Study on the Influence of Phase Change of CO₂ on the Injection Capacity During the CO₂ Reinjection Process in the Depleted Gas Field

There are more than 1.2 billion m3 CO2 could be recycled and injected annually in South Sea of China, and the formation pressure of the depleted gas reservoirs could be increased by CO2 injection. As a result, the development of the gas reservoirs can be improved and the CO2 emission can be significantly reduced. Due to the influence of temperature, presure and purity, the phase state of the injected CO2 through the wellbore and the formation are varied, and the density and viscosity are varied consequently. The variation of phase state in turn affects the injectivity of CO2. According to the geology development characteristics and practical CO2 injection conditions of the offshore gas field, the influence of phase variation on CO2 injectivity is studied in this work based on theoretical calculation, simulation experiment and numerical simulation. Core testing experiments show that at a certain temperature, the CO2 injection capacity increases with the increase of injection pressure before the CO2 partial pressure reaches saturation pressure. When the pressure is higher than saturation pressure, the CO2 injection capacity decreases with the increase of injection pressure. When the injection pressure increases from 7.5 to 10 MPa at 80 ℃, the injection index decreases by 12.1%. Under a certain pressure, an increase in CH4 content injected into CO2 will reduce the CO2 partial pressure, thereby increasing the injection capacity of the mixed gas. The injection index at a CH4 content of 10% increased by 15.2% compared to pure CO2. Numerical simulations of the gas reservoir show that: the initial injection of CO2 injection capacity and formation permeability is nearly linear positive correlation. At the later stage of injection, the pressure of gas reservoir increases, and the trend of CO2 injection capacity increasing with the increase of formation permeability becomes slower, showing a power exponential relationship.