Noble gases identify the mechanisms of fugitive gas contamination in drinking-water wells overlying the Marcellus and Barnett Shales

Abstract

Horizontal drilling and hydraulic fracturing have enhanced energy production but raised concerns about drinking-water contamination and other environmental impacts. Identifying the sources and mechanisms of contamination can help improve the environmental and economic sustainability of shale-gas extraction. We analyzed 113 and 20 samples from drinking-water wells overlying the Marcellus and Barnett Shales, respectively, examining hydrocarbon abundance and isotopic compositions (e.g., C2H6/CH4, δ(13)C-CH4) and providing, to our knowledge, the first comprehensive analyses of noble gases and their isotopes (e.g., (4)He, (20)Ne, (36)Ar) in groundwater near shale-gas wells. We addressed two questions. (i) Are elevated levels of hydrocarbon gases in drinking-water aquifers near gas wells natural or anthropogenic? (ii) If fugitive gas contamination exists, what mechanisms cause it? Against a backdrop of naturally occurring salt- and gas-rich groundwater, we identified eight discrete clusters of fugitive gas contamination, seven in Pennsylvania and one in Texas that showed increased contamination through time. Where fugitive gas contamination occurred, the relative proportions of thermogenic hydrocarbon gas (e.g., CH4, (4)He) were significantly higher (P < 0.01) and the proportions of atmospheric gases (air-saturated water; e.g., N2, (36)Ar) were significantly lower (P < 0.01) relative to background groundwater. Noble gas isotope and hydrocarbon data link four contamination clusters to gas leakage from intermediate-depth strata through failures of annulus cement, three to target production gases that seem to implicate faulty production casings, and one to an underground gas well failure. Noble gas data appear to rule out gas contamination by upward migration from depth through overlying geological strata triggered by horizontal drilling or hydraulic fracturing.

Keywords: groundwater contamination; isotopic tracers; methane; noble gas geochemistry; water quality.

Fig. 1.

A diagram of seven scenarios that may account for the presence of elevated hydrocarbon gas levels in shallow aquifers (see discussion in text). The figure is a conceptualized stratigraphic section and is not drawn to scale. Additional scenarios (e.g., coal bed methane and natural-gas pipelines leaking into aquifers) are unlikely in our specific study areas (Figs. S2 and S3).

Fig. 2.

The ratios of CH₄/³⁶Ar [ratios are in units (cm³ STP/L)/(cm³ STP/L); A and C] and ⁴He/²⁰Ne (B and D) vs. Cl⁻ of domestic groundwater wells. The samples were collected in the Marcellus (MSA) (Left) and Barnett (BSA) (Right) study areas at distances >1 km (triangles) and <1 km (circles) from unconventional drill sites (Tables S1 and S2). [CH₄] is shown using grayscale intensity [0–60⁺ cm³ ([CH₄]) STP/L]. The dashed lines in the MSA are the regressions of all points collected >1 km from drill sites. In the MSA, all samples >1 km from drill sites had [CH₄] at or below saturation and showed significant correlations between Cl⁻ and CH₄/³⁶Ar (r² = 0.72; P < 0.01) or ⁴He/²⁰Ne (r² = 0.59; P < 0.01) defined as the normal trend. For samples <1 km from drill sites, one subset was consistent with the “normal trend” (P = 0.31; Chow test), whereas the other anomalous subset had supersaturated [CH₄] and high CH₄/³⁶Ar and ⁴He/²⁰Ne, even at low [Cl⁻] (green-rimmed circles in A and B). The natural Salt Spring in Montrose, PA, is shown as a square in all MSA figures, and samples targeted for microbial-sourced gases are distinguished by diamonds. In the BSA, 15 samples had [CH₄] at or below saturation and significant correlations between Cl⁻ and CH₄/³⁶Ar (r² = 0.59; P < 0.01) or ⁴He/²⁰Ne (r² = 0.48; P < 0.01) (dashed lines in C and D). Five samples, including two that changed between the first and second sampling periods (Fig. S4), had substantially higher CH₄/³⁶Ar and ⁴He/²⁰Ne independent of [Cl⁻]. The anomalous subset of samples from both locations with elevated CH₄ that do not fall along the normal trend (>1 km) regression lines are consistent with a flux of gas-phase thermogenic hydrocarbon gas into shallow aquifers.

Fig. 3.

²⁰Ne (Top), N₂ (Middle), and CH₄ (Bottom) vs. ³⁶Ar in the MSA (Left) and BSA (Right) at distances >1 km (triangles) and <1 km (circles) from drill sites. All normal trend samples have ³⁶Ar and N₂ within 15% of the temperature-dependent ASW solubility line (cyan lines). Conversely, a subset of wells with elevated [CH₄] <1 km from drill sites (green-rimmed circles) shows significantly stripped ASW gases (²⁰Ne, ³⁶Ar, N₂), which result from extensive partitioning of dissolved ASW gases into a large volume of migrating gas-phase hydrocarbons (i.e., a fugitive gas). Note that domestic wells labeled previously elevated CH₄ were vented to remove methane from the water before our sampling. Consistent with the MSA, most BSA samples (15 of 20) also have normal ASW composition, but five anomalous samples, including the two that displayed pronounced changes between the initial and later sampling events (Fig. S5), have significantly stripped ASW gas composition (green-rimmed circles).

Fig. 4.

⁴He/CH₄ vs. ²⁰Ne/³⁶Ar (Upper Left) and ⁴He/CH₄ vs. δ¹³C-CH₄ (Lower Left) and C₂H₆+/CH₄ vs. δ¹³C-CH₄ (Upper Right) and ⁴He/⁴⁰Ar* vs. ⁴He/²⁰Ne (Lower Right) of produced gases and groundwater in the MSA (Left) and BSA (Right) at distances >1 km (triangles) and <1 km (circles) from drill sites. Normal trend groundwater samples in the MSA display ⁴He/CH₄ and ²⁰Ne/³⁶Ar values that increase with [CH₄] and that are significantly higher than Marcellus-produced gases. These data suggest natural geological migration of gas under relatively low V_gas/V_water conditions (scenario 2). Samples <1 km from drill sites with evidence for fugitive gas migration (green-rimmed circles) plot along a trend between Marcellus (black box) and Upper Devonian-produced gases (pink hatched box) consistent with Scenarios 4 (annulus) or 5 (production casing) (B). A cluster of groundwaters near a gas well that experienced an underground blowout (circled in A and B) displays significant stripping and enrichments in both ⁴He/CH₄ and ²⁰Ne/³⁶Ar, consistent with modeled solubility fractionation vectors (red dashed lines) for gas migration through the water-saturated crust (e.g., along faults or fractures) (scenario 6), but likely results from a well packer failure at depth (scenario 5). The Strawn- and Barnett-produced gases include data reported in ref. and collected as part of the present study (Table S3). The molecular ratio of aliphatic hydrocarbons (C₂H₆+/CH₄) (C) and noble gases (⁴He/⁴⁰Ar* and ⁴He/²⁰Ne) (D) in samples with evidence of fugitive gas contamination (green-rimmed circles) are significantly greater than other natural groundwaters in the area. The similarity between the C₂H₆+/CH₄, ⁴He/⁴⁰Ar*, and ⁴He/²⁰Ne composition of the five impacted wells, including the two that changed between the first and second samplings (Fig. S6), and Strawn-produced gases, suggests an intermediate depth Strawn gas (scenario 4) as the most likely cause for the fugitive gas contamination observed in Texas.