Method for injecting an aqueous polymer solution into an underground formation

A method for injecting a sodium or potassium or ammonium 2-acrylomido-2-methylpropanesulfonate homopolymer P aqueous solution and/or its salts, the homopolymer P having an average molecular weight by weight greater than 1 million daltons, the homopolymer P aqueous solution being injected into an underground formation for the assisted extraction of hydrocarbons or hydraulic fracturing, said method comprising the following successive steps:

• in a location A:
• preparing, in a mobile unit U1, of sodium or potassium or ammonium 2-acrylamido-2-methylpropanesulfonate in crystalline form (AMPS.S),
• sodium 2-acrylamido-2-methylpropanesulfonate in crystalline form having a powder X-ray diffraction pattern comprising peaks at 11.70°;
• 12.20°;
• 13.2°;
• 13.5°;
• 15.60°;
• 16.80°;
• 17.80°;
• 18.5°;
• 19.1°;
• 20.6°;
• 21.4°;
• 23.3°;
• 25.1°;
• 25.8°;
• 26.9°;
• 29.10°;
• 29.50°;
• 31.0°;
• 33°;
• 33.6°;
• 34.4°;
• 35.2°;
• 35.9°;
• 37.1°;
• 38.4°;
• 39.6°;
• 41.10°;
• 42.90°;
• 45.10°;
• 46.0°;
• 47.2°;
• 47.6°, 2-theta degrees,
• potassium 2-acrylamido-2-methylpropanesulfonate in crystalline form having a powder X-ray diffraction pattern comprising peaks at 13.10°;
• 14.4°;
• 16.3°;
• 19.8°;
• 23.50°;
• 24.3°;
• 26.9°;
• 27.6°;
• 29.3°;
• 30.6°;
• 31.6°;
• 34.3°;
• 36.1°;
• 41.7°;
• 44.6°;
• 46.7°, degrees 2-theta,
• ammonium 2-acrylamido-2-methylpropanesulfonate in crystalline form having a powder X-ray diffraction pattern comprising peaks at 10.10°;
• 13.10°;
• 13.2°;
• 17.5°;
• 15.60°;
• 18.10°;
• 19.50°;
• 20.0°;
• 21.4°;
• 22.8°;
• 23.6°;
• 24.8°;
• 25.1°;
• 25.8°;
• 26.2°;
• 27.4°;
• 28.10°;
• 29.4°;
• 30.2°;
• 30.8°;
• 32.0°;
• 32.9°;
• 33.2°;
• 33.9°;
• 34.4°;
• 35.0°;
• 36.2°;
• 37.7°;
• 38.8°;
• 39.6°;
• 40.4°;
• 41.4°;
• 41.9°;
• 42.8°;
• 43.5°;
• 44.8°;
• 45.5°;
• 45.7°;
• 46.5°;
• 47.0°;
• 47.4°, degrees 2-theta,
• collecting of an aqueous solution S1 of sodium or potassium or ammonium 2-acrylamido-2-methylpropanesulfonate AMPS.OS, coming from the purge of the AMPS.S preparation in the mobile unit U1,
• polymerizing, in a unit U2, the AMPS.OS solution S1 collected in the mobile unit U1 to obtain a polymer P′ aqueous solution S2,
• transferring by pipes the solution S2 from unit U2 of location A to a mobile unit U3 of a location A′,
• in the location A′:
• preparing, in a mobile unit U3, a composition CA′ containing an AMPS.S solution S3 and at least some of the polymer P′ solution S2, the mass ratio [P′]/[AMPS.S] being of between 0.05 and 0.20, with [P′] concentration by weight of polymer P′ in the composition CA′ and [AMPS.S] concentration by weight of AMPS.S in the composition CA′,
• preparing, in the mobile unit U3, a homopolymer P by gel polymerisation of AMPS.S aqueous solution S3 in composition CA′,
• dissolving, in a mobile unit U4, the polymer P in a saline solution SS1 containing at least 1 g/L of alkaline and/or alkaline earth salts in water to obtain a polymer P solution SM,
• transferring the solution SM from the location A′ to a location B,
• in the location B:
• diluting, in a mobile unit U5, of at least a factor 4 and as a maximum a factor 60 of the solution SM by a saline solution SS2 containing at least 1 g/L of alkaline and/or alkaline earth salts in water to obtain a solution SF containing less than 0.6% by weight of polymer P, and
• injecting of the solution SF into the underground formation.