Factors influencing fracking-related seismicity in Oklahoma
The depth of a hydraulic fracturing well in Oklahoma, USA, among other factors, increases the probability that fracking will lead to earthquake activity, reports the Bulletin of the Seismological Society of America. The researchers from Miami University in Ohio, USA, hope their findings, published as part of an upcoming BSSA special issue on observations, mechanisms and hazards of induced seismicity, will help oil and gas operators and regulators in the state refine drilling strategies.
During hydraulic fracturing, a pressurized liquid fractures the rock layer and allows natural gas or petroleum to flow more freely. A growing number of studies suggest that this process can induce seismic activity large enough for people to feel, possibly by increasing fluid pressures within the rock that relieve stress on faults and allow them to slip.
In one rock layer examined in the BSSA study, the likelihood that hydraulic fracturing triggered seismic activity increased from 5 to 50% as well operations moved from 1.5 to 5.5 kilometers deep, the researchers found. Similar effects were seen in Lancashire in the UK during fracking work.
Oklahoma has seen an increase in earthquake activity over the past decade, mostly caused by oil and gas companies injecting drilling wastewater back into deeper rock layers. Hydraulic fracturing is associated with a magnitude 4.6 earthquake in Canada and a magnitude 5.7 earthquake in China, although fracking-induced earthquakes tend to be smaller in magnitude than those caused by wastewater disposal. As a result, oil and gas operators and regulators would like to establish why some wells trigger seismic activity, and how to adjust their operations.
The scientists used publicly available data on injected volume at well sites, the number of wells on a drilling pad, what kind of fluid was injected, and the vertical depth of the well, among other features. Surprisingly, the total volume of injected liquid at the Oklahoma wells did not affect the probability of seismic activity near the well. Other studies of induced seismicity have shown a link.
Most of the wells in the current study are single wells, which may explain the difference. In some places in western Canada and Texas, where there is a link between the injected volume and
The researchers also compared the probability of seismic activity in wells where the injected liquid was a gel versus ‘slickwater’, water with chemicals added to increase flow. They found a lower level of seismicity in gel operations compared to slickwater, although the difference was not as statistically significant as the other trends.