A Study of Stress Change and Fault Slip in Producing Gas Reservoirs Overlain by Elastic and Viscoelastic Caprocks

Geomechanical simulations were conducted to study the effects of reservoir depletion on the stability of internal and boundary faults in gas reservoirs overlain by elastic and viscoelastic salt caprocks. The numerical models were of a disk-shaped gas reservoir with idealized geometry; they mimic the structure of a gas field in the northern Netherlands which has experienced induced seismicity during gas production. The geomechanical simulations identified the area of the internal fault most sensitive to fault reactivation as coinciding with the epicenters of the largest seismic events associated with gas production. Depletion-induced shear slip is initiated at the depth of the reservoir, in the fault areas where the vertical fault throw ranges between 0.5 and 1.5 times the reservoir thickness. The extent of reactivated area differs depending on whether the caprock is viscoelastic or elastic: when it is viscoelastic, there is more down-dip shear displacement. High initial horizontal stresses in the rock salt and lower stresses in the elastic side-seal and the reservoir promote unloading of the internal and reservoir-bounding faults even before the reservoir is depleted. Particularly prone to fault reactivation are the fault zones along the interface between the reservoir rock and the salt caprock, which may already be critically stressed before depletion and are likely to be reactivated early during gas production. Stress relaxation and associated geomechanical changes affecting fault stability and ground surface deformation may continue long after production has ceased, due to the viscous behavior of the salt.