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The Barnett Shale is an area of natural gas in Johnson County, Texas. The company, Chesapeake Energy, is drilling natural gas wells through the horizontal drilling method. There is question as to whether the drilling is responsible for recent earthquakes.

Sizing up seismic activity

By: Julie Rambin

Posted: 6/25/09

The small town of Cleburne, in North Texas, has been experiencing something unprecedented in its 142-year history - earthquakes.

"It's a little unnerving," said Cleburne Mayor Ted Reynolds. "We've had five in recent days."

The earthquakes have been between 2.0 and 3.0 magnitude - just enough to be felt, but not enough to damage property, Reynolds said.

"Some people haven't felt them at all. No damage, no injuries," he said. "Most people are curious rather than scared."

Beneath Cleburne is the Barnett Shale formation, one of the largest onshore natural gas fields in the nation. In the last several years, the city has received between $20 million and $25 million in drilling royalties, and some people are asking whether the drilling and the earthquakes are connected, Reynolds said.

"We have enjoyed a wonderful relationship with the drilling company so far," Reynolds said. "We hope that's not what it is."

Geophysicists from Southern Methodist University were interested in the possible causes of the earthquakes, and have begun a research study of the area, said geophysics research projects director Chris Hayward.

"Cleburne was very interested in finding out more about it and we were very interested in doing the research," Hayward said. "We'll try to decide where the earthquakes are, how deep they are. Right now we don't have any evidence one way or another. We just don't know."

There is currently no evidence to show that natural gas drilling causes earthquakes, Hayward said.

"There are some papers in the State of Texas talking about induced seismicity in West Texas, suggesting that if you drill [an oil] field for a long period of time, you might induce earthquakes," Hayward said.

Chesapeake Energy, the company responsible for much of the drilling in Cleburne, denies any connection between drilling and earthquakes.

"Often earthquakes are located many miles below drilling sites," said Chesapeake Energy public relations manager Jerri Robbins. "There is no proof yet that drilling activities and seismic activities are related."

However, drilling and seismic activity can be related, according to some geophysicists.

"The shale is pretty brittle. [Drilling] changes the pressure from the natural gas field to the rock itself," said Christopher Mathewson, a Texas A&M University regents geophysics professor. "So the rock fails."

Earthquakes are comparable to a whoopee cushion, Mathewson said.

"When you sit on it, you increase the load at the surface to the point that you push the air out of the cushion. When you're expelling the air from the cushion, you're sinking further into the seat," Mathewson said. "You moved down, didn't you? That's subsidence, because of the change of pressure."

Subsidence is the downward movement of a field, Mathewson said, and this term is more appropriate for the Cleburne earthquakes.

"They're caused by changing the stress," Mathewson said. "They're not associated with a major fault displacement."

The seismic events in Cleburne may continue and might augment the available quantity of gas in a particular field, Mathewson said.

"Sometimes the rock breaks and it increases the amount of gas you can get out of them," Mathewson said. "So gas producers like them."

Though no damage has resulted, it is still necessary to learn more about the unexpected seismic activity, Hayward said.

"If I were in the town, I would not be particularly worried about them," he said. "The big question is, how deep are they? One of the reasons we're doing the study is to find out."