Geomechanical Modelling of the Induced Seismicity for a Gas Field

Abstract

Gas production from a reservoir may lead to reactivation of nearby faults, which is likely to cause local seismic events. This paper is focusing on the calculations of stress evolution during depletion of a gas reservoir in order to investigate the possibilities for the reactivation and slip on normal faults, thrust faults and bedding planes at the contact between reservoir and overburden. The program DIANA was used for finite element calculations for a gas field under consideration. Several calculation were run with various initial stress regimes and frictional parameters for faults. The possibility of reactivation of thrust faults in the overburden of the depleting gas reservoir is largely determined by the in situ stress regime in the subsurface. In a normal faulting stress regime, when the vertical effective stresses are greater than the horizontal stresses, the shear stress on thrust faults decreases during depletion reducing the chance of fault reactivation. In a thrust faulting stress regime, with the lateral stress ratio greater than one, the shear stress on thrust fault increases during depletion rising the chance of fault reactivation. In a normal faulting stress regime, reservoir depletion will likely cause reactivation of two normal, wedge-forming faults at reservoir level. A slip on bedding planes, which mark the contact between the reservoir and the overburden, may occur locally during depletion as the consequence of reactivation of normal faults.