NUMERICAL MODELING EXPERIMENT REVEALS OPPORTUNITIES AND CHALLENGES FOR CO2 SEQUESTRATION IN LOW-VOLUME BASALT FORMATIONS

Geologic carbon sequestration in basalt-hosted reservoirs is predicated on laboratory experiments demonstrating that CO₂-water-rock interactions are energetically favorable for permanent carbon trapping on a time scale of 10² – 10³ days. While these laboratory experiments suggest that basalt-hosted reservoirs may be ideal targets for long term CO₂ disposal, little work has been done to understand the site scale impacts of commercial CO₂ injections into basalt-hosted reservoirs. Among the many questions that remain open, the influence of spatially distributed formation heterogeneity is a fundamental challenge for characterizing and modeling CO₂ injections into a basalt reservoir. In the work presented here, we use a Monte Carlo numerical modeling experiment of CO₂ injections into a low-volume basalt reservoir—with properties based on data from the east Snake River Plain (ESRP) in southern Idaho—to investigate how a priori unknown heterogeneous property distributions influence injection pressure accumulation, geomechanical changes in the target reservoir, and vertical CO₂ migration. The target reservoir is modeled as a three-dimensional bimodal stochastic continuum (2.88M gridblocks) using 50 equally probable synthetic reservoirs. Supercritical CO₂ is injected into each reservoir model at a constant mass rate of ~682,000 metric tons/year for 20 years. Results from this work suggest that 1) formation heterogeneity strongly influences the rate and magnitude of injection pressure accumulation within the first month of injection; 2) for an extensional stress regime (as exists within the ESRP), shear failure is unlikely for minimum horizontal compressive stress (S_h) greater than 60% of the vertical stress (S_v), and; 3) the mean vertical CO₂ mass flux is less than 5×10^-4 kg/s at 800m depth after 20 years suggesting that carbonate precipitation rates described in the literature may be adequate to trap CO₂ prior to widespread escape.