Caprock integrity in steam-assisted gravity drainage (SAGD) has been evaluated by various researchers. However, most of these caprock integrity studies assume that the caprock is not fractured, which is not a valid assumption in most cases. This paper provides a comprehensive analysis of caprock integrity in the Athabasca oil sands with presence of natural fractures of various dip-angles in the Clearwater shale.
Geological studies have shown that natural fractures exist in the Clearwater shale formation, which acts as the caprock for the McMurray oil sands. Based on some publicly available data in the Athabasca oil sands, we developed a numerical simulation model for a SAGD pad where reservoir and mechanical properties were characterized. A discrete fracture network model with natural fractures of different dip-angles was developed for the Clearwater shale. Coupled reservoir and geomechanical simulations were performed to evaluate the stress changes and deformations caused by steam injection in the reservoir. The impact of such stress changes on the natural fracture deformations was evaluated.
It is observed that with steam injection, the temperature increases, steam chamber grows—which causes horizontal stresses to increase in the reservoir. When the steam chamber reaches the caprock, the temperature at the bottom of the caprock also increases due to thermal conduction. Such temperature increase results in horizontal stress increase there. Shear slip is induced in the low-angle fractures at the bottom of the caprock due to the stress changes. This plastic shear deformation increases with time. The top part of the caprock is less disturbed and the low-angle fractures there remain stable. High-angle fractures that are present from the bottom to the top of the caprock tend to slip as well. These plastically deformed fractures of different dip-angles provide a network of higher hydraulic conductivity conduits for fluid to more easily pass, and therefore are a potential risk to caprock integrity.
The behavior of natural fractures in the caprock and their impact on caprock integrity in SAGD operations have rarely been addressed in the literature. This paper aims to fill the gap by presenting an analysis of natural fractures in the Clearwater shale and a comprehensive approach of simulating their shear slip tendency and thereafter the impact on caprock integrity in SAGD.