Experimental study on the effects of half cycle slug size and miscibility condition on the performance of tertiary carbon dioxide-water alternating gas injection

Water Alternating Gas (WAG) process is one of the most common methods employed in Enhanced Oil Recovery (EOR). WAG process combines two traditional technologies, i.e., waterflooding and gas injection, and is expected to improve the oil recovery due to the expected increase in macroscopic displacement efficiency. However, since the understanding of this process is far from complete, the field performance of WAG is often much less than expected.;WAG injection is in fact affected by several important factors, such as reservoir heterogeneity (stratification and anisotropy), rock wettability, fluid properties, miscibility condition, and WAG parameters, including WAG ratio and half cycle slug size. The goal of this work is to study the roles of half cycle slug size (HCSS) and miscibility condition, which have never been experimentally investigated before.;Coreflood experiments are performed in Berea sandstone core, from which the WAG performance, such as percent oil recovery, tertiary recovery factor, and CO2/Gas utilization factor are determined. The results show that there is an optimum HCSS, by which the oil tertiary recovery reaches a maximum value and the CO2 utilization factor reaches a minimum value. Immiscible flooding does not perform as well as near-miscible and miscible floodings, but near-miscible and miscible floodings perform comparably in water wet system.