A coupled reservoir-geomechanical simulation study of CO_2 storage in a nearly depleted natural gas reservoir

摘要

Atzbach-Schwanenstadt natural gas field located in Upper Austria Molasse Foreland basin was one of the four European sites selected for subsurface CO_2 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 CO_2 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 CO_2 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 Mohr- Coulomb shear failure analysis was carried out for the gas reservoir undergoing reservoir pressure depletion and then re-pressurisation due to CO_2 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.