Abstract
Based on the properties of high temperature and salinity reservoir, the water-soluble polymer with good heat resistance and salt tolerance can be obtained through copolymerization between 2-acrylamide-2-methyl sulfonate monomer (AMPSN) and acrylamide monomer (AM) in water. The star shaped stable complexes (STARPAM) with the star nucleus of β-CD are prepared by living radical polymerization, which can improve the viscosity and change the percolation characteristics of the polymer in porous media. In the article, the performance of the STARPAM (star-shaped polymer) with heat resistance and salt tolerance was evaluated by comparing the viscosification property, heat and salt resistance, calcium and magnesium tolerance, and long-term thermal stability of STARPAM (star-shaped polymer) with those of HPAM (partially hydrolyzed polyacrylamide) and MO-4000 (linear polymer). The results of physical simulation experiment showed that the viscosity of the STARPAM is 3.3 times that of MO-4000 and 4 times that of HPAM under the conditions of mineralization degree of 20000 mg/L, concentration of 1500 mg/L, and 75°C, which indicated that heat resistance and salt tolerance of the STARPAM are excellent. Oil displacement experiments showed that STARPAM can enhance oil recovery by 20.53% after water flooding, and the effect of oil displacement is excellent. At present, 19 wells were effective with a ratio of 95.2%. Compared with before treatment, the daily liquid production increased by 136 m3, daily oil production increased by 44.6 t, water cut decreased by 4.67 percentage points, and flow pressure decreased by 1.15 MPa.
Highlights
The heterogeneity of the reservoir and the unfavorable mobility ratio are two important factors that affect the sweep efficiency and oil recovery of the water flooding
When the content of Ca2+ and Mg2+ is higher, and the degree of hydrolysis of polyacrylamide is more than 40%, molecules of HPAM will combine with Ca2+, Mg2+, and other polyvalent ions, resulting in flocculation and sedimentation
The hydrolysis reaction of an amide group in HPAM is very rapid under acid and alkaline conditions, and the rate of hydrolysis under neutral conditions is accelerated with the increase of temperature, which makes HPAM do not have the characteristics of heat resistance and salt tolerance [7,8,9]
Summary
The heterogeneity of the reservoir and the unfavorable mobility ratio are two important factors that affect the sweep efficiency and oil recovery of the water flooding. Hydrolysis degree of the polymer molecules used for tertiary recovery must be less than 40% In this way can the polymer have the characteristics of heat resistance and salt tolerance in oil field application. The polymers with heat resistance and salt tolerance for tertiary oil recovery have been developed at home and abroad, including HPAM of super-high molecular weight, amphoteric ion polymer, monomer copolymer, hydrophobically associating polymer, multiple composite polymer, comb polymer, and star-shaped polymer. What is more important is that the hydrophobic cavities of βCD have supramolecular inclusion for a wide range of guest molecules, which can change the seepage characteristics of the star-shaped polymers in reservoirs [22,23,24]. Through the active free radical polymerization, a stable star-shaped complex with β-CD as the core is formed, which can improve the viscosity and change the seepage characteristics of the polymer in porous media. The precipitate was filtered off and purified by reprecipitation with chloroform-methanol and dried in vacuo to give polymer as a white powder
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