Mechanistic study of immiscible and near-miscible CO2 water-alternating-gas process
Abstract
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.