Discussion on the Main Control Effect of Geological Structures on Coal and Gas Outburst.

Coal and gas outbursts are great natural disasters in underground coal mines, seriously threatening the lives of miners and coal mine safety. The study of the relationship between coal and gas outbursts and geological structures has always been an important component of gas geological work, and it also serves as a foundation for coal mines to develop gas outburst prevention measures. This study considers groups F and E coal seams, mainly mined in the southwest wing of Likou syncline in the Pingdingshan mining area, as the research object. Based on actual outburst data, this study investigates the distribution and intensity characteristics of coal and gas outbursts and the correlation between coal and gas outbursts and geological structures using gas geological analysis methods and taking structural control as the main line. The results depict that the coal and gas outbursts in the study area have obvious district and subband characteristics. The outburst district mainly occurred in the East, and the four outburst zones, I, II, III, and IV, were the concentrated occurrence subband of coal and gas outbursts. The district and subband of coal and gas outburst are mainly controlled by and related to the geological structure. The eastern fold structure has developed more than the middle and western districts, which is why the eastern part of the coal and gas outburst is more severe than in the middle and western regions. Under the structural background of the coal and gas outburst district, the subband outburst characteristics were mainly controlled by local geological structural conditions.

Discrete Element Simulation of Fracture Evolution around a Borehole for Gas Extraction in Coal Seams.

The stability analysis of underground caverns, tunnels, and boreholes is of great significance to underground engineering. In order to study the stress change of the coal around a hole and the evolution law of fractures during the coal seam drilling process, the discrete element simulation of the coal seam drilling process was carried out by using the particle flow code (PFC2D). The results show that during the drilling process, under the influence of confining pressure and drilling disturbance, the coal stress field around the hole and the development of fractures around the hole have the characteristics of zoning and dynamic evolution. In the axial direction of the borehole, it is divided into front and rear areas, and in the vertical axial direction, it is divided into the drilling disturbance zone and the confining pressure main control zone. During the drilling process, the direction of the maximum principal stress in the front zone gradually changes from the vertical hole axis to the direction parallel to the hole axis, and tension fractures are mainly developed along the drilling direction. In the rear zone, the principal stress direction tends to be stable and the principal stress value undergoes dynamic changes, and a large number of vertical hole axis tension fractures are developed. The drilling disturbance zone appears near the hole wall and has an important influence on the stability of the hole wall, while the confining pressure main control zone determines the antireflection effect around the hole and the influence radius of the hole. This work helps the understanding of the damage range and failure characteristics of the surrounding rock during the drilling process and has great significance for the guidance of drilling design.

Analysis Method of the Occurrence Law of Coalbed Gas Based on Gas-Geology Units: A Case Study of the Guhanshan Mine Field, Jiaozuo Coalfield, China.

The correct understanding of the occurrence law of coalbed gas (CBG) is the premise of gas disaster prevention, outburst risk prediction, and gas exploitation. The factors affecting gas occurrence in different gas-geology units are different, so the correct division of gas-geology units is the foundation for studying the occurrence law of CBG. In view of this, this paper defined the division principle of gas-geology units. A gas-geology unit is an area with the same gas-geology characteristics. Based on the division of tectonic units, gas-geology units can be divided by integrating the differences of in situ stress, geological factors, and gas distribution of each tectonic unit. Then, taking the Guhanshan mine field in the Jiaozuo coalfield as an example, the analysis method of the occurrence law of CBG based on gas-geology units was expounded. Taking the EW section, NE section, and their extension lines of the Tuanxiang fault as the boundary, the Guhanshan mine field was divided into four tectonic units, and the factors affecting the gas occurrence of each tectonic unit were analyzed. Finally, according to the difference of the bedrock thickness and the CBG distribution in the four tectonic units, combined with the fracture development degree of the coal seam, surrounding rock, and the development degree of deformed coal, the Guhanshan mine field was divided into three gas-geology units, and their occurrence law of CBG was analyzed.

Study on the Quantitative Characterization and Seepage Evolution Characteristics of Pores of Loaded Coal Based on NMR.

Quantitative characterization of the pore structure and gas seepage characteristics of loaded coal is of great significance to the study of high-efficiency gas drainage in coal seams. Aiming at the problem of imperfect characterizations of coal seepage characteristics based on nuclear magnetic resonance (NMR), a calculation method for the pore permeability of coal with different pore diameters is proposed. The pore structure and seepage characteristics of coal have been quantitatively studied using a nuclear magnetic resonance (NMR) system. The results show that with increasing external load, the proportion of the pore volume of the coal sample in the range of 0.01-0.52 µm gradually decreases, while that in the range of 5.11-352.97 µm increases. In this process, the porosity increases from 0.9967 to 1.0103%, the connectivity increases from 0.1718 to 0.2391, and the permeability increases from 2.64 × 10-6 to 8.20 × 10-6 µm2. The calculation of the coal sample connectivity and permeability using the improved NMR permeability component proves that 94.37-352.97 µm pores are the main channel of fluid flow. When the axial pressure increases, the coal body permeability in the aperture range of 94.37-352.97 µm rapidly increases. The improved permeability component calculation model can better reflect the variation law of pore permeability of the loaded coal body.

Pore Structure Characterization of Tailing Bed and Dewatering Mechanism at nm-µm Scales Under Shearing.

The preparation of high-density tailings is a prerequisite for cemented paste backfill technology, and the flocculated fine tailings of sealed water leads to challenges in the slurry thickening of tailings. Shearing conditions can compact the micro floc structure to improve the underflow concentration. The nm-µm scales of pore characteristics and connectivity are essential for the dewatering process. The computed tomography (CT) results show that the underflow concentration increases from 62.3 wt% to 68.6 wt% after undergoing rake shearing at 2 rpm, and the porosity decreases from 42.7% to 35.54%. The shearing conditions reduces the spheres and sticks by 43.14% and 43.3%, respectively, from the pore network model (PNM). The seepage flow states were affected by the changes in the pore structure. The maximum surface velocity and the maximum internal pressure decrease after undergoing shearing. Shearing conditions can break the micro floc structures, and the fine particles can fill in the micron-scale pores by gravity and shearing conditions, resulting in the forced drainage of water into the pores. Shearing conditions can break the thickening floc network structures; natural fine particles can fill the micron-scale pores by gravity and shearing conditions. The upward seepage of sealed water along the µm-scale pore channel causes a higher bed concentration. However, the sealed water in the nm-scale pores cannot flow upward due to water cohesion and particle adhesion resistance.

Micro-Nanostructure of Coal and Adsorption-Diffusion Characteristics of Methane.

The adsorption and diffusion characteristics of coal are important parameters for coalbed methane (CBM) extraction and mine gas control. However, the adsorption test can only obtain the apparent adsorption amount, and it cannot obtain the actual adsorption amount, which leads to a large error during the calculation of the coal diffusion coefficient. Taking the anthracite coal in the Jiulishan Mine as the research object, the micro-nanostructure and instantaneous apparent methane adsorption isotherms of the primary structure coal and tectonic coal were determined by low-temperature CO2 adsorption, mercury intrusion and methane diffusion kinetics tests, and the instantaneous apparent adsorption isotherms of methane were corrected to the instantaneous actual adsorption isotherm by the Langmuir model. The results demonstrate that the micro-nanopore, Density Function Theory (DFT) pore volume and specific surface area values below 1-2 nm in tectonic coal are larger than those in the primary structure coal, which is the fundamental reason why the ultimate adsorption capacity of tectonic coal is larger than that of the primary structure coal. The apparent adsorption amounts of the tectonic coal and the primary structure coal reach the maximum at 8 MPa and 10 MPa, respectively. Thereafter, the instantaneous isotherms of the apparent adsorption amount decrease with increasing of gas pressure. However, the instantaneous isotherms of the actual adsorption amount tend to be stable. The diffusion coefficient undergoes a rapid decay with time under low gas pressure, and undergoes a slow decay with under the high gas pressure.

Micro-Nano Fine Characterization of Coal Fracture Evolution During the Triaxial Compression Creep.

The production and evolution of fractures during coal creep will directly affect the occurrence, extraction and flow law of gas in a coal seam. The coal fracture evolution under creep conditions was studied by qualitative analysis and quantitative characterization. At a room temperature of 24 °C, triaxial compression creep tests of coal samples from the Zhaogu No. 2 coal mine in Jiaozuo were carried out under different loading conditions (0 MPa, 6 MPa, 9 MPa and 12 MPa), and low field nuclear magnetic resonance technique tests and industrial CT scanning experiments were performed. The obtained CT images were analyzed with the MATLAB software for equalization and binary image processing. The development and distribution of fractures in coal samples under different loading conditions were studied. The results show that the internal fractures are unevenly distributed and controlled by the main fracture, and the expansion direction of fractures is parallel to the direction of the maximum effective compressive stress. The number of fractures shows an increasing trend with the increase of axial stress, and the pace of growth of new fractures accelerates. The primary fractures in the coal body expand and generate new fractures, which improves the connectivity of the fractures in the coal body. The research results can provide a basis for studying the gas flow rule around the borehole and determining the influence range of the borehole.

Fourier Transform Infrared Spectroscopy Evidence of the Nanoscale Structural Jump in Medium-Rank Tectonic Coal.

Coal is a pressure-sensitive organic rock. The effect of tectonism on the structural evolution of medium-rank coal has been confirmed by the change in the crystal state of tectonic coal, but the organic molecular level response has not been reported. In this paper, three sets of medium-rank tectonic coals and symbiotic nontectonic coals were selected. The distributions of their functional groups and their molecular structure evolution were assessed using Fourier Transform Infrared Spectroscopy (FTIR), and their structural parameters were determined from the curve-fitting analysis. The nanoscale structural jump characteristics and mechanisms of medium-rank tectonic coal were revealed. Compared with symbiotic nontectonic coal, tectonism accelerated the exfoliation of side chains (groups) in the macromolecular structure, enlarged the aromatic system, and removed the unstable groups such as associative hydrogen bonds at first, which indicated that the molecular structure of tectonic coal was affected by nanoscale deformation, showing obvious advanced evolution characteristics. For the fat coal, the removal of side chains (groups) during the formation of tectonic coal makes the aromatic ring condensation obvious. For the coking coal, the formation of tectonic coal is dominated by cycloaliphatic dehydrogenation and aromatization, accompanied by the condensation of the aromatic rings. The tectonic coal formed from lean coal shows obvious aromatization characteristics. The molecular depolymerization and chemical tailoring caused by tectonism promotes the removal of hydrophobic side chains (groups) and activates some polar structure sites in coal. It is considered that the nanoscale structural jump of medium-rank tectonic coal is the result of the competition between the aromatic system and aliphatic structures.