by Carolyn Gramling Thursday, January 5, 2012
A saltwater disposal well, a part of the natural gas production process, may have been responsible for triggering a series of minor earthquakes in the Dallas-Fort Worth area of Texas in 2008, according to a recent study.
From Oct. 31 to Nov. 1, 2008, several minor earthquakes rattled the walls and shook the furniture of numerous residences in the Dallas-Fort Worth area. The earthquakes, with magnitudes between 2.5 and 3.0, prompted questions among the residents about whether drilling for natural gas in the nearby Barnett Shale was responsible for the shaking. A second series of earthquakes, with the largest a magnitude 3.3, occurred on May 16, 2009; a third occurred on June 2, 2009.
Natural gas production involves multiple steps, including drilling a natural gas well, pumping pressurized fluids into the well to crack open the rock (hydraulic fracturing), and then extracting the natural gas and used fluids. Once the gas and fluids are extracted, the fluids are reinjected back into the ground via a different well, called a saltwater disposal well, located some distance away from the production wells.
Following the quakes in the Dallas-Fort Worth area in 2008, seismologists installed six seismographs at different sites in the area and monitored activity from Nov. 9, 2008, through Jan. 2, 2009. There were no earthquakes during that period strong enough to be felt, but the seismographs recorded numerous smaller quakes. A team of researchers, led by Cliff Frohlich, a seismologist at the University of Texas at Austin, and Brian Stump, a seismologist at Southern Methodist University in Dallas, studied the seismograph data, including the arrival times of the seismic waves, and pinpointed the location of the events: They occurred at a depth of about 4 kilometers beneath the surface and along a line trending southwest-northeast. The epicenters were also generally all within half a kilometer of a 4-kilometer-deep saltwater disposal well. Injection into that well began in September 2008, seven weeks before the earthquakes began.
One mystery that the researchers puzzled over was why the seismicity only appeared to be associated with one saltwater disposal well, as there are more than 200 such wells active in the Barnett Shale production region. Frohlich and his team found a possible connection between the well and the earthquakes: a previously mapped fault that seemed to coincide with the southwest-northeast trend of the recorded quake events. Given both the spatial and the time correlations between the earthquakes and injections, and the fault and the well’s location, the authors wrote in The Leading Edge, a publication of the Society of Exploration Geophysicists, “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 [Dallas-Forth Worth] earthquakes.”
The most likely model for inducing earthquakes through fluid injection, Frohlich says, is that if there are many little faults under the surface that are locked and not slipping, pumping fluid in could force the sides of the faults apart, allowing them to slip. “It’s like an air hockey table — once you turn the air off, the puck stops. But when you pump the air in, it slips just fine.” However, he notes, there is no way to absolutely prove that this is what caused the Dallas-Fort Worth earthquakes.
Given this uncertainty, Chesapeake Energy, the company that operates the saltwater disposal well located near the earthquake events, denies any connection between their production activities in the Barnett Shale and the series of quakes. “Chesapeake maintains that a direct, causal relationship between saltwater disposal wells and seismic activity in the Dallas-Fort Worth area has not been scientifically proven, and plans no changes to our current operations,” says Julie Wilson, vice president for urban development at Chesapeake Energy. Chesapeake discontinued operations at that saltwater disposal well in August 2009, but says that is because there are other available disposal wells in the region.
The question of whether human activities can induce earthquakes — whether intentionally or unintentionally — has a decades-long history. “Most seismologists didn’t believe in induced earthquakes until the 1960s,” Frohlich says. In 1962, he says, the U.S. Army began injecting chemical waste into a deep well near Denver, Colo. Those injections were discontinued in 1966, however, following a series of earthquakes, some as strong as magnitude 5.3, that began shaking the area shortly after the injections began and raised fears that the events were connected.
Other researchers have suggested the possibility of intentionally inducing earthquakes as a form of earthquake control — by, for example, pumping water into the ground to produce many small quakes as an alternative to one big one. Whether that would work in the intended manner is unclear, and given the potential for liability and lawsuits, “the community as a whole never took it that seriously,” Frohlich says.
Lawsuits have still come looking for some geologists involved in energy production, however. Last December, Swiss geologist Markus Haering, CEO of Geothermal Explorers International Ltd., was tried in a criminal court for triggering damaging earthquakes near Basel, Switzerland, during the construction of a geothermal power plant. Haering was acquitted shortly after, because the court found he had not intended the damage — but his geothermal drilling project was put on hold. In a statement on his company’s Web site, Haering acknowledged his surprise at the strength of the tremors, and the fact that they were a setback for deep geothermal energy in the region.
Frohlich notes that the Texas and Switzerland cases have some significant differences. Unlike the Dallas-Fort Worth area, he says, Basel has historically had very large earthquakes, including one powerful quake in the 14th century that destroyed the city. Seismic networks in that part of Europe show numerous small earthquakes there, he adds. “It’s an area known to have seismic activity. If they had asked seismologists before the project, ‘Is this a totally safe place to inject fluids?,’ they’d have said it’s not the safest place. It’s quite different than a place like Dallas.”
In Dallas, he says, if you stand on the streets and feel shaking, “you’ll think it’s a cattle stampede.” So when it comes to drilling for natural gas in Texas, he adds, “I’m not terribly worried. However, there are several industries important to this country — including the petroleum industry, geothermal energy and carbon sequestration — that are doing an increasing amount of injection. So this is a phenomenon that we ought to study and understand as much as possible, to mitigate any risks.”
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