Three-Dimensional Pore Networks in Miocene Stevens Sandstone of California: Implications for CO₂ Geologic Storage

The Miocene Stevens Sandstone in the San Joaquin Basin of California is increasingly recognized as a promising candidate for CO₂ geological storage due to the enormous storage capacity, proven sealing, and existing infrastructure. In this study, computed microtomography imaging and pore network modeling were employed to investigate the influence of pore geometry and wettability on the CO₂ injectivity and residual trapping. Image analysis revealed that a significant fraction of the cement and matrix consists of microporous regions. The microporosity can substantially increase the overall pore space, yet its contribution to permeability remains modest, particularly in samples with low permeability. The intrinsic heterogeneity of turbidite reservoirs further complicates the reservoir properties among different layers. Two-phase flow simulations under varying wettability conditions (water-wet, weak water-wet, and neutral-wet) demonstrated that the CO₂ injection is predominantly controlled by macropores. CO₂ invades microporous regions only after these larger pores are filled. The presence of microporosity leads to a decrease in both initial and residual CO₂ saturations, with the magnitude of the reduction being influenced by wettability. Neutral-wet scenarios exhibit higher CO₂ mobility and thus lower residual trapping than water-wet scenarios. The results imply that heterogeneity in pore geometry and cement distribution across different layers can result in stratified CO₂ flow pathways, complicating efforts to predict injection performance. Overall, the Stevens Sandstone shows considerable promise for CO₂ geologic storage, but effective implementation will require detailed characterization of the pore structure as well as the integration of reactive fluid flow to account for potential mineral dissolution and fines migration.