Optimization of SAGD Wind-down and Blow-down for Athabasca Oil Sands

Athabasca is the largest resource accumulation among three major Canadian oil sands reservoirs. Through more than twenty years of research and studies, Steam Assist Gravity Drainage (SAGD) has been proven as an economic viable commercial method to develop Athabasca oil sands. However, because very few of industrial SAGD projects have matured to date, Wind-Down (WD) and Blow-Down (BD) operation strategies are not well understood and more research and study efforts are needed. In this paper the beginning time and gas injection and production rates in WD and BD with pure methane are optimized for an Athabasca oil sands reservoir with numerical simulation. A simplified economic method and assumptions for optimization are presented and applied in this study. Six type wells are selected to capture the complexity of geological architecture (including pay thickness, and permeability and porosity heterogeneities) and fluid characteristics (thief zones) of the investigated reservoir. The optimization is based on the net values of WD and BD with 2D layer cake simulation models. The sensitivity of the net values to gas and bitumen prices is also analyzed. The strategies to optimize WD and BD for compartment areas or full fields are explored in this paper. For the areas without geomodels, a weighting method is proposed and applied. For areas with geomodels single model simulation is recommended if the areas are not large while the type well/pad methodology is suggested if the compartment areas or full fields are too large to simulate with one model. The results of this study indicate that understanding reservoir geology and optimizing WD beginning time are important and necessary to have positive WD economic values. In some cases compromise has to be made in optimization simulation due to the complexity of reservoir, availability of model and limitation of resources, and a practical optimization method with proper simplifications have to be taken. With the optimized WD and BD operation and management strategies the economy of SAGD projects can be improved significantly.