Adsorption behavior and prediction model of coalbed methane under the influence of temperature and pressure: a case study of the Zhijin Block, Western Guizhou, South China

In recent years, significant breakthroughs have been achieved in the exploration and development of deep coalbed methane in northern China. These have revealed marked differences in gas content between deep and shallow coalbeds. The Zhijin Block in western Guizhou, southern China, was one of the earliest regions to achieve commercial CBM development in southern China. As development targets extend progressively to greater depths, the region is now transitioning from shallow beds (< 1000 m) to deep beds (> 1000 m). However, the gas-bearing property and enrichment patterns in these deep coalbeds remain poorly understood. This study focused on the coal-bearing formation of the Longtan Formation in the Zhijin Block to address this issue. Six high-rank coal samples from different regions underwent isothermal adsorption experiments at 30 °C, 45 °C, and 60 °C. The study analyzed the response patterns and key controlling factors of coal sample adsorption behavior to temperature-pressure conditions, establishing a predictive model for adsorption gas content in deep coalbeds. Results indicate: (1) The adsorption behavior of coal samples towards CH₄ conforms to the supercritical Langmuir model. Adsorption capacity decreases with increasing temperature and increases with increasing pressure. (2) A coupled temperature-pressure prediction model for gas adsorption in deep coalbeds was established based on experimental data. Integrating parameters such as the geothermal and fluid pressure gradients revealed that the gas adsorption content initially increases and then decreases with burial depth, indicating the existence of a critical depth in the vertical direction. (3) The spatial distribution characteristics and primary controlling factors of the critical depth for adsorption gas content were elucidated. Both the geothermal gradient and the fluid pressure gradient exhibit negative correlations with the critical depth. (4) Coal adsorption ability exhibits a positive correlation with fixed carbon content, maximum vitrinite reflectance and vitrinite content, and a negative correlation with inertinite content. The research findings provide a theoretical basis for evaluating deep coalbed methane resources, predicting favorable zones, and enabling efficient development in the Zhijin block and other areas of southern China with similar geological conditions.