The Impact of Viscoelastic Caprock on Fault Reactivation and Fault Rupture in Producing Gas Fields

In The Netherlands gas is produced from over 150 onshore gas fields. In several fields induced seismicity has been recorded during production. These seismic events are interpreted as induced by pore pressure changes in the reservoir rocks, resulting in stress changes on faults within and in close vicinity of the gas fields. Understanding the underlying processes of production-induced seismicity is crucial for the assessment and mitigation of seismic hazards during ongoing production in the onshore gas fields. In this study, we use a numerical geomechanical model in FLAC3D to analyze the relation between changes in reservoir pore pressures and fault stress changes. We address the effects of fault strength, reservoir and fault geometry and the presence of a viscoelastic caprock on the timing of fault reactivation, the nucleation of seismic events and the main characteristics of the fault rupture process. Results of our models show that the presence of viscoelastic caprock can strongly influence the timing and extent of fault reactivation and rupture. Faults with offset are generally reactivated at an early stage of reservoir depletion and involve relatively small slip displacements, stress drops and rupture lengths. The presence of a viscoelastic caprock even further promotes fault slip and nucleation of a seismic events at an early stage of depletion. Faults without offset are reactivated during later stages of depletion and involve larger slip displacements, stress drops and more extensive slip lengths than in case of early reactivation.