In northeast Alberta, Canada, steam-assisted gravity-drainage (SAGD) is used to produce bitumen from the Athabasca oil sands. In some regions, bitumen resources are located within 100 m of the ground surface and have average caprock thicknesses of about 50 m. The caprock unit overlying the bitumen reservoir is comprised mainly of mudstones and shales from the Early Cretaceous, Wabiskaw Member and Clearwater Formation. These shales form a baffle, impeding vertical steam migration and confining the stress and strain fields developed during the SAGD process, which are crucial for maintaining caprock integrity at SAGD operations.
In May 2006, an unexpected, catastrophic failure of the caprock seal occurred at the Joslyn Creek SAGD project, resulting in a steam release, which caused considerable caprock and ground surface deformation. Because of this failure, the Alberta Energy Resources Conservation Board modified the existing application guidelines and directives to include an assessment of caprock integrity at the maximum operating pressure specified for the thermal project applications.
The authors recently completed a core based, caprock integrity assessment for a proposed, shallow, low pressure SAGD, pilot project in northeast Alberta. This paper describes the information reviewed, assessment method and results of this study. From this evaluation, information about the geomechanical characteristics, the potential for containing the induced stress and strain fields and the geologic risks associated with caprock integrity for the proposed pilot project were determined. An assessment of the underburden seal was also performed as this seal can influence caprock properties. Results of the underburden seal assessment are also presented.