UBC Theses and Dissertations
A field investigation on the transport and fate of fugitive gas from energy wells in glaciogenic sediments Chao, Jessie Tse-Hua
Recent petroleum resource development has generated concerns regarding the release of fugitive natural gas (comprised primarily of methane) from compromised wellbores into the shallow subsurface. Free-phase fugitive gas can move vertically to the ground surface, contributing to greenhouse gas emissions, or it can accumulate in confined spaces, creating explosive hazards. When free-phase gas dissolves into groundwater, it can be oxidized via microbially-mediated reactions that may deteriorate groundwater quality. To assess environmental risks posed by subsurface fugitive gas, it is critical to understand the transport and fate of fugitive gas in the shallow subsurface, particularly in petroleum resource development regions. The subject of this thesis is a controlled release field experiment to investigate the migration, dissolution behavior, and biogeochemical activity of fugitive gas as well as the hydrogeological controls on its migration in a shallow (<30 m) groundwater system. The study site is located in northeastern British Columbia, in the western portion of Western Canada Sedimentary Basin (WCSB). The site’s geology consists of heterogeneous glaciogenic sediments representative of much of the WCSB, where extensive petroleum resource development occurs. Approximately 82 kg of a synthetic natural gas, representative of local shale gas, was injected at 26 m depth over 69 days. The evolution of the injected gas and biogeochemical changes in groundwater chemistry were tracked through 55 depth-discrete monitoring points over 760 days. The key outcomes of this study are: (1) the demonstration that for regions of similar glaciogenic sediments, fugitive gas will likely be retained in the subsurface by the relatively continuous veneer of low-permeability diamict, a feature common across the WCSB; (2) evidence of significant heterogeneity in fugitive gas distribution and dynamic gas partitioning in the glaciofluvial groundwater environment, both of which affect the assessment of subsurface gas migration; (3) the approximation of time scales for methane dissolution; and (4) evidence for a low potential for anaerobic biogeochemical methane oxidation and limited changes in groundwater quality to occur. This thesis advances our understanding of the migration and fate of fugitive gas in a representative heterogeneous shallow groundwater system, providing insights into environmental practices at sites impacted by fugitive gas.
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