Site-Specific Assessment of CO2 Storage Potential in Chalk: The Harald East Experience

This research provides a site-specific assessment of the viability and safety of long-term CO2 storage within chalk formations, utilizing the Harald East Field in the Danish Central Graben as a case study. By leveraging over three decades of production data, 4D seismic monitoring, and subsidence records, we established a multiphysics modeling framework to evaluate injectivity, storage capacity, containment, and geomechanical stability. The simulations suggest that the highest capacity for CO2 storage, limited by the pre-depletion pressure of the reservoir, can be attained while ensuring that the structural and stratigraphic integrity of the plume is preserved during both the injection and post-injection stages. Geomechanical modeling indicates a minimal risk of fault reactivation, even under stress regime transitions induced by repressurization. While chalk's low permeability poses injectivity challenges, its high porosity enables substantial storage potential, provided injection strategies are carefully optimized. Although the low permeability of chalk presents challenges for injectivity, its high porosity allows for significant storage potential, contingent upon the careful optimization of injection strategies. Reactive transport modeling indicates localized porosity reduction near the injection wells due to rock-fluid chemical interactions; however, injectivity is not compromised due to the extended horizontal well sections. In the caprock, long-term geochemical reactions, especially under scenarios involving impure CO2 injection, facilitate secondary mineral precipitation, which enhances sealing capacity. Diffusion simulations conducted over a 10,000-year timeframe confirm minimal migration of CO2 and impurities into the caprock, with porosity reductions attributed to mineral trapping and selfsealing processes. While thermal effects are spatially constrained, they do affect fluid behavior and reaction kinetics in proximity to the wellbore, and the cooling effects on the caprock merit further investigation. In summary, the findings affirm the technical feasibility of CO2 storage in depleted chalk reservoirs and provide a transferable modeling framework for assessing analogous carbonate systems, thereby contributing to a more comprehensive understanding of CO2 dynamics in formations characterized by low permeability and high porosity.