Prediction of Critical Desorption Pressure of Coalbed Methane in Multi-coal Seams Reservoir of Medium and High Coal Rank: A Case Study of Eastern Yunnan and Western Guizhou, China

In the process of multiseam combined drainage, the critical desorption pressure ( P cd ) is the basic measurement index for determining whether a multigas-bearing system can be combined with drainage, and it is the basic index for identifying effectively the contributions of productive strata. In actual exploration and development processes, the P cd is constrained by geological factors, engineering factors and economic factors, and the P cd of some key coal seams cannot be determined effectively, which restricts the efficient development and utilization of coalbed methane. Accurate prediction of the P cd of multiple coal seams under formation conditions has become a key requirement. In this study, medium-rank and high-rank coal samples were collected from the main synclines in Eastern Yunnan and Western Guizhou. By introducing the fractal dimension using nuclear magnetic resonance and the Pearson–Spearman correlation coefficient, the influences of coal metamorphism, pore structure, coal quality, temperature ( T ), and others on coal adsorption capacity were revealed. The results showed that, affected by the hydrocarbon generation and evolutionary processes of coal, fractal dimensions of adsorption pore ( D 3 ) and seepage pore ( D 4 ), the ratio of vitrinite to inertinite (V/I), the Langmuir volume ( V L ) and Langmuir pressure ( P L ) showed segmentation as the degree of metamorphism increased significantly. Bounded by random reflectance R r = 1.30%, before R r = 1.30%, with increase in metamorphic grade, V L and R r showed “U” type change due to the change of molecular structure, maceral content, and structure of seepage pore, and then the evolution of hydrocarbon generation was weakened mainly by the influence of coal and rock components, showing linear change. The slopes of P L and R r were larger before R r = 1.30% than after R r = 1.30%. The variation in P L with metamorphic degree was controlled mainly by the seepage capacity of coal rock, followed by the macerals of coal rock. Based on these results, the Levenberg–Marquardt algorithm was used with R r and T as independent variables, V L and P L as dependent variables, and the R 2 as the judgment value. A piecewise equation for calculating adsorption parameters with a high-fitting degree was obtained. Combined with the Langmuir equation, the prediction equation of P cd can be calculated under the conditions of R r and T and the measured gas content of known key coal seams. The correlation between the prediction equation and the measured parameters was high, and the identification template of P cd of medium–high-rank coal in the study area was given. The new calculation method is more convenient for obtaining parameters and can be applied effectively to coal seams without the need to perform isothermal adsorption tests.