432,88 1 or 4,520,182 mentioned here by way of reference; 
     presence of the insoluble hydrophobic monomer in the form of a fine suspension (heterogeneous copolymerization) according to “HILL A., CANDAU F., SELB J., Macromolecules, 1993, 4521-4531, Properties of Hydrophobically Associating Polyacrylamides: Influence of the Method of Synthesis”, mentioned here by way of reference; 
     copolymerization in solution in the presence of a solvent or of a mixture of solvents of both monomer types. 
     The following, non limitative examples describe various processes for obtaining the copolymer according to the invention. Example 1 gives a process allowing to obtain a polyacrylamide (PAM) whose characteristics are compared to those of the copolymer according to the invention. 
     EXAMPLE 1 
     21 g of acrylamide and 669 g of purified water are fed into a I liter reactor. The reaction medium is placed in an inert atmosphere (argon) and brought to a temperature of 50° C. 
     0.15 g of potassium persulfate dissolved in 10 g of water are fed into the reactor. The reaction medium is kept under stirring at 50° C. for 6 hours. The reaction medium is then precipitated in a large methanol excess. The polymer obtained is filtered and washed three times with methanol, then vacuum dried at 50° C. 
     20.4 g of polymer bearing reference PAM is obtained, whose average molecular weight of 2.5·10 6  g.mol −1  is measured by light scattering in formamide. 
     EXAMPLE 2 
     Micellar Copolymerization 
     20.5 g of acrylamide, 669 g of purified water and 20.3 g of dodecyl sodium sulfate are fed into a 1 liter reactor. 
     0.5 g of 3-methacryl-oxypropylbis(trimethylsiloxy) methylsilane are added to the reaction medium while stirring. The reaction medium is placed in an inert atmosphere (argon) and brought to a temperature of 50° C. 
     0.15 g of potassium persulfate dissolved in 10 g of water are fed into the reactor. The reaction medium is kept under stirring at 50° C. for 6 hours. The copolymer obtained is recovered and purified according to the method described in example 1. 
     19.8 g of copolymer bearing reference M05 and containing 0.5% by mole of hydrophobic groups are obtained, with an average molecular weight of 3.2·10 6  g.mol −1  measured by light scattering in formamide. 
     EXAMPLE 3 
     Micellar Copolymerization 
     If in example 2, all other things being equal, we copolymerize 20.25 g of acrylamide and 0.75 g of 3-methacryl-oxypropylbis(trimethylsiloxy) methylsilane, we obtain 19.5 g of a copolymer bearing reference M075, containing 0.75% by mole of hydrophobic groups and whose average molecular weight, measured by light scattering in formamide, is 2.9·10 6  g.mol −1 . 
     EXAMPLE 4 
     Heterogeneous Copolymerization 
     Example 2 is repeated without using a surfactant. The copolymer obtained after purification, bearing reference H05, contains 0.5% of hydrophobic monomers and its average molecular weight is 3.0·10 6  g.mol −1 . 
     EXAMPLE 5 
     Heterogeneous Copolymerization 
     If in example 4, all other things being equal, we copolymerize 20 g of acrylamide and 1 g of 3-methacryl-oxypropylbis(trimethylsiloxy) methylsilane, we obtain 18.75 g of a copolymer bearing reference M075 and containing 1% by mole of hydrophobic groups, its average molecular weight measured by light scattering in formamide being 3.1·10 6  g.mol −1 . 
     EXAMPLE 6 
     Rheological Properties 
     The various polymers obtained in examples 1 to 5 are brought into solution in water at different concentrations ranging between 100 and 10,000 ppm. 
     The absolute viscosities of these solutions are measured by means of a Couette type low-shear viscometer at a temperature of 30° C. and at velocity gradients ranging between 0.017 and 128 s −1 . The reduced specific viscosity VSr in cm 3 .g −1  is calculated and plotted against the polymer concentration in ppm in FIG.  1 . Curves 1, 2, 3, 4 and 5 represent copolymers M075, M05, H05, H1 and PAM respectively. 
     For a given concentration, the copolymers obtained according to the invention by micellar copolymerization (M075 and M05) are much more viscous than the polyacrylamide (PAM). It can be noted that the viscosifying power of the copolymer solution is all the higher as the percentage by mole of hydrophobic monomer increases. 
     The products obtained by heterogeneous copolymerization (H05 and H1) also have enhanced viscosifying properties compared to the polyacrylamide (PAM). It can be noted that the percentage by mole of hydrophobic monomer does not substantially modify the viscosity of the solutions. 
     7) Adsorption Properties 
     In order to evaluate the adsorption properties, a comparative study has been made between polyacrylamide and copolymers according to the invention in solution in contact with a clay (homo-ionic montmorillonite in which there are only potassium counterions left after a calcium ion exchange) by means of the remainder method. The adsorption isotherms are obtained by measuring the amount adsorbed on mixtures of montmorillonite dispersed in the water and of polymer solution. The polymer concentration varies whereas the solid/liquid ratio remains constant (3.6·10 4 ). All the isotherms have been obtained under the same conditions: T=40° C., [KCl]=20 g/l, i.e. 0.27 mol/l, pH=8 and t=24 hours. 
     In FIG. 2, the polymer equilibrium concentration Ce (ppm) is laid off as abscissa and the adsorbed amount A (in milligram of polymer per gram of rock) is laid off as ordinate. Curves 6, 7 and 8 relate to M075, M05 and PAM respectively. It is clear that the copolymers according to the invention have a markedly increased adsorption power in relation to the PAM. 
     Such a copolymer can be used in aqueous solution as a drilling, completion, workover, fracturing or cementing fluid. More generally, it can be used in applications where it is necessary to control the rheology of an aqueous fluid containing a certain proportion of solid particles, under the pressure and temperature conditions which can be encountered in petroleum wells. 
     Of course, the applications of the copolymer according to the invention are not limited to those described above. This product can also be used as a thickening or viscosifying agent.