ABSTRACT
The underground storage of CO₂ has been proposed as an option to reduce atmospheric emissions of this greenhouse gas. Retention of CO₂ depends on fine-grained geologic units that overlie target storage formations and act as barriers or “caprock seals” to impede CO₂ migration. Assessment of storage sites depends on acquiring key data to build confidence and knowledge on whether the sealing behavior of a caprock will be acceptable. A challenge is the identification and characterization of local, relatively high permeability features such as fractures. This study presents an investigation of the seal integrity and transport properties of the Kirtland Formation, which is a regional aquitard and seal in the San Juan Basin, USA. In addition to multi-scale petrographical and petrophysical analyses on core samples and fracture information from well logs, our methodology incorporates natural noble gas data from preserved core samples. X-ray diffraction data, sub-micron digital reconstructions of pore geometries, mercury capillary injection pressure, and other petrophysical properties indicate a high quality sealing matrix. Natural and fully mineralized natural fractures are present in core, observed on image well logs, and were formed from early soil-forming processes, differential compaction, and tectonic events. Fracture mineralization includes barite, quartz, and calcite cements, and the fractures may provide potential migration pathways. Noble gas data is used to assess large scale diffusional or advectional transport through the Kirtland; although, uncertainties from possible degassing or stripping of some noble gases before or during preservation of core complicates interpretations. The author acknowledges the U.S. Department of Energy’s National Energy Technology Laboratory for sponsoring this project. The Kirtland overlies the coal seams of the Fruitland Formation into which CO₂ has been injected as a Phase II demonstration project of the Southwest Regional Partnership on Carbon Sequestration.