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Guizhou Province has many multi-layer, thin-thick coal seams; however, complex geology, incomplete reservoir characterization, and sweet-spot selection technology prevent large-scale coalbed methane (CBM) development. This study evaluates the CBM reservoir properties within the Dahebian block using logging data, coal sample analysis, and well-testing data and develops a 3D static reservoir properties model to analyze their spatial and vertical propagation. A sweet spot evaluation model was established using a multi-level fuzzy method based on 9 parameters extracted from a 3D static reservoir properties model. The coal measure has 22 coal seams, and seams >2 m thick have 2 or 3 thin non-coal layers intercalated. Coal seams 1#, 7#, and 11# are thin to thick, deeply buried, widely distributed, and have high gas content and saturation. Undeformed and cataclastic coal predominates the coal seam 1# and 7#, whereas coal seam 11# is dominated by cataclastic and granulated coal. The southern and central parts of coal seam 7# and 11# have less tectonically deformed coal (TDC). Coal seams 1# and 7# have low permeability relative to seam 11# and are localized, while coal seams 11# have high permeability, are extensively distributed, and contain substantial gas concentrations. Comparative analysis of evaluation scores and CBM production statistics shows that high scores indicate sweet spots for CBM development. Sweet-spot potential was classified as high, medium, and low. Scattered sweet spots are found in single layers, while combined development (1# + 7#+11#) reveals a wider high-potential area in the south-central region. This area, featuring deep, thick coal seams, high permeability, gas saturation, reservoir pressure, and low TDC proportion, indicates significant development potential. This study validates CBM development statistics, identifies future development areas, and guides the development of geologically complex Guizhou CBM.

As an important component of coal structure, the macropores have a great influence on CBM recovery. In this paper, the macropores characteristic of two coal samples collected from 3# coal seam in the south of Qinshui Basin, China was analyzed on the basic of mercury intrusion porosimetry, and a fully coupled triple-porosity/double permeability mathematical model for CBM recovery was established according to the physical structure of coal and the non-Darcy flow of methane in macropores. Then, the various factors affecting the macropores permeability were discussed and the influence of size distribution and connectivity of macropores on CBM recovery was investigated. The following conclusions have been drawn from these efforts: (1) in 3# coal seam of the south of Qinshui Basin, the macropores have an extremely heterogeneous pore size distribution with the high variation, and their connectivity is not good because they are mainly composed of the conical and cylindrical pores with one dead end and the open pores, the structure characteristics of macropores are not conducive to CBM recovery; (2) the fully coupled triple-porosity/double permeability mathematical model containing the non-Darcy flow of methane in the macropores includes the methane occurrence-migration field, hydraulic field, thermal field and stress field as well as the complex intercoupling between them, and the model was verified by the fitting of methane production history, with an average error of 3.24%; (3) the macropores permeability is closely related to the Knudsen number controlled by methane pressure and temperature in macropores and the intrinsic permeability which is an internal attribute of macropores affected by size distribution and connectivity of pore; (4) the pressure drop of reservoir plays a major role in the macropores permeability, which promotes the increase of macropores permeability with time, and the high intrinsic permeability of macropores corresponding to the good pore size distribution and connectivity is more conducive to the improvement of fracture permeability and methane production rate of coal reservoir during CBM recovery. It is recommended that coal seams including the macropores with uniform size distribution and good connectivity should be preferentially used for the development of CBM.