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Fugitive gas migration from leaking oil and gas wells Forde, Olenka Noelani

Abstract

Fugitive gas migration (GM) due to leaking oil and gas wells poses a major risk to the environment. The unintentional and uncontrolled release of methane (CH₄, the primary component of natural gas) from a compromised well can lead to aquifer contamination, explosive conditions, and greenhouse gas emissions to the atmosphere. Currently, there is a lack of knowledge on the occurrence, distribution, fate, and transport of fugitive gas. This has limited the development of effective and reliable monitoring techniques at oil and gas sites. This thesis presents results from: i) GM monitoring across well pads in Northeastern British Columbia (BC), and ii) two large-scale field experiments in Borden, Ontario and Northeastern, BC. Soil gas effluxes and stable carbon isotope ratios identified considerable variation in the spatiotemporal distribution of fugitive gas on well pads. The field experiments were designed to characterize the transport and fate of fugitive gas via controlled subsurface injections of natural gas into: i) the saturated zone of a shallow unconfined sand aquifer (Borden) and; ii) a thick vadose zone of glacio-lacustrine deposits (Northeastern, BC). Results from the experiment at Borden suggest that the distribution of fugitive gas is affected by the rate of natural gas leaking from a well and, characteristics of the subsurface lithology. Subtle heterogeneities led to extensive lateral GM and episodic effluxes to the atmosphere. A higher rate of gas leakage allowed greater lateral and vertical GM. Cessation of gas leakage was associated with enhanced CH₄ dissolution, greater oxidation, and lower effluxes. Geochemical impacts were caused by processes induced by natural gas leakage, and CH₄ oxidation. At the field site in Northeastern, BC, changes in barometric pressure directly controlled the magnitude and temporal distribution of fugitive gas from the thick vadose zone. Despite active gas release, increases in barometric pressure suppressed CH₄ effluxes. Decreases in barometric pressure allowed gas to break through low-permeability soils, even after injection stopped. The findings from these studies have significant implications to improve oil and gas well site monitoring to identify the occurrence and risks of GM.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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