MULTIPHASE FLOW MODELING OF CO2 INJECTED INTO DEEP SALINE FRACTURED RESERVOIRS IN KNOX GROUP, BLACK WARRIOR BASIN, ALABAMA

Subsurface storage of CO₂ in geological formations is a promising tool for reducing global atmospheric CO₂ emissions. Numerical simulation of injected supercritical CO₂ has become increasingly important for understanding CO₂ flow and migration, as well as assessment of possible CO₂leakage. While previous studies have focused on porous geological media, fractured reservoirs receive much less attention. Because fracture reservoirs commonly exhibit higher heterogeneity and more complicated flow pathways than pure porous media, it is important to conduct such simulation studies.

The Knox Group in central Alabama is composed of a thick sequence of dolomite and dolomitic limestone. Laboratory core analysis indicates that porosity is typically 3 to 5% and that matrix permeability is low (~ 0.003-0.005 mD). However, injection tests have demonstrated large injectivity, indicating fractures are the primary source of permeability. Recent analysis of FMI logs has confirmed the existence of abundant open fractures. Two sets of fractures have been identified in the FMI log from a deep test well, and fracture orientation in the Knox Group is consistent with that in surface strata, indicating a similar structural origin. The basic statistical properties of the Knox fracture networks were used to stochastically generate multilayer DFN models.

To determine the viability of CO₂storage in fractured Knox carbonate, we have developed multiphase flow models with a new algorithm for characterizing effective fracture permeability (EFP) that includes integration of discrete fracture network (DFN) models. Preliminary results predict significant permeability anisotropy that favors development of a northeast-elongate CO₂ plume. The saturation model simulating 1.5 years of injection further predicts significant heterogeneity of CO₂ saturation within the plume. The diffusive spreading front of the CO₂ plume shows strong viscous fingering effects. Open fractures, moreover, make a significant contribution to plume buoyancy. Post-injection simulation indicates significant lateral spreading of CO₂ at the top of the fractured layers. Addional simulations are required to evaluate long-term storage security, and preliminary simulations suggest that leakage through low-permeability upper Knox carbonate is minimal.