Temporal variations in near-wellbore pressures during CO2 injection in saline aquifers

Numerical simulations of carbon dioxide injection, via a fully penetrating well, into a homogeneous confined saline aquifer were conducted using TOUGH2 to study temporal variations in near-wellbore pressures. The effect of contrasts in fluid properties on near-wellbore pressure was studied by comparing the predicted pressure histories of carbon dioxide injection to that of water injection into a confined saline aquifer. Simulation results predict an initial jump followed by subsequent decline in near-wellbore pressure over time under isotropic and weakly anisotropic conditions due to phase separation between the less dense and highly compressible carbon dioxide-rich (gas) phase and weakly compressible brine. Conversely, near-wellbore pressure increased monotonically during water injection because the differences between the viscosities, densities, and compressibilities of resident brine and water are relatively small. Sensitivity studies on the effects of the compressibility and viscosity of carbon dioxide and permeability anisotropy suggest that temporal variations in near-wellbore pressures depend strongly on the contrast in density between carbon dioxide and brine, and on the ratio between vertical and horizontal permeabilities of the aquifer (permeability anisotropy). These results suggest that the monitoring near-wellbore pressures during carbon dioxide injection is crucial for maintaining the integrity of the caprock and thereby warrants geomechanical studies.