Abstract
Carbon dioxide (CO2) injection has become an important technology to enhance oil recovery in ultra-low permeability reservoirs. Compared with other CO2 flooding technologies, CO2 miscible flooding has a better development effect, and the minimum miscible pressure (MMP) is a key parameter to realize miscible flooding. Therefore, it is very important to accurately predict the MMP. The prediction methods of MMP generally include laboratory experiment method and theoretical calculation method. In this study, a long-slim-tube displacement experiment method was used to determine the MMP in the study area, and the experimental temperature and pressure were consistent with those under reservoir conditions. The research results show that the recovery ratio increased gradually with the increase of experimental pressure, but the increase amplitude gradually decreased. According to the relation curve between crude oil recovery ratio and experimental displacement pressure, when the experimental pressure was larger than 29.6 MPa, the recovery ratio did not increase significantly with the increase of displacement pressure, which indicates that the interfacial tension between crude oil and CO2 disappeared under this pressure and they reached a miscible state. It is speculated that the MMP between crude oil and CO2 system in the study area predicted by the long-slim-tube displacement experiment method was 29.6 MPa. The results of this study help to realize miscible flooding in ultra-low permeability reservoirs and thus enhance oil recovery.
Highlights
Enhanced oil recovery (EOR) by carbon dioxide (CO2) injection has been widely applied by the U.S, Russia, Canada, China, and other countries (Mogensen, 2016; Azizkhani and Gandomkar, 2020)
When the experimental pressure was 20 and 25 MPa, the recovery ratio increased slightly after the CO2 injection amount was greater than 1.0 PV
When the experimental pressure was 30, 35, 35, and 40 MPa, the recovery ratio increased insignificantly after the CO2 injection amount was greater than 0.9 PV
Summary
Enhanced oil recovery (EOR) by carbon dioxide (CO2) injection has been widely applied by the U.S, Russia, Canada, China, and other countries (Mogensen, 2016; Azizkhani and Gandomkar, 2020). Some CO2 can be stored underground by CO2 flooding to reduce greenhouse gas emissions (Qin et al, 2015; Ahmadi et al, 2017; Birdja et al, 2019; Zhang et al, 2020a). Theoretical calculation methods are mainly empirical formula method, equation of state method, and simulation calculation method (Zhang et al, 2008; Yang et al, 2019; Zhang et al, 2020b; Kaufmann and Connelly, 2020) This method is an effective one-dimensional flow displacement experimental model. When the interfacial tension under different gas components and different pressures is determined by an interfacial tension tester, and the interfacial tension is extrapolated as zero, the corresponding pressure is the minimum miscible pressure (MMP) (Zhang et al, 2006; Peng et al, 2007; Gunde et al, 2010; Ghorbani et al, 2014; Li et al, 2016)
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