Distributions and evolution model of water-soluble organic acids for coals with different thermal maturations

Abstract

Soxhlet extractions of fresh natural coal samples with different thermal maturations representing two typical coal-accumulation periods in China were conducted using boiling ultrapure water to obtain water-soluble organic acids (WSOAs) that have great significance for fluid-rock interactions and acid fracturing technology. The goals of this project were to look at the distributions of WSOAs as well as the key factors affecting their release and to establish an evolution model of WSOAs. Total concentrations of WSOAs range from 0.078 to 1.705 mg/g TOC. The proportion of monocarboxylic acids is notably higher than that of dicarboxylic acids. Monocarboxylic acids are dominated by formate and acetate, and the dicarboxylic acids mainly include oxalates. The diversity of WSOA types tends to decrease with progressive thermal maturation. The WSOA concentrations are positively and linearly proportional to oxygen index (OI) and O/C atomic ratio, respectively, and correspond to more intense peaks for O-containing functional groups from infrared spectra to thermal maturation. There are remarkable differences in WSOA components between different organic matters. Specifically, the formate/acetate ratio is greater than 1 for type-III organic matter, such as coals, but is less than 1 for type-II organic matter, such as shales. This ratio shows the opposite tendency with ongoing thermal maturity for different organic matters, thus indicating that formate is relatively enriched in coals but acetate is relatively abundant in shales as thermal maturity increases. Additionally, there is a positive correlation for coals but a negative correlation for shales between the ratios of formate/acetate and oxygen index/hydrogen index (OI/HI). Taken together, these results suggest the WSOA compositions are closely related to the intrinsic macromolecular structure of organic matter. The evolution model of WSOAs can be divided into four stages based on their concentrations, types and generation mechanisms, corresponding well to the immature stage, oil window, gas window and graphitization.