Earthquakes that are strong enough to be felt may continue to occur for years after reductions in the rate at which wastewater from continental oil and gas production is pumped below ground. The model, published in Nature Communications, suggests that following wastewater injection rate reductions in Oklahoma and Kansas, the frequency of high-magnitude earthquakes may decrease more slowly than the overall earthquake rate.
Wastewater from oilfields is generally discarded by pumping it into deep geologic formations via salt water disposal wells. This process causes the pressure in basement rocks to increase until a threshold is reached and the pressure is released via seismic activity. Across the mid-continental United States, particularly Oklahoma and Kansas, this causes so called injection-induced earthquakes, typically 4 - 8km below ground.
Ryan Pollyea and colleagues show in a numerical model that wastewater may have a higher density than basement rock fluids and therefore migrates deeper into rocks than previously thought. The model predicts that this delays pressure recovery significantly and can lead to increasing fluid pressures long after wastewater injection rate reductions. The authors propose that reducing or stopping wastewater injections may not necessarily reduce or stop seismic activity and that it could continue for another decade after the process has been curbed. The findings may have implications for long-term planning and seismic hazard assessments in active oil and gas field regions, they conclude.