A comparison of seismicity rates and fluid-injection operations in Oklahoma and California: Implications for crustal stresses

Fluid injection into deep wellbores can increase pore pressure, reduce effective stress, and trigger earthquakes. The extent of the seismogenic response to injection provides insight into how close faults are to failure in the injection-affected area. The seismogenic response to injection operations in hydrocarbon basins is examined in California and Oklahoma. Changes in spatial and temporal seismicity rates are tested for significant variations, and timing and location of such variations are determined based on nonparametric modeling of background seismicity rates. Oklahoma has experienced a recent surge of seismic events, which exceeded the 95% confidence limit of Poissonian background rates in c. 2010. Annual injection volumes in Oklahoma increased systematically between 1998 and 2013 and have been connected to several earthquake sequences. In California, injection volumes increased monotonically between 2001 and 2009; however, the seismogenic response was limited and was devoid of large-scale background rate increase. A detailed comparison of injection parameters in Oklahoma and California included well density, wellhead pressures, peak and cumulative rates, and injection depths. No detectable difference was found that could readily explain the observed changes in seismicity rate in Oklahoma and the lack thereof in California. A strongly different seismogenic response to similar pressure perturbations indicates that the injection parameters considered are only of secondary influence on the resulting earthquake activity. The primary controls on injection-induced earthquakes might be the specific geologic setting and the stress state on nearby faults