Factors influencing porosity and permeability in coal reservoirs of western Guizhouc

Western Guizhou is rich in coalbed methane (CBM) and is the most extensive coal production base in southern China, making it a vital replacement area for CBM and even nonconventional gas exploration and development in China. However, the complex genetic types of coal seam pores and fractures, wide pore size distribution, and substantial heterogeneity in this area restrict the pace of CBM exploration and development in this region. To accelerate the commercial exploitation of CBM in this area, 43 coal samples from the Zunyi, Panguan, Bijie, and Zhina mining areas were used in this study to systematically investigate the main factors controlling the porosity and permeability of the coal in this area through coal petrology, , microfracture, and low-temperature nitrogen pore structure analyses. The results reveal that the coal and rock porosity in western Guizhou is generally between 1.6% and 8.1%, with an average of 4.18%. The porosity of coal is mainly dictated by three coalification steps and is minimal near 1.2%, which is the average random vitrinite reflectance of the coal. At the same time, mineral filling reduces the porosity of the coal. The permeability of coal in western Guizhou is highly heterogeneous and is generally below 5 × 10 −3 μm 2 , whereas the permeability of most coals is within 1 × 10 −3 μm 2 . Permeability shows a strong positive correlation with porosity, following a power-law trend. The high-rank coal in western Guizhou is mainly microporous, with a porosity below 6%. Its permeability mainly depends on the degree of fracture development and is little affected by porosity. In contrast, the permeability of medium- and low-rank coals with vitrinite reflectance less than 1.2% is affected not only by fractures but also by pores, even micropores and small pores. • The innovation of this article lies in the systematic use of coal samples from four major coalfields in western Guizhou for physical property description. • The porosity of coal is mainly dictated by three coalification steps and when the vitrinite reflectance value of coal is about 1.2%, its porosity reaches the minimum value. • the permeability of the high-rank coal in western Guizhou mainly depends on the degree of fracture development and is little affected by porosity. In contrast, the permeability of medium- and low-rank coals with vitrinite reflectance less than 2.1% is affected not only by fractures but also by pores, even micropores and small pores.