Measurements of Atmospheric Methane Emissions from Stray Gas Migration: A Case Study from the Marcellus Shale.

Understanding emissions of methane from legacy and ongoing shale gas development requires both regional studies that assess the frequency of emissions and case studies that assess causation. We present the first direct measurements of emissions in a case study of a putatively leaking gas well in the largest shale gas play in the United States. We quantify atmospheric methane emissions in farmland >2 km from the nearest shale gas well cited for casing and cementing issues. We find that emissions are highly heterogeneous as they travel long distances in the subsurface. Emissions were measured near observed patches of dead vegetation and methane bubbling from a stream. An eddy covariance flux tower, chamber flux measurements, and a survey of enhancements of the near-surface methane mole fraction were used to quantify emissions and evaluate the spatial and temporal variability. We combined eddy covariance measurements with the survey of the methane mole fraction to estimate total emissions over the study area (2,800 m2). Estimated at ∼6 kg CH4 day-1, emissions were spatially heterogeneous but showed no temporal trends over 6 months. The isotopic signature of the atmospheric CH4 source (δ13CH4) was equal to -29‰, consistent with methane of thermogenic origin and similar to the isotopic signature of the gas reported from the nearest shale gas well. While the magnitude of emissions from the potential leak is modest compared to large emitters identified among shale gas production sites, it is large compared to estimates of emissions from single abandoned wells. Since other areas of emissions have been identified close to this putatively leaking well, our estimate of emissions likely represents only a portion of total emissions from this event. More comprehensive quantification will require more extensive spatial and temporal sampling of the locations of gas migration to the surface as well as an investigation into the mechanisms of subsurface gas migration. This work highlights an example of atmospheric methane emissions from potential stray gas migration at a location far from a well pad, and further research should explore the frequency and mechanisms behind these types of events to inform careful and strategic natural gas development.

Detecting anomalous methane in groundwater within hydrocarbon production areas across the United States.

Numerous geochemical approaches have been proposed to ascertain if methane concentrations in groundwater, [CH4], are anomalous, i.e., migrated from hydrocarbon production wells, rather than derived from natural sources. We propose a machine-learning model to consider alkalinity, Ca, Mg, Na, Ba, Fe, Mn, Cl, sulfate, TDS, specific conductance, pH, temperature, and turbidity holistically together. The model, an ensemble of sub-models targeting one parameter pair per sub-model, was trained with groundwater chemistry from Pennsylvania (n=19,086) and a set of 16 analyses from putatively contaminated groundwater. For cases where [CH4] ≥ 10 mg/L, salinity- and redox-related parameters sometimes show that CH4 may have moved into the aquifer recently and separately from natural brine migration, i.e., anomalous CH4. We applied the model to validation and hold-out data for Pennsylvania (n=4,786) and groundwater data from three other gas-producing states: New York (n=203), Texas (n=688), and Colorado (n=10,258). The applications show that 1.4%, 1.3%, 0%, and 0.9% of tested samples in these four states, respectively, have high [CH4] and are ≥50% likely to have been impacted by gas migrated from exploited reservoirs. If our approach is indeed successful in flagging anomalous CH4, we conclude that: i) the frequency of anomalous CH4 (# flagged water samples / total samples tested) in the Appalachian Basin is similar in areas where gas wells target unconventional as compared to conventional reservoirs, and ii) the frequency of anomalous CH4 in Pennsylvania is higher than in Texas + Colorado. We cannot, however, exclude the possibility that differences among regions might be affected by differences in data volumes. Machine learning models will become increasingly useful in informing decision-making for shale gas development.

Methane concentrations in streams reveal gas leak discharges in regions of oil, gas, and coal development.

As natural gas has grown in importance as a global energy source, leakage of methane (CH4) from wells has sometimes been noted. Leakage of this greenhouse gas is important because it affects groundwater quality and, when emitted to the atmosphere, climate. We hypothesized that streams might be most contaminated by CH4 in the northern Appalachian Basin in regions with the longest history of hydrocarbon extraction activities. To test this, we searched for CH4-contaminated streams in the basin. Methane concentrations ([CH4]) for 529 stream sites are reported in New York, West Virginia and (mostly) Pennsylvania. Despite targeting contaminated areas, the median [CH4], 1.1 µg/L, was lower than a recently identified threshold indicating potential contamination, 4.0 µg/L. [CH4] values were higher in a few streams because they receive high-[CH4] groundwaters, often from upwelling seeps. By analogy to the more commonly observed type of groundwater seep known as abandoned mine drainage (AMD), we introduce the term, "gas leak discharge" (GLD) for these waters where they are not associated with coal mines. GLD and AMD, observed in all parts of the study area, are both CH4-rich. Surprisingly, the region of oldest and most productive oil/gas development did not show the highest median for stream [CH4]. Instead, the median was statistically highest where dense coal mining was accompanied by conventional and unconventional oil and gas development, emphasizing the importance of CH4 contamination from coal mines into streams.

Exploring How to Use Groundwater Chemistry to Identify Migration of Methane near Shale Gas Wells in the Appalachian Basin.

Methane (CH4) enters waters in hydrocarbon-rich basins because of natural processes and problems related to oil and gas wells. As a redox-active greenhouse gas, CH4 degrades water or emits to the atmosphere and contributes to climate change. To detect if methane migrated from hydrocarbon wells (i.e., anomalous methane), we examined 20 751 methane-containing groundwaters from the Upper Appalachian Basin (AB). We looked for concentrations (mg/L) that indicated AB brine salts (chloride concentrations ([Cl]) > 30; [Ca]/[Na] < 0.52) to detect natural methane, and we looked for concentrations of redox-active species ([SO4] ≥ 6; [Fe] ≥ 0.3) to detect anomalous methane. These indicators highlight natural contamination by methane-containing brines or recent onset of microbial oxidation of methane coupled to iron- or sulfate-reduction. We hypothesized that only waters recently contaminated by methane still exhibit high iron and sulfate concentrations. Of the AB samples, 17 (0.08%) from 12 sites indicated potential contamination. All were located in areas with high densities of shale-gas or conventional oil/gas wells. In contrast, in southwestern Pennsylvania where brines are shallow and coal, oil, and gas all have been extracted extensively, no sites of recent methane migration were detectable. Such indicators may help screen for contamination in some areas even without predrill measurements.

Detecting and explaining why aquifers occasionally become degraded near hydraulically fractured shale gas wells.

Extensive development of shale gas has generated some concerns about environmental impacts such as the migration of natural gas into water resources. We studied high gas concentrations in waters at a site near Marcellus Shale gas wells to determine the geological explanations and geochemical implications. The local geology may explain why methane has discharged for 7 years into groundwater, a stream, and the atmosphere. Gas may migrate easily near the gas wells in this location where the Marcellus Shale dips significantly, is shallow (∼1 km), and is more fractured. Methane and ethane concentrations in local water wells increased after gas development compared with predrilling concentrations reported in the region. Noble gas and isotopic evidence are consistent with the upward migration of gas from the Marcellus Formation in a free-gas phase. This upflow results in microbially mediated oxidation near the surface. Iron concentrations also increased following the increase of natural gas concentrations in domestic water wells. After several months, both iron and SO42- concentrations dropped. These observations are attributed to iron and SO42- reduction associated with newly elevated concentrations of methane. These temporal trends, as well as data from other areas with reported leaks, document a way to distinguish newly migrated methane from preexisting sources of gas. This study thus documents both geologically risky areas and geochemical signatures of iron and SO42- that could distinguish newly leaked methane from older methane sources in aquifers.