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Scientist: Drilling Caused Small South Texas Quakes
A geophysicist with the U.S. Geological Survey (USGS) believes at least two minor earthquakes in South Texas, including a 3.0-magnitude temblor on Feb. 4, were caused by oil and natural gas drilling.
Art McGarr, who works at the USGS Earthquake Science Center in Menlo Park, CA, told NGI that the Feb. 4 quake and another on Oct. 20, 2011, which registered 4.6 on the Richter Scale, both occurred in the Fashing Gas Field, which has been actively drilled since 1958.
“Judging by the age of the gas field, I would guess the wells that they’re using to extract the natural gas have been in place for quite some time,” McGarr said Friday. “Production seems to have been pretty steady over the years. As you produce the natural gas, the formation that holds the natural gas compresses to fill in the missing volume and releases energy, at least part of which is converted into earthquakes.”
McGarr said he didn’t believe wastewater injection wells, such as the ones allegedly tied to earthquakes in Ohio and Arkansas, were responsible for the quakes in Texas (see Shale Daily, Jan. 4; July 29, 2011).
“There are some wastewater injection wells in the area, but they haven’t really been all that active,” McGarr said. “That’s one of a number of reasons why we thought it was more likely associated with the natural gas production there.”
In 1986 researchers from several oil and gas companies claimed that seismic activity in South Texas appeared to begin around 1973 and were the result of fluid withdrawal without massive subsidence. They said the activity at the time centered on Fashing and the Imogene Oilfield, near Pleasanton.
“The slip on the bounding faults may be due either to differential compaction of limestones containing the hydrocarbons or to continued slip along previously creeping faults,” Wayne Pennington, Scott Davis, Steven Carlson, James DuPree and Thomas Ewing reported for the August 1986 issue of Bulletin of the Seismological Society of America.
McGarr said he agreed with the 1986 findings. “There are some [unnamed] faults that helped to form that gas field,” he said. “The earthquake last October has a mechanism indicating that this earthquake may well have occurred on that fault. So the gas production probably imposed stresses on the fault, which in turn slipped to produce the earthquake. Whether this smaller one [Feb. 4] is the same mechanism, I certainly couldn’t say, but it seems to be located very close to both the gas field and the [epicenter] of the October quake.”
A series of small earthquakes also shook parts of North Texas in 2009, which is where the Barnett Shale is centered (see Daily GPI, July 13, 2009). Initial findings concluded that there was a “possible correlation” between the quakes and a reinjection well used to dispose of saltwater (see Daily GPI, Aug. 17, 2009).
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