Abstract
The physical properties of thick coal seams show strong vertical heterogeneity; thus, an accurate characterization of their pore structure is essential for coalbed methane (CBM) exploration and production. A total of 18 coal samples, collected from a thick coal seam in the Yili Basin of NW China, were tested by a series of laboratory experiments to investigate the peat mire evolution and pore structure characteristics. The results show that the No. 4 coal seam has undergone multiple stages of evolution in the peatification stage, and was divided into four water-transgression/water-regression cycles according to the regular cyclic changes of the vitrinite/inertinite ratio, structure preservation index, gelification index, vegetation index, trace element ratios, and stable carbon isotopes of organic matter. The changes of pore structure characteristics with the changes of coal deposition cycles are also analyzed. It is concluded that pore structure characteristics of the four cycles are quite different. In each water-transgression cycle, the vitrinite gradually increased and the inertinite gradually decreased, resulting in a decrease of the porosity, pore volume, specific surface area, and fractal dimension. While in each water-regression cycle, the vitrinite gradually decreased and the inertinite gradually increased, leading to an increase of the porosity, pore volume, specific surface area, and fractal dimension. A strong relationship exists between the porosity, pore volume, specific surface area, fractal dimension, and submacerals, with fusinite and semifusinite which contained more pores having a positive correlation, desmocollinite and corpovitrinite which contained few pores having a negative correlation.
Highlights
Coal seams whose minable single-thickness layers are 3.5–8.0 m are considered thick coal seams (Yang and Dexin, 1979)
There are still relatively few studies related to the vertical pore structure characteristics of thick coal reservoirs at present, which hinders further understanding the heterogeneity of thick coal reservoirs and their effect on the development of coalbed methane
Screening pulverized coal samples of less than 1 mm and about 20 g to prepare polished grain mounts 60– 80 mesh (0.20–0.25 mm) pulverized coal samples were selected for N2 adsorption/desorption experiments, and pulverized coal samples below 200 mesh were used for inductively coupled plasma mass spectrometry (ICP-MS) measurement of the trace elements in samples
Summary
Coal seams whose minable single-thickness layers are 3.5–8.0 m are considered thick coal seams (Yang and Dexin, 1979). When the long-term accommodation rate (AR) outpaces the peat production rate (PPR), the water table of the mire gradually rises and the coal-forming environment becomes wetter and more reductive. It is not difficult to find that the physical characteristics have strong vertical heterogeneity and the pore structure parameters even differ by an order of magnitude according to the current situation of describing the characteristics of thick coal reservoirs(Hou et al, 2017; Wang, 2018); this often brings difficulties to the selection of coalbed methane exploitation interval. There are still relatively few studies related to the vertical pore structure characteristics of thick coal reservoirs at present, which hinders further understanding the heterogeneity of thick coal reservoirs and their effect on the development of coalbed methane
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