Thermo‐Mechanical Creep Behavior of Clearwater Caprock

Abstract

Alberta’s oil sands deposits are the third-largest proven crude oil reserve in the world. However, only 20% of the bitumen in Alberta’s oil sands are shallow enough to be recoverable by surface mining techniques; the remaining 80% can only be extracted using in-situ techniques that involve the injection of steam at high temperatures, such as Steam Assisted Gravity Drainage (SAGD) and Cyclic Steam Stimulation (CSS). In order to guarantee the safety of the projects, the isolation of the injected fluid in the subsurface should be assured over the lifetime of a project. Caprock integrity has been identified as one of the main concerns that allow communication of the reservoir with the upper geological layers in thermal operations in the oil sands production. Clearwater Formation is the main caprock that hydraulically isolates the reservoir with the upper permeable zones and ground surface in the Athabasca area. Clearwater clay shales are typically classified as a “hard soil/ soft rock”, representing the complexity when characterizing the mechanical behavior of the material. This research is aimed to enrich the geomechanical characterization of Clearwater caprock through laboratory testing. Intact and reconstituted Clearwater samples test results served to identify the intrinsic properties of the material, the effects of material structure on caprock behavior, and the approximate degree of overconsolidation and yield stress. These studies evidenced that creep dominates the time-dependent behavior of Clearwater caprock. Time-dependent tests were used to evaluate the best way to analyze and understand the creep behavior of Clearwater for different laboratory conditions, and to find significant material properties and parameters that can be used to characterize and predict the long-term behavior of Clearwater clay-shale caprock. Finally, thermal creep tests supplement creep findings for the particular case of thermal recovery applications. From the latter tests, it was studied how temperature affects long-term deformation of Clearwater clay-shale at in-situ effective stress conditions. Furthermore, key parameters required to obtain quality tests when high temperature is involved were identified. Models used in geomechanical simulation should honor the experimental results that the materials described in these tests, since having a proper understanding of the geomechanical response to thermal recovery of the reservoir and overlaying formation is crucial to avoid caprock integrity issues.