ON CO2 SEQUESTRATION: CHANGES IN CO2 ENTRY PRESSURE AND ADSORPTION CAPACITY BY HEAT

Abstract

Geological sequestration of CO2 requires a deep permeable geological formation where captured CO2 can be stored and an overlying impermeable caprock that prevents buoyant CO2 from leaking upward to the surface. Shale formations are good seals due to their abundancy and typically ultralow permeability. Shale's excellent sealing ability makes it a good caprock formation. The correct evaluation of the CO2 sequestration ability of shale caprocks, also referred to as sealing capacity, must be conducted under high-pressure and high-temperature conditions representative of in situ conditions. The CO2 sequestration quality of shale caprocks, sealing capacity, is usually evaluated through measurements of CO2 capillary entry pressure into shale caprocks. In this work, CO2 capillary entry pressure in different shales under a wide range of temperatures (298-428 K) is measured using an incremental pressure transmission methodology. The impact of temperature on the physicochemical and petrophysical properties of shale is addressed, and their indirect effect on CO2 capillary entry pressure is analyzed. Furthermore, the influence of temperature on interfacial tension between CO2 and shale's pore fluid and contact angle is examined. Also, the effect of temperature on CO2 sorption on clay sites is analyzed. The experimental outcome exhibited that capillary entry pressure increases steadily and appreciably for shales (A and B) up to a certain temperature (around 358.15 K), after which the behavior of capillary entry pressure of CO2 for both shales changes markedly. For shale (A), the increase in measured capillary entry pressure for temperatures above 358.15 K slows down significantly, ranging between 841 and 851 psi compared to its measured value of 835 psi at 358.15 K. As for shale (B), the reaction is completely different, where measured capillary entry pressures of CO2 for temperatures above 358.15 K decreased noticeably as measured capillary entry pressures dropped from 534 psi at 358.15 K to 473 psi at 428.15 K. Results show that temperature has a profound impact on CO2 capillary entry pressure when interacting with shales through the alteration of shale's physicochemical and petrophysical properties. Data suggests that pore throat size could be altered by heat through a phenomenon called pore dilation. It was also clearly shown that higher temperature causes higher interfacial tension, which indeed causes higher capillary entry pressures when CO2 interacts with shale. Moreover, results indicate that high temperature could energize active swelling clays and invoke a transition of inactive clays to active swelling clays. High temperature causes clay swelling and may produce hydrational and expansive stresses in shale, especially when confined, which could induce fissures and microfractures. The creation of fissures and microfractures could enhance shale's average pore radii and permeability, thus reducing the capillary entry pressure. In addition, results suggest that the adsorption capacity of CO2 on clay sites decreases with increasing temperature and that the presence of smectite clay greatly enhances the swelling potential of shale upon exposure to CO2.