Lean zones have been reported in Nexen's Long Lake project and Suncor's Firebag project in Canada. With high steam-oil ratio and low oil recovery caused by lean zones, developing a heterogeneous oil sand reservoir is a considerable challenge. To study the mechanism of how lean zones affect thermal recovery performance, the role of the huge volume of mobile water cannot be neglected. In this paper, we propose a new mass transfer model for the thermal recovery process in oil sands with mobile water. Further discussions based on the model showed the following conclusions: (1) Reservoir regions during thermal recovery are divided into four regions based on initial water saturation. An increase in the initial water saturation may cause the oil bank to disappear. (2) The initial mobile water saturation significantly reduces the width of the two-phase flow region once it is larger than the frontal water saturation of this region. (3) High water saturation leads to a long breakthrough time in a hot driving region and low oil recovery. (4) Large connate water saturation and porosity and small heated oil viscosity benefit the oil recovery before the breakthrough of the steam chamber.

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