Impact of Miscibility on Gas-Oil Gravity Drainage in Fractured Reservoirs

Abstract

Abstract A significant proportion of the world remaining oil reserves is residing in fractured reservoirs. Conventional recovery mechanisms may result in low oil recovery efficiencies in such reservoirs. Gas injection may be one of the few techniques, which can boost production from these resources. However, higher capillary pressures associated with the matrix blocks coupled with the extreme hydraulic contrast between the matrix and the fractures may result in ultimate recovery factors significantly lower than the equivalent non-fractured systems. Miscible or near-miscible gas injection may substantially boost the ultimate oil recovery as the interfacial tension and hence the capillary pressure vanishes or diminishes. Furthermore, the viscosity of the oil can be significantly reduced by mixing and interacting with the injectant, leading to further increase in oil drainage rate. Due to the nature of the problem that originates from the extreme contrast between matrix and fracture, numerically it is considered to be a difficult problem. Therefore, it is important to design an appropriate grid system for this class of reservoir systems. We first validate our model by conducting black oil simulations to match the previously performed gas-oil gravity drainage (GOGD) experiments. We then generate a three-component synthetic oil to perform compositional simulations of gas injection at different enrichment conditions; immiscible, near-miscible and first-contact miscible. We show that ultimate recovery and oil drainage rate significantly increase once the miscibility is developed. In addition, we study the effect of various block boundary conditions that are aligned with various geological artifacts, like horizontally-oriented impermeable shale layers on recovery efficiency. We show that impermeable layers significantly reduce the performance of the gas-oil gravity drainage process for immiscible gas injection, while increasing the recoveries substantially for multiple- and first-contact miscible gas injection cases.