The impact of model complexity on CO2 saturation and mass balance

Abstract:

When modeling geologic carbon sequestration, predicting the saturation of carbon dioxide (CO₂) over space and time and the distribution of CO₂ mass (free, trapped or dissolved) over time are primary concerns. Modeling may be done to address short-term concerns such as determining the saturation of CO₂ at the end of the injection period or long-term concerns such as estimating the mass of dissolved CO₂ hundreds of years after the injection period. Model complexity describes the physics included in the flow model and encompasses how simply key input data can be described-- homogeneous or heterogeneous permeability, homogeneous or heterogeneous capillary pressure, simple or hysteretic relative permeability, and simple or hysteretic capillary pressure.

Starting with a simple 3D model and building to a complex model in a stepwise manner, the effects of additional complexity on the short-term and long-term CO₂ saturation and mass balance (free supercritical fluid, trapped, and dissolved) are being evaluated. Initial results suggest that the CO₂ plume footprint, plume extent and mass balance vary significantly with model complexity over the short- and long-term, and that representing full complexity (i.e., heterogeneity and hysteresis in the characteristic relationships for capillary pressure and relative permeability, as they vary within the reservoir’s geologic architecture) may be critical to properly representing CO₂ dynamics in some candidate reservoirs. Ultimately, we seek to demonstrate the significance of various pore-scale processes to the continuum-scale distribution of CO₂.