Investigation of mechanical failure of unconventional rocks and transport in resulting fractures with implications for supercritical carbon dioxide storage and utilization as a stimulation agent
Keywords: Energy Resources Engineering.

The exponential increase of atmospheric carbon dioxide (CO2) emissions motivates deep consideration of CO2 capture, utilization, and storage processes to limit the serious effects of global warming. There are at least two significant options for injection of CO2 into geological units: waterless fracturing of shale and other unconventional rock formations and CO2 storage in various geological units. By injecting fluids at elevated pressures, subsets of fractures are initiated and propagated. Likewise, any existing fractures or faults may be activated by shear displacement. The current practice of injecting so-called slick water (to stimulate natural gas production or enhance recovery of geothermal energy) consumes large quantities of fresh water and is only partially successful. Such applications drive the need to explore the mechanics of rocks exposed to new stimulation fluid candidates. Likewise, many storage formations where CO2 might be sequestered are overlain by a shale caprock. Here, the caprock needs to remain unfractured and intact. Both applications require a thorough understanding of the geomechanical and flow capacity of high conductivity conduits (cracks/fractures/faults) in shale under CO2 conditions to evaluate their effectiveness and risk. To address these motivations, we utilized a novel high pressure high temperature triaxial cell for an experimental campaign to evaluate the mechanisms of shale rock and fracture mechanics when CO2 is present. In doing so, the implications of cracks and fractures on transport are investigated. A series of breakdown pressure tests were conducted to investigate the fracturing behavior accompanying supercritical carbon dioxide (sc-CO2) injection compared to water. The high pressure high temperature triaxial cell was utilized to fracture intact unconventional rock samples under reservoir-like conditions. Furthermore, the experimental setup allowed continuous monitoring of in-situ details using X-ray Computed Tomography (CT). Here, CT images were utilized for the first time to investigate and confirm the breakdown pressure using fast iterative digital volume correlation that permits visualization of in-situ deformation and strain. Results demonstrated a two to three times greater breakdown pressure for sc-CO2 when compared to water for the samples studied. Under isotropic horizontal stresses, sc-CO2 induces fractures propagating almost independent of bedding planes. Fracture slippage experiments were additionally carried out to evaluate the friction coefficient under sc-CO2 conditions and the transport implications of fracture slip. The two major items of interest in this portion of the thesis are frictional strength and permeability change of the crack. The sc-CO2 generally did not alter the friction coefficient over the time scale of experiments, but there is a negative correlation with clay-content. Saturating cracks with sc-CO2 substantially decreased permeability, while slip resulted in various permeability responses. Overall, the combined impact of saturation and slip reduced fault permeability for all tests. Finally, experiments were conducted to evaluate the influence of sc-CO2 saturation on shale sample elastic and time-dependent deformation parameters. The power-law model for creep demonstrated an excellent fit to measured behavior. Power-law model parameters reflect significant ductility of Green River shale and very small time-dependent deformation. Short-term saturation with sc-CO2 (three days) significantly reduced static moduli by 19% to 38%. From a viscous creep perspective, sc-CO2 exposure causes a reduction in time-dependent deformation