THE ROLE OF GEOLOGICAL FACTORS IN DETERMINING THE SUITABILITY OF GEOLOGICAL FORMATIONS FOR LONG-TERM CO₂ STORAGE – EXAMPLES FROM ACID GAS INJECTION OPERATIONS IN WESTERN CANADA

Geological sequestration of CO₂ has been proposed as a technologically feasible means to reduce atmospheric emissions of anthropogenic CO₂, thereby contributing to the mitigation of increasing atmospheric concentrations of greenhouse gases. The fundamental concept of geological sequestration is based on the availability in the subsurface of geological materials and accessible pore space, which can take up and retain significant amounts of CO₂ through physical and/or chemical trapping mechanisms. Knowledge of the geometry (e.g., shape, thickness, extent, structural characteristics and present burial depth), nature (e.g., lithology, mineralogical composition), and properties (e.g., porosity, permeability, heterogeneity, geomechanical stability) of geological formations provides the framework for the continuous evaluation of their suitability and effectiveness for CO₂ storage throughout the lifetime of a storage project, from the site selection stage to the syn- and post-injection monitoring stages.

Three primary types of subsurface geological formations have been identified as targets for future long-term CO₂ storage: oil and gas reservoirs, deep saline aquifers, and unmineable coal seams. Current acid gas injection operations in western Canada utilize depleted oil and gas reservoirs and saline aquifers as injection units and present a useful analogue to future large-scale geological storage of CO₂. The purpose of this paper is to demonstrate, based on examples from acid gas injection operations in western Canada, the role of geological factors in the evaluation of the storage capacity, injectivity and integrity of a variety of storage targets, as well as the integrity of the overlying or surrounding permeability barrier(s). Furthermore, the use of geological factors in a methodology for the selection of an acid gas injection monitoring site and their importance in the site-specific design and implementation syn- and post-injection monitoring programs will be addressed.