| Several experimental studies and field observations have indicated that gravity drainage results in very high oil recoveries. In this investigation, it is demonstrated that a very large percentage of low viscosity waterflood residual oil can be recovered by gravity drainage, assisted by low pressure inert gas injection, from both water-wet and oil-wet samples. A semipermeable membrane made with a mixed-wet paste assures the production of both oil and water, but prevents gas production and eliminates capillary end effects. Stacks of short core plugs can be produced equally as a single core if a coarse version of the mixed-wet paste is used between adjacent plugs. In water-wet cores, displacement of waterflood residual oil requires a positive spreading coefficient of oil on water in gas. Parallel-type macroscopic heterogeneities in the formation may have relatively little effect on the course of oil recovery, but they prevent the establishment of a uniform saturation in the core cross-section. Because the oil is redistributed in the smallest pores during gravity drainage, microscopic pore scale heterogeneities decrease the recovery efficiency. Semipermeable membranes have been used to selectively measure the pressure in the oil and aqueous phases at various heights during gravity drainage in a glass bead pack and a Berea sandstone core. Pressure equations for two-phase displacements were developed and employed to interpret the data for gravity drainage of waterflood residual oil, assuming as a first approximation that the displacement of oil by gas and the displacement of water by oil take place independently. In the regions of a water-wet core where the oil is produced by film flow, the gas-oil capillary pressure stays constant as a function of time although the saturation is continually decreasing. Therefore, if a mixed-wet semipermeable membrane is used at the producing well, extremely small residual oil saturations can be achieved by gravity drainage with very low pressure gas injection, even in shallow reservoirs. |
