In the oil sands of western Canada, caprock integrity has become a central issue in projects using steam injection recovery processes such as SAGD (steam-assisted gravity drainage). Caprocks contain steam and fluids within the reservoir and understanding the integrity of the caprock over the life of the operation is critical in order to ensure a safe and economically viable project.
A multi-disciplinary study was undertaken to evaluate geological facies as potential caprock for Ivanhoe Energy’s Tamarack project. This examination of the historical performance of operating SAGD projects correlated the maximum vertical growth of the steam chamber with geology and the steam-injection operating pressure. The study found that SAGD steam chambers are being constrained by geological facies grading upward to poorer reservoir quality rather than being constrained by shallower, regionally extensive, massive, low-permeability barriers.
Geomechanical reservoir simulations of Ivanhoe Energy’s proposed Tamarack SAGD Project predict that the steam chamber will be similarly constrained as reservoir quality degrades upward. The simulations show the pressure and stress gradients in the formations above the steam chamber as a function of time and operating conditions, allowing for a more accurate assessment of steam containment and the risk for shear and/or tensile failure.
The findings are significant because they confirm that the vertical growth of SAGD steam chambers has been effectively halted by facies consisting of interbedded sands and mudstones. These effective caprock facies have higher fracture pressures than the regionally extensive low permeability barriers because these facies are found at greater depths. The higher fracture pressure justifies a higher maximum operating pressure, with its associated higher reservoir temperatures resulting in much lower bitumen viscosities. As a result, SAGD well productivity and project economics are greatly improved, particularly for shallow SAGD projects.