Managing seismic hazard from induced earthquakes in central Arkansas through cooperation between scientific and regulatory agencies

Abstract

ABSTRACTAfter the first saltwater disposal (SWD) wells associated with the development of the Fayetteville Shale became operational in April 2009, central Arkansas experienced an increase in the rate of earthquakes with M ≤4.7. This seismic activity in the Fayetteville Shale development area (FSDA) included both natural and induced earthquakes that often occurred in spatial and temporal clusters. The Center for Earthquake Research and Information (CERI) and the Arkansas Geological Survey (AGS) closely monitored the area using a permanent network of seismograph stations, augmented by portable real-time broadband stations, to focus on specific sites of suspected induced seismicity. Any concerns were communicated directly to the Arkansas Oil and Gas Commission (AOGC), the agency charged with regulating SWD wells in the state.The largest and most significant cluster of induced earthquakes in the FSDA occurred between August 2010 and July 2011, when thousands of earthquakes (M ≤4.7) occurred on the Guy-Greenbrier fault, which had not been mapped prior to the earthquakes. Since a 13-km-long fault can potentially produce a M 6 earthquake if it slips as a single event, CERI and AGS staff testified at an AOGC hearing in July 2011 that continued fluid injection into SWD wells near the fault could potentially trigger a damaging earthquake. Operators voluntarily shut down and plugged three nearby SWD wells. Another SWD well operator was ordered (by the AOGC) to shut down and plug their well. Earthquake activity dramatically decreased on the Guy-Greenbrier fault and ceased soon after shut-in, thus providing strong supporting evidence that the earthquakes were induced. The AOGC also established a moratorium that prohibited SWD well operations in the area surrounding the fault and rules governing proximity of new SWD wells to mapped faults and proximity of SWD wells to each other. When the price of natural gas rises to make development cost effective, we anticipate the new rules will help to minimize the occurrence of felt and potentially damaging earthquakes.Although the Guy-Greenbrier earthquake sequence ended in 2011, earthquakes still occurred within the FSDA. While some of these earthquakes were within 10 km of active SWD wells, many other earthquakes were not close to any SWD well. We identified 156 earthquakes M ≤3.4 from the regional catalog that occurred between 2012 and 2016 that were within 5 km of a natural-gas production well during the hydraulic fracturing window posted by the AOGC. This suggests that these earthquakes may have been induced by hydraulic fracturing.