Induced seismicity from hydraulic fracturing and large volume wastewater injection: Revising the assessment criteria using results from recent swarms

English Abstract:

The United States midcontinent has experienced a dramatic increase in seismicity over the past seven years, led by Oklahoma, and accompanied by unusual earthquakes in England and Canada. During this time frame, the new technologies (and the expanded use of existing technologies) used to increase hydrocarbon production have significantly increased the fluid volume injected into the subsurface both for hydraulic fracturing and for wastewater disposal. Studies of these recent earthquake sequences have increased our understanding of fluid triggering. Rare cases also motivate modification of the commonly used criteria (Davis and Frohlich, 1993) for induced seismicity, specifically in the expectations for temporal and spatial correlations between injection rates, well locations, and seismicity. Recent potentially-triggered swarms have grown to cover hundreds of square kilometers: In the well-documented case of seismic triggering in the Paradox Valley region of Colorado, earthquakes occur up to 16 km from injection wells (King et al., 2014). Similarly, hydrogeologic models for central Oklahoma indicate that the region affected by increased pore fluid pressure (sufficiently high to trigger earthquakes) likely extends tens of kilometers from disposal wells (Keranen et al., 2014). Earthquakes in both cases have migrated and continued over 24 and 7 years, respectively. These swarms deviate substantially from existing criteria, though the majority of other cases of apparently triggered earthquakes near disposal wells and hydraulic fracturing operations are more closely aligned. Rapid, near-well seismic responses near hydraulic fracturing operations (e.g., Skoumal et al., 2014; Holland et al., 2012) imply high hydraulic connectivity between the fault zone and injection wells. In contrast, sustained fluid injection at disposal wells, without accompanying fluid extraction, can create a migrating zone of perturbed fluid pressure capable of either activating seismogenic faults near the well on similarly short time frames, or at greater distances over years. Though challenging to compose criteria to encompass both scenarios, modifying to include long-term effects and net volume changes will aid regulators and be relevant for seismicity in regions of production, gas storage, and carbon sequestration.