Experimental Investigation of Miscible Secondary Gas Injection

Abstract

Abstract Our earlier experimental work1 on tertiary mode gas injection indicated that the gas injection strategy, either continuous (CGI) or alternating with water (WAG), did not have a significant effect on recovery during immiscible gas injections. This paper examines the effect of gas injection strategy on recovery in miscible gas injection projects. Here we compare the effects of injection strategy (namely secondary and tertiary) on horizontal type, miscible continuous gas injection (CGI) and Water-Alternating-Gas (WAG) processes. The secondary miscible corefloods demonstrated high oil recoveries in both CGI and WAG mode of injection, attributable to higher initial oil saturations and the high capillary numbers achieved due to miscibility. The flood performance was analyzed using the conventional recovery plots as well as the tertiary recovery factors, TRF, defined as the oil recovery per unit volume of gas injected. Comparison of the TRF plots for secondary and tertiary gas injections, clearly demonstrate the beneficial recovery effects of gas injection early in the life of the flood. On the other hand, the WAG floods demonstrated higher gas utilization factors than those in CGI under secondary mode of injection. The water shielding effect, responsible for delayed oil breakthroughs in tertiary floods, was observed to be minimal in the secondary WAG floods. This work reconfirms that the optimal injection strategy consists of a combination of CGI and WAG modes of injection, which appears to be independent of whether the gas injection is carried out as a secondary or tertiary process.