Analysis of near-miscible CO2-WAG displacements: The distinction between compositional and interfacial tension effects

CO₂ Water-Alternating-Gas injection (CO₂ WAG), which involves complex phase and flow behaviour, is still a challenging task to simulate and predict accurately. In this paper, we focus specifically on the regime of viscous fingering flow in CO₂ WAG in heterogeneous systems because of its importance. We investigated two key physical processes that occur during near-Miscible WAG (nMWAG) processes, namely oil stripping (Mechanism 1, M1) and low-interfacial-tension (IFT) film flow effects (Mechanism 2, M2). The low IFT effects in M2 manifest themselves in an increased mobility of oil phase due to film flow process (discussed below). The importance of properly simulating the interaction of viscous, compositional (M1), and low-interfacial-tension effects (M2) is clearly demonstrated in this study. Our specific aim is to improve the modelling of CO₂ displacement in the transition from immiscible to miscible flows in CO₂ WAG processes. We simulated both immiscible and near-miscible CO₂ WAG and also continuous CO₂ displacements with unfavourable mobility ratios for 1D and 2D systems. 2D heterogeneous permeability fields were generated with certain Dykstra-Parsons coefficients and dimensionless correlation ranges. IFT (σ_go) was calculated by the simulator as part of the compositional simulation using the McLeod-Sugden equation. The consequent IFT effects on relative permeability was imposed using two commonly used models, i.e. Coats (1980) and Betté et al. methods (1991), which have been implemented in many commercial software packages, such as CMG/GEM and E300. Thus, we identify two clear mechanisms of oil recovery that may occur in nearmiscible CO₂ (or other gas) injection which we denote as, M1 for oil stripping or compositional effects, and M2 for low-IFT effects which are described by an enhanced hydrocarbon relative permeability. We tested various combinations of oil-stripping effects (M1) and IFT effects (M2) to evaluate the potential impact of each mechanism on the flow behaviour such as the local displacement efficiency, the tracking of tracer flow and the ultimate oil recovery. Oil bypassed by viscous fingering/local heterogeneity, can be efficiently recovered by WAG in the cases where both M1 and M2 are taken into account (as opposed to either mechanism being considered alone). Through tracer analysis, we found that a major recovery mechanism in near-miscible displacement was viscous crossflow between non-preferential (bypassed) flow-paths and preferential flow-paths (i.e. fingers). Indeed this is the key to the recovery of the bypassed oil, which can be strongly amplified/enhanced by the combined mechanisms of M1 and M2 in the case of WAG, but not under continuous CO₂ injection alone. This is because during continuous CO₂ injection, gas fingers are dominant in the preferential flow paths, and therefore significantly suppress oil flow from the nonpreferential (bypassed) paths into the preferential routes. In contrast, the relatively stable displacing front of WAG is able to make full use of M2, leading to viscous crossflow of bypassed oil into the main preferential flow paths (fingers) and be efficiently produced through M1 by subsequent injections. Eventually, these overall combined mechanisms greatly improve the displacement performance in the case of near-miscible WAG. Due to the significance of IFT effects (M2), we comment on the discrepancy between two of the IFT-dependent relative permeability models (Bette and Coats) and its impact on the simulation of the flow behaviour.