Declining High-Pressure Sorption Isotherms on Shale and Coal: Systematic Comparison of the Contributing Factors

Showing a maximum followed by a decline in the CH4/CO2 high-pressure sorption isotherms measured on shale and coal is often considered abnormal, and it also fails to form a consistent view on the reason resulting in this abnormity. Thus far, various factors affecting this phenomenon were proposed, such as the error produced during the void volume calibration, the sorption-induced swelling effect, the physical properties of the measured gas, the difference between the excess and absolute adsorbed amount, etc. In this study, a systematic classification for these factors is first provided on the basis of the relevant theoretical and technical backgrounds, including three categories: ( 1) measurement procedure, (2) data processing procedure, and (3) other factors. Then, high-quality CH4 and CO2 high-pressure sorption isotherms are obtained in our laboratory and published literature. On this basis, the effects of these factors on the isotherm are compared by quantitative or qualitative analysis. The results show that, as a result of the technical limitation during processing sorption data, the deviation at 20 MPa would exceed 28% for the CH4 isotherm and 142% for the CO2 isotherm within the measured temperature range, which is considered as the main reason resulting in the decline in the high-pressure sorption isotherms on shale and coal. Some additional evidence, such as the temperature dependence of the decline phenomenon, may also indicate the rationality of this conclusion. In comparison to the effect of the data processing procedure, the effect of the measurement procedure is expected to be relatively low, and the third category of factors only indirectly controls the display of the decline in the sorption isotherm. In addition, our analyses further support the necessity of the correction of the high-pressure sorption isotherm via blank measurement, especially for a low-sorption sample.

Automated summary

This study provides a systematic classification and comparative analysis of high-quality CH4 and CO2 high-pressure sorption isotherms, revealing that technical limitations during data processing can lead to deviations of up to 28% for CH4 and 142% for CO2 at 20 MPa. This is considered a main reason for the decline in high-pressure sorption isotherms on shale and coal. Additional evidence, such as the temperature dependence of the decline phenomenon, supports this conclusion.