Caprock and overburden processes in geological CO2 storage: An experimental study on sealing efficiency and mineral alterations

A comprehensive set of experimental and analytical methods has been used to characterise the sealing and fluid -transport properties of fine-grained (pelitic) sedimentary rocks under the pressure and temperature conditions of geological CO₂ storage. The flow experiments were carried out on cylindrical sample plugs of 28.5 or 38 mm diameter and 10-20 mm length. The capillary sealing effici ncy of the lithotypes was determined by repetitive gas breakthrough experiments to test for reproducibility and to detect petrophysical changes of the rock samples resulting from CO₂/water/rock interactions. These tests were performed with both, Helium and sc CO₂ on the initially water-saturated sample plugs. Although molecular diffusion is not considered as an efficient leakage mechanism it represents a rate-determining step in mineral reactions and reactive transport. Therefore repetitive CO₂ diffusion experiments were carried out on selected samples in the water-saturated state. These measurements provide information on the molecular mobility of CO₂ and its hydrolysis products and on the physical and chemical storage capacity of the rock for these species. Before and after each experiment a steady -state fluid flow of water was established across the samples by applying high pressure gradients. This procedure ensured a defined state of saturation. Permeability coefficients derived from these tests were used to detect changes in the transport properties resulting from exposure to CO₂. The fluid transport experiments were complemented by petrophysical (BET specific surface area, mercury porosimetry) and mineralogical analyses (X-ray diffraction; XRD ) of the original and post -experiment samples. The experiments revealed significant changes in the transport properties and the sealing efficiency of the samples. The gas breakthrough tests resulted in reduced capillary entry pressures and increased effective gas permeability as a result of repetitive exposure to CO₂. Repeated diffusion tests revealed a faster diffusive transport in the second experiment. An increase in water permeability was consistently observed after both, capillary breakthrough tests and diffusi on experiments with CO₂. The BET and mercury porosimetry results were not significantly affected by the CO₂ treatment. XRD measurements before and after CO₂ treatment revealed significant variations in the mineral compositions of the samples upon exposure to CO₂.