Free Phase Gas in Shallow Groundwater
Date
2022-08
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Abstract
The unintended migration of natural gases and saline fluids from deep formations into shallow aquifers can increase the risk of explosion during groundwater extraction, induce biogeochemical changes in aquifers containing potable groundwater, and increase greenhouse gas emissions into the atmosphere. Hence, long-term monitoring of gases present in shallow aquifers is essential to evaluate the occurrence and the potential environmental impacts associated with the presence of fugitive natural gas migration. However, accurate sampling and monitoring of dissolved and free-phase gases (FPG) is particularly challenging in gas-charged groundwater wells. In addition, the fate of fugitive natural gases in shallow aquifers around oil and gas wells with gas migration is not yet fully understood. This thesis investigates the best approaches for accurate monitoring and sampling of dissolved gases and FPG in shallow aquifers. In addition, a multidisciplinary field investigation was conducted around an oil and gas well with integrity failure to investigate the key mechanisms that govern transport and attenuation of fugitive gases in the shallow groundwater zone. Monitoring of Total Dissolved Gas Pressure (PTDG) and dissolved gas concentrations in a 'gassy' groundwater well demonstrated that bubble exsolution (i.e., FPG formation), bubble-driven micro advection, and thermally driven convection are key mechanisms controlling degassing in gas-charged groundwater wells. In addition, laboratory experiments indicated that the combined measurement of water pressure, electrical conductivity, and PTDG can accurately identify the occurrence of FPG in groundwater wells under non-flowing and flowing conditions. Finally, high-resolution monitoring of fugitive gases and hydrogeological conditions in the shallow groundwater zone around an oil and gas well with gas migration demonstrated that the transport of fugitive gases at the study well is controlled by the presence of preferential pathways along the well casing, the distribution of lenses of unconsolidated sediments with higher silt and clay content, and the groundwater flow direction. Together, these findings provided insights into the key mechanisms controlling in-well degassing in gas-charged wells, the usage of water pressure and continuously measured field parameters to detect the occurrence of FPG in groundwater wells, and the transport of fugitive natural gases in shallow aquifers around 'leaky' oil and gas wells.
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Keywords
Free phase gas, In-well mixing and degassing, Dissolved gas, Groundwater gas detection, Water pressure, Field Parameters, Total dissolved gas pressure
Citation
Morais, T. A. (2022). Free phase gas in shallow groundwater (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.