| Popis: |
Large-scale fluid injection into the subsurface is important for a number of industrial applications, including the injection and storage of CO2 for the mitigation of anthropogenic climate change. A vast amount of seismic data suggests that vertical heterogeneity dominates the distribution of fluids in layered aquifers. However, most studies of fluid injection typically neglect layering. This thesis aims to characterise the fundamental dynamics of subsurface fluid injection and migration in a layered aquifer system. This goal is achieved via the derivation of a novel theoretical model that is both accurate and computationally efficient. This vertically integrated sharp-interface model utilises a weakly compressible gravity current formulation that accounts for vertical flow of water and gas across a stratified system consisting of relatively thick higher permeability aquifers, alternating with thinner and much lower permeability seals. The coupling with vertical water leakage, and the corresponding vertical dissipation of pressure out of the injection aquifer, is found to have a leading-order role on lateral gas migration. Results demonstrate that vertical pressure dissipation acts to significantly reduce the lateral extent of the plume. Furthermore, vertical pressure dissipation has a significant impact on gas leakage which occurs when a capillary entry threshold is exceeded by controlling the magnitude, and style, of capillary pressure buildup. Results further indicate that distributed CO2 leakage during CO2 injection into layered aquifers such as at Sleipner is more likely than previously considered. As the model developed here retains the plume shape and pressure field at the end of injection, the model can be used to provide unique insights into post-injection migration. Results suggest that due to the strongly tongued shape present at the end of CO2 injection, a period of stagnant lateral migration occurs after an initial phase of depressurisation, and before the transition towards late-time self-similar spreading. |
