Date of Award:
Master of Science (MS)
James P. Evans
The interface between a reservoir and a caprock is generally considered a flow boundary where capillary trapping at the base of the caprock and low permeability of the seal resists the upward migration of fluids. Joints and faults may act as mechanisms for seal bypass, allowing fluids to escape from the reservoir.
The injection and storage of carbon dioxide (CO2) in subsurface porous sandstone with effective top seals is a proposed method for reducing the amount of anthropogenic CO2 in the atmosphere in the system of carbon capture utilization and storage (CCUS). Uplift and erosion of the San Rafael Swell in south-central Utah has exposed the Jurassic Navajo Sandstone and Carmel Formation reservoir-seal pair that is analogous to potential CO2 injection targets. The outcrops show evidence of seal failure through mesoscopic opening-mode fractures and faults in the caprock.
We focus on three sites of different structural position with exposures of the reservoir-seal pair that were analyzed for fracture characteristics. Deformation-band
faults, joints, and joint clusters are present in the Navajo Sandstone and deformation-band faults are present in the Page Sandstone where faulting has occurred. The structures in the reservoir lithologies penetrate the interface and transition into opening-mode fractures in the caprock. The fractures in the caprock show evidence of seal failure which include multiple mineralization events and bleaching, likely from reducing fluids within the fractures. Seal failure is most likely where fracture density is highest. Fracture density data acquired from scan-line measurements shows that fracture density is highest in fracture clusters, in the syncline hinge where curvature is highest, and near faults. The fracture distributions are related to the structural settings in which transmissive fractures have predictable orientations.
The opening-mode fractures and faults of the study may compromise the seal integrity of the caprock and are undetectable using traditional seismic techniques. Mesoscopic subseismic seal-compromising features may allow CO2 to leak though the caprock slowly, which could be significant over the thousands of years necessary for CCUS to be feasible.
Flores, Santiago L., "Mesoscale Deformational Features Near Outcrop Analogs of a Reservoir-Seal Interface: Implications for Seal Failure" (2014). All Graduate Theses and Dissertations. Paper 2794.
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