Leakage Risk Assessment in Rollover Structures of the Sable Subbasin, Offshore Nova Scotia: Implications for CCS

On the Scotian Margin, most of the successful commercial gas discoveries occur in simple closures within rollover structures of the Sable Subbasin where they have been produced from the high net-to-gross Missisauga Formation reservoirs. In the absence of the simple closure in this stratigraphic interval, a favorable juxtaposition of strata with the presence of extensive thick shales play a role in guaranteeing the success of key reservoirs that have been intercepted by faults.

This paper uses a 3D geocellular model of two Structure drilled offshore Nova Scotia, populated with key reservoir parameters to demonstrate potential risks associated with fault leakage in the rollover. Using a previously constructed static model taken to dynamic modelling in these structures, a hypothesis that the residual saturations on logs will match the residual saturations in the dynamic models and will correspond to large water tests with very little gas is tested.

Results from the dynamic modeling demonstrate an upward and outward cross-fault, stair stepping migration of gas through the reservoirs in the high net-to-gross shallow to intermediate sections of one of the structures after a minimum of 25 to 50 years of steady injection and equilibration. Post simulation, the presence of some residual gas saturation after the injection and equilibration phase supports initial observations of upto 50% gas saturation from log analysis of key reservoirs. In the other modelled structure, there is very little movement of residual fluids across the fault when geomechanical modelling is explored.

In conclusion, results of the dynamic modelling confirm that residual trapping holds some significance in the sequestration of CO2 in faulted rollover anticlines where fault leakage may be a risk. Modelling applications done on the Rollover Structures in this study may be applicable to prospective margins where CO2 sequestration in rollover anticlines is being considered such as the Brazilian Margin and its conjugates in the Southern Atlantic.