Analysis of mechanics of fault reactivation in depleting reservoirs

Abstract

Pressure depletion of hydrocarbon reservoirs can cause re-activation along fault planes of weakness, resulting in geomechanical hazards such as seismicity. The article presents analytical solutions of conditions leading to mechanical fault reactivation. The developed methodology starts by defining various fault categories present in the Dutch subsurface based on reservoir setting and fault properties, such as throw, dip, strike and transmissibility. For all fault categories initiation criteria for fault re-activation have been developed based on a new Coulomb stability relationship. The depletion-induced poro-elastic stress changes depend on the reservoir geometry and the fault transmissibility while the induced shear stresses are determined from differential vertical displacement profiles. The induced shear stress is either caused by the fault throw or by the pressure difference caused by the (partly) sealing fault or at the boundary fault. Analysis of the Coulomb stability function allows determination of the relative importance of poro-elastic stressing relative to differential compaction. The critical reservoir pressure depletion for fault re-activation to occur has been determined for open, intra-reservoir faults with or without a throw, (partly) sealing intra-reservoir faults and boundary faults. Satisfying this Coulomb criterion is the first necessary mechanical condition for seismicity to occur. These results can be used to determine the initiation pressures for seismicity to occur in depleting gas reservoirs in The Netherlands and can in addition explain why the observed seismicity has been attributed to steeply dipping fault planes.