Can natural fluid pore pressure be safely exceeded in storing gas underground?

Underground gas storage (UGS) and CO2 sequestration (CCS) are strategic practices to address the growing demand of energy and reduction of greenhouse gas emission. There is an interest from the energetic, economic, and environmental viewpoint to store as much gas as possible consistent with the requirement of a safe disposal. A transversely isotropic geomechanical model is developed and calibrated using the vertical and horizontal displacements measured by SAR-based interferometry over an exhausted gas field located in Northern Italy where UGS is active since 1986. The predictions show that a maximum storage pressure up to 140% pi, pi being the virgin fluid pore pressure, may yield a 400% increase of the gas stored relative to pi provided that an accurate assessment of the parameters defining the yield surface, i.e. friction angle and cohesion in the Mohr–Coulomb criterion, is performed for the reservoir formation. No appreciable risk for the integrity of the sealing layer is ever expected, along with a negligible impact on the ground structures. Land motion does not exceed few centimeters with the differential displacements safely below the bound required by structural safety.