The effect of inorganic salt precipitation on oil recovery during CO2 flooding: A case study of Chang 8 block in Changqing oilfield, NW China

Static experiments and dynamic displacement experiments were conducted to quantitatively determine the amount of precipitate generated by the CO2-formation water reaction at different temperatures, pressures, and scaling ion concentrations during CO2 flooding in the Chang 8 block of Changqing Oilfield, the influence of precipitate on the physical properties of reservoirs was investigated, and the corresponding mathematical characterization model was established. The mathematical characterization equation was used to correct the numerical simulation model of E300 module in Eclipse software. The distribution pattern of inorganic salt precipitates during continuous CO2 flooding in Chang 8 block was simulated, and the influence of inorganic salt precipitates on oil recovery was predicted. The inorganic salt precipitate generated during CO2-formation water reaction was mainly CaCO3, and the pressure difference and scaling ion concentration were proportional to the amount of precipitate generated, while the temperature was inversely proportional to the amount of precipitate. The rate of core porosity change before and after CO2 flooding was positively correlated with temperature and flooding pressure difference. The core porosity increase in the CO2-formation water-core reaction experiment was always lower than that of CO2-distilled water-core reaction experiment because of precipitation. The area around the production wells had the most precipitates generated with the injection of CO2. The oil field became poor in development because of the widely distributed precipitate and the recovery decreased to 33.45% from 37.64% after 20-year-CO2 flooding when considering of precipitation.