Recent studies suggest that continental flood basalts may be suitable for geologic carbon sequestration due to fluid-rock reactions that mineralize injected CO₂ on relatively short time-scales. Flood basalts also possess a permeability structure favorable for injection, with alternating high-permeability (flow margin) and low-permeability (flow interior) layers. However, little information exists on the behavior of CO₂ as it leaks through fractures characteristic of the flow interior, particularly at conditions near the critical point for CO₂. In this study, a two-dimensional 5 × 5 m model of a fracture network is built based on high-resolution LiDAR scans of a Columbia River Basalt flow interior taken near Starbuck, WA. Three-phase CO₂ flow is simulated using TOUGH3 (beta) with equation of state ECO2M for 10 years simulation time. Initial conditions comprise a hydrostatic pressure profile corresponding to 750-755 m below ground surface and a constant temperature of 32° C. Under these conditions, the critical point for CO₂ occurs 1.5 meters above the bottom of the domain. Matrix permeability is assumed to be constant, based on literature values for the Columbia River Basalt. Fracture permeability is assigned based on a lognormal distribution of random values with mean and standard deviation based on measured fracture aperture values and in situ permeability values from literature. In order to account for fracture permeability uncertainty, CO₂ leakage is simulated in 50 equally probable realizations of the same fracture network with spatially random permeability constrained by the lognormal permeability distribution. Results suggest that fracture permeability uncertainty has some effect on the distribution of CO₂ within the fractures, but network geometry is the primary control in determining flow paths. Fracture permeability uncertainty has a larger influence on fluid pressure, and can affect the location of the critical point within ~1.5 m. Uncertainty in fluid pressure was found to be highest along major flow paths below channel constrictions, indicating permeability at a few key points can have a large influence on fluid pressure distribution. / Master of Science / Geologic carbon sequestration (GCS) is a means of reducing greenhouse gas emissions using currently available technology. It consists of trapping carbon dioxide (CO<i>2</i>) released by the burning of fossil fuels at a large emitter, such as a coal fired power plant, and injecting it deep beneath the earth’s surface for permanent storage. This research builds on an increasing body of evidence that suggests that the Columbia River Basalt Group (GRBG), a large lava formation located in the northwestern United States, may be a suitable target for GCS. This is largely because CO<i>2</i> reacts with basalt rocks within a few years of injection to form stable minerals, after which it is permanently immobilized. This basalt province also contains alternating layers of rock, some of which have high permeability, meaning that they can accept CO<i>2</i> injections, and some of which have low permeability, meaning that they would block CO<i>2</i> rising from the injection layers. Layers with low permeability are called confining layers, and in the CRBG, they contain fractures that formed when the lava initially cooled. While some information about these fractures is known, it is impossible to know how easily fluid might flow through them at any given point (permeability) at the depths of interest for GCS. This study seeks to quantify the effects of that uncertainty, by building a model of CO<i>2</i> flow through a CRBG fracture set, and running that same model 50 times with all variables held constant, except the exact location of permeability values within the fracture network. Chemical reactions are not considered, so this model represents behavior in the network very soon after CO<i>2</i> is injected, before minerals start to form. The results of this model suggest that uncertainty in permeability values within fractures influences predictions of fluid pressure within the confining layer. This is important, because fluid pressure has a large influence on whether or not CO<i>2</i> will leak through the confining layer. This research will be useful in informing the model design of future researchers attempting to simulate GCS efforts in the CRBG and similar geologic formations.