An experimental study of the effects of CO₂ injection on gas/condensate recovery and CO₂ storage in gas-condensate reservoirs

The application of non-hydrocarbon gas injection for enhanced gas and condensate recovery (EGCR) is still in a developmental stage as the mixing/interaction between the injected gas and resident reservoir fluid is yet to be extensively understood and the inability to optimize the recovery process has led to limited pilot trials. Carbon dioxide (CO₂) injection into gas-condensate reservoirs for improved recovery and CO₂ storage provides additional and favorable changes in phase and fluid flow behaviour making it economically more attractive compared to other injection gases. However, to make an informed decision, adequate phase and flow behaviour analysis are required to better forecast the reservoir performance under CO₂ injection. In this research, appropriate experimental phase behaviour, EOS modeling, and unsteady-state flow tests have been conducted to determine the level of CO₂/gas-condensate interaction including condensing/vaporizing mechanisms during CO₂ Huff-n-Puff (HnP) injection. A CO₂ HnP injection technique was followed to identify the best CO₂ flooding conditions. A total of four HnP injection cycles with incremental CO₂ volumes of 20, 40, 60, and 80 % of the initial resident fluid volume prior to depletion was considered. CO₂ injection pressure and volume are optimized below the saturation pressure. The analysis is based on evaluating the level of interaction between CO₂ and resident fluid at the maximum condensate saturation of the corresponding CO₂-gas-condensate fluid mixture as determined in a phase equilibria cell. Appropriate experimental phase behaviour and core flood data were generated and analyzed to identify and quantify the level of condensing/vaporizing mechanisms which are vital for adequate optimization of the injection pressure and amount of injected CO₂ for both enhanced gas and condensate recovery and CO₂ storage purposes. The amount of gas, condensate, and CO₂ produced at each core flood stage was recorded. These data allow bridging the gap between conflicting reports on the trend and level of CO₂/gas-condensate fluid interactions at pressures below the dew point pressure (Pdew). They also provide a better knowledge of the governing mechanisms during CO₂ flooding, which are required for designing appropriate CO₂ HnP injection for reservoir engineering applications.