Early 1990 La Plata County Studies of Groundwater-Entrained Methane

 

Shortly after the onset of CBM production in La Plata County in the late 1980’s, a local citizens group voiced concern about an alleged increase in natural gas contamination of domestic water wells. U.S. Representative Ben Nighthorse Campbell initiated the formation of a committee to address the concerns. As an outgrowth of the Campbell Committee, the U.S. Geological Survey began a study in July 1990. This study focused on documenting the occurrence of natural gas in near-surface ground water and in soils adjacent to gas wells in the Animas River Valley in the San Juan Basin between Durango, Colorado, and Aztec, New Mexico. From analysis of water chemistry in samples collected from near-surface aquifers at domestic water wells, the study sought to identify and map the occurrence, determine potential sources, and suggest possible pathways through which natural gas might migrate to near-surface aquifers. Included was the investigation of the relationship between methane concentrations and mapped geologic fractures.

This study (Chafin, 1994) showed measurable concentrations of methane (greater than the detection level of 0.005 mg/L) in 34 percent of the samples tested, with bedrock wells exhibiting higher concentrations than alluvial wells. Hydrogen sulfide was often found associated with elevated concentrations of entrained methane. On the basis of a thermogenic isotopic signature (Appendix C: Chart 10) and molecular composition of the gas isolated from the water of some domestic wells showing similarity to gas collected from producing horizons, the latter were depicted as probable sources of the methane. (The isotopic character of carbon atom distribution in water-entrained methane can be confounded by the fact that methylotrophic bacteria often oxidize methane. Following methane oxidation, the stable isotopic ratio of the carbon atom population tends to indicate a false maturity and may result in misleading assumptions. The deuterium isotope and chemical composition of the entrained gas can be utilized to minimize confusion over this issue.) Shuey (1990) reviewed gas composition data of samples drawn from domestic water wells and seeps between Bondad, Colorado, and a few miles south of Aztec, New Mexico. He concluded that approximately half of the samples contained gas similar in character to that produced from Fruitland Formation coalbeds. Beckstrom and Boyer (1991) determined that the gas isolated from three conventional gas well surface casings (bradenhead gas) was chemically and isotopically consistent with Fruitland coalgas and hypothesized that gas migration had occurred upward from the Fruitland Formation along uncemented well-bore annuli of conventional gas wells. Proposed methane migration pathways to water wells having a thermogenic gas signature include deficiencies in well casing integrity, a lack of adequate annular isolation through the Fruitland coal horizons, cathodic protection wells, seismic test holes, bedrock water wells, and natural joints and fractures.

Conversely, methane isolated from water wells having carbon isotopic signatures reflecting biogenic sources was attributed to microbial action in near-surface regimes such as sewage lagoons, septic fields, swampy areas, or within the groundwater aquifer itself. While the accumulation of methane in these domestic water wells may represent environmental hazards, the implicated sources are not under the auspices of oil and gas regulatory agencies. Therefore, investigations into biogenic methane sources have been excluded.

With the rapidly increasing CBM development in the years 1989 to 1991, La Plata County residents expressed concern that anticipated increases in drilling activity and production from the Fruitland Formation coals might adversely affect their water wells. In response, the San Juan Basin Oil and Gas Coordinating Committee (with representation from state, local, and Federal agencies, the Southern Ute Indian Tribe, gas industry operators, citizen groups and private citizens) was formed in 1989 to study the effects of oil and gas development, with an emphasis on groundwater quality issues. The need for a baseline of water quality was recognized, and in February 1991, the Colorado Oil and Gas Conservation Commission (COGCC) established a Groundwater Task Force to initiate a study to provide baseline data in La Plata and Archuleta Counties (Velez, 1993). This Groundwater Task Force was comprised of the COGCC, BLM, San Juan Citizens Alliance, Southern Ute Indian Tribe, La Plata County, San Juan Health Department, Colorado Division of Water Resources – Water Quality Control Division and State Engineers Office, Office of Senator Ben Nighthorse Campbell, Colorado Department of Local Affairs, and private citizens. The State of Colorado, Department of Local Affairs Energy Impact Assistance Fund and gas industry contributions to the COGCC Environmental Response Fund provided the needed monies. A total of 324 wells were sampled in 1991, with analyses being completed by several laboratories. Headspace methane concentrations were reported in parts-per-million (ppm) in contrast to the USGS study, which reported in milligrams of methane per liter of water. The latter has become the accepted standard for reporting methane entrainment in groundwater.  

Unfortunately, quality control split-samples for methane concentration differed substantially between laboratories. Credibility of the study suffered. Nevertheless, analyses showed 81 wells to have methane above the detection limit of 7 ppm in the headspace. The study also identified specific water wells devoid of measurable methane contamination, establishing a baseline at these locations. Twenty-eight water wells having in excess of 1000-ppm methane in the headspace were isolated and samples from sixteen of these were submitted to the USGS laboratory in Denver for stable carbon isotope determination. Using a breakpoint of -55 per mil (⁰/₀₀ ), ten samples appeared to be of biogenic origin while six indicated thermogenic origins with potential relationship to gas producing horizons.

A Colorado Western Slope groundwater quality monitoring study (Schenderlein, 1993) evolved out of an agreement between the Colorado Department of Health and the Office of the Colorado State Engineer, Department of Natural Resources, Division of Water Resources. A grant from the Environmental Protection Agency funded this 1992 study to determine the extent of groundwater contamination attributable to non-point source activities on the Western Slope Area of Colorado. During this study twenty water wells and springs were sampled in the San Juan Basin in the summer of 1992; nineteen wells were re-sampled in the fall of 1992. Seventy percent of the first round of samples and eighty-five percent of the second round of samples exhibited quantifiable methane concentrations above the lower detection limit of 0.005 milligrams methane per liter of water. Twenty-five percent of the first round of samples and sixteen percent of the second round of samples divulged concentrations exceeding one milligram methane per liter of water, with two revealing concentrations of methane greater than ten milligrams methane per liter of water. No isotopic determinations were reported.



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