Propensity for fugitive gas migration in glaciofluvial deposits: An assessment of near-surface hydrofacies in the Peace Region, Northeastern British Columbia

Jessie Tse-Hua Chao; Aaron G Cahill; Rachel M Lauer; Cole J C Van De Ven; Roger D Beckie

doi:10.1016/j.scitotenv.2020.141459

Propensity for fugitive gas migration in glaciofluvial deposits: An assessment of near-surface hydrofacies in the Peace Region, Northeastern British Columbia

Sci Total Environ. 2020 Dec 20:749:141459. doi: 10.1016/j.scitotenv.2020.141459. Epub 2020 Aug 3.

Authors

Jessie Tse-Hua Chao¹, Aaron G Cahill², Rachel M Lauer³, Cole J C Van De Ven⁴, Roger D Beckie⁴

Affiliations

¹ Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, BC V6T 1Z4, Canada. Electronic address: tchao@eoas.ubc.ca.
² Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, BC V6T 1Z4, Canada; The Lyell Centre, Heriot-Watt University, Edinburgh, Scotland EH14 4AS, UK.
³ Department of Geoscience, University of Calgary, Calgary, AB T2N 1N4, Canada.
⁴ Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, BC V6T 1Z4, Canada.

PMID: 33370906
DOI: 10.1016/j.scitotenv.2020.141459

Abstract

Petroleum resource development has generated a global legacy of millions of active and decommissioned energy wells. Associated with this legacy are concerns about wellbore integrity failure and leakage of fugitive gas into groundwater and atmosphere. The fate of fugitive gas in the shallow subsurface is controlled by sediment heterogeneity, hydrostratigraphy and hydraulic connectivity. We characterized the shallow subsurface at a site in northeastern British Columbia, Canada; a region of extensive petroleum resource development. We collected 13 core profiles, 9 cone-penetrometer profiles, 58 sediment samples and 4 electrical resistivity profiles. At the site, a ~ 12 m thick layer of low-permeability diamict (10^-8 m/s) overlays a more permeable (10^-6 - 10^-4 m/s) but highly heterogeneous sequence of glacigenic sand, clay and silt. We develop a conceptual hydrostratigraphic model for fluid flow in this system in the context of fugitive-gas migration. Driven by buoyancy forces, free-phase gas will move upward through discontinuous permeable zones within the Quaternary sediments, until it encounters lower permeability interbeds where it will pool, flow laterally or become trapped and dissolve into flowing groundwater. The vertical extent of gas migration will be significantly limited by the relatively continuous overlying diamict, a feature common across the Western Canadian Sedimentary Basin. However, intra-till lenses observed embedded within the diamict may provide pathways for gas to move vertically towards ground surface and into the atmosphere. This study provides one of the few investigations examining geological and hydrogeological heterogeneity in the shallow subsurface at scales relevant to gas migration. For glaciated regions with similar surficial geology, such as Western Canada Sedimentary Basin, gas that is released into the subsurface from an energy wellbore, below a surface diamict, will likely migrate laterally away from the wellbore, and be inhibited from reaching ground surface and emitting to atmosphere.

Keywords: Fugitive gas; Gas migration; Heterogeneity; Hydrogeology; Quaternary sediments.