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Faulty oil and gas wellbores are a primary pathway of concern for gas migration from the deep subsurface into shallow freshwater aquifers. Leaked gases migrating vertically along wellbores either collect and build a pressure at the wellhead known as sustained casing pressure (SCP) or escape into the atmosphere as surface casing vent flow (SCVF). SCP and SCVF are valuable indicators of integrity loss that provide insight into the potential for groundwater contamination through gas migration. Previous models of SCP and SCVF have focused on offshore wells and have not considered the relationship between SCP/SCVF and gas migration away from onshore wells. We present the first modeling framework for SCP and SCVF that is applicable to onshore oil and gas wells constructed with outermost annuli that are hydraulically connected or open to the surrounding formation. Our results show that SCP behavior is unique in onshore wells with open annuli, because steady state SCP is not achieved in an open annulus unless gas escapes the wellbore via gas migration (Darcy Flow), a wellhead leak, or a controlled bleed-off test. We show that appropriately modeling gas leakage along the wellbore and SCP/SCVF helps to constrain methane leakage fluxes from faulty wells and could be integrated with subsurface flow and transport models of stray gas contamination. By relating gas fluxes to SCP/SCVF, our model can provide information used to support regulatory actions informed by SCP/SCVF, which are easy to measure and monitor.

Plain Language Summary Oil and gas wells can leak gas into groundwater if they are improperly constructed or become faulty over their lifetime. Gas contamination degrades groundwater quality and poses an explosion or asphyxiation hazard if allowed to degas in a confined space. Gas leakage in an oil and gas well can be detected easily at the surface by monitoring for undesirable pressure or gas flow at the wellhead, which are referred to as sustained casing pressure (SCP) and surface casing vent flow (SCVF), respectively. In this study, we present process-based models for SCP and SCVF, which can be used to develop a relationship between measured values and gas leakage from the well into groundwater. We demonstrate the validity of our model based on comparisons to SCP field data and illustrate its applicability for diagnosing gas leakage based on several case studies in the Wattenberg Field in Colorado. We also identify well construction practices that increase the risk of groundwater contamination and suggest better management practices for faulty oil and gas wells that will help to protect groundwater.