Groundwater-quality implications of methane leakage from hydrocarbon wellbores

Abstract

Loss of hydrocarbon wellbore integrity can lead to impaired groundwater quality. Methane is the most probable groundwater contaminant from hydrocarbon wellbore leakage due to its buoyancy and natural abundance. Here, a synthesis of the literature is conducted to investigate groundwater-quality hazards of methane leakage from hydrocarbon wells. As informed by this synthesis, three-dimensional, multiphase (gas and liquid), multicomponent (methane, water, salt), numerical modeling (in TOUGH2 EOS7C) is used to evaluate methane leakage from a natural-gas wellbore that migrates upwards towards groundwater. This work focuses on processes with the potential to slow down methane migration since older wells, completed before modern regulations, are more likely to leak. Potentially delaying processes simulated here include (i) multiphase flow, which allows orders of magnitude slower flow as compared to single-phase models due in part to changes in relative permeability, (ii) geostatistical variation of intrinsic permeability, and (iii) dual-domain mass transfer (DDMT), which permits storage of solutes in less-mobile pore space for long periods of time. In these simulations, variation in multiphase parameters (i.e., those impacting relative permeability and capillarity) leads to substantial changes in the flow rates of methane reaching groundwater, with a greater impact than the changes in flow rates associated with variability in intrinsic permeability. Multiphase parameters must, therefore, be measured, or carefully estimated, with more general approaches (e.g., setting parameters at reasonable or literature values) likely to produce significant errors in models of methane migration. DDMT substantially increases methane flow rates to groundwater, and leads to peaks in methane concentration at a downstream well, which occur decades after leakage ends. Therefore, in these simulations, legacy wells with wellbore integrity loss present a current hazard to groundwater resources, even following remediation of leaking wells.