Since the cessation of injections at two natural gas saltwater disposal wells in the Fayetteville Shale nearly two weeks ago, a swarm of earthquakes there that began last summer seems to have died down, a researcher said. However, it will be at least a few more weeks before any link between the wells and quakes might be seen, and with how much certainty remains unclear.
The operators of the injection wells -- Oklahoma City-based Chesapeake Operating Inc. and Little Rock, AR-based Clarita Operating LLC -- agreed March 4 to stop injections (see Shale Daily, March 8). Seismic activity has moderated since then, the Arkansas Geological Survey's (AGS) Scott Ausbrooks, geohazards supervisor, told NGI's Shale Daily.
"You can definitely see a marked difference, especially in the larger events, the 2.5 or greater," he said. "We're still having the normal, smaller microquakes, but even those, to a degree, have settled down a little bit."
Ausbrooks said there are two schools of thought on the current series of quakes, which are part of what is known as the Guy-Greenbriar earthquake swarm in Faulkner County, AR. One is that the current calm period is a product of nature, similar to what was seen after the initial outburst of the Enola earthquake swarm in Arkansas during the 1980s.
The Enola swarm was initiated by a magnitude 1.2 earthquake recorded on Jan. 12, 1982 near the town of Enola in Faulkner County, according to the AGS website. "Since then, over 40,000 seismic events have been recorded in the Enola area," the survey said. Most of the seismic events have been microquakes, but at least 93 earthquakes have been felt in the local area by at least one person during the first year of seismic activity. The Enola swarm was not linked to natural gas activity.
"The second scenario [for the current swarm] is this could be a response to the [disposal] wells being shut down," Ausbrooks said. "I think what will happen is time will tell.
"We do expect even if it's natural or related to the wells that this possibly could go on for weeks or months afterward. Naturally, the Enola Swarm went on...after it's initial six months of really intense activity, it went on for almost two years afterward. If it's related to the injections, we should see a marked decrease in activity..."
Clarita parent company True Energy Services LLC has strongly objected to the assertion that its well is linked to the earthquakes, noting that quakes began in the area before the well was drilled.
"If our company owners and investors believed there was the slightest chance our well was inducing or triggering earthquakes that might damage persons or property, we would voluntarily plug our well," True Energy partner Mickey Thompson said in a recent statement. "We would not expose our employees, our customers, or our neighbors to such a threat."
The shut in of the Clarita well has crippled his company, Thompson said, noting that unlike Chesapeake, Clarita has no oil and gas production in the state and relies on "the trucking and natural gas producer customers who haul oilfield fluids to our facility for proper and necessary disposal."
As for Chesapeake, "We do not have any additional information to share or comments to make," said spokesman Mark Raines when contacted by NGI's Shale Daily Wednesday.
There is a precedent for a causal link between injection wells and seismic activity that dates back to the 1960s in Colorado, Ausbrooks noted. Whether it has relevance to the Fayetteville quakes is undetermined.
In 1961 a 12,000-foot well was drilled at the Rocky Mountain Arsenal, northeast of Denver for disposal of waste fluids from Arsenal operations, according to the U.S. Geological Survey (USGS) website. Injections began in March 1962 and "an unusual series of earthquakes erupted in the area shortly after."
Thousands of earthquakes were recorded from then into the mid-1960s, according to the website.
"Then a year and half after the Rocky Mountain Arsenal waste dumping practice stopped, the strongest and most widely felt shock in Denver's history struck that area on Aug. 9, 1967, at 6:25 in the morning," the website said.
More modest shocks were felt during 1968. "In September of that year the [U.S.] Army began removing fluid from the Arsenal well at a very slow rate, in hope that earthquake activity would lessen," according to USGS.
Earthquakes continued into the 1970s, but Ausbrooks said researchers "were able to establish a definitive causal relationship between the earthquakes and the [disposal] well."
Ausbrooks said if the trend of decreased seismic activity in the Fayetteville continues, "it would be a good indicator" that the disposal wells and the quakes are related. However, "I don't think I'd draw any conclusions just on that alone. I think you have to use the preponderance of data that we're currently compiling..." he said.
When the research is done AGS will present its findings to the Arkansas Oil and Gas Commission (AOGC), which will decide what, if anything, to do about the wells. A hearing is scheduled for March 29 at the AOGC at which Chesapeake and Clarita will be able to present evidence.
According to AOGC data, Faulkner County is the fifth most productive county in the state when it comes to Fayetteville Shale gas sales, moving 101.9 Bcf from 2006 through 2010. While Faulkner's sales are less than one-fifth those of leader Van Buren County (562.5 Bcf), Faulkner leads No. 6, Independence County (9.1 Bcf), by a wide margin.