Imperial Oil's heavy oil operation in northern Alberta utilizes Cyclic Steam Stimulation (CSS) to extract bitumen from the oilsand reservoir (~400 mKB). The operation currently produces over 120,000 bpd from approximately 3000 deviated wells.
During CSS, the well casings are subjected to thermal stresses due to the cyclic, high-pressure, high-temperature nature of the process. In addition, shear stresses develop in the overburden due to volumetric dilation of the reservoir during steam injection. The combination of thermal fatigue and shear deformations can occasionally result in casing failures at some location along the wellbore, but typically at the top of the oilsand reservoir.
Imperial Oil and CANMET developed the application of passive seismic systems to monitor a volume of the overburden shale for casing failures, as well as for other events. As a result of this monitoring, a model for the seismic signature of well casing failures was developed. The seismic energy and radiation pattern of casing failure events has enabled the passive seismic systems to detect the occurrence of casing failures at the reservoir top, beyond the intended monitoring zone in the overburden shale. Subsequent casing checks have identified that the model detection is correct 88% of the time. In addition, event source location accuracy can be established based on the location of the actual casing failures determined from well workovers. This tool has allowed Imperial Oil to decrease well downtime and develop steaming strategies to reduce the occurrence of casing failures at the top of the reservoir.
Imperial Oil uses Cyclic Steam Stimulation (CSS) at its heavy oil operation in Cold Lake, Alberta. In the CSS process, the same wells are used for high-pressure steam injection, (typically 10-12 MPa), and for production. A typical stratigraphic section for Cold Lake is shown in Figure 1. Steam injected into the Clearwater Sand (CW) is accommodated by hydraulic fractures and non-linear dilation, which result in deformation of the CW1,2. Numerical modeling has demonstrated that shear stresses, which develop at the boundary between the CW sand and the overlying shale, occasionally exceed the expected shear strength of the boundary3. In addition, thermal stresses on the casing impact its ability to withstand shear loads4. These factors are manifested in the field as casing deformations and failures located at or near the top of the CW.