Miscible injection of carbon dioxide (CO2) has seen a significant increase in interest for the purpose of enhanced oil recovery in conventional oil reservoirs. However, naturally fractured reservoirs, which are among the largest oil reserves in the world, are considered poor candidates for this process due to presumed low performance efficiency. This paper presents the results of an experimental study on the effect of connate water saturation, matrix permeability, and oil viscosity on the performance of gravity drainage from matrix (into fracture) when it is surrounded by a CO2-filled fracture. Experiments were performed in an experimental model under different operating pressures to cover both immiscible and miscible conditions. Experiments were conducted using synthetic oil (nC10) and light crude oil in two Berea cores having large differences in permeability. In addition, the effect of connate water saturation was studied by performing experiments in an initially brine saturated Berea core and comparing the results with those obtained when the core was 100% saturated with oil. The experimental results showed that matrix permeability had a significant effect on the rate of gravity drainage when CO2 was injected under immiscible conditions. When experiments were performed at immiscible conditions, production rate by gravity drainage was nearly 5 times greater in the Berea core with 1000 md permeability compared to the core with permeability of 100 md. The ratio of production rate was not similar to the permeability ratio, indicating the important role of capillary pressure in the gravity drainage mechanism. However, ultimate oil recovery was less sensitive to the matrix permeability at pressures near or above minimum miscibility pressure. The observations were more interesting when experiments were performed in the presence of connate water saturation. The ultimate oil recovery from a core saturated with oil in the presence of connate water saturation was less at immiscible conditions. However, at near miscible and miscible conditions, the presence of connate water was beneficial to the gravity drainage mechanism in that it lead to higher ultimate oil recovery. This study also shows that miscible CO2 injection in fractured reservoirs is a viable option for both oil recovery and storage purposes. However, under immiscible conditions, when CO2 is injected at pressures below the minimum miscibility pressure (MMP) and above the supercritical condition, it is not beneficial for improving oil recovery via gravity drainage. This was clearly seen when gravity drainage experiments using crude oil were performed and MMP was not achieved at the maximum possible operating pressures. The results obtained from this study address the knowledge gap in the best practices for utilizing CO2 for improving oil recovery from fractured reservoir environments and demonstrate the effects of key parameters on the gravity drainage mechanism.
Primary production of light crude oils by natural energy of the reservoirs may produce up to one third of the recoverable oil. Secondary oil recovery processes including gas and waterflooding methods may increase the oil recovery to two thirds of initial recoverable oil-in-place in non-fractured reservoirs if applied adequately.