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 (σgo) 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.
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
A study was made of the effect of interfacial tension (IFT) on the immiscible displacement of oil by water through water-wet-porous media, commonly found in practice. This was done through the comparison of the displacement efficiencies of two binary liquid-systems: OIL-WATER (immiscible) and SUCROSE SOLUTION-WATER (Miscible). The liquids were chosen and prepared such that both systems had identical viscosity ratio and density difference.
Results of the study have shown that the displacement efficiency was markedly higher in the immiscible case. This suggests that IFT should be increased to improve the recovery of oil under water-wet-conditions. This conclusion, which is in agreement with that of Newcombe et al., is explained by the fact that the capillary force, due to IFT, is in the direction of the driving force, under the water-flooding condition. The results also indicate that the size of the porous bed is also an important factor determining the porous bed is also an important factor determining the magnitude of the IFT effect.
A number of studies has been made to investigate the role played by interfacial tension (IFT) in the entrapment of residual oil during the oil-water displacement process through porous beds. This study has been found necessary because the high costs of oil exploration make it imperative that water flooding be conducted under conditions favouring most efficient oil recovery.
In the study of immiscible displacement of oil by water, it is important to distinguish two kinds of reservoirs:
a) Water-wet reservoirs: most commonly found in practice. In those systems, water-flooding is a displacement process of a non-wetting fluid (oil) by a wetting fluid (water).