Multi-phase coalbed methane content model in deep strata of eastern margin of Ordos Basin

The deep coalbed methane resources in the Ordos Basin are abundant. Accurately understanding the laws and underlying mechanisms of coalbed methane content phase change is of great significance for achieving large-scale and efficient development of deep coalbed methane. Currently, prediction models for deep multi-phase coalbed methane content remain inadequate, and the mechanism of coalbed methane phase change is not well understood. These have become the key problems restricting the efficient development of deep coalbed methane. A multi-factor coupled prediction model for the variation of multiphase coalbed methane content in the deep (>1 500 m) is established by sorting out the controlling factors of coalbed methane content and phase changes in the eastern margin of the Ordos Basin. The general laws and underlying mechanisms of the variation of coalbed methane content phase state with burial depth are summarized and revealed. The results show that: ① The variation of adsorbed gas content in deep coalbed methane is influenced by reservoir temperature (T), reservoir pressure (P), Ro,max and water saturation (S_w); The free gas storage space is affected by the combined effects of adsorbed gas and water molecules; The dissolved gas volume has a good correlation with temperature, pressure, and mineralization degree; ② An adsorption gas content model (R²=0.924 6) is developed based on multivariate factors. A free gas content model is established considering adsorption-induced expansion, while a dissolved gas content model account for variations in solubility with temperature, pressure, and salinity. By integrating these sub-models, a predictive model is constructed to estimate the variation of deep multi-phase coalbed methane content in the eastern margin of the Ordos Basin. The calculated total gas content shows strong agreement with measured values across different blocks. ③ Taking the critical depth (2 000 m) as the boundary, the content of adsorbed gas gradually decreases, while the content of free gas and dissolved gas gradually increases. The gas occurrence state changes from “adsorbed gas” to “adsorbed gas + free gas”, and finally to “adsorbed gas + free gas + dissolved gas”. The findings clarify the evolutionary patterns and occurrence mechanisms of phase states and content of deep coalbed methane, providing an important theoretical basis for assessing the resource potential of deep coalbed methane and promoting its large-scale and efficient development in the Ordos Basin.