NUMERICAL ANALYSIS OF GROUNDWATER QUALITY IMPACTS FROM NATURAL GAS WELLBORE LEAKAGE BELOW FRESHWATER AQUIFERS

The development of directional drilling and stimulation of reservoirs by hydraulic fracturing makes it economically feasible to recover unconventional oil and gas resources from shale formations, coal beds, and tight sand reservoirs. Hydraulic fracturing presents a set of water-quality challenges, including increased competition over water resources and the potential for air, surface water, and groundwater contamination. In this project, we use a three-dimensional, multiphase, multicomponent numerical model to investigate hydrogeologic conditions that could lead to groundwater contamination from a leaking hydraulic fracturing wellbore. This work explores the fate of methane that enters a well annulus, possibly from an intermediate formation or from the production zone via a flawed cement seal, and leaves the annulus below a freshwater aquifer. This leakage scenario is largely ignored in the current scientific literature, where focus has been on leakage directly into freshwater aquifers, despite modern regulations requiring steel casings and cement sheaths at these depths. To investigate leakage at depths below which steel casings and cements sheaths are required, we perform a three-stage sensitivity analysis, examining (1) hydrogeologic parameters of media surrounding a methane leakage source zone beneath a freshwater aquifer, (2) geostatistical variations in intrinsic permeability in the same region, and (3) methane source zone pressurization. Results indicate that in all cases methane leakage reaches the base of the freshwater aquifer, driven by buoyancy and source zone pressure. The degree of impact is strongly dependent on mean matrix permeability and source zone pressure and, to a lesser degree, varies as a function of heterogeneity and anisotropy in subsurface permeability. These results can be used to inform assessment of aquifer vulnerability to hydrocarbon well leakage.