Adsorption capacity and controlling mechanisms of Paleozoic shales in Yangtze region

As a gas-rich shale layer, the Paleozoic in Yangtze region is a main target for shale gas exploration and development in China. Adsorption state is one of the most important occurrence modes of shale gas; hence it is very necessary to study the adsorption capacity and controlling mechanisms of shales. We collected some shale samples from Paleozoic in Yangtze area, and then carried out TOC analysis, Rock-Eval, XRD and water content analyses, isothermal adsorption experiments, and ultra-high pressure isothermal adsorption experiments. The adsorption properties of shales vary during different ages and in different areas due to the combined effects of TOC content and mineral composition. The TOC content and methane adsorption of shales do not have a positive relationship as proposed by previous researchers, because that shale samples are limited and are in the high-mature and over-mature stages. The methane adsorption isotherms of kerogens in different ages show that older kerogens have a stronger methane adsorption capacity. Removing the effects of organic matter abundance and maturity, the kerogens of type Ⅲ adsorb more methane than those of type Ⅱ. If organic matter abundance and type are the same, the methane adsorption amount of high maturity kerogen is more than that of low maturity kerogen. Soluble organic matter can dissolve and adsorb methane, and hence improve the methane adsorption capacity of shales. The relationship between clay mineral and methane adsorption which is normalized by TOC is not obvious, this is mainly because the samples contain water. Maturity, porosity and permeability may affect the maximum methane adsorption of shales. Compared with low pressure, the methane adsorption characteristics under high pressure have a certain continuity. There are several influencing factors, which demand deep research to reveal the influence of each single factor on shale adsorption characteristics.