Prediction model and evolution patterns of multiphase methane content in deep coal reservoirs

Deep coalbed methane reservoirs contain substantial gas resources, but the depth-dependent evolution of adsorbed, free, and water-soluble methane remains insufficiently quantified. This study developed a multiphase methane content prediction model by integrating a multi-factor Langmuir adsorption model, free-gas formulation, and a methane solubility model considering pressure, temperature, and formation-water salinity. Geological and experimental data from deep coalbed methane wells in the eastern Ordos Basin were used for model construction, validation, and sensitivity analysis. Model validation using 43 measured gas content samples showed good agreement between predicted and measured values, with MAE, RMSE, MAPE, and R2 values of 1.084 m3/t, 2.044 m3/t, 6.84%, and 0.876, respectively. The results indicate that adsorbed gas reaches its principal peak at approximately 1,700–2,200 m, whereas total gas content reaches its main enrichment zone at approximately 2,800–3,000 m. Free gas contribution increases progressively with burial depth but does not universally replace adsorbed gas. Water saturation mainly reduces total gas content by compressing free-gas storage space, whereas coal rank enhances methane enrichment through increased adsorption capacity. This work provides a practical multiphase framework for evaluating deep coalbed methane enrichment.