Induced Seismicity of CO2 Injection in a Depleted Chalk Field with a Thermo-Hydro-Mechanical (THM) Model

Abstract

Chalk fields, particularly in the Danish North Sea, are being considered for carbon capture and storage (CCS). This study aims to address seismic risks and fault reactivation during CO2 injection in the Harald East chalk field which is under investigation as part of the Bifrost project. Returning to the virgin reservoir pressure during the injection phase, in a highly depleted chalk reservoir is challenging. Fault reactivation becomes a concern because the stress regime changes during injection. This is due to significant plastic deformation that occurred during the production phase. In this study, two scale models are used to assess the fault reactivation and induced seismicity during the injection. First, in a field-scale model, the critically stressed faults are determined with a coupled thermo-hydro-mechanical analysis. Then, in a fault scale model, different scenarios are explored to assess the risk of seismic events for the selected faults. A frictional model is applied to the faults in the fault scale model to capture the slip behavior and stress distribution along the fault surfaces, as well as the potential for fault reactivation under changing stress conditions. When injection starts, compacted reservoir unloads and slip tendency decreases on fault plane until reaching the hydrostatic condition. As injection continues, stress regime changes from normal fault to reverse fault and slip tendency increases promoting fault instability in some scenarios considering low friction coefficients.

Keywords: CCS; Chalk; Slip tendency; Induced seismicity; Fault reactivation