Omori Decay of Hydraulic Fracture Induced Seismicity

ABSTRACT: An important consideration for managing induced seismicity is the characterization of seismicity following operational changes made in an attempt to mitigate seismicity. When operational changes are made to control seismicity, ongoing activity associated with earlier activated structure can continue to generate seismicity regardless of the current operational status. It is therefore important to distinguish between ongoing ‘relaxation’ activity associated with a return to geomechanical equilibrium versus continued seismicity associated with ongoing activation and additional perturbation of the seismically active fault. In this paper, three case studies are described demonstrating that seismicity follows an Omori decay, consistent with earthquake aftershock sequences. The Omori decay characteristics thereby provides an important framework to distinguish between fault ‘relaxation’ seismicity and additional activation. 1. INTRODUCTION Induced seismicity is an important consideration for many industrial activities, including mining, reservoir impoundment, geothermal extraction, petroleum production and subsurface injections. Unconventional reservoir development has recently resulted in an increase in reported cases of induced seismicity (see for example, Schultz et al., 2020), particularly in regions of North America that were previously seismically quiescent. Specifically, hydraulic fracture stimulations and subsurface disposal of produced connate saltwater have both resulted in anomalous seismicity in several regions. Injection induced seismicity results from increased subsurface pore pressure reducing the effective normal clamping stress on preferentially oriented, pre-existing faults (Ellsworth, 2013), triggering slip and associated seismicity along these reactivated faults. In response, regulatory agencies and industry have developed operational guidelines to ensure safe operations, and industry best practices have emerged to mitigate induced seismicity. Seismicity patterns associated with hydraulic fracturing and saltwater disposal (SWD) typically have distinct patterns (Rubenstein and Mahani, 2015). SWD injections are generally located in the general vicinity of unconventional wells so that water coproduced with hydrocarbons can be safely stored in deep, permeable intervals. SWD wells normally operate for an extended time period resulting in pressure plumes within the target intervals which in some case encounter critically stressed faults that are activated resulting in associated seismicity. Seismicity can occur several kilometers from the causal injection well with variable onset times relative to the start of injection. Cumulative injection effects from interaction of different injection wells can further extend the spatial reach and timing of fault activation and associated seismicity. In general, SWD induced seismicity tends to be relative long lasting, since elevated pressure can persist after injections stop and may appear as a diffuse regional effect around groups of injection wells.