ANALYSIS OF SEISMICITY COINCIDENT WITH HYDRAULIC FRACTURING OF A WELL IN SOUTHWEST OKLAHOMA

In the case of a hydraulically fractured well in Southwest Oklahoma, there is a strong temporal correlation between injection parameters and the occurrence of earthquakes that is clearly distinct from the background rates of seismicity. While Southwest Oklahoma has a low level of background seismicity, about 10 located events per year, there have been 53 earthquakes located in the area during 2014. Of the recent events, 32 coincide, in time and space, with the four-stage, 2-day, vertical hydraulic fracturing of this well while an additional 52 events have been indicated through cross-correlation. The majority of events that have been manually located are under magnitude 2.5ML, occur at depths of ~ 3.5 to 8.5 km, and are within 1-9 km from the well. The largest, event located was a magnitude 3.2ML, located ~ 4.5 km southwest of the well, at ~7.3 km depth, and occurred approximately 100 minutes after fracturing began. By plotting the pressures measured at wellhead and the discharge rates through time, we are able to see a strong correlation with the seismicity and hydraulic fracturing of this well. Our strong temporal correlation between injection parameters and the occurrence of earthquakes, distinct from the background rate of seismicity, along with the relatively close spatial proximity to the well suggests a causal link. What remains to be explained is the apparent significant spatial offset between the stimulated well and the earthquakes coupled with the short time between the stimulation of the well and the onset of seismicity. Greater geotechnical information will be required to both address our earthquake location uncertainties as well as geomechanical considerations as to how this sequence of earthquakes may have been triggered by hydraulic fracturing and what information we can ascertain about Earth properties in the area from this occurrence.