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Groundwater‐quality hazards of methane leakage from hydrocarbon wells: A review of observational and numerical studies and four testable hypotheses

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Methane leakage from hydrocarbon wells plays an important role in the groundwater‐quality impacts of hydrocarbon development and presents a more likely hazard than hydraulic fracturing or formation fluids. Methane released from contaminated water wells has been linked with combustion risks and degraded water quality. Potentially, methane can serve as a precursor to other fluids associated with hydrocarbon extraction, such as volatile organics. In this review, we surveyed studies relating to contamination of drinking‐water aquifers by methane gas from leaking hydrocarbon wells. Challenges associated with linking methane in groundwater to hydrocarbon extraction are identified, highlighting the need for groundwater‐quality and well‐integrity databases. Science‐based policy recommendations are made, including deeper surface casings and greater cement coverage for wells with deviated wellbores, remediation of faulty abandoned wells, and increased gas‐migration monitoring. We suggest four hypotheses to quantify risks to groundwater quality from methane leakage. First, differentiation between thermogenic methane occurring in groundwater due to natural migration and thermogenic methane present due to hydrocarbon development can be used to alleviate the need for baseline measurements of methane in groundwater. Second, methane newly discovered in freshwater aquifers is unlikely to have originated from leaks beginning decades ago. Third, pertaining to the zone separating methane leakage from groundwater, relative permeability will have a larger impact on plume diameter than heterogeneity in intrinsic permeability. Fourth, thermogenic methane in groundwater will serve as a precursor to benzene, toluene, ethybenzene, and xylene (BTEX) under conditions where methane and BTEX coexist in a hydrocarbon reservoir and leakage is transported primarily in the aqueous phase. This article is categorized under: Engineering Water > Sustainable Engineering of Water Science of Water > Water Quality
Overview of potential methane migration pathways (in green) to groundwater that can be linked to oil and gas operations. Well 1 shows stray gas migrating through fractures to an existing fault, which connects to groundwater (e.g., Cahill et al., ; Vengosh, Jackson, Warner, Darrah, & Kondash, ). Well 2 shows migration from a deep hydrocarbon reservoir or production casing leak through an open annulus and/or faulty annular cement to groundwater (e.g., Birdsell, Rajaram, Dempsey, & Viswanathan, ; Jacquet, ; Prpich, Coulon, & Anthony, ; Torres, Yadav, & Khan, ). Well 3 shows gas migration from an intermediate‐depth gas‐bearing formation (nonproduction) to groundwater via an open annulus and/or faulty annular cement (e.g., US Environmental Protection Agency ‐ US EPA, ). Methane leakage into the matrix below groundwater is also shown, which can lead to groundwater contamination via buoyant advection through pores or fractures. Figure is not shown to scale
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(a) Major well components are identified and mechanisms for gas migration along faulty or degraded hydrocarbon wellbores are illustrated. Scenarios associated with methane leakage to groundwater shown in Figure are highlighted, including migration from deep hydrocarbon reservoirs (Scenario 2) and gas migration from an intermediate‐depth gas‐bearing formation (Scenario 3). (b) Gas well integrity loss can lead to either surface‐casing pressure (SfCP) or surface‐casing‐vent flow (SCVF), depending on gas venting at the surface
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