Precise identification and threshold inversion of pores and fissures in CT digital coal rock based on Bi-PTI model

In the research area of the CT digital coal petrography, the selection of gray threshold directly affects the accuracy of spatial structure remodeling. To allow the pores and fissures space remodeling data to more accurately represent the real structure and improve the reliability of research on microscopic seepage of coal rock, the mapping relation between the CT digital coal rock porosity and gray threshold is established. Also, the distribution rule of gray threshold is adopted to conduct a quantitative analysis of the development features of pore, fissure and matrix. The Bi-PTI (Biphasic Pore Threshold Inversion) model is established for the identification of pore and fissure structures. The value inversion of the optimal gray threshold of pore and fissure based on the CT scanning data of metamorphic coal, and the comparison of porosity obtained with the model against the mercury injection test value are conducted. On the basis of the optimal gray threshold inversion results, the spatial structure of pore and fissure is remodeled and compared with the Otsu model through the parameters of spatial structure and topological structure. Research results indicate that the porosity based on the CT digital coal petrography is in asymmetrical S-shaped distribution as the threshold value increases, with the features of exponential increase and logarithmic climb at different sections. The Bi-PTI model can fairly reflect the mapping relation between porosity and gray threshold, and represent the development features of pore, fissure and matrix. The porosity calculated through the Otsu model is as high as over 70%, the porosity calculated by the MP-Otsu model is greatly affected by the mineral content while that calculated with the Bi-PTI model ranges from 0.40% to 16.22%, more close to the mercury injection test data. The Otsu model is most likely to neglect the topological structure with partial pores and fissures. The MP-Otsu model is not effective for constructing the topology of coal samples with low mineral content, while the Bi-PTI model can better identify the topological features in the locations of the defect and faithfully restore the pore throat abundance and connectivity in the pore and fissure topology

Research on overlying strata movement and dangerous area of coal spontaneous combustion in goaf in dual-system coal seam mining

Aiming at the engineering problem of large air leakage in the goaf of 8106 fully mechanized mining face of Yongdingzhuang Mine in Datong Mining Area, numerical simulation method is used to study the movement breaking form and the dynamic evolution of cracks and the expansion range of the double-system coal seam under multiple mining. The goaf connection mechanism reveals the distribution law of the coal spontaneous combustion danger area in the goaf. The results show that the goaf of Jurassic coal seam group is manifested as roof fall, large floor heave and shear failure of the floor near the coal pillar; the mining of Carboniferous 3-5# coal seam will successively cause the breakage and caving of the basic roof, the lower key layer and the upper key layer. After the upper key layer breaks and subsides, the double-system goaf will be connected and lead to air leakage. The breakage period of the lower key layer is 60 m. In the cross section of the crack zone formed between the key layers, the main air leakage channels are divided into dynamic cracks and edge cracks; when there is external air leakage, the coal spontaneous combustion danger area in the goaf is mainly distributed between 100-175 m from the working face, and when there is no external air leakage, the relative area of the coal spontaneous combustion danger zone is greatly reduced and the overall position is moved forward about 30 m in the direction of the working face

Experimental Research on the Law of Energy Conversion during CO₂ Sequestration in Coal

CO2 sequestration in coal is mainly attributed to adsorption. The adsorption experiments of CO2 were conducted at injection pressures ranging from 1 to 3 MPa on coal samples with five kinds of particle sizes. The fitting degree of four classical adsorption models to experimental adsorption data was systematically compared. The adsorption properties of CO2 were comprehensively discussed. The temperature changes of coal samples at different positions during CO2 adsorption were measured by using the improved adsorption tank, and then the energy conversion law was obtained. The results showed increasing gas injection pressure can effectively increase the adsorption capacity of CO2 on coal samples. The BET equation had the best fitting accuracy for CO2 adsorption on various size coal samples. There was a significant exothermic effect during CO2 adsorption and storage. With the rise of injection pressure, the peak value of the rising temperature of coal samples increased, but the change rate decreased. The maximum temperature rise of coal samples was up to 13.6 °C at 3 MPa, which should be of great concern for the prevention of coal spontaneous combustion. During the sequestration process of CO2, the adsorption resulted in a decrease in coal surface free energy and then partial conversion to heat, leading to the rise of coal temperature. In addition, the CO2 adsorption on the pore surface caused the expansion and deformation of coal

Research on measurement of specific energy of crushing based on particle compression experimen

In order to better characterize the strength characteristics of coal, according to P. R. Rittinger’s new surface theory, a determination method of specific energy of crushing based on particle compression experiment is proposed. Based on this method, the specific energy of crushing of primary coal and different types of tectonic coal in Yangquan Mining Area is systematically measured. The results show that the specific energy of crushing of tectonic coal in Yangquan Mining Area is 18.14-87.39 J/m2, which is 1-2 orders of magnitude lower than that of primary coal(about 1 098.46 J/m2); at the same time, the specific energy of crushing of broken coal (64.26-87.39 J/m2) is significantly greater than that of mylonitic coal(about 18.14 J/m2), which is 3.54-4.82 times that of mylonitic coal

Molecular Simulation on Competitive Adsorption Differences of Gas with Different Pore Sizes in Coal

Micropores are the primary sites for methane occurrence in coal. Studying the regularity of methane occurrence in micropores is significant for targeted displacement and other yield-increasing measures in the future. This study used simplified graphene sheets as pore walls to construct coal-structural models with pore sizes of 1 nm, 2 nm, and 4 nm. Based on the Grand Canonical Monte Carlo (GCMC) and molecular dynamics theory, we simulated the adsorption characteristics of methane in pores of different sizes. The results showed that the adsorption capacity was positively correlated with the pore size for pure gas adsorption. The adsorption capacity increased with pressure and pore size for competitive adsorption of binary mixtures in pores. As the average isosteric heat decreased, the interaction between the gas and the pore wall weakened, and the desorption amount of CH4 decreased. In ultramicropores, the high concentration of CO2 (50–70%) is more conducive to CH4 desorption; however, when the CO2 concentration is greater than 70%, the corresponding CH4 adsorption amount is meager, and the selected adsorption coefficient SCO2/CH4 is small. Therefore, to achieve effective desorption of methane in coal micropores, relatively low pressure (4–6 MPa) and a relatively low CO2 concentration (50–70%) should be selected in the process of increasing methane production by CO2 injection in later stages. These research results provide theoretical support for gas injection to promote CH4 desorption in coal pores and to increase yield

Molecular Simulation of the Adsorption Characteristics of Methane in Pores of Coal with Different Metamorphic Degrees

In order to study differences in the methane adsorption characteristics of coal pores of different metamorphic degrees, 4 nm pore structure models based on three typical coal structure models with different metamorphic degrees were constructed. Based on the molecular mechanics and dynamics theory, the adsorption characteristics of methane in different coal rank pores were simulated by the grand canonical Monte Carlo (GCMC) and molecular dynamics methods. The isothermal adsorption curve, Van der Waals energy, concentration distribution, and diffusion coefficient of methane under different conditions were analyzed and calculated. The results showed that at the same pore size, the adsorption capacity of CH4 is positively correlated with pressure and metamorphic degree of coal, and the adsorption capacity of CH4 in high metamorphic coal is more affected by temperature. The relative concentration of CH4 in high-order coal pores is low, and the relative concentration at higher temperature and pressure conditions is high. The CH4 diffusion coefficient in high-rank coal is low, corresponding to the strong Van der Waals interaction between CH4 and coal. The research results are of great significance for further exploration of the interaction mechanism between CH4 and coal with different metamorphic degrees and can provide theoretical support for the selection of gas extraction parameters

Pressure Relief Mechanism and Gas Extraction Method during the Mining of the Steep and Extra-Thick Coal Seam: A Case Study in the Yaojie No. 3 Coal Mine

Gas disasters, such as coal and gas outburst and gas overflow, always occur during the mining of the steep and extra-thick coal seam in the horizontal, fully mechanized, top coal slice caving (HFMTCSC) method. To solve these issues and guarantee the safe and efficient mining in the Yaojie No. 3 coal mine, 3DEC software was used in this work to investigate the overburden movement and collapse law as well as the stress redistribution and coal-seam deformation characteristics below the goaf. The results show that a pressure arch structure and a hinge structure are formed in succession in the overburden rock, which induces stress redistribution in the coal below the goaf. During the mining of the upper slice, more than 75% of the coal in the lower slice is located at the effective pressure relief zone; therefore, the steep and extra-thick coal seam can then be protected slice by slice. Meanwhile, with the increase of mining depth, the efficient pressure relief range expands. Based on this pressure relief mechanism, crossing boreholes and bedding boreholes were reasonably designed to efficiently extract the pressure relief gas during the mining of the steep and extra-thick coal seam in the Yaojie No. 3 coal mine

Pressure Relief Mechanism and Gas Extraction Method during the Mining of the Steep and Extra-Thick Coal Seam: A Case Study in the Yaojie No. 3 Coal Mine

Gas disasters, such as coal and gas outburst and gas overflow, always occur during the mining of the steep and extra-thick coal seam in the horizontal, fully mechanized, top coal slice caving (HFMTCSC) method. To solve these issues and guarantee the safe and efficient mining in the Yaojie No. 3 coal mine, 3DEC software was used in this work to investigate the overburden movement and collapse law as well as the stress redistribution and coal-seam deformation characteristics below the goaf. The results show that a pressure arch structure and a hinge structure are formed in succession in the overburden rock, which induces stress redistribution in the coal below the goaf. During the mining of the upper slice, more than 75% of the coal in the lower slice is located at the effective pressure relief zone; therefore, the steep and extra-thick coal seam can then be protected slice by slice. Meanwhile, with the increase of mining depth, the efficient pressure relief range expands. Based on this pressure relief mechanism, crossing boreholes and bedding boreholes were reasonably designed to efficiently extract the pressure relief gas during the mining of the steep and extra-thick coal seam in the Yaojie No. 3 coal mine

Molecular Simulation of the Effects of Cyclic Organic Compounds on the Stability of Lccbm Hydrates

CH4 can be separated from low-concentration coal bed methane (LCCBM) by using the hydrate-based gas separation (HBGS) method. To study the contribution of different cyclic organic compounds to the separation of CH4 in LCCBM, an LCCBM hydrate model was constructed. Based on the Monte Carlo and molecular dynamics theory, we simulated the effect of three cyclic organic compounds—cyclopentane (CP), cyclopentanone (CP-one), and cyclopentanol (CP-ol)—on the stability of the LCCBM hydrate at P = 2 MPa, various temperatures, and discussed the structural stability of the hydrate in depth in terms of final snapshots, radial distribution function, mean square displacement, diffusion coefficient, and potential energy change. The results showed that for the CH4-N2 LCCMM gas mixture, CP showed the best facilitation effect compared to the other two cyclic compounds by maintaining the stability of the LCCBM hydrate well at T = 293 K. The promotion effect of CP-one is between CP and CP-ol, and when the temperature increases to T = 293 K, the oxygen atoms in the water molecule can maintain the essential stability of the hydrate structure, although the orderliness decreases significantly. Moreover, the structure of the hydrate model containing CP-ol is destroyed at T = 293 K, and the eventual escape of CH4 and N2 molecules in solution occurs as bubbles. The research results are important for further exploration of the mechanism of action of cyclic promoter molecules with LCCBM hydrate molecules and promoter preferences