Integrated Evaluation of Alleged Natural Gas Contamination of the Trinity Aquifer by Horizontal Gas Wells Completed in the Barnett Formation, Southern Parker County, Texas

During 2010, the Texas Railroad Commission and the US Environmental Protection Agency (EPA) responded to complaints by a landowner that his residential water well near Weatherford, Texas was contaminated with natural gas caused by production from two nearby horizontal gas wells (Butler #1H and Teal #1H) operated by Range Production Company. We used geological, geochemical, and petroleum engineering data obtained from those gas wells and from 26 nearby water wells to determine that gas dissolved in the Twin Mountains (Trinity) aquifer is not related to Barnett gas production. Instead, gas present in Pennsylvanian Strawn sandstones migrated across an angular unconformity into the aquifer. Thermal gas is present in shallow Strawn sandstones in Parker County: e.g., the Strawn Center Mills gas field is located ~8500 ft southeast of the landowner’s property. In addition, gas was flared from a water well drilled in 2005 only ~800 ft from the landowner's property -- four years before Range drilled the Butler and Teal wells. Furthermore, a water well drilled in 2003 by the nearby Lake Country Acres water district was abandoned when it flowed >100 Mcf of natural gas/day, and that district's water storage tanks are aerated to remove dissolved natural gas. The migration of natural gas from Pennsylvanian strata into the overlying Trinity aquifer independently is supported by the presence of ~500-1800 ppm total dissolved solids in water samples from some water wells. In addition, gas fingerprinting confirms that gas produced from the Barnett Formation by the Range wells is not the origin of gas dissolved in the Trinity aquifer. Barnett gas samples obtained from the Butler and Teal wells contain only ~1.2 mol% N₂. But most gas samples produced from Pennsylvanian reservoirs in this area contain ~2.0-4.5 mol% N₂. Similarly, the natural gas component of headspace gas samples collected from the landowner’s water well and from a nearby water well contains ~2.5-5.0 mol% N₂: a match to Strawn gas. The EPA erroneously relied only on the similarity of the C and H isotopic composition of methane in gas samples from the landowner’s water well and the Butler gas well to conclude that Range was responsible for contaminating the Trinity aquifer. But C isotope data cannot distinguish Barnett gas from Strawn gas: e.g., methane produced by the Range gas wells, the 2005 water well that flared gas, and the landowner's water well has the same C isotopic composition.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California