Effect of Carbon Dioxide Dissolving in Formation Water on the Mechanism of Oil Displacement

Abstract

In order to study the effect of CO2 dissolving in formation water on the process of CO2 injection, a thermodynamic model of CO2-hydrocarbon-formation water has been used to study the dissolving law of CO2 in formation water and oil-gas-water phase behavior considering the CO2 dissolution in formation water. Based on compositional model considering the dissolution of CO2 in formation water, a dimensional model for long core CO2 displacement is presented to simulate the oil displacement efficiency, gas/oil ratio, oil or gas saturation profile, the solubility of CO2 in in-place oil or formation water with grid distance in the process of long core displacement of CO2 injection. The results show that the solubility of CO2 in formation water increases with increasing pressure and decreases with increasing temperature, and that the solubility of CO2 in formation water is usually about 5.5 wt% at the condition of reservoir pressure and temperature. In addition, oil saturation and gas phase saturation with the consideration of CO2 dissolving in formation water lag behind those without consideration of CO2 dissolution, which results in later breakthrough time for CO2 injection. Moreover, residual oil saturation with considering CO2 dissolving in formation water is higher than that without considering CO2 dissolution at the condition of the same hydrocarbon pore volume for CO2 injection, and the ultimate recovery for the former is lower than that for the latter when pouring into 0.3 hydrocarbon pore volume of CO2.