CaCO₃, its reaction and carbonate rocks: terahertz spectroscopy investigation

Carbonate-rich rocks cover a primary part of the earth's petroleum geology reservoir. The study of carbonate has special significance and more effective study methods are now needed. In order to improve the availability of carbonate rock detection, terahertz (THz) spectroscopy was employed to investigate relevant materials in Na₂CO₃  +  CaCl₂  =  CaCO₃  +  2NaCl, which is often used to generate CaCO₃. By comparing the materials composited with different ions, it can be revealed that Ca²⁺, ${\text{CO}}_3^{2-}$, Na⁺ and Cl⁻ have respective absorption features at different frequencies. Furthermore, by utilizing a conservation equation it can be observed that the average refractive indices of Na₂CO₃ as well as CaCl₂ equal those of CaCO₃ and NaCl in the entire range. Combining the absorption and refractive effect of the materials in the reaction can comprehensively characterize the different substances and reveal the inner interaction during the reaction. THz spectra can deduce the process of molecule rearrangement in the chemical reaction of long-term rock evolution. Besides, the absorption features of the real carbonate rock collected from the nearest town of Sinan county, Guizhou province in Yunnan–Guizhou plateau validate the peaks' central frequencies of ions and the principal components of carbonates, which can be in agreement with the SEM-EDS analysis. This research will supply a spectral tool to identify the particles in the rock and deduce an evolution of petroleum carbonate reservoir. (paper)