Li et al. (2004) described three zones at the edge of steam chambers based on drainage conditions: 1. drained, 2. partially drained, and 3. undrained. In the drained zone, the pore pressure is controlled by injection pressure and fluid mobility within this region is sufficient to drain additional pore pressures due to shear dilation and pore fluid thermal expansion. The undrained zone lies beyond the partially drained zone and extends to virgin reservoir far beyond the chamber. In this zone shearing behaves under undrained condition; by this they mean no volume change occurs but shear lead to changes in pore pressure. Li et al. (2004) proposed that the boundaries of these zones are based on bitumen viscosity which relates to the temperature distribution beyond the steam interface. Since drained/undrained conditions impact the geomechanics at the edge of the chamber, we investigate whether Li et al.’s assumption that there is no volume change within the sheared zone is correct and is supported by field data. Here, we establish the physics associated with the undrained zone at the edge of SAGD steam chamber and explore the pressure front versus temperature front of different oil sand field projects. The results reveal that the drained zone governed by pressure front advancement is greater in extent than the sheared zone.. The thermodynamics of the undrained zone is discussed to derive a new theory for mechano-thermal phenomena at the edge of the chamber. The results from the theory show that the drained zone extends beyond the temperature front and thus, from a geomechanical point of view, the system solely consists of the drained and partially drained zones.

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