Geomechanical numerical simulations were conducted to analyze the stability of faults during gas production. A FLAC3D model of a fault intersecting a producing gas reservoir was developed which incorporates the fully dynamic behavior of the fault and surrounding rock mass, and a fault frictional behavior based on a slip-weakening law. The simplified reservoir and fault geometry of the model are representative for the Dutch gas fields. Simulated fault slip displacements and fault slip lengths were used to calculate moment magnitudes of induced seismic events. In addition, results of the fully dynamic model with slip-weakening frictional behavior were compared to results of a static geomechanical model with slip-weakening. Comparison of model results shows that, for a first-order assessment of induced seismicity, the static models can be used as a simplified and computationally less expensive alternative to the fully dynamic fault rupture models.
The operation of a gas field causes dynamic changes of the pore pressure and therefore changes in the stress state of the reservoir and surrounding rocks. Production-induced stress changes can destabilize faults which transect or bound the reservoir, or are located in the vicinity of the reservoir, causing associated seismicity. Whether or not faults are (seismically) reactivated during reservoir depletion depends on a complex interplay of many factors, such as the combination of initial stress state of the reservoir and faults, the magnitude of pore pressure changes, the geometry of the reservoir and faults, such as orientation, dip and fault offset, and the geomechanical properties of the rocks and faults.
In many onshore producing gas fields in the Netherlands seismicity has been observed during the depletion of the gas reservoirs. Induced seismicity has been recorded in 26 out of 150 producing gas fields, with maximum magnitudes of the seismic events up to Mw 3.6. In all of the gas fields pressure depletion was significant prior to the onset of recorded seismicity (Van Wees et al., 2014). The marked delay between the start of reservoir depletion and the onset of seismicity has been interpreted as an indication that the in-situ stress conditions in the northern part of the Netherlands are non-critical.