Effect of CO2 Chemical Reactions on Rock Pore Surface Morphology – A Laboratory Study

Abstract

CO2 capture and subsurface sequestration (CCS) is a viable solution to reduce the greenhouse gas effect on global warming. It is known that CO2 in water chemically reacts with rocks during the process of CCS (injection, migration, plume, and long-term storage). The objective of this study is to better understand the dynamics of this interaction, and to develop measurements capable of monitoring changes of rock properties during CCS. As changes in rock properties originate from chemical reactions between pore-surface minerals and CO2, characterizing changes in pore-surface texture and geometry is essential for predicting subsequent changes of other rock properties relevant for CCS. As such, the methods used in this laboratory study include laser scanning confocal microscopy (LSCM) for measuring pore-surface roughness, Brunauer-Emmett-Teller (BET) adsorption isotherms for measuring the specific surface area, and nuclear magnetic resonance (NMR) relaxation for measuring pore-size, pore-connectivity, and surface-relaxivity (a function of wettability and fluid-surface interactions). In this study, five brine-saturated carbonate rocks (including three outcrops and two reservoir rocks) were exposed to supercritical CO2 (scCO2) under various ageing conditions. Specifically, we exposed the carbonate rocks to scCO2 under increasing pressure, temperature, and salinity, and measured LSCM, BET, and NMR after each of the total five ageing steps. By comparing with the initial non-ageing measurements, data indicate that the scCO2 exposure increases both surface-relaxivity and surface-roughness, particularly for the reservoir rocks. At the final step of ageing, the scCO2 exposure increases both pore-size and pore-connectivity for the reservoir rocks and some outcrops. Our findings may have direct impacts on planning and executing CCS projects, especially in carbonates. Changes in pore surface roughness and wettability can directly affect CO2 injection because it affects the reactive surface of the pores. Once significant surface erosion occurs, other macroscopic properties may change as well, as observed from the increase in pore connectivity in certain cases. Dissolution and precipitation change the pore-size and connectivity, thereby capillary pressure and permeability, which may also affect caprock's integrity. Our study shows that quantifying the changes caused by CO2 chemical reactions with rock minerals is crucial for CCS projects, including site selection and storage capacity assessment. Further, this study shows that NMR could be a valuable downhole tool to capture and monitor these changes, such as assessing changes of rock properties due to CO2-rock chemical reactions and contributing in validating dynamic chemical reaction models and help to adjust for prediction models.