Post-injection seismicity associated with hydraulic stimulation has
posed great challenges to hydraulic fracturing operations. This work
aims to identify the causal mechanism of the post shut-in ML 2.9
earthquake in August 2019 at the Preston New Road, UK, amongst three
plausible mechanisms, i.e., the post shut-in pore pressure diffusion,
poroelastic stressing on a non-overpressurised fault, and poroelastic
stressing on an overpressurised fault. A 3D fully-coupled poroelastic
model that considers the poroelastic solid deformation, fluid flow in
both porous rocks and fracture structures, and hydraulic fracture
propagation was developed to simulate the hydromechanical response of
the shale reservoir formation to hydraulic fracturing operations at the
site. Based on the model results, Coulomb stress changes and seismicity
rate were further evaluated on the PNR-2 fault responsible for the
earthquake. Model results have shown that increased pore pressure plays
a dominant role in triggering the fault slippage, although the
poroelastic stress may have acted to promote the slippage. Amongst the
three plausible mechanisms, the post shut-in pore pressure diffusion is
the most favoured in terms of Coulomb stress change, seismicity rate,
timing of fault slippage and rupture area. The coupled modelling results
suggested that the occurrence of the post shut-in ML 2.9 earthquake was
a three-staged process, involving first propagation of fracture tips
that stimulated surrounding reservoir formations, then hydraulic
connection with and subsequent pore pressure diffusion to the
partially-sealing PNR-2 fault, and eventually fault activation primarily
under the direct impact of increased pore pressure.