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Methane leakage due to compromised hydrocarbon well integrity can lead to impaired groundwater quality. Here we use a three-dimensional, multiphase (vapor and aqueous), multicomponent (methane, water, salt), numerical model (TOUGH2 EOS7C) to investigate hydrogeological conditions that could result in groundwater contamination from natural gas wellbore leakage that migrates upward toward a freshwater aquifer. The conceptual model used for the simulations assumes methane leakage at 20-30 m below groundwater. We perform 180 simulations for a sensitivity analysis, examining (1) multiphase flow parameters related to storage, capillarity, and relative permeability, including porosity (phi), initial fluid-phase saturation (S-L), and van Genuchten n and alpha, (2) geostatistical variations in intrinsic permeability (k(i)), and (3) methane source-zone pressure. Simulated mean k(i) values are 10(-18) and 10(-13) m(2) with variances of 1 and 5 m(4). Simulated source-zone pressures range from just over ambient hydrostatic pressure at the depth of leakage (100 kPa) to the maximum pressure that steel casings are commonly rated to withstand (20,340 kPa). k(i), initial S-L, phi, and van Genuchten's n and phi were the most important parameters in determining the volume of methane reaching groundwater during a given time period. Multiphase parameterization of formations underlying freshwater aquifers and overlying hydrocarbon production zones is fundamental to assessing aquifer vulnerability to methane leakage.

Plain Language Summary Methane leakage from oil and gas wellbores below freshwater aquifers may impact groundwater quality. The scope of the problem is such that millions of kilograms of methane could reach groundwater in the case of a long-term, persistent leak. However, flow rates at the base of the aquifer are slow, and changes in methane concentrations may go undetected. In this paper, we use numerical modeling to investigate hydrogeological conditions that could result in groundwater contamination from natural-gas wellbore leakage that migrates upward toward a freshwater aquifer. Measurement or careful estimation of parameters impacting both gas- and liquid-phase flow and transport are needed in determining volumes and flow rates of methane reaching groundwater and, thus, aquifer vulnerability to methane leakage.