FLUID COMPOSITION AFFECTS INJECTION-INDUCED EARTHQUAKE OCCURRENCE, PERSISTENCE & HAZARD

It now widely known that oilfield wastewater disposal causes earthquakes. Although several mechanisms have been proposed to explain injection-induced earthquakes, there is little doubt that they originate with pressure transients caused by wastewater injections into deep geologic formations. This study shows that, in addition to pressure transients related to pumping, there are pressure transients caused by high-density wastewater displacing comparatively lower density basement fluids. We model this phenomenon for a typical high-rate injection well in Alfalfa County, Oklahoma, where wastewater is characterized by dissolved solids concentration exceeding 200,000 ppm and mean annual earthquake depth has been increasing each year since the onset of seismicity. Our models show that density-driven pressure fronts migrate deeper at the same rate as mean annual hypocenter locations during and after injection operations. Moreover, we find that density-driven pressure transients remain in the environment over much longer timescales than pressure transients caused by pressure diffusion. To understand the broader implications of density-driven pressure transients, we analyze earthquake data from the Anadarko Shelf from 2013 to 2018, and find that the relative proportion of high-magnitude earthquakes increases below 8 km depth. Thus, for regions where wastewater is characterized by higher density than host rock fluids (i) injection-induced earthquakes may persist long after wastewater injection rates decline, and (ii) the frequency of high-magnitude earthquakes may decay more slowly than the overall earthquake rate.