An Analytical Solution to Pressure Build-Up for CO2 Injection into a Saline Aquifer: The Impact of Stress-Dependency of Permeability

Abstract

Aquifer pressure build-up caused by CO2 injection has been an important issue for evaluating the safety of CO2 geological sequestration in saline aquifers. Although several analytical solutions to aquifer pressure evolution have been developed, an important factor, the stress dependency of the permeability, has been ignored in them. This study develops a new analytical solution by incorporating the dependency based on the commonly used quasi steady-state flow approximation. Following some previous studies, CO2 plume evolution is calculated from a principle that for a given CO2 injection rate, the plume evolves in such a way that the injection pressure at the injection well is in its minimum. The stress-dependent permeability is expressed with the two-part Hook's model. The analytical solution gives the thickness of CO2 plume and aquifer pressure as functions of time and location. It also provides bottom hole pressure of injection well as a function of time for a given CO2 injection rate. The impact of stress-dependency of permeability depends on several factors, including the aquifer stress sensitivity and pore pressure changes during the CO2 injection. The dependency generally reduces the pressure build-up for a given injection rate because the increase in pore pressure enhances aquifer permeability.