A logging calculation method for shale adsorbed gas content and its application

Abstract

A large part of shale gas occurs in organic matter and clay mineral surfaces in an adsorption state (20–85%). In a shale reservoir, the pore surfaces of organic matter and clay minerals are the main locations for shale gas adsorption. However, calculating the adsorption gas content is a difficult problem in logging. In order to build a calculation model for adsorption gas, a series of experiments was conducted to understand the relationship between adsorption gas content, mineral type, and content in the Zhaotong Longmaxi formation. These experiments show that organic matter has the most specific surface area and is the most important adsorption matter in the reservoir, especially under low pressure. Clay minerals become more important as the pressure increases. The KIM formula, which is widely used in coalbed methane, was introduced in this work. At a certain temperature and pressure in a reservoir, the gas adsorption specific surface can be simplified as a function of total organic carbon and clay mineral content. The calculated results from the Zhaotong A well are in good agreement with the experiments. The well logging information is used to further calculate the clay minerals of the Zhaotong A well profile, the total organic carbon profile, and the adsorption volume profile. The adsorption gas and total gas of the Zhaotong A well Longmaxi group in a lower place had similar reductions, which is consistent with the field desorption experiment.