Experimental observation of permeability changes in dolomite at CO ₂ sequestration conditions

Injection of cool CO₂ into geothermally warm carbonate reservoirs for storage or geothermal energy production may lower near-well temperature and lead to mass transfer along flow paths leading away from the well. To investigate this process, a dolomite core was subjected to a 650 h, high pressure, CO₂ saturated, flow-through experiment. Permeability increased from 10^-15.9 to 10^-15.2 m² over the initial 216 h at 21 C, decreased to 10^-16.2 m² over 289 h at 50 C, largely due to thermally driven CO₂ exsolution, and reached a final value of 10^-16.4 m² after 145 h at 100 C due to continued exsolution and the onset of dolomite precipitation. Theoretical calculations show that CO₂ exsolution results in a maximum pore space CO₂ saturation of 0.5, and steady state relative permeabilities of CO₂ and water on the order of 0.0065 and 0.1, respectively. Post-experiment imagery reveals matrix dissolution at low temperatures, and subsequent filling-in of flow passages at elevated temperature. Geochemical calculations indicate that reservoir fluids subjected to a thermal gradient may exsolve and precipitate up to 200 cm³ CO₂ and 1.5 cm ³ dolomite per kg of water, respectively, resulting in substantial porosity and permeability redistribution.