Appraising Carbon Geological-Storage Potential in Saline Aquifers Using Two-Phase Rate-Transient Analysis

Rate transient analysis (RTA) is a practical and cost-effective method for CO₂ injection data analysis and storage capacity evaluation of saline aquifers. However, the nonlinear behavior introduced by two-phase CO₂–brine flow, coupled with pressure-dependent fluid and rock properties, significantly limits the applicability of conventional production-based RTA models, which typically assume single-phase flow and rely solely on pressure propagation. To overcome these limitations, this study presents a novel two-phase RTA method that, for the first time, explicitly incorporates both pressure evolution and CO₂ front migration, while accounting for the nonlinearities associated with brine displacement and pressure-sensitive reservoir behavior. First, a two-phase flow model is developed for CO₂ injection in saline aquifers, and an analytical solution is derived by introducing new definitions of pseudo-pressure and pseudo-time that capture the effects of both multiphase flow and pressure-dependent properties. By introducing distinct definitions of the radius of investigation (ROI) for the pressure front and the CO₂ front, average pressure and saturation are evaluated based on their respective controlling regions and incorporated into the estimation of pseudotime. Second, a two-step RTA approach is proposed to analyze CO₂ injection data, including flow regime identification using a two-phase diagnostic plot and subsurface properties estimation using a specialty plot. Finally, we provide a workflow that integrates formation properties evaluation with storage capacity prediction under the more realistic conditions of variable injection. The proposed method is validated using synthetic data from numerical simulations and a field example from the Illinois Basin Decatur Project (IBDP). The close estimation of CO₂ storage capacity, reservoir pore-volume, and permeability confirm the accuracy of the proposed model and demonstrate the method's reliability compared with numerical simulations for rapid assessment of storage potential and analytical RTA methods for considering the specific nonlinearities. With the proposed approach, the scattered CO₂ injection pressure and rate data are, for the first time, transformed into clear straight-line behaviors with unique slope, revealing the underlying two-phase flow characteristics during CO₂ injection up to the injection limit.