‘Hindcasting’ helps identify causes of induced earthquakes in Delaware Basin
Using a method that works backwards based on a series of observed earthquakes to test seismic models that fit those observations, researchers in the Delaware Basin were able to determine whether earthquakes in the region since 2017 were caused by oil and gas operations.
The new study published in Seismological Survey Letters also indicates whether seismicity in a specific part or “block” of the basin, which extends through western Texas and New Mexico, was caused by hydraulic fracturing or shallow or deep wastewater runoff.
Iason Grigoratos of ETH Zurich, Swiss Seismological Service and colleagues at the University of Texas at Austin calculated the likelihood that one or both of these processes caused seismicity in each block, and found that at least 68% of earthquakes in the Delaware Basin in their study occurred in blocks where oil and gas activities were the likely cause with a 95 percent confidence interval. Grigoratos conducted the research when he was a postdoctoral researcher at the UT’s Jackson School of Geosciences.
The SRL research found that hydraulic fracturing was linked to more seismic events than previously suspected, especially in Reeves County in Texas. However, shallow (about one to two kilometers below ground level) wastewater discharge is linked to more earthquakes overall, Grigoratos and colleagues conclude.
Seismicity around the town of Pecos appears to be affected by both hydraulic fracturing and wastewater runoff, the researchers found. In Texas’s Culberson County, and around the epicenter of the 5.0 magnitude earthquake in Mentone, Texas, in 2020, the triggering mechanism appears to be the drainage of sewage from deep wells.
Historically, the Delaware Basin has experienced very low seismicity, with only three events above magnitude 4.8 in the past 100 years. But the level of seismicity has been gradually increasing in the area since 2009 and is increasing further after 2013. These changes are related to an increase in injection volume in both hydraulic fracturing and wastewater drains in the area since 2013.
In their paper, Grigoratos and colleagues “hindcast” magnitude 1.5 and larger earthquakes that occurred in the Delaware Basin after 2017, using data on hydraulic fracturing or wastewater discharge in the region as input to their forecasting model. They compared these speeds to those expected under regular tectonic conditions in the region.
The hindcasting method works well at disentangling the possible causes of seismicity in areas where there are multiple types of overlapping oil and gas activities, Grigoratos said.
Looking at changes in well operations over a specific time and space interval may be useful in studying “isolated bursts of seismicity in areas of low well density, such as in hydraulic fracturing earthquakes in areas of non-overlapping oil and gas operations.” ,” he noted, but “they are not well suited to wastewater runoff, which often shows multi-year delays to the onset of distant and gradual seismicity.”
Hindcasting seismicity rates over time within specific spatial blocks can account for changes in injection rates and how pore pressure changes caused by injection diffuse over time and space through subsurface formations, he added.
In places where both hydraulic fracturing and wastewater runoff occurred, the method doesn’t identify which could be more important in causing earthquakes, Grigoratos said. “Seismicity in areas affected by both hydraulic fracturing and wastewater runoff is in the ‘shallow’ sedimentary layers and has not reached magnitudes above magnitude 3.8 since 2017,” he explained. “Therefore, as long as the earthquake faults don’t get bigger and don’t migrate very close to exposed assets of critical importance or high vulnerability, regulatory constraints don’t seem very likely.”
The block map of the region could help oil and gas operators and regulators identify places where future earthquake mitigation or monitoring measures should be taken, the researchers noted.
Study maps stress changes around fault triggered by hydraulic fracturing
Iason Grigoratos et al, Distinguishing the Causative Factors of Induced Seismicity in the Delaware Basin: Hydraulic Fracture or Wastewater Discharge?, Seismological Survey Letters (2022). DOI: 10.1785/0220210320
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