Explaining long-range fluid pressure transients caused by oilfield wastewater disposal using the hydrogeologic principle of superposition

Injection-induced earthquakes are now a regular occurrence across the midcontinent United States. This phenomenon is primarily caused by oilfield wastewater disposal into deep geologic formations, which induces fluid pressure transients that decrease effective stress and trigger earthquakes on critically stressed faults. It is now generally accepted that the cumulative effects of multiple injection wells may result in fluid pressure transients migrating 20–40 km from well clusters. However, one recent study found that oilfield wastewater volume and earthquake occurrence are spatially cross-correlated at length-scales exceeding 100 km across Oklahoma. Moreover, researchers recently reported observations of increasing fluid pressure in wells located ~90 km north of the regionally expansive oilfield wastewater disposal operations at the Oklahoma-Kansas border. Thus, injection-induced fluid pressure transients may travel much longer distances than previously considered possible. This study utilizes numerical simulation to demonstrate how the hydrogeologic principle of superposition reasonably explains the occurrence of long-range pressure transients during oilfield wastewater disposal. The principle of superposition states that the cumulative effects of multiple pumping wells are additive and results from this study show that just nine high-rate injection wells drives a 10-kPa pressure front to radial distances exceeding 70 km after 10 years, regardless of basement permeability. These results yield compelling evidence that superposition is a plausible mechanistic process to explain long-range pressure accumulation and earthquake-triggering in Oklahoma and Kansas.