Segmentation of multi-coal seam pore structure in single well profile and its sedimentary control: a case study of Well Y1 in Panguan syncline, western Guizhou, China

Abstract

The pore structure of coal reservoirs is crucial for the occurrence and production of coalbed methane (CBM). Taking as an example the upper Permian coal-bearing strata profile of Well Y1 in Panguan syncline of western Guizhou, mercury intrusion porosimetry (MIP) and low-temperature N2 adsorption/desorption (LT-N2GA) experiments were carried out on samples from six main coal seams. Hysteresis coefficients of MIP and LT-N2GA were proposed. Fractal dimension of pore structure of each coal seam was calculated. The segmentation of pore structure of multi-coal seams was found. We found the total pore volume (TPV) and specific surface area (SSA) of coal were increased and then decreased as the increase of coal seam buried depth, which is characterized by two-segment distribution. The upper segment included the 7#, 10#, and 12# coal seams, whereas the lower segment contained the 18#, 19#, and 24# coal seams. The TPV, SSA, and MIP hysteresis coefficients and fractal dimension of seepage pore of the upper segment coal seams are larger than those of lower segment coal seams on the whole, while the LT-N2GA hysteresis coefficient, fractal dimension of adsorption pore, and fractal dimension of high-pressure stage of the upper segment are smaller than those of the lower segment. The coal seams in upper segment have more open pores, while the coal seams in the lower segment have more fine neck bottle pores or ink bottle pores. Taken together, these data demonstrated that the coal seams in the upper segment have higher adsorption and seepage capacity, and are more favorable for CBM development. The fundamental reason for the two-segment variation of TPV and SSA of six main coal seams is the symmetry of sedimentary facies in coal-bearing strata; moreover, the ash yield controls the TPV and SSA of coal seams, and the ratio of vitrinite to inertinite (V/I) controls the development of pores.