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1、263 Chemistry and Technology of Fuels and Oils, Vol. 47, No. 4, September, 2011 (Russian Original No. 4, July-August, 2011) RESEARCH STUDY OF COPOLYMER PERFORMANCE IN POLYMER FLOODING OF THE DAQING OIL FIELD Guijiang Wang, Jian Ouyang, and Xiaoling Yi _ Research Institute of Petroleum Exploration Cl
2、ass II reservoir; polymer; molecular weight; salt resistance. The Daqing oil field has now entered the high water cut stage, with a growing imbalance between reserves and the volume of oil production. It is of major importance to develop enhanced oil recovery (EOR) technologies to improve ultimate r
3、ecovery and increase recoverable oil reserves. Polymer flooding has become a major EOR technology to maintain oil production. The second-class reservoirs in Daqing oil field are characterized by fine river sand, multiple thin layers, and low permeability. The reservoirs are extremely heterogeneous b
4、oth areally and vertically 1. Transport of 264 polymers through porous media will be limited by pore structure, geometry and the size of pore throats. A polymer of a certain molecular weight can only penetrate corresponding porous media. Matching studies of the relationship between reservoir permeab
5、ility and polymer molecular weight showed that for optimal results, polymers of the corresponding molecular weight should be used for reservoirs of different permeabilities. The high-molecular-weight polymer HPAM cannot be injected into these formations because they are larger than the pore throats.
6、 However, a medium-molecular-weight polymer (with molecular weight of 15 million daltons) can match the pore size of the formation, but its viscosity is low and proper viscosity has to be achieved by increasing the polymer concentration. Therefore developing a new salt-resistant polymer for the seco
7、nd-class reservoirs is important. A new copolymer called KP was successfully developed which has a molecular weight of about 15 million daltons and is a very good match for the second-class reservoir. In this polymer, a small number of hydrophobic groups are introduced into the partially hydrolyzed
8、polyacrylamide chains. Because of the hydrophobic groups, in aqueous solution the polymer molecules can have a certain physical strength but reversibly associate due to electrostatic, hydrogen bonding, or van der Waals forces between molecules. The formation of a large three-dimensional network can
9、dramatically improve the viscosity of the solution. The viscosity of a KP copolymer solution is close to the viscosity of a solution of an HPAM polymer of molecular weight 30 million daltons, but it is far higher than the viscosity of a solution of HPAM polymer of molecular weight 15 million daltons
10、. Therefore, the salt-resistant polymer KP is an ideal option for the second-class reservoirs in the Daqing oil field. The following reagents were used: acrylamide, sodium formate, potassium peroxydisulfate, sodium bisulfite, sodium carbonate, and the surfactant Triton X-100. All these reagents were
11、 Fig. 1 Effect of sodium formate concentration on copolymer molecular weight. Molecular weight (in million daltons) Sodium formate concentration, mg/L Molecular weight (in million daltons) Fig. 2 Effect of surfactant Triton X-100 concentration on properties of copolymer KP solution. Surfactant conce
12、ntration, mg/L Water-insoluble matter, % 265 industrial grade. The hydrophobic monomer AMC12 was synthesized in-house. Mineralized water (total salt content 4000 mg/L, including 80 mg/L Ca2+ and Mg2+) was used in the experiments. Synthesis of the copolymer KP: Add measured amounts of Na2CO3, AMC12,
13、and the surfactant Triton X-100 to deionized water and stir for 30 minutes. At the appropriate temperature, add acrylamide crystals to the mixture and transfer to a flask placed in a thermostatted water bath. Purge the flask with N2 and then add ammonia solution and urea to the air-tight flask. Cont
14、inue purging with N2 for 20 minutes, then add measured amounts of potassium peroxydisulfate and sodium bisulfite. Continue purging for another 10 minutes and then raise the temperature to 80C. Let the hydrolysis reaction run for 8 hours. The gel product was removed from the flask and placed on a tra
15、y, baked, milled, and sifted. A white powder was obtained as the final product, KP 2. Late in the reaction, fewer monomers remain in the reaction medium and the free radicals produced by the initiators attack the C-H bonds at the tertiary carbon of the free radicals forming the polymer. This results
16、 in tertiary carbon free radicals which subsequently lead to C-C self-crosslinking of the polymer chains. This is why the polymer becomes insoluble and crosslinked in alkaline medium. Furthermore, a small number of tertiary carbon free radicals can initiate some monomers, forming branched-chain free radicals. At the same time, C-C crosslinking can occur by means of the branched-chain radicals
