Challenges and enablers for large-scale CO2 storage in chalk formations

The past two decades of research on Carbon Capture and Storage (CCS) seem to have finally become fruitful as global leaders and energy-intensive industries are cooperating to materialize CCS projects and reach the promised reduction in CO2 emissions. Traditionally, CCS projects targeted mostly high permeability sandstone formations, despite the numerous carbonate fields undergoing CO2 injection for Enhanced Oil Recovery (EOR) in the United States or Canada. Because of the reactivity between calcite minerals and CO2 saturated water, chalk formations, characterized by high porosity and low permeability, have been previously portrayed as infeasible CO2 storage sites. Although previous laboratory investigations were carried out to assess the performance of CO2EOR in North Sea chalk fields, these studies did not result in any field-scale demonstration projects; this may soon change since a positive movement towards CO2 storage in depleted oil fields has been recently initiated. In this work, we reviewed existing studies on CO2 injection in chalk to address the suitability of this type of formation for CCS. Although the evidence on the thermo-hydro-mechanical-chemical behaviour of chalk in the presence of CO2-saturated aqueous solutions is mixed, the majority of flooding tests performed on reservoir core samples do not support further weakening relative to water injection conditions nor significant changes in the petrophysical properties. Along with the weakening effect and using the Danish North Sea chalk fields as a case study, we addressed events that impact the storage site safety such as fault reactivation, and caprock and well integrity. Furthermore, monitoring techniques relevant to offshore locations are also discussed. Based on studies on other types of carbonates, and considering the characteristics of chalk (e.g., permeability, wettability, and reactivity) we analysed the relevance of different trapping mechanisms (i.e., solution, capillary, and mineral) but also several effects (i.e., chemical, biological, mechanical) that can lead to loss of injectivity. The main observations and conclusions in this work can be easily extrapolated to other chalk formations worldwide.