Experimental and Numerical Investigation on the Performance of Gas-oil Gravity Drainage at Different Miscibility Conditions (SPE 154368)

Description Fractured reservoirs have been traditionally considered poor candidates for gas injection processes as highly conductive fractures rapidly transport the gas to the wellbore and consequently majority of oil in the matrix is bypassed. In fractured reservoirs, matrix recovery is achieved by interactions between matrix blocks and fractures. A comprehensive study of the controlling mechanisms (e.g. capillarity, gravity, phase behavior) can lead to optimized recovery. We describe a set of gas injection experiments conducted at different enrichment conditions(immiscible, near-miscible, and miscible) using CO2, nitrogen and flue gas. Moreover, we study the effect of block boundary conditions that are aligned with geological artifacts, e.g., horizontally-oriented impermeable shale layers on the recovery efficiency. A compositional numerical model is developed to simulate gas injection processes at different miscibility conditions. Application When the capillary-driven counter-current imbibition is hampered due to non-water-wetting nature of the reservoir, the injection of gas can be considered as an alternative for recovery from fractured reservoirs. Injection of a miscible gas improves the ultimate recovery, because the miscibility adds the advantage of single-phase flow and interfacial tension elimination. Moreover, injection of an immiscible gas before injecting the miscible gas can be considered to optimize the economics of the project. Results Results reveal that although ultimate oil recovery increases considerably once miscibility is reached, increasing the pressure postpones the oil recovery. This can be attributed to the lower density difference between gas in the fracture and oil in the matrix. The impermeable layer impairs the performance of the gas-oil gravity drainage process for immiscible gas injection, however, it improves the recoveries for miscible gas injection. Significance This study addresses important fluid exchange processes (gravity, mass diffusion and capillary diffusion) which occur between fracture and matrix block at various miscibility conditions.