The origin of methane found in private water wells in shale gas regions of the US has been discussed for many years. Several possibilities or mechanisms have been suggested, such as 1) in situ naturally generated methane (e.g. in peat and/or swamp regions), 2) naturally migrated methane from the deep sub-surface over a geological timescale, 3) migration of methane through the hydraulic fracturing process or 4) gas migration caused by leaks in shale gas production wells themselves. Now, using noble gas isotopic compositions in groundwater, a new study by Darrah et al (2014), shows that in their study area of wells, poor well integrity is responsible for the found methane contamination. The aim of the research was to identify the sources and mechanisms of contamination to improve the environmental and economic sustainability of shale gas extraction.

Investigated domestic groundwater wells in the Marcellus study area (113 samples) are located at 35-90 m depth (Marcellus shale is located 800-2,200 m subsurface) and in the Barnett area (20 samples) at 60-75 m depth (Barnett shale is located 1,950-2,500 m subsurface). For their investigation, Darrah et al (2014) chose water wells far from drill sites, and in areas that are known for methane contamination.

Their results showed two mechanisms that explain the presence of methane in groundwater:

Natural migration of methane

• Elevated concentrations of natural brine components (e.g. Cl^- and ⁴He) co-occur alongside methane (CH₄) in groundwater samples observed >1 km from shale gas wells;
• Noble gas composition from groundwater within 1 km and a subset of samples <1 km, all had similar diagnostic noble gas composition (CH₄/³⁶Ar at or below CH₄ saturation).

These data suggest salt- and gas-rich groundwater derived from a deep gas-rich brine that migrated over geological time (natural migration); in simple words: There are many wells (> 1 km and <1 km away from gas wells) that have a high methane level but also exhibit elevated levels of natural crustal brine components (Cl^- and ⁴He), evidence of an influence of deeper formations on the shallow ground water over geological time.

Anomalous sample set

• A subset of samples (<1 km) showed significantly higher levels of thermogenic gases and other hydrocarbons relative to noble gases (i.e. high CH₄/³⁶Ar and ⁴He/²⁰Ne) and a lack of correlation with Cl^-.

These data suggest a thermogenic hydrocarbon gas that has separated from the brine-meteoric water mixture and migrated into the gas phase (i.e., a fugitive gas) as the result of compromised well integrity.

With this comprehensive geochemical investigation of the origin of methane in groundwater, the authors were able to exclude the direct migration of gases upward through overlaying strata following hydraulic fracturing. Darrah et al (2014) explained this by the lack of fractionation of the diagnostic gas isotope ratios that must occur during migration through the water-saturated crust.

Click here for more information.