Implications of fracture heterogeneity for geologic carbon sequestration in reactive basalt aquifers

Basalt reservoirs have been suggested as attractive formations for long term CO2 disposal on the basis of rapid CO2-water-rock interactions, which result hi permanent Mineral trapping. Although mineral trapping represents the long term storage mechanism for CO2 disposal in basalt reservoirs, the suitability of these systems for geologic CO2 sequestration depends, to a large extent, on understanding (1) the physical distribution of CO2 within years of injection, i.e. prior to the onset of mineralization and (2) the accumulation and redistribution of injection pressure. Understanding these processes is complicated by incomplete knowledge of in situ fracture distributions and their scale-dependent hydraulic properties at depths of interest (> 800 m) for long term CO2 disposal. In this dissertation, I confront this reservoir uncertainty and develop a process-based model of CO2 injections into basalt formations typical of the East Snake River Plain (ESRP), Idaho. The conceptual model for ESRP reservoir characterization is discussed in Chapter 1, along with a low-cost spatial sampling strategy for inferring the correlation structure of fracture distributions observed in outcrop. In Chapter 2, the field site is revisited with a terrestrial light detection and ranging (LiDAR) scanner and a new algorithm--called surface roughness by orthogonal distance regression--is developed for extracting high resolution (cm scale) fracture maps from LiDAR data; the accuracy of this algoritm is demonstrated experimentally in Chapter 3. The field data described in Chapters 1 and 2 are employed in Chapter 4 to develop a numerical CO2 injection model that investigates the physical processes governing supercritical CO2 injections within synthetic basalt reservoirs. Results suggest that the spatial distribution of high permeability rubble zones and the connectivity length of these units from the injection well into the target reservoir is a primary factor influencing the rate of injection pressure accumulation within the first year of injection. This impies that a successful field implementation of CO2 sequestration in fractured basalt reservoirs will depend on the ability to demonstrate sufficient fracture connectivity from the injection well into the target reservoir.