Impacts of stray gas migration on shallow groundwater: insights from laboratory experiments and numerical modelling

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The combination of horizontal drilling and multi-stage hydraulic fracturing has created a significant shift in global energy production, increasing the use and availability of natural gas through new technologies that allow the extraction of gas from previously inaccessible formations. Unconventional natural gas (shale gas) resources exist across Canada, but expansion has been accompanied by concern over potential threats to water resources. Research concerning these potential threats has not kept pace with the surge in development, and public concern, along with gaps in our understanding of controlling processes and the associated risks, is a significant impediment to further development in Canada. One potential risk is the leakage of gas (mostly methane) from deeper zones to shallow aquifers by gas flow outside the surface casing of an inadequately sealed or damaged wellbore. This free gas can dissolve into the surrounding groundwater leading to chemical and biological reactions that reduce groundwater quality. Focused research is needed to better understand factors influencing stray gas migration and dissolved gas transport to develop best practices for risk management and monitoring for potential effects on groundwater quality. This study will use a combination of high-resolution laboratory experiments and advanced numerical models to investigate the effects of stray gas migration on shallow groundwater. One-dimensional and two-dimensional experiments will track gas flow and dissolution using visual techniques as well as the analysis of gas and water samples. Analysis will include stable isotopes to assess reactions occurring in both the saturated and unsaturated zones. Numerical simulations using two different approaches will be compared to the experimental results and to each other, to assess model capabilities and develop new techniques. Additional simulations will investigate larger-scale, longer-term field scenarios to develop monitoring strategies and establish a framework for risk assessment under different geological and hydrogeochemical conditions.