Texas earthquakes prompt regulators to take action

Oct. 21, 2014
A series of unusual earthquakes in North Texas is prompting state regulators to look closely at a potential link between seismic activity and wastewater injection wells.

Rachael Seeley, Editor

A series of unusual earthquakes in North Texas is prompting state regulators to look closely at a potential link between seismic activity and wastewater injection wells.

The Texas Railroad Commission (RRC) has hired a seismologist and proposed rules that would require permit applications to include a seismic analysis and grant the agency authority to adjust permitted injection volume or close down wells linked to seismic activity.

The RRC unveiled the rules in August. They come after a swarm of small earthquakes began last year around the North Texas towns of Reno and Azle, along the Parker and Tarrant-county line (Fig.1). US Geological Survey (USGS) data show more than 25 earthquakes greater than magnitude 2.0 have struck the area since Nov. 1, with the two strongest quakes measuring magnitude 3.6. Earthquakes above magnitude 3.0 are fairly small but powerful enough to be felt by humans on the surface.

The proposed rule change comes after more than 30 residents from the Reno-Azle area traveled to the state capital in January to ask the RRC to shut down area injection wells as a precautionary measure. The agency declined, citing a lack of information.

Expert hired

In March, RRC added a seismologist to its staff. David Porter, one of RRC's three commissioners, said the hiring of David Craig Pearson-a seismologist and former team leader for Los Alamos National Laboratory-will enable the agency to "further examine any possible correlation between seismic events and oil and gas activity and gain a more thorough understanding of the science and data available."

Christi Craddick, a fellow RRC commissioner, said: "It is important that sound science be our guide in determining if there are any links to seismic activity."

Including the activity near Reno and Azele, more than 50 small quakes have struck within 120 miles of Dallas since 2008, correlating roughly with the emergence of the Barnett shale as a commercial gas play.

USGS data show no quakes were recorded in North Texas prior to 2007. The agency maintains an online database with records of all quakes greater than magnitude 2 that have occurred in the continental US since 1973.

Complex geology

Wastewater injection activity in North Texas has increased in conjunction with the rise of hydraulic fracturing of Barnett shale wells, which generate flowback and waste water. Barnett shale production averaged 4.9 bcfd in the first 6 months of 2014 (see figure).

RRC's proposed rules would require applications for new waste-water disposal wells to include a seismic analysis of the surrounding area.

RRC is calling for injection well permit applications to include information such as logs, geologic cross sections, and structure maps for wells proposed in areas where "conditions exist that may increase the risk that injected fluids will not be confined to the injection interval," according to a memorandum issued Aug. 5.

RRC said risky conditions are determined by "complex geology, [the] proximity of baserock to the injection interval, transmissive faults, and/or a history of seismic events in the area as demonstrated by information available from the United States Geological Survey."

Applicants would also be required to calculate the estimated radius of each well's 10-year, 5-psi pressure-front boundary then cross-reference that zone against the USGS online database to determine if any earthquakes have occurred within that area.

The rule change would also grant RRC the authority to modify the volume and pressure of injection activity or even terminate a permit for an injection well if data link it to seismic activity.

More than 34,000 wastewater disposal wells are operating in Texas, and more than 50,000 Class II injection wells have been permitted in the state since the 1930s. The Class II designation applies to injection wells used for disposing waste that arises from oil and gas production, such as flowback water and waste water from hydraulic fracturing. It also applies to injection wells used for enhanced oil recovery and wells used to store liquid hydrocarbons.

The US Environmental Protection Agency estimates there are 144,000 Class II injection wells across the US.

Justin Scott, an associate with the law firm Baker Hostetler, thinks the hiring of a seismologist and the proposed seismic reporting requirements signal that the RRC is responding to public concerns.

"I think that's their means of trying to collect that data and do a more thorough analysis of it so they can go back to the public and say 'Hey, we've been looking into this, and here is what we have determined,'" Scott told UOGR.

Monitoring quakes

Southern Methodist University (SMU), Dallas, has installed a temporary seismic network in the Reno-Azle area to closely monitor seismic activity. From December 2013 to May 2014, the network-which is capable of detecting earthquakes weaker than the magnitude 2.0 threshold detected by USGS equipment-recorded more than 300 earthquakes large enough to be felt at multiple seismic stations and thousands of very small events during periods of earthquake swarm activity.

Researchers will use the data to precisely identify the locations of the quakes and characterize the sizes, mechanisms, and accelerations associated with them. The North Texas Earthquake study is expected to take more than 1 year to complete.

Other published studies have identified a link between seismic activity and a handful of injection wells in Texas and Oklahoma. None of the research found a link to hydraulic fracturing.

In 2012, researchers at the University of Texas at Austin (UT) identified a correlation between small quakes and injection wells in the Barnett shale region. Most of the recorded quakes were within 2 miles of one or more injection wells, and the majority were too weak to be felt at the surface-ranging in magnitude from 1.5 to 2.5.

The UT study identified eight distinct earthquake groups and found that all of the injection wells nearest to the earthquake groups had high injection rates, exceeding 150,000 bbl/month of water. Yet no earthquake activity was identified in other areas near wells with similarly high injections rates.

Author Cliff Frohlich, senior researcher at UT's Institute for Geophysics, said the results suggest an injection well can trigger an earthquake only if injected fluids reach and relieve friction on a nearby fault that is ready to slip. "You can't prove that any one earthquake was caused by an injection well," Frohlich said. "But it's obvious that wells are enhancing the probability that earthquakes will occur."

A joint study published by researchers at SMU and UT in 2010 found a saltwater disposal well was a "plausible cause" for a series of small earthquakes near the Dallas-Ft. Worth Airport (DFW) that occurred from October 2008 to May 2009. The quakes stopped after Chesapeake Energy Corp. shut down one of two injection wells on airport property. Seven weeks before the first earthquake was recorded, wastewater injection began into a zone beneath the Barnett shale.

The researchers found a northeast-trending fault intersects the Dallas-Tarrant County line roughly where the airport quakes occurred. According to the study, "It is plausible that the fluid injection in the southwest saltwater disposal well could have affected the in-situ tectonic stress regime on the fault, reactivating it and generating the DFW earthquakes."

In neighboring Oklahoma, a USGS study published this year found that a magnitude 5.7 earthquake that struck near Prague, Okla., in 2011 and led to hundreds of smaller aftershocks was likely triggered by a smaller quake, or foreshock, linked to wastewater injection wells near a known fault. The earthquake, which occurred on Nov. 6, 2011, was the most powerful ever recorded in the state (UOGR, May-June 2014, p. 20).

Rulemaking process

RRC was expected to publish its proposed rules in the Texas Register on Aug. 29, followed by a 30-day public comment period. If no comments are received, Scott said, the rules could go into effect when the comment period passes. If RRC receives comments that indicate something warrants further examination, the agency could take longer to do more research and potentially modify the proposed rules.

In the meantime, SMU researchers will continue their investigation into the cause of recent earthquakes in North Texas and look for a potential link to wastewater fluid injection.

"Understanding if and/or how the injection of fluids into the crust reactivates faults has important implications for seismology, the energy industry, and society," SMU researchers said in a recent progress report.