Significance of Conceptual Model Uncertainty in Simulating Carbon Sequestration in a Deep Inclined Saline Aquifer

In modeling geologic carbon sequestration in a deep inclined aquifer in Wyoming, the impact of geologic, engineering, and environmental uncertainty factors on parameter importance and prediction uncertainty is evaluated. Given site characterization data, a suite of geologic model families were built to represent aquifer permeability heterogeneity at increasing complexity. With each family, the same CO2 experiment was simulated. Over a period of 50 years, 17 million tons of CO2 is injected into the aquifer at an approximate depth of 3,750 m. Postinjection simulation is then carried out for a total simulation time of 2,000 years. Based on the design of the experiment, a screening sensitivity analysis was first conducted for all families, systematically varying uncertain input parameters. Parameters with firstorder impact on CO2 performance metrics (i.e., trapped gas, dissolved gas, brine leakage, storage ratio) are identified, which vary with time and modeling choice. When the model is of low complexity, engineering and environmental factors are identified as the most significant; when the model increases in complexity, geologic factors that influence aquifer heterogeneity become more important. Given the screening test outcome, a response surface analysis was carried out for each family to create prediction envelopes of the CO2 storage ratio. By the end of injection, all families predicted similar uncertainty in the storage ratio. After injection ceases, prediction envelopes of the families deviated gradually from one another as a result of different large-scale heterogeneity experienced by each family because the plume migrated continuously along dip. For this inclined aquifer, resources should be devoted first to characterize geologic uncertainty factors (i.e., porositypermeability transform and facies correlation structure) that influence permeability magnitude and connectivity. The effect of these factors on CO2 flow, trapping, and storage becomes overwhelmingly important compared with engineering and environmental factors. Under conditions of low formation temperatures and high formation fluid pressures, representative CO2 plumes corresponding to end member CO2 storage ratios become gravity-stable. DOI: 10.1061/(ASCE)HZ.2153-5515.0000246. © 2014 American Society of Civil Engineers. Author keywords: Carbon storage; Deep saline aquifer; Geologic modeling; Uncertainty; Gravity-stable flow.