Recovery of Adsorbed and Free Oil in Shale Formations by CO₂ Injection: An Experimental Study Using 1D- and 2D-NMR

Abstract

Carbon dioxide (CO 2 ) injection has become particularly attractive for its effectiveness in shale oil recovery and additional potential for CO 2 sequestration. However, recovery mechanisms of shale oil in different occurrence states, such as adsorbed and free oil, are still unclear. In this study, we conducted CO 2 injection experiments on an oil-bearing shale sample collected from the Lucaogou Formation under several temperatures and injection pressures. The CO 2 injection experiment contained initial injection, soaking, and depressurization, which simulated one cycle of CO 2 huff-n-puff. The response of shale oil to CO 2 injection was captured by the combination of T 2 , T 1 −T 2 nuclear magnetic resonance (NMR) and mass balance analysis. Our results showed that free oil was easily recoverable by CO 2 injection. However, its recovery was not sensitive to the operating temperature since it increased slightly with elevated temperatures. As CO 2 injection pressure rose from 10 to 15 MPa under the operating temperature of 80 °C, the free oil recovery decreased by 6.7%. Adsorbed oil was only produced under a high temperature of 100 °C, and its content in shale increased by 9.9% when CO 2 was injected at 15 MPa. As residual water in the oil-bearing shale can be extracted by CO 2 , the shale oil recovery was overestimated by mass balance and T 2 NMR analysis. Our findings contribute to a better understanding of shale oil recovery by CO 2 injection.