Impacts of Gas Trapping and Capillarity on Oil Recovery by Near-Miscible CO2-WAG

Abstract

Summary CO2 Water-Alternating-Gas injection (CO2-WAG) under near-miscible conditions is a multifaceted process due to the complex interaction of thermodynamic phase behaviour, multi-phase flow behaviour and the heterogeneity of the porous medium. The central objective of this study is to improve the fundamental understanding of fluid behaviour in the process of near-miscible CO2-WAG. This work presents a detailed simulation study of CO2-WAG displacements with unfavourable mobility ratios in a 2D areal heterogeneous system to trigger the fingering flow regime. In our previous work ( Wang et al., 2019 a; 2019b ; 2020 ), we have successfully developed a new mechanistic synthesis of near-miscible WAG, incorporating compositional effects (the MCE mechanism) and interfacial tension effects (the MIFT mechanism). Here, we extend our study to include additional key multiphase flow mechanisms, such as gas trapping and capillarity, to reflect better the flow physics in a 3-phase system. We identify that the effect of gas trapping reduces the oil recovery due to the degraded displacement performance in the “non-preferential” flow routes (areas between gas fingers). This is because the trapping mechanism greatly hampers the MIFT mechanism acting during the secondary water injection cycle. The viscous crossflow between the non-preferential routes and preferential routes (gas fingers) is restricted, which leads to a lowered sweep efficiency. On the other hand, the effect of the capillary force is more complex. In a water-wet system, the oil production increases at the early stage of displacement but approaches the plateau more quickly. In this case, capillary pressure creates entry barriers for gas flowing into low-permeability zones, which gives rise to more severe gas fingers and a larger amount of bypassed oil. The oil recovery drops by over 7% compared to the zero capillary pressure case. For the oil-wet system with capillarity, the production life is much extended by the capillary forces compared to the water-wet case. Although the production rate is reduced at the early stage of the displacement, the oil-wet capillary pressure function enables gas to imbibe into low-permeability zones (under near-miscible conditions), which mitigates the effect of the dominant gas fingers. The improved sweep efficiency maximizes the benefits of the combined MCE and MIFT mechanisms, particularly at the late stage of the displacement. The oil recovery in the oil-wet case can be almost as good as in the base case provided the final water cycle is long enough.