Depth-Dependent CO2 Storage Efficiency: A Numerical Simulation Study of Saline Aquifers

Abstract Estimation of CO2 storage efficiency with depth is important for assessing the feasibility, scalability, and long-term performance of carbon capture and storage (CCS) projects. Previous studies have reported inconsistent observations regarding the variation of storage efficiency with depth, including a nonlinear trend in which structural storage efficiency declines to zero near 2400 m, nonzero simulated storage efficiencies by the DOE-NETL, and claims of completely leak-free storage at greater depths due to phase-density reversal. A consistent and validated understanding of depth-dependent storage efficiency based on exhaustive reservoir fluid dynamics and reactive phase equilibrium is unavailable. This study presents a comprehensive numerical investigation of CO2 storage efficiency in saline aquifers as a function of depth using 2-D CMG-GEM simulations conducted over 1000 years, with all key flow dynamics and thermodynamic parameters defined as depth-dependent. Results show that total CO2 storage efficiency decreases from ∼0.92 at 800 m and 0.99 at 1200 m to ∼0.76 at 2500 m and remains within a narrow band at greater depths. Total CO2 leakage efficiency increases from ∼0.02 at 800 m to a maximum of ∼0.35 at 4000 m, and then declines to ∼0.22 at 11,000 m. The CO2 storage capacity per unit pore volume increases continuously with depth, reaching ∼980 kg/m3 at approximately 11,000 m, with ∼860 kg/m3 stored in the aqueous phase, ∼20 kg/m3 as supercritical CO2, and ∼100 kg/m3 in mineral form. Beyond ∼11,000 m depth, distinct liquid and gas phases disappear under supercritical pressure and temperature conditions, indicating the absence of density reversal within the 800–12,000 m range modeled here and nonzero leakage of CO2 in the supercritical water–CO2 mixture. Sensitivity analyses further show that the overall trends in storage and leakage efficiency are only weakly affected by reservoir heterogeneity, anisotropy, grid size, and variations in flow dynamics parameters.