Studies of Critical Gas Saturation During Gas Injection

Abstract

Summary Results of studies of critical gas saturation in a long, porous medium have been compared with those obtained in a short system. In general, the relationship of rate vs. critical gas saturation for gas flow into an oil-saturated core can be divided into two regions in which capillary forces and viscous forces govern flow, respectively. At low gas injection rates (below about 10−3 cm3/cm2·s [0.39×10−3 in.3/in.2-sec] for oil of 1.4-mPa·s [1.4-cp] viscosity), capillary processes govern distribution of the gas and result in increasing critical gas saturation as the flow rate is increased from very low values. At high injection rates, viscous forces become prominent and result in very low critical gas saturation, probably as a result of viscous fingering. In a long system, the saturation distribution includes high gas saturation values (>8%) near the inlet end and low values (~2%) near the outlet. The latter are the critical gas saturation. For the case of 1.4-mPa·s [1.4-cp] oil in a short core, the increase of fingering with increasing flow rate produces more regions of low gas saturation. This tends to counterbalance the increase in saturation occurring because of capillary forces, with the result that a maximum is eventually observed in average saturation for the short core. In very viscous (63-mPa·s [63-cp]) oil, no maximum is observed. Studies of saturation profiles during gas injection at high rates indicate that no stabilized zone is formed, but further work, probably with longer cores, could clarify the displacement mechanism.