Oil recovery performance and permeability reduction mechanisms in miscible CO2 water-alternative-gas (WAG) injection after continuous CO2 injection: An experimental investigation and modeling approach

Asphaltene deposition and inorganic deposition are a serious problem because it may lead to the clogging of the reservoir and the reducing of oil production during miscible CO2 water-alternative-gas (CO2-WAG) injection or continuous CO2 injection process. In this study, first of all, the minimum miscibility pressure (MMP) of the crude oil-CO2 system was measured by using the slim-tube apparatus and obtained to be 22.95 MPa. Then, the composite long coreflood experiment was conducted under miscible condition (i.e., Pop>MMP) to determine the ultimate recovery factor (RF) and the permeability reduction caused by asphaltene deposition and inorganic deposition in a tight sandstone reservoir. The results of the experiment found that, after the CO2 breakthrough (BT) of continuous CO2 flooding, the ensuing miscible CO2-WAG injection is able to enhance effectively the ultimate RF from 51.97% to 73.15% under miscible condition. It was obtained that the permeability reduction of the reservoir core as a result of asphaltene deposition is represented an increased wave-like pattern along the flow direction in the CO2-WAG injectionafter continuous CO2 injection process, which is comparable to the predicted results calculated by the solubility model. In addition, the permeability reduction due to inorganic deposition is also shaped as a wave-like mode along the flow direction, which is corresponding to the results of the content change of mineral ions in brine. Moreover, it was also found that the permeability reduction due to asphaltene deposition is much larger than the permeability reduction as a result of inorganic deposition and mainly occurs in the middle and rear of the reservoir in the miscible CO2-WAG injection after the continuous CO2 injection. Thus, for the ensuing miscible CO2-WAG injection after the continuous CO2 injection, such as Jilin reservoir, the injection of chemical inhibitor into the middle and rear of the reservoir is recommended as an effective and economical means to reduce the risk of asphaltene deposition and then sustain the oil production.