Permeability Reduction Produced by Grain Reorganization and Accumulation of Exsolved CO₂ during Geologic Carbon Sequestration: A New CO₂ Trapping Mechanism

Carbon sequestration experiments were conducted on uncemented sediment and lithified rock from the Eau Claire Formation, which consisted primarily of K-feldspar and quartz. Cores were heated to accentuate reactivity between fluid and mineral grains and to force CO₂ exsolution. Measured permeability of one sediment core ultimately reduced by 4 orders of magnitude as it was incrementally heated from 21 to 150 °C. Water-rock interaction produced some alteration, yielding sub-μm clay precipitation on K-feldspar grains in the core’s upstream end. Experimental results also revealed abundant newly formed pore space in regions of the core, and in some cases pores that were several times larger than the average grain size of the sediment. These large pores likely formed from elevated localized pressure caused by rapid CO₂ exsolution within the core and/or an accumulating CO₂ phase capable of pushing out surrounding sediment. CO₂ filled the pores and blocked flow pathways. Comparison with a similar experiment using a solid arkose core indicates that CO₂ accumulation and grain reorganization mainly contributed to permeability reduction during the heated sediment core experiment. This suggests that CO₂ injection into sediments may store more CO₂ and cause additional permeability reduction than is possible in lithified rock due to grain reorganization.