Modeling the dynamics of remobilized CO2 within the geologic subsurface

Abstract

Long after CO2 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) and predict the fraction of CO2 that will be remobilized as a result. We then model the dynamics of this remobilized CO2 in two distinct steps: (1) vertical rise within the reservoir, followed by (2) spreading of mobile CO2 into the far-field of the domain and justify this approach from a scaling analysis of the governing equations. We show that a model of relative permeability that takes account of insights from percolation theory near the minimum CO2 saturation leads to much more rapid rise and subsequent radial spreading of remobilized CO2 than a traditional empirical correlation such as the Brooks-Corey model. Furthermore, we find that over a broad range of remobilized CO2 mass fraction and Bond number, the radial extent of the mobile plume does not exceed a factor of 1.8 times the radius of the original immobilized CO2 region.