Dynamic Modeling and Risk Assessment of Carbon Dioxide Sequestration in Saline Aquifers

Geological carbon capture and storage (CCS) in deep saline aquifers is a critical strategy for mitigating climate change and demands robust understanding of subsurface complexities. This thesis advances the knowledge required for secure and effective CO2 geological sequestration by addressing key challenges in fracture characterization, CO2 solubility modeling, and the dynamic influence of fractures on CO2 migration. First, recognizing the critical impact of natural fractures on subsurface fluid flow, Chapter 1 develops and validates a Variable Shape Distribution (VSD) method for modeling a fracture size distribution. Compared to commonly used power law and scale-dependent methods, the VSD method demonstrates a superior fit across wide fracture size ranges, including small and large apertures, which are often truncated or inaccurately represented by conventional approaches. This advancement provides a more reliable estimation of fractal dimensions and fundamental fracture properties, crucial for accurate reservoir modeling. Next, Chapter 2 addresses limitations in CO2 solubility estimation by introducing an improved Henry's law formulation incorporated into a compositional simulator. This refined model accounts for variations in temperature and salinity, crucial factors often simplified in previous studies. Findings highlight that lower salinity significantly enhances CO2 solubility and injectivity, while temperature's impact is more complex, influencing both solubility and CO2 density, which affects buoyancy. This analysis identifies optimal aquifer conditions for CO2 storage with respect to solubility. Finally, Chapter 3 integrates these understandings into a comprehensive investigation of the dynamic effects of various fracture states on CO2 storage performance, employing a coupled geomechanical-compositional simulation model, represented by Barton-Bandis model. Three distinct vertical fracture scenarios were analyzed: confined, permanently open, and pressure-activated. Results demonstrate that confined fractures primarily re-route CO2 migration pathways but have limited impact on an overall plume radius. In contrast, permanently open fractures act as highly efficient conduits, causing significant CO2 leakage. Crucially, pressure-activated fractures, which are more geologically realistic, exhibit a dynamic transition from a barrier to a conduit under elevated reservoir pressure, facilitating leakage that is less severe than from permanently open fractures. The overarching conclusion is that the physical state of a fracture (confined, open, or pressure-activated) is a more critical factor governing CO2 plume behavior, sweep efficiency, and storage security than its simple presence or proximity to a well. This research underscores the indispensable need for integrating geomechanical coupling in future CO2 storage models to accurately predict dynamic fracture behavior, thereby enabling more robust risk assessments and optimizing long-term sequestration strategies in complex geological environments.