CO₂ storage in chalks: What are we afraid of?

Carbon Capture and Storage (CCS) has been recognized as an effective strategy to limit the temperature rise to 1.5 °C by 2050 under the Paris Agreement. As a result, more than 50 CCS pilot or large-scale projects have been commissioned in recent decades, targeting mainly sandstone reservoirs for their favourable petrophysical properties. Although many discussions and practical procedures have been developed, large-scale CO₂ storage in carbonates has not been implemented due to practical risks associated with rapid geochemical interactions. This study attempts to evaluate the feasibility of CO₂ storage in carbonate chalk formations by focusing on changes in storage capacity (porosity) and injectivity (permeability) over time. A series of laboratory tests were carried out on Stevns Klint chalk from Denmark after exposure to CO₂ for 37 days at a fluid pressure of 15 MPa and a temperature of 50 °C. The results obtained indicated a large removal of Ca⁺² ions from CO₂ saturated water solution and strong precipitation of secondary calcite in the pore structure, which reduced the porosity and permeability of the samples. It seems that CO₂ injection into chalk should be done very carefully, as the progressive dissolution of calcite and saturation of the formation water will initiate large secondary calcite precipitation in the long term, leading to a reduction in injectivity and storage capacity over time.