Forensic numerical analysis of gas venting in Southwestern Ontario

A forensic modeling study examines the potential causes of gas and water venting observed during the excavation of a landfill in a 40 m thick clayey till deposit in southwestern Ontario, Canada. The clayey till is known to be underlain by permeable, natural gas bearing, rock and gas has been diffusing through the clayey deposit over about the last 13,000–15,000 years. Attention is focused on 2D modeling of hydrofracturing as well as gassy soil behavior. The model examines the predicted location of venting with and without the presence of gassy sand lenses and a discontinuous layer of basal till between the bedrock and the low permeability till. The modeling shows that hydrofracturing alone can explain the occurrence and location of the observed venting in one of the sub-cells. In the other sub-cell, hydrofracturing explains the occurrence but not the location of the observed venting or the presence of the sand in the venting water at this location. Modeling of appropriately placed gassy sand lenses is shown to explain the occurrence and location of the observed venting as well as the presence of both waters with different geochemistry from the bedrock and the till in the venting waters.