HYDRAULIC PARAMETER UPSCALING AND SENSITIVITY IN GEOLOGIC CARBON SEQUESTRATION SIMULATIONS

Geologic carbon sequestration (GCS) is a process to capture and inject CO₂ into underground porous rock formations. Assuming the continued use of fossil fuels, GCS is considered to be the most promising storage option for the CO₂ produced due to its high potential volume of sequestration. Hydraulic properties of variably-saturated porous media are crucial for modeling multiphase flow and transport in many energy and environmental related applications such as GCS. Upscaling small scale measurements to a suitable scale for numerical modeling has been a research focus for decades.

Numerical simulations are widely used in feasibility studies for GCS. Storage potentials and efficiencies of the GCS process must first be assessed before reactive transport simulation can be evaluated. Accurate estimates of hydraulic parameters are needed as inputs for these simulations. However, only few experimental values are available for CO₂ – brine systems. Sensitivity and scaling analyses were conducted using the STOMP numerical code (PNNL, US Department of Energy) for supercritical CO₂ injected into a model confined deep saline aquifer. The influence of hydraulic parameters, i.e. intrinsic permeability (k), porosity, pore compressibility, and capillary pressure – saturation / relative permeability parameters on CO₂ injection rates and costs were evaluated and the parameters were ranked based on normalized coefficients of variation. The two most influential parameters were intrinsic permeability k and the van Genuchten m value (which is related to the pore-size distribution). The porosity, mostly used in CO₂ storage capacity calculations, ranked fifth out of the seven parameters investigated.

Scale effects were investigated by comparing point and height- averaged van Genuchten capillary pressure-saturation parameters. The point parameters reduced the cost of the injections significantly relative to the average parameters.

Findings highlight the need for accurate estimates of capillary pressure – saturation / relative permeability parameters for CO2 – brine systems at reservoir-relevant scales for GCS simulations, in addition to porosity and intrinsic permeability.