Analysis of Seismicity Coincident with Hydraulic Fracturing of a Well in Southern Oklahoma

Abstract:

A recently completed, hydraulically fractured, well in Southern Oklahoma revealed a strong temporal correlation between injection parameters and the occurrence of earthquakes distinct from the background rates of seismicity. While this area of Southern Oklahoma has a low level of background seismicity, about 10 located events per year, there have been 43 earthquakes located in the area during 2014. Of the recent events, 29 coincide, in time and space, with the 2-day hydraulic fracturing of this well, while an additional 31 events have been identified through waveform cross-correlation and extend the magnitude of completeness down to 1.5ML. The majority of events that have been manually located are under magnitude 2.5ML, at depths of ~ 3.5 to 8.5 km and are within 1-7 km of the well. There are 6 stations within 90 km from the center of the seismicity with fair azimuthal coverage, the nearest seismic station is 25 km away, and while not ideal, the locations appear to be well constrained. The largest event located was a magnitude 3.2ML, located ~ 4.5 km southwest of the well, ~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 the stages of hydraulic fracturing. Our strong temporal correlation between injection parameters and the occurrence of earthquakes, distinct from the background rates of seismicity, along with the relatively close spatial proximity to the well do suggest 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 or improve 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.