Experimental study of enhanced oil recovery with CO2slug+N2flood in low permeability reservoir

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Abstract
Gas injection has become a significant enhanced oil recovery (EOR) measure to develop low permeability reservoirs. CO2 injection can achieve miscible displacement, but its cost is relatively high and CO2 is corrosive. N2 injection is cheap, but it is difficult to achieve miscible displacement in the formation with a limited EOR efficiency. Based on these problems, a feasibility analysis of CO2 slug+N2 to boost the recovery in low permeability reservoirs is studied through experiment. In this paper, with slim-tube test and core flood experiment, an analysis of minimum miscibility pressure, recovery of different gas injection scheme, and sensitivity of oil displacement efficiency of CO2 slug+N2 are obtained. The results show that the scheme of CO2 slug+N2 gets low miscibility pressure and can achieve miscible displacement with high ultimate recovery approximate to the scheme of continuous injection of CO2. Besides, injection timing, slug size of injected CO2, and formation pressure recovery before CO2 slug injection greatly influence the ultimate recovery. This study is significant for the understanding of EOR mechanism of CO2 slug+N2, optimization of injection parameters, and associated numerical simulation in low permeability reservoirs.

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