| dc.description.abstract | Carbon capture and storage (CCS) is an important technology developed to mitigate human-induced increase in atmospheric carbon dioxide concentrations. Injecting fluid carbon dioxide in geological reservoirs is one of the most promising methods for effective and safe carbon storage. Fractures in rocks govern fluid flow in many geological reservoirs. When injecting carbon dioxide, stress within the reservoir will change, leading to the nucleation, growth and coalescence of fractures. This thesis investigates the effect of changing stress conditions on fractures, and how this process influences fluid flow through the fracture. A naturally fractured core sample of tight sandstone from a natural carbon dioxide migration system (Entrada formation, Utah, USA) was confined in a rubber membrane and set in an X-ray translucent isotropic pressure cell. Pore pressure was controlled through both top and bottom inlets located on the two extremities of the sample, and permeability could therefore be measured at various effective pressures. The sample was pressurised and depressurised stepwise in three cycles, with CO2 as pore fluid in the first cycle, and brine as a pore fluid in the two others. Lastly, CO2 was injected into the brine saturated sample. The sample was X-ray scanned for 3D computed tomography (CT) images at every pressure configuration step and every step of injection of CO2 into brine. Acoustic velocity and electric resistivity were monitored throughout the test. Qualitative and quantitative approaches are used and discussed to characterize the relationships between mechanical strain, permeability and aperture from CT and pressure data. A method for extracting the fracture strain from CT data in lack of successful fracture segmentation is applied. Strain is calculated at detailed spatial resolution from CT scans. Distribution of CO2 injected in the brine saturated sample is characterised through CT images, albeit with limited success. Responses in acoustic wave velocity and electric resistivity from stress, strain and injection are discussed. Correlation is found between the hydraulic aperture retrieved from pore fluid data and strain retrieved from CT data for the initial closure of the fracture. In a second cycle of effective pressure loading and release, hydraulic aperture remains sensitive to effective pressure, while strain is low. Local strain within the fracture is suggested as an interpretation. The findings highlight the importance of local fracture strain, and methods for further research is tested and suggested for better understanding change in fluid flow through complex, natural fractures. | eng |