Sensitivity of Effective Medium Models for Estimating Elastic Properties Using Inverted Versus Deterministic Pore Aspect Ratio in Columbia River Basalts

ABSTRACT To mitigate the changes in the world's climate due to increasing CO 2 concentration and its far‐reaching implications on humans, habitats and the exploitation of available energy resources, a global strategy is being implemented, focusing on smart capture at source and long‐term geological storage. CO 2 sequestration in basalts has been demonstrated as a promising candidate due to the deposition of CO 2 as a stable carbonate mineral and due to its widespread availability. While numerous studies have investigated geochemical interactions between basalt, CO 2 and brine, the applicability of effective medium models to basalt formations remains largely unexplored. This study looks into modelling the elastic properties of basalt flows and pore characterisation, establishing a baseline model of basalt reservoir for CO 2 sequestration. In this study, we evaluated KT (Kuster‐Toksöz), SCA (self‐consistent approximation), differential effective medium (DEM) and SCA–DEM models for predicting the elastic properties of basalt. While all models predict the bulk modulus reasonably well, KT consistently overpredicts the shear modulus at all porosities due to its low porosity, isolated pores assumption. SCA underpredicts shear at high porosity and overpredicts at low porosity, whereas SCA–DEM performs well unless secondary, more compliant minerals because of weathering or alteration are present. Additionally, sensitivity analysis is also performed to assess the impact of pore aspect ratio variations on different models, where KT is most sensitive to change in pore aspect ratio in bulk modulus and SCA is more sensitive to shear modulus, providing valuable insights for selecting appropriate rock physics models based on the micropore structure of the formation. Considering both reliability and sensitivity, we found DEM as best to map the elastic properties in basalt flows with varying heterogeneity across all porosity range. The DEM model is then used to optimise the pore aspect ratio, a key parameter for understanding reservoir heterogeneity, hence CO 2 injectivity.