Investigation of Mass Transfer and Sorption in CO₂/Brine/Rock Systems via In Situ FT-IR

Abstract

CO2 geological storage in deep saline formations is considered a promising method to mitigate anthropogenic CO2 emissions and, thereby, minimize changes to the Earth’s atmosphere. A fundamental understanding of CO2 mass transfer and sorption phenomena in brine-saturated reservoir formations is necessary to understand the long-term fate of injected CO2 as it is subjected to different (physical, dissolution, and mineral) trapping mechanisms. In this work, we investigate CO2 sorption in brine-saturated and dry Mt. Simon sandstone samples via in situ Fourier transform infrared spectroscopy (FT-IR) at elevated pressures, ranging from 0.3 to 8.3 MPa, at a temperature of 50 °C. The FT-IR spectra of bulk-phase CO2 were simultaneously recorded under the same conditions. For bulk-phase CO2, we observed, in agreement with past studies, a doublet peak at 2361 and 2336 cm–1 and another peak (ν2 bending mode) at 667 cm–1. With increasing pressure, the position of the peak at 667 cm–1 remains invariant; however, when crossing into the supercritical region, the doublet peak degenerates onto a single peak at 2336 cm–1 with a barely visible shoulder at 2361 cm–1. The bulk CO2 data provide a perfect fit for Beer’s law for the whole range of pressure conditions. For the dry sample, the IR spectrum is experimentally indistinguishable from the bulk CO2 spectrum, signifying that if physical adsorption occurs to any significant extent, the adsorbed CO2 molecules are not substantially more rotationally constrained than the dense bulk CO2 molecules. For the brine-saturated sample, we observe a strong band centered at 2342 cm–1 and a small companion peak at 2360–2361 cm–1 that degenerates into a barely visible shoulder peak at higher pressures. The 2342 cm–1 band has been previously observed by other investigators for CO2 dissolved in bulk water/brine as well during its adsorption on a variety of other wet natural porous media. We observe no peaks corresponding to bicarbonate or carbonate bulk species, which correlates well with the prior literature on similar low-pH aqueous solutions. The integrated peak area for the CO2 sorbed in the brine-saturated sample correlates linearly with its solubility in the same bulk brine, as measured separately via a PVT-cell approach. This validates the accuracy of both techniques and the potential of the FT-IR method to be used in the study of mass transfer and adsorption in such systems. To that effect, a simple mathematical model is presented to analyze the FT-IR data to determine the CO2 effective diffusivity in the brine-saturated sandstone sample.

Topics & Concepts