MIGRATION BEHAVIOR OF FUGITIVE METHANE IN SHALLOW AQUIFERS: MULTI-PHASE LAB- AND FIELD-SCALE NUMERICAL MODELLING WITH DUMUX

Oil and gas development and abandoned or poorly sealed energy wells have been associated with risks of fugitive methane emissions into shallow aquifers. A better understanding is needed to identify these risks and develop monitoring and mitigation strategies. In this study, we apply the multiphase numerical model DuMux in order to better understand the migration behavior of methane from deep shale formations into shallow aquifers, and to provide insight into potential impacts on shallow groundwater resources. The simulations include the migration of water, gas, and dissolved phase methane, applied to conceptual models at the laboratory and field scale.

The primary transport mechanisms simulated with DuMux are groundwater flow, transport of free phase methane subject to viscous, capillary and gravity forces, and advective-diffusive transport of dissolved phase methane. The DuMux multi-phase simulations are based on capillary pressure-saturation curves which are calibrated using the Brooks - Corey model.

The model is first calibrated to a series of 2D laboratory experiments on gas phase injection and dissolution performed at Queen's University under a background flow gradient and including homogeneous and heterogeneous structures. Comparisons are made with respect to the simulated and observed gas saturations and concentrations over time, and using the breakthrough of dissolved phase methane at selected monitoring points.

Field-scale simulations are then carried out based on conceptualized hydrogeological systems at the Saint Édouard site in southern Quebec where an exploration well was drilled into the Utica shale. Conceptual models of additional selected sites include emissions from methane sources into confined, unconfined and partially confined aquifers. Intrinsic parameters of the porous medium, in particular its permeability and porosity, as well as the Brooks - Corey parameters, specifically the entry pressure and the residual gas saturation, are shown to control gas migration. The impact of spatial property distributions on methane migration is also discussed, in particular gas pooling below low-permeability layers. The results will help in the design of effective strategies for monitoring methane migration.