Uniaxial Compression Analysis and Microdeformation Characterization of Kevin Dome Anhydrite Caprock

Abstract:

The Department of Energy currently manages the Regional Carbon Sequestration Partnership (RCSP) in efforts to develop techniques to characterize promising CO₂ storage sites, efficient and durable technology for injection, and suitable regulations for future CO₂ storage. Within the RCSP, the Montana State University-Bozeman led Big Sky Carbon Sequestration Project has focused on potential CO₂ storage sites, including the Kevin Dome in northern Montana. The 750mi² large dome lies along the north-southwest trending Sweetgrass Arch and is a natural CO₂ reservoir with the potential to produce one million tonnes of CO₂. The Project intends to extract and reinject this one million tonnes of CO₂back into the water-leg of the Dome within the dolomitic, middle Duperow Formation to monitor impacts on the surrounding environment and communities. The caprock system includes extremely low porosity dolomite in the upper Duperow that is overlain by the anhydrite-dominated Potlatch caprock. Core was extracted by the Project from the Wallawein 22-1 well. Six 1"-diameter sub-samples were taken at depths of 3687 and 3689' of the 4"-diameter core in both vertical and horizontal directions.

Unconfined uniaxial compression tests were conducted at room temperature using an Instron 4483 load frame with a 150kN load cell operated at a strain rate of 6.835^-5mm per second. Samples were instrumented with four strain gages to record elastic moduli and characterize fracture behavior. The Potlatch anhydrite has demonstrated to be both strong and stiff with an average uniaxial compressive strength of 150.62±23.95MPa, a Young’s modulus of 89.96±10.22GPa, and a Poisson’s ratio of 0.32±0.05. These three variables are essential to developing geomechanical models that assess caprock responses to injection during CO₂ sequestration. Petrographic characterizations of the fractured samples reveal an 80% groundmass of subeuhedral anhydrite crystals measuring 97-625µm and 20% 0.12-1mm wide veins comprised of 9-35µm wide dolomite grains that become increasingly anhedral toward vein centers. Petrographic observations of tightly aligned anhydrite grains support the porosity of 7.5% calculated from sample densities. Such microscopic observations are key to understanding fracture propogation and permeability responses on a reservoir scale.