Caprock integrity determination at the Christina Lake Thermal Recovery Project, Alberta

Abstract The Cenovus-ConocoPhillips Christina Lake oilsands project uses Steam-Assisted Gravity Drainage (SAGD) to recover bitumen from the Lower Cretaceous McMurray Formation at depths between 300 and 400 m. The McMurray Formation at the Christina Lake Field consists of fluvial to estuarine sands and muds capped by marine shales and sands of the Wabiskaw member of the Clearwater Formation. The SAGD process involves injecting steam under pressure to mobilize the bitumen, which is then pumped to the surface. The increased reservoir pressure is expected to be contained by the caprock. In the event of a failure of the caprock to contain these pressures the steam and oil may escape to shallower units and possibly the surface. This is not a desired outcome. We describe a process to characterize overburden and evaluate caprock strength. The minimum in situ principal stress is shown to be five times reservoir operating pressure and 2.5 times startup pressure. Compressive strength at the reservoir confining pressure of 4.5 MPa is six times operating pressure and three times startup pressure. The Wabiskaw member is shown to contain laterally continuous and impermeable semiconsolidated muds. Porosities and permeabilities are low and capillary entry pressure is high. A program of subsurface and surface monitoring is described.