A Probabilistic Methodology for Evaluating Fluid Injection-induced Seismic Risk in Faulted Geothermal Reservoirs and Application to a Field Case Study in Iceland

Abstract

Fault re-activation and associated seismicity pose a potential threat to industrial processes involving fluid injection into the subsurface. This paper presents the development of a probabilistic methodology to evaluate fluid injection-induced seismic susceptibility in faulted reservoirs. This methodology assumes that fluid injection-induced seismicity results from slippage of pre-existing fractures primarily driven by fluid pore pressure diffusion. The fracture criticality, defined in this study as the gradient of critical fluid pressure change to trigger seismicity (?pc/h, h being the reservoir depth), is used as the metric for fault slip susceptibility in the evaluation. The probabilistic injection-induced seismic risk evaluation is carried out by integrating seismic observations and hydrogeological modelling of fluid injection operations to obtain the probabilistic distribution of fracture criticality, which is in turn used to calculate the likelihood of seismic event occurrence within a certain region during a fluid injection period. The proposed seismic risk evaluation method considers the injection-driven fluid pressure increase, the variability of fracture criticality, and regional fracture density. Utilising this methodology, the probabilistic distribution of fracture criticality was obtained to evaluate the potential for injection-induced seismicity in both fault and off-fault zones at the Hellisheiði geothermal site, Iceland over a two-year period. Fault zones around five geothermal fluid re-injection wells at the site were estimated to have relatively high probability of seismic event occurrence, and these regions experienced high levels of induced seismicity over the microseismic monitoring period. The seismotectonic state estimated for each zone is generally consistent with the forecasted susceptibility to seismicity based on statistics of fracture criticality.