Earthquakes in North Texas
North Texans know a lot about tornadoes and golf ball-size hail storms, but when it comes to earthquakes, many Texans are experiencing them for the first time. Our first instinct is to ask what is causing them and what do we need to do to stop them. With oil and gas operations near the areas of the recent earthquakes near Eagle Mountain Lake, some have naturally turned to questioning whether these activities are somehow involved with oil and natural gas activities.
At a January 2014 town hall meeting in Azle, organized by Commissioner David Porter of the Railroad Commission of Texas, some speakers pointed to drilling, hydraulic fracturing and disposal wells as possible culprits.
While drilling, hydraulic fracturing and disposal wells are often lumped together, it is important to distinguish between these different aspects of shale gas development. The process begins with a drilling rig, which can usually drill a Barnett Shale gas well in about two weeks. The drilling rig drills about 8,000 feet to the Barnett Shale and then turns the drill bit 90 degrees and drills a horizontal “lateral” another 3,000 to 5,000 feet.
After the drilling rig has completed its job, it leaves. Next, hydraulic fracturing equipment, consisting mostly of trucks with high-pressure pumps mounted on them, arrives to hydraulically fracture the well. The process of “fracking” takes nine days. A completion/work-over rig and crew then arrives to finish setting final casing and to install other production equipment such as wellheads, dehydrators and water storage tanks.
Once the natural gas well is put into production, water is also produced, which goes into on-sight water storage tanks. Water tanker trucks visit the site as necessary to unload the water tanks. This produced water is then taken to saltwater disposal wells, which are deeper than 10,000 feet and encased in steel and concrete.
Researchers who have been studying the earthquakes in North Texas have concluded that neither drilling nor hydraulic fracturing is causing earthquakes in the area. Geophysicist Bill Ellsworth with the U.S. Geological Survey said, “We don’t see any connection between fracking and earthquakes of any concern to society.” One reason is that fracking occurs at depths much shallower than the epicenters of the earthquakes. Barnett Shale natural gas wells are typically 7,000 to 8,000 feet deep while the earthquakes are originating at depths of 3 to 3.5 miles.
If Barnett Shale natural gas drilling and hydraulic fracturing are not contributing to seismic activity, how about saltwater disposal wells? Saltwater disposal wells, or SWDs, are completely different from natural gas wells. SWDs are drilled for the sole purpose of disposing of the saltwater that is produced along with the natural gas, called produced water. SWDs are much deeper than Barnett gas wells, typically 12,000 feet deep and drilled into the Ellenburger formation, which is a saltwater-bearing sand. The produced water is injected into the SWD at varying pressure.
There are five SWDs in the Azle area, and researchers have installed seismic equipment in an attempt to see if the epicenters of the seismic activity are near any of these SWDs; however, there has not been any seismic activity since January 28, 2014, so researchers have not had any new data to work with. We will await their reports.