Schlumberger

Technical Paper: Caprock Integrity Analysis of Thermal Operations: An Integrated Geomechanics Approach

Society: Other
Paper Number: WHOC11-609
Presentation Date: 2011
 

Abstract

Caprock integrity is as critical as it is complex in thermal injection process such as steam-assisted gravity drainage (SAGD) and cyclic steam stimulation (CSS). Continuous steam injection triggers complex coupled thermal and hydraulic processes, which can dramatically alter the in-situ stresses, reduce rock strength, induce fractures or re-activate existing fractures posing continued risk of containment breach of caprock. This can ultimately lead to breach in well or reservoir integrity, providing pathways for bitumen or steam to flow to the shallower fresh aquifers or surfaces, both of which pose significant risk to safety and the environment. Traditionally, fracture pressure typically measured from mini-frac testing is considered as the upper limit of the net injection pressure; however, several cases of compromising the integrity of caprock have been reported despite keeping the net injection pressure below the fracture pressure. Shear failure of rock has been one of the major reasons for breaching the caprock in most of these cases. In this paper, we present an integrated geomechanics approach to evaluate caprock integrity in thermal operations. The alteration of in-situ stresses caused by steam injection cannot be easily predicted by basic analytical methods, it requires sophisticated numerical modeling of the reservoir and the surrounding rock; hence, as part of this integrated approach, we use coupled modeling between dynamic reservoir modeling and geomechanical modeling to see the effect of steam injection on caprock integrity. To demonstrate the capability of this approach, this study presents an analysis of the steam release incident at Joslyn Creek, in Alberta, Canada, identifying possible reasons for the breach of caprock and how the integrated approach, detailed in this study, can be a proactive plan for operators to avoid such catastrophic incidents.

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