Modeling the Dynamics of Remobilized CO2 within the Geologic Subsurface

Long after CO$_{\mathrm{2}}$ is injected into a brine aquifer, most reservoir-scale fluid dynamic simulations predict large fractions of the original plume will become immobilized via capillary trapping and dispersed throughout the formation. We begin our analysis with a reservoir in this state and consider the effects caused by a depressurization of the domain (e.g. from a nearby production well or newly formed fracture between neighboring reservoirs). Using supercritical CO$_{\mathrm{2}}$ density data from NIST and an assumed knowledge of the minimum residual saturation of CO$_{\mathrm{2}}$, we demonstrate that even a large decrease in reservoir pressure is likely to only result in a small mass fraction of remobilized CO$_{\mathrm{2}}$. Once mobile, this volume of CO$_{\mathrm{2}}$ will rise in the reservoir and concentrate beneath the caprock of the domain. We show that a model of relative permeability that takes account of insights from percolation theory near the minimum CO$_{\mathrm{2}}$ saturation leads to much more rapid rise of remobilized CO$_{\mathrm{2}}$ than a traditional empirical correlation such as the Brooks-Corey model.