Methane, the major component of natural gas, is a key component of the Texas economy. This has prompted interest in the state in a recently released study by researchers at the University of Texas at Austin, who found that hydraulic fracturing has little impact on methane in drinking water wells in the Fort Worth area.
Texas is estimated to hold about 23 percent of the nation’s natural gas reserves. Of the state’s three major natural gas fields, the Barnett Shale play, which surrounds and extends into the Fort Worth metropolitan area, is the largest. By the end of 2015, there were about 20,000 natural gas wells in the region. Given that Barnett’s gas reserves are “tight,” hydraulic fracturing is a common practice to separate gas from the shale formations.
High Levels in 11 Wells
The research was intended to examine the source and extent of methane in water wells. The researchers analyzed samples from more than 450 wells across 12 counties in the western Barnett Shale. The vast majority of samples—85 percent—showed very low methane levels in groundwater, less than 0.1 milligrams of methane per liter of water. However, a cluster of 11 wells in Parker County and Hood County had methane levels above 10 milligrams per liter of water, a level that can trigger venting of well water systems to ensure the flammable gas does not become hazardous.
Shallow and Deep Gas
Methane is produced two ways: (1) thermogenically from the breakdown of organic material under elevated temperature and pressure and (2) biogenically by microbial activity. Biogenic methane is generally generated at shallow depth. Thermogenic methane is always produced at depth, although gas can sometimes migrate over geologic time to shallower areas.
The researchers used carbon isotope analysis to determine that the methane found in well water was thermogenic, which ruled out biogenic sources, but didn’t pinpoint whether the gas came from the deeper Barnett or shallower deposits, called the Strawn. Additional analysis of the samples, conducted in conjunction with a research team from the University of Michigan, linked the methane to the shallow natural gas deposits of the Strawn. The results were complemented by a study from 2015 that found nitrogen isotopes associated with the Strawn.
Number of Wells Not a Factor
“In addition to chemical evidence, the researchers found a strong correlation between water well proximity to the Strawn Group and high methane levels,” states a news release from UT’s Jackson School of Geosciences. “There is no correlation between high-methane wells and the distance to the nearest hydraulically fractured wells. The researchers also note that Parker and Hood counties have a relatively low number of hydraulic fracturing wells compared with other counties in the Barnett Shale, such as Denton, that have water wells with low methane levels.”
Jackson School adds that while the findings suggest that methane from the Strawn Group is the most likely source for the methane in water wells in Parker and Hood Counties, the researchers cannot completely rule out that some of the methane may have come from leaks caused by hydraulic fracturing operations.
“In fact, the researchers suggest that leaks from deep reservoirs might help explain certain cases recorded by other studies where methane levels in water wells are increasing over time and cases where methane is present in water wells where it used to be absent,” says Jackson School.
Information on the study is here.