Mechanistic study of immiscible and near-miscible CO2 water-alternating-gas process
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CO2 Water-Alternating-Gas injection (CO2-WAG) under near-miscible conditions is a multifaceted process due to the complex interaction of thermodynamic phase behaviour, multiphase flow behaviour and the heterogeneity of the porous medium. The central objective of this study is to improve the fundamental understanding of fluid behaviour during the injection process of CO2-WAG, with an emphasis on the transition from immiscible to miscible conditions. This work presents a detailed simulation study of both continuous and WAG displacements with unfavourable mobility ratios for 1D and 2D areal systems. Various flow regimes were investigated, including viscous fingering and channelling displacements within heterogeneous random correlated fields. The key novelty/contribution of this thesis is to bring a new synthesis, which incorporates both compositional effects (MCE) and interfacial effects (MIFT), to improve the numerical simulation of near-miscible processes. Based on this newly developed synthesis, this study was then further extended to investigate a range of key physics, including gas trapping, water hysteresis and capillarity, all of which may occur in 3-phase systems. For the first time, the significance of these mechanisms has been clearly identified with the use of very-fine scale compositional simulations (Δx=0.05m), where all the physics of interest can be fully represented. The efforts of studying the key processes separately leads to a greater insight into how these physical processes positively or negatively affect the sweep and local displacement efficiency. Issues such as front stability, local displacement efficiency, formation of fingering/channelling and viscous crossflow during CO2 near-miscible displacement can lead to behaviour that is significantly different from immiscible flooding in these systems. From the modelling perspective, it demonstrates convincingly that field-scale reservoir models should properly address these small-scale effects to lay claim to reasonable accuracy in forecasts of flow/reservoir behaviour. The complete dataset and results of this study are available online as a model case example for compositional flows in heterogeneous systems. DOI is 10.17861/fc1c90bb-9d3f-4a6c-9170-7b7fe10ec7b9.