ABSTRACT
The Cold Lake reservoir is an unconsolidated sand containing extremely viscous bitumen. Steam injectivity during cyclic steam stimulation can be achieved only by injecting at pressures high enough to mechanically fail the formation. Simulation of the complex fracturing and reservoir deformation behavior that results is very challenging. In addition, the reservoir exhibits water-oil relative permeability hysteresis, which must also be properly modeled. This paper describes enhancements made to a thermal reservoir simulator to incorporate these Cold Lake physics.
Rigorous geomechanical modeling is not economical, so an empirical approach has been developed that is consistent with the behavior of unconsolidated sands. Fracturing is modeled by allowing the permeability in a. plane of gridblocks to increase rapidly when the pressure exceeds a specified fracture pressure. Reservoir deformation in all blocks is modeled by first allowing dilation, during which porosity increases when the pressure exceeds a specified failure pressure. Subsequent pressure decline causes the reservoir to recompact, and porosity decreases. However, recompaction is not the reverse of dilation, and a fraction of the total dilation is permanent. While all gridblocks have similar deformation properties, the history of each individual block plays a role in determining its exact behavior.
This geomechanical representation allows the simulator to match field observations that are otherwise difficult to reproduce, including injection pressures, flowback times, and production pressures. Also, the model appropriately handles the recompaction process which provides drive energy in the Cold Lake reservoir.
The water-oil relative permeability hysteresis model is based upon laboratory measurements. Bounding imbibition and drainage curves are input; gridblock relative permeabilities, which depend upon both saturation and saturation history, are determined such that calculated values always lie on or between the bounding curves. The hysteresis model makes it possible to use laboratory-derived relative permeabilities when simulating cyclic steam stimulation and still match field water-oil ratios.
