Hydraulic Fracture Injection Strategy Influences the Probability of Earthquakes in the Eagle Ford Shale Play of South Texas

Seismicity in the Eagle Ford play grew to 33 times the background rate in 2018. We identified how hydraulic fracturing (HF) contributed to seismicity since 2014 by comparing times and locations of HF with a catalog of seismicity extended with template matching. We found 94 M-L >= 2.0 earthquakes spatiotemporally correlated to 211 HF well laterals. Injected volume and number of laterals on a pad influence the probability of seismicity, but effective injection rate has the strongest effect. Simultaneous stimulation of multiple laterals tripled the probability of seismicity relative to a single, isolated lateral. The 1 May 2018 M-W 4.0 earthquake may have been the largest HF-induced earthquake in the United States. It occurred similar to 10 km from a M-W 4.8 earthquake in 2011 and was thought to be induced by fluid extraction. Thus, faults in this area are capable of producing felt and potentially damaging earthquakes due to operational activities. Plain Language Summary We investigated the recent increase in seismicity rate in the Eagle Ford oil and gas field of south Texas in 2018 that grew to 33 times higher than previous years and how hydraulic fracturing (HF) contributed. We compared times and locations of HF wells with a catalog of seismicity we enhanced through seismogram similarity detection. Over 200 HF wells had seismicity nearby during operation with similar to 90 earthquakes having magnitudes =2.0, indicating seismicity from HF is more common in this area than previously thought. We found that HF strategy affects the probability of earthquakes. Seismicity was twice as likely when operators inject into multiple nearby wells simultaneously compared to when they inject into multiple wells one at a time. The simultaneous strategy was three times more likely to produce seismicity compared to a single well strategy. Of the similar to 2,400 HF-induced earthquakes we identified, a magnitude 4.0 is one of the largest reported in the United States, and it occurred similar to 10 km from the largest (magnitude 4.8) earthquake in south Texas, thought to be due to fluid extraction in 2011. This study demonstrates that faults in this area are capable of producing felt and potentially damaging earthquakes due to ongoing HF.