(302b) Rate Dependent Oil Recovery in Fractured Basement Reservoirs

In fractured basement reservoirs, where oil is primarily stored in fractures, only large aperture fractures are visible through seismic reflection surveys. Considerable reserves may exist in sub-seismic fractures. A discrete fracture network (DFN) simulation study is undertaken to examine the effect of rate on oil and water displacements. The displacement of oil from fractures in waterflooding is controlled by gravity, viscous, and inertial forces. Multiphase displacement in fractures is controlled by fracture dip and continuity of flow paths in the direction of buoyant flow. Higher capacity for flow in seismic fractures can lead to high water production before sub-seismic oil is recovered. High water cut in oil-water basement reservoirs may make oil production economically unrealistic. The effects of threshold capillary pressure, wettability, relative permeabilities, and storage ratios on rate dependent recovery were investigated in this paper. Results indicate that a high rate of production is ideal for optimal oil recovery when time is considered. Comparing pore volumes injected, equivalent recovery is achieved with fewer pore volumes for lower water injection rates. Delayed water production occurs at low rate; however, overall recovery does not increase. Rate dependent relative permeabilities would make low recovery rates more beneficial, however the physical basis for rate-dependent relative permeabilities in oil-water systems is not known at this time. The viscous forces are dominant in high rate simulations. A Darcy flow assumption in the DFN simulations neglects inertial forces which may become important if a Reynolds number > 1 is achieved at higher displacement rates. Results from this study may be used to develop operational guidelines for waterflooding in fractured basement reservoirs. History matching using a DFN approach will improve the understanding of flow interaction in seismic and sub-seismic fractures.