Evolution Characteristics Of Microscopic Pore Structure Of Hard-to-drain Coal Seams In Pingdingshan Coalfield And Their Influences On Gas Storage And Transportation

Coal seam gas extraction is of great significance to mine disaster prevention and control,clean resource recovery and low-carbon green production.Coal seams where gas extraction is difficult(hereafter referred to as GED coal seams)are widely distributed in China.In such coal seams,gas extraction is of a fluctuant and quickly attenuating concentration and low efficiency,which greatly restricts the safe and efficient development of coal and gas resources.The Pingdingshan area is a typical low-permeability and GED mining district in China.The GED coal seams in Pingdingshan mining area have undergone complex geological evolution,resulting in extremely complex pore structures.The coal seams often contain highly saturated and high-pressure gas,and have extremely poor permeability,forming‘high storage,low porosity and low permeability'reservoir characteristics.The microstructure of coal,especially the pore structure,plays a key role in controlling gas storage and migration.Especially for low-porosity and low-permeability reservoirs,the complex pore structure may be the main reason for the rapid decline of borehole flow rate and difficulty in gas extraction.In this paper,the evolution characteristics of the microscopic pore structure of GED coal seams in the Pingdingshan mining area and their influence on gas storage and transportation were analyzed in depth by means of theoretical analysis,physical experiments and numerical simulation,in order to deeply reveal the microscopic mechanism of gas storage and transportation in GED coal seams.The research provides a new understanding for solving the gas drainage problem of GED coal seams.The main research results are as follows:(1)The gas generation and storage mechanism of GED coal saems in the Pingdingshan mining area was explored from the perspective of chemical functional groups and pore structure evolution.The following findings were obtained:In the coalification stage?(R_o=0.6-1.3%),the number of oxygen-containing functional groups and hydrochemical bonds of aliphatic hydrocarbons drops sharply,resulting in a significantly decreasing the aliphatic hydrogen rate(H_al),a dramatically shortened aliphatic side chain length(CL_al)and a rapidly increased in aromaticity(AR).As dehydrogenation and deoxygenation becomes more intense,hydrocarbon gas is generated in large quantities.Pingdingshan GED coal seam is at its peak of hydrocarbon generation where the hydrocarbon generation potential falls significantly.Meanwhile,a large number of aliphatic structure pores of GED coal near the second coalification jump point(R_o=1.3%)disappear,and a large number of generated asphaltenes block the pores,so that the pore volume plunges to the lowest level in the whole coalification stage.Since meso-pores and macro-pores in Pingdingshan GED coal are underdeveloped and not open,the intensely generated methane loses migration channels.Under high stress,the methane gets sealed in micro-nanopores,even in macropores and microcracks,leading to high saturation,even supersaturation of high pressure gas.(2)The influence of coal macromolecular morphological structure on the formation and evolution of micro-pores and meso-pores was investigated,and the microscopic mechanism of pore evolution of GED coal seams in Pingdingshan mining area was revealed.The ductility of the microcrystalline basic structural unit(BSU)L_ais linearly positively correlated with the micropore volume.As L_a increases,the interlayer voids of the aromatic layer in the BSU and the defects formed by the dislocation arrangement of BSUs both increase.Resultantly,the micropore volume expands.The defect degree R₁ and L_a/L_c(the ratio of BSU ductility L_a to stacking height L_c)have a remarkable influence on the volume of mesopores.The development degree of mesopores is related to the growth and arrangement characteristics of large-scale BSUs and molecular orientation domains.Before the second coalification jump,the degradation of aliphatic structure and asphaltene blocking dominate the pore evolution,and the pore volume falls rapidly with the decrease in the macromolecular structure defects.After the jump,the disordered microcrystalline structure evolution(the increase in L_a)begins to dominate the pore evolution,and the growth of BSUs contributes to the turning increase in the pore volume.The pores in tectonically deformed coal develop because its structural defects increase greatly through intensifying BSU growth and disordered arrangement.(3)The evolution characteristics of full-scale pore size distribution under coalification were studied,and the accessibility evaluation and classification method of coal pore structure based on NMR-Mn Cl₂ was proposed.It is found that the coalification effect significantly alters the pore size distribution,especially the meso-pore distribution.Under the action of coalification jump,mesopores in Pingdingshan GED coal experiences deteriorated development and disappearance in the pore size distribution.The free fluid porosity and nuclear magnetic permeability of medium-rank GED coal are extremely low,only 1.16%and 10^-4 m D,respectively.By analyzing the response of NMR T₂ spectrum to paramagnetic ion Mn²⁺diffusion,the pore structure access characteristics of coal samples were inverted.According to the accessibility characteristics,the pores in coal fall into four types,i.e.,accessible pores,less accessible pores,inaccessible pores and isolated pores.The proportion of accessible pores in GED coal is significantly lower than that of non-GED coal,while the proportion of isolated holes is much higher than that of non-GED coal.The average size of pores in Pingdingshan GED coal is small.Micro-nanopores that are smaller than5?m are developed in isolated groups.A small number of connected pores are distributed in a banded agglomeration.Overall,the pore structure is of poor connectivity.(4)The 3D visualization model of multi-scale complex pore-fracture network of GED coal was established,and the topological spatial structure characteristics of micro-nanopores and their influence on micro-scale flow were obtained.Based on the deep learning of convolutional neural network,the intelligent segmentation and accurate extraction of coal pore-fracture network structure were achieved.Furthermore,the equivalent pore network model(PNM)was established through the maximum sphere algorithm,and the quantitative parameters of pore topological structure were obtained.The maximum connected group size of the pore space of GED coal is rather small.Different from non-GED coal,GED coal features the development of small-scale equivalent pore cavities.In addition,the pore cavities are of a small coordination number,and the pore throats are of a small diameter,a short length and a small number.The FIB-SEM results show that the nanopore groups of GED coal feature a spatially cross-linked and connected network between large-scale pore cavities and small-scale pore throats.The pore throats below 30 nm constitute the main channel of gas diffusion.Micro-scale flow simulation and microstructure streamline field simulation based on PNM were carried out.The simulation results disclose that GED coal has an obvious pressure gradient mutation of nano-scale equivalent structure fluid and short flow field streamline.Consequently,fluids are trapped in the microstructure.Small-scale pore clusters,narrow pore throats and the low coordination number of pore cavities are the main causes for inefficient flow.(5)The visualization experimental platform of gas migration was established based on the micron CT system,and a visualization study was conducted on the influence of microstructure on gas diffusion.The results suggest that both GED coal and non-GED coal contain a dark black low-permeability region distributed along the bedding direction.The low-permeability region in GED coal accounts for 42.24%,much larger than 8.14%of non-GED coal.The Kr gas diffuses slowly from fractures to the surrounding matrix in the low-permeability region,with the average rate within 2-50 h being only 1.04?m/h.Moreover,the visualization of Kr gas diffusion process and the quantitative analysis on normalized concentration distribution in the low-permeability region were realized through image registration,gray correction calculation,threshold segmentation and 3D volume rendering.The desorption experiment demonstrates that GED coal corresponds to a low desorption rate.Consequently,a great amount of Kr gas is trapped in the micro-nanopore groups in the low-permeability region.The spatial pore structure in the low-permeability region is characterized by notably isolated distribution.The content,distribution and micro-nanopore connectivity of the low-permeability region significantly influence gas diffusion.(6)The characteristics of gas storage and migration in GED coal seams in Pingdingshan mining were revealed,and methods to enhance gas extraction in GED coal were pointed out.The high gas,low permeability,heterogeneity and easy mutation characteristics of medium-rank GED coal are caused by high-pressure gas agglomeration occurrence,micro-nanopore solitary development and pore size malformation distribution.Besides,borehole gas extraction presents characteristics such as instantaneous explosive emission,short-term mutation desorption and long-term slow flow.Finally,in light of the characteristics of large adsorption capacity of low-pressure gas and large equivalent matrix scale of low-permeability region in GED coal seams,it is proposed to promote low-pressure gas desorption and accelerate gas diffusion and migration by deep pressure relief and strengthened permeability enhancement,so as to realize efficient extraction.There are 121 Figures,24 Tables and 215 References in this paper.