In early 2016 the Oklahoma Corporation Commission (OCC), the state regulator for oil and gas wells in Oklahoma, observed anomalous seismicity near oil and gas wells with active hydraulic fracturing operations. In response to well completion-associated seismicity, the OCC in the summer of 2016 implemented mandatory submission of Hydraulic Fracturing Notices ("Frac Notices"), and followed with a series of seismicity protocols related to well completions beginning in 2016, that were further refined through 2018. In near real-time, the OCC cross-references Frac Notices with seismic events in the Oklahoma Geological Society (OGS) seismic network to create a catalog of well completions and potentially associated seismicity, the "Frac Notice – Seismicity Match Catalog." In this paper, we review the Catalog for a time period between October 25, 2016 and August 6, 2019 and present the amount, location and rate of seismicity associated with hydraulic fracturing in the State of Oklahoma. To be included in the OCC's Frac Notice – Seismicity Match Catalog, an earthquake must have occurred within 5 kilometers of a well and between the start of that well's hydraulic fracturing operations and seven days following the onset of well flowback. During the study period we identified 826 unique well-seismicity matches, or 19.2% of the wells for which Frac Notices had been submitted. We further refined the Catalog to 624 wells by removing wells with associated seismicity where the maximum magnitude is <2. Assigning individual seismic events to a single nearby well further culled the dataset to 333 wells with hydraulic fracturing-related seismicity.
The major findings of this study are:
7.7% of the wells fracture stimulated during the study period in the state of Oklahoma have associated seismicity magnitude ≥2.
The rate of well-seismicity occurrence and amount of seismic energy matched to a well completion is relatively constant over the study period.
There are areas in Oklahoma with a significantly higher concentration of wells associated with seismic events. In the Central Area of the Well Completion Area of Interest outlined by the OCC, 19.5% of the wells have associated seismicity with magnitudes greater than or equal to 2.0.
We identified 960 earthquakes with magnitude ≥ 2 in the Frac Notice – Seismicity Match Catalog. Of those, 57 events had a magnitude ≥ 3. The largest earthquake associated with a well completion occurred in July 2019 in Kingfisher County; the OGS earthquake catalog recorded the magnitude as a ML 3.9 and the USGS NEIC catalog listed the event as a Mw 3.6.
Seismic waveform template matching using the OGS network data has helped increase the sensitivity of the OGS regional seismic array with additional earthquake detections. The OCC will continue to assess its usefulness in assessing the success of the operator's mitigation strategies during hydraulic fracturing operations associated with induced seismicity.