Effect of CO2 Sequestration on Carbonate Formation Integrity – Aspects of Static Interactions

Saline aquifers are often preferred candidates for CO2 sequestration as they have large storage capacities. However, as CO2 dissolves in formation brine, a lower pH carbonic acid solution is formed. The interactions between carbonic acid and rock matrix might result in deterioration of formation integrity and/or changes in key petrophysical properties. The main objective of the current study is to investigate the effect of CO2 sequestration on the integrity of carbonate outcrop rocks as analogs for subsurface carbonate aquifers, focusing on static interactions between rocks and fixed amounts of CO2-saturated water, with limited discussions on the effect of dynamic contact between rock pore surfaces and CO2-saturated water. CO2 solubility in water was investigated by dissolving CO2 in brine and then aging the carbonate rock samples in the CO2-saturated brine. The aging was conducted at 1,500 psi and 90°C for 1-, 3-, and 6-month periods, in two scenarios: static and dynamic aging. For static aging, the carbonate samples were aged in a static pressurized vessel with CO2-saturated brine. For dynamic aging, the carbonate samples were continuously flooded with CO2-saturated brine in a controlled coreflooding system. Petrophysical measurements of porosity, permeability, ultrasonic velocity, resistivity, and nuclear magnetic resonance (NMR) were executed to capture the effect of CO2-saturated brine on the rock samples’ petrophysical parameters. Sister samples were used for mineralogy assessment using X-ray diffraction (XRD), and thin sections were prepared for petrographic studies. High-resolution images were also captured on rock chips that were prepared and treated under the same conditions to provide insights into the effects of the CO2-saturated brine on rock pore surface morphology. Results indicate that static aging in CO2-saturated brine up to 6 months has minimal effect on rock porosity, permeability, and resistivity. The changes, however, were more pronounced during the dynamic aging for rock porosity, permeability, and resistivity. The NMR-T2 distribution, for both aging scenarios, showed a clear change in apparent pore-size distribution, indicating a possible change in pore surfaces and/or surface relaxivity. For dolomite-rich samples, the stiffness of the rock samples was basically maintained under both static and dynamic aging, as illustrated by the limited change in dynamic shear modulus, a promising aspect for CO2 sequestration in dolomite-rich carbonate aquifers. For the calcite-rich samples, however, the ultrasonic measurements showed changes in rock stiffness. These changes were more pronounced during the dynamic aging.