EVIDENCE THAT GEOLOGY PLAYS A PRIMARY ROLE IN INJECTION INDUCED SEISMICITY (Invited Presentation)

The sharp increase in earthquakes in the Central and Eastern United States over the past decade has mostly been attributed to large volume wastewater disposal (WD) into deep sedimentary layers. Although microseismicity with magnitude (M) < 1 is an inherent component of rock-fracturing during hydraulic fracturing (HF), recent evidence has demonstrated that HF can induce larger magnitude events along pre-existing faults and has been the dominant source of seismicity in some areas. To investigate the causes of injection induced seismicity, we have employed waveform similarity techniques to improve the completeness of seismicity catalogs and compared them with detailed records of HF and WD activity. Through coordination, collaboration, computational developments, and field deployments, we have documented cases of seismicity induced by HF and WD in Oklahoma, Texas, Ohio, West Virginia, and Pennsylvania. The cases studies have helped to reveal how geology plays a primary role in the probability of seismicity being induced by injection activities. The proximity of injection to seismogenic faults is one of the most influential factors, with several studies illustrating this relationship. In the Eagle Ford, nearly all seismicity occurs along a mapped fault of the Karnes Trough, including the largest reported HF induced earthquake in the US (M 4.0), despite HF injection occurring over a widespread area. This principle is also demonstrated in the Appalachian Basin where the probability of seismicity was substantially higher for WD and HF wells in close proximity to the Precambrian basement where faulting is more pervasive. In fact, detailed studies of HF and WD induced seismicity in Ohio have revealed the different frequency-magnitude patterns of more mature basement roots versus those on the immature flower structures in the overlying sedimentary section. However, higher probabilities of seismicity for deeper HF wells in the Anadarko Basin are not correlated with basement proximity, and instead point to increased overpressure deeper in the basin raising the likelihood of seismicity. Nevertheless, geology is not the only key factor, as studies also demonstrate that the injection rate, strategy (e.g., zipper HF), and fluid viscosity each have an influence on the probability of seismicity as well. Together, these findings provide clues about the physical mechanisms involved, but ultimately broader integration with geomechanical, hydrological, and structural data will be needed to determine the best practices for mitigating the hazard.