­3D modeling of fault reactivation during CO2 injection

Abstract:

Induced seismicity plays a crucial role for the success of geological storage of carbon dioxide. No felt seismicity has been reported in current CO₂ projects, but the overpressure due to the injection may increase the risk of fault reactivation. Although a site can be designated to stay away from major and regional faults, undetected faults may still be a great concern. Previous 2D numerical results showed that such undetected faults could produce felt seismic events, whose magnitude is only limited by the size of the fault itself. However, 2D models overestimate overpressure acting on the fault. Thus, 3D models are necessary to reproduce the actual overpressure evolution.

In this work we analyze the reactivation of a minor, undetected fault, simulating two different cases of injection through a vertical and a horizontal well, respectively. Results show that for a vertical well the fault pressurizes faster and more locally, resulting in a smaller seismic event. For a horizontal well the pressure is more distributed along the fault and requires longer time to reach the critical value for reactivation, thus resulting in a larger event.

The fault reactivation also produces changes in damage zone and fault core permeability, allowing the CO₂ to leak from the injection zone thereby to escape upward towards shallower depths. Although the calculated fault permeability enhancement is similar for the two cases, results show a slightly higher leakage rate for the case of the vertical well in a region close to the well, while the leakage resulting from injection through a horizontal well is smaller but more distributed in space.