Experimental and simulation study of gas diffusion effect during gas injection into naturally fractured reservoirs

Abstract

Naturally Fractured Reservoirs (NFRs) contain a large portion of oil reserves. Most of NFRs are subjected to secondary and tertiary recovery schemes due to their low primary recovery. Gas injection into NFRs has received much attention in recent years. The main mechanisms contributing to oil recovery during gas injection into NFRs are viscous flow, gravity drainage and diffusion. The viscous flow rarely happens in NFRs due to low pressure drop and early breakthrough of injected gas in high conductive fractures. Gravity drainage is also important in fractured reservoirs with tall matrix blocks where gravity force overcomes capillary force. Diffusion is another recovery mechanism that is found to have a significant role in recovery of oil from NFRs. This has been affirmed by numerous experimental and simulation investigations in the literature. However, sole effect of this mechanism is not well recognized especially in NFRs with very low matrix permeability.In the current work, we have experimentally studied the effect of diffusion and viscous flow on oil recovery from NFRs with tight matrix blocks. The relative importance of each mechanism which reflects the impact of convection versus diffusion is investigated. We have also studied the sole effect of diffusion on final oil recovery using two types of gas (helium and nitrogen) with different diffusion coefficients. Results show that effect of diffusion in tight reservoirs is significant and comparable to effect of viscous flow. Numerical simulation also was conducted to verify results obtained in four experiments involving nitrogen injection. Simulation results reveal that current models for diffusion do not take into account the role of diffusion in ultimate oil recovery properly.