A coupled reservoir-geomechanical simulation study of CO2 storage in a nearly depleted natural gas reservoir

Atzbach–Schwanenstadt natural gas field located in Upper Austria Molasse Foreland basin was one of the four European sites selected for subsurface CO2 storage feasibility/performance evaluation in the recently completed EU-funded research project CASTOR. The objectives of the coupled reservoir-geomechanical modelling effort at Aztbach-Schwanenstadt gas field were: 1) evaluation of the hydro-mechanical response of the reservoir rock and overburden formations to historical and current gas production rates, different CO2 injection scenarios and its long-term storage; and 2) assessment of the potential for shear failure and/or re-activation of pre-existing faults as a result of changes in the reservoir pressure due to natural gas production and CO2 injection. The simulation results in terms of changes in the reservoir stresses and associated reservoir compaction/uplifting are presented and their likely impact on reservoir integrity discussed. The widely used MohrCoulomb shear failure analysis was carried out for the gas reservoir undergoing reservoir pressure depletion and then re-pressurisation due to CO2 injection, in particular under the regional strike-slip fault stress regime relevant to the gas field. An equation was derived to estimate the maximum sustainable pore pressure under different horizontal/vertical stress ratios and rock strength properties, with reference to the stress path hysteresis during reservoir re-pressurisation.