CONCEPTUAL MODELS FOR LONG TERM METHANE EMISSIONS TO PRIORITIZE PLUGGING OF ORPHANED OIL AND GAS WELLS

Methane (CH₄) emissions from orphaned oil and gas wells are a significant source of greenhouse gas (GHG) emissions and a growing policy issue as governments seek to reduce anthropogenic GHG releases, reduce CH₄ emissions, and limit public hazards. CH₄ is the second most abundant GHG in the atmosphere after carbon dioxide (CO₂), with a much higher 20-year Global Warming Potential of 82.5 relative to CO₂. However, CH₄ has a shorter atmospheric lifetime than CO₂, so management of CH₄ emissions will have more immediate impacts to our climate. Although the locations of more than 100,000 orphaned wells are currently known in the United States, estimates for unknown orphaned wells range in the millions. Only a few hundred orphaned wells have been measured for methane emissions and documented in the literature, causing many uncertainties about the occurrence and magnitude of their emissions. To address concerns about orphaned wells, the U.S. Congress included $4.6 billion to states, tribes, and federal agencies to plug abandoned and orphaned wells across the nation in the Infrastructure Investment and Jobs Act of 2021. The initiative for plugging orphaned wells has highlighted the need for tools to prioritize known wells, while little is understood about the geologic and engineering factors that contribute to the highest CH₄ emitters.

To better understand the causes of CH₄ emissions in orphaned wells, we have developed conceptual geologic models and analyzed petroleum drilling practices to identify contributing factors that could lead to CH₄ emissions. Our conceptual models provide possible explanations of methane emissions from orphaned wells in three scenarios: a steady gas recharge into the reservoir, migration of thermogenic and biogenic coalbed methane from coal seams and nearby coal mines, and conditions and processes that compromise wellbore integrity (corrosion, heat, and pressure) that could lead to additional CH₄ release from other reservoirs and formations. The highest CH₄ emitting orphaned wells may contain a combination of these influences, as well as possibly other factors. The goal of our work is to create a framework for studying wells at high risk for current or future emissions and aide prioritization of known orphaned wells for plugging.