Mechanistic simulation of fracture effects on miscible CO2 injection

Abstract

Miscible injection of carbon dioxide (CO2) into oil reservoirs as an Enhanced Oil Recovery (EOR) method has proved to be highly advantageous. According to the volume of the world's recoverable oil resides inside the fractured reservoirs, investigation of the controlling parameters in the efficient injection of miscible CO2 is of paramount importance, mainly owing to the intricacies and complexities associated with this process. This complexity in fractured reservoirs arises due to the presence of two distinct media for fluid transfer (i.e., matrix and fracture network) and the corresponding differences in fluid velocities. Accordingly, performance of miscible carbon dioxide injection in these reservoirs was investigated through mechanistic simulation model in the form of dual-porosity (DP), and dual porosity-dual permeability (DPP). Moreover, due to limited supply and high injection costs of this gas in its pure form, performance of the miscible CO2 injection combined with C1, N2, and H2S was also surveyed and compared to pure gas injection case. A sensitivity analysis was also performed based on fracture porosity, fracture horizontal and vertical permeability, matrix horizontal permeability, block height shape factor, matrix capillary pressure, and impure injected components in DP and DPP models, showing that matrix horizontal permeability and capillary pressure have the greatest, and porosity has the lowest impact on miscibility performance and oil recovery in these models. In the end, after investigating the effect of different injection cases on miscibility performance and oil recovery, it was concluded that the highest oil recovery in miscible gas injection obtained through optimization of a gas composition having the lowest minimum miscibility pressure (MMP) and the lowest density.