Pengfei Ji

The pulsed nitrogen injection for methane displacement (PNI-MD) technology is a novel approach aimed at reducing greenhouse gas emissions and enhancing the extraction efficiency and utilization of methane from low-permeability coal seams. By conducting experiments with PNI-MD, the influence of nitrogen injection duration on the methane displacement effect and the coal deformation characteristics is analyzed. The gas flow state and the mechanism of blockage-unclogging and permeability-increasing are investigated, and the field application mode of PNI-MD technology in low-permeability coal seams is constructed. The results show that with the prolongation of nitrogen injection time, the methane displacement efficiency increases while the methane displacement ratio decreases. During the nitrogen injection stage (Stage A), the high-pressure nitrogen gradually affects the gas flow state within the coal and at the outlet end. The gas flow state within the coal in Cycle 1 is divided into five stages. High-pressure pulsed nitrogen generates periodic tensile and compressive stresses with strong alternating pressure, which promotes repeated changes in the mechanical properties of coal, and the cumulative effect of damage leads to fatigue deterioration of the coal. The blockage-unclogging and permeability-increasing effects throughout the entire process of PNI-MD are divided into four stages. Under varying nitrogen injection durations, the increase in coal permeability after the completion of PNI-MD ranges from 0.50% to 3.63%. The PNI-MD technology effectively leverages the dual role of multi-gas displacement for desorption and energy injection for changing coal properties to achieve the objectives of blockage-unclogging and permeability-increasing. The dominant mechanisms during the Stage A include carrying methane with charging energy (CCWCE), promoting methane desorption by sharing pressure (PCDBSP), enlarging pores with injecting energy (EPWIE), and expanding pores with injecting energy (EPWIE1). The dominant mechanisms during the stop-injection stage (Stage B) include promoting methane diffusion by dilution its concentration (PCDBDIC) and enlarging pores with promoting methane desorption (EPWPCD). The damaging pore structure/spread fracture network by pulse gas flow to increase the methane flow (DPSSFNBPGF) throughout the entire process of PNI-MD. According to the properties of methane occurrence in low-permeability coal seams in China, the construction technology and equipment for underground PNI-MD technology are summarized. A flexible, simple and multi-selectable method for optimizing the parameters of the PNI-MD process is proposed, which is applicable for varying methane occurrence characteristics in different coal seams, thereby contributing to the precise extraction of methane from low-permeability coal seams, as well as carbon peak and carbon neutrality in China.

More accurate prediction of CO₂/CH₄ adsorption selectivity coefficients in the CO₂ Enhanced Coal Bed CH₄ Recovery (CO₂-ECBM) project can help to judge the CO₂ adsorption concentration and the desorption purity of CH₄ during the CO₂ injection process, and to achieve the maximization of CO₂ sequestration as well as the optimization of the CH₄ recovery rate. To this end, a coal molecular slit model with 16 sizes including micro-, meso-, and macropores was constructed in this study, and the competitive adsorption characteristics of CO₂ and CH₄ gas mixtures in bituminous coal molecules were investigated using molecular dynamics and giant canonical Monte Carlo simulations. The CO₂/CH₄ adsorption selectivity coefficients (S_c) as a function of gas ratio, gas pressure, pore size, and temperature were analyzed using a large amount of adsorption isotherm data. Based on the simulation results, considering the neglect of pressure and component changes when calculating the adsorption selectivity coefficient using the traditional extended Langmuir (E-L) model, a correction term regarding the pressure of the mixed gas and the mole fraction of CO₂ is set, and a modified equation is proposed. The results show that the adsorption potential energy of CO₂ is significantly higher than that of CH₄, giving it an absolute advantage in the competition. Through multiple regression analysis, the ranking of the influence weights of the four factors on S_c is as follows: pore size > mixed gas pressure > molar fraction of CO₂ > temperature. The negative exponential function can describe the variation of S_c with four factors. The fitting degree between the modified prediction model and the S_c data obtained through simulation reaches 0.84, and the model effect is good. The research results provide theoretical guidance for the optimization of gas injection parameters in the CO₂-ECBM project.

The pore structure of coal plays a key role in controlling the storage and migration of CH₄/N₂. The pore structure of coal is an important indicator to measure the gas extraction capability and the gas displacement effect of N₂ injection. The deformation characteristic of coal during adsorption–desorption of CH₄/N₂ is an important factor affecting CH₄ pumpability and N₂ injectability. The pore structure characteristics of low-permeability coal were obtained by fluid intrusion method and photoelectric radiation technology. The multistage and connectivity of coal pores were analyzed. Subsequently, a simultaneous test experiment of CH₄/N₂ adsorption–desorption and coal deformation was carried out. The deformation characteristics of coal were clarified and a coal strain model was constructed. Finally, the applicability of low-permeability coal to N₂ injection for CH₄ displacement technology was investigated. The results show that the micropores and transition pores of coal samples are relatively developed. The pore morphology of coal is dominated by semi-open pores. The pore structure of coal is highly complex and heterogeneous. Transition pores, mesopores and macropores of coal have good connectivity, while micropores have poor connectivity. Under constant triaxial stress, the adsorption capacity of the coal for CH₄ is greater than that for N₂, and the deformation capacity of the coal for CH₄ adsorption is greater than that for N₂ adsorption. The axial strain, circumferential strain, and volumetric strain during the entire process of CH₄ and N₂ adsorption/desorption in the coal can be divided into three stages. Coal adsorption–desorption deformation has the characteristics of anisotropy and gas-difference. A strain model for the adsorption–desorption of CH₄/N₂ from coal was established by considering the expansion stress of adsorbed gas on the coal matrix, the compression stress of free gas on the coal matrix, and the expansion stress of free gas on micropore fractures. N₂ has good injectability in low-permeability coal seams and has the dual functions of improving coal seam permeability and enhancing gas flow, which can significantly improve the effectiveness of low-permeability coal seam gas control and promote the efficient utilization of gas resources.

In deep underground mine engineering, the critical warning signals before the sudden failure of coal are crucial to predict coal or rock dynamic catastrophes and to help the coal industry grow sustainably. Therefore, with the objective of accurately identifying the precursor signals of coal fracture, a uniaxial compression test was adopted. Tests were performed on multiple sets of raw coal samples, and acoustic emission (AE) technology was used to capture the deformation and destruction courses of the coal samples. Furthermore, the signal intensity of AE energy was discussed. Based on the critical slowing down theory, the AE energy sequence was processed. The results indicate that there are significant discrepancies in the strength of coal affected by initial pore fissures. During the whole loading process, the AE energy signals showed obvious stage characteristics, and there was a high risk of rapid coal energy storage during the unstable rupture development (URD) stage, which predicted the imminent destruction of the coal. The variance mutation point that was not affected by the lag step selection was easier to identify than that of the autocorrelation coefficient, and the precursor points were all in the URD stage, which is more accurate than using the AE cumulative energy curve slope.

Overburden strata fracture evolution is critical to dynamic disaster prevention and gas-relief drainage, so it is important to accurately determine the evolution relationships with mining disturbance. In this paper, experiments and numerical simulation were adopted jointly to characterize the time-varying fracture area of overlying strata. The experimental results showed that the roof strata gradually broke and collapsed with coal mining, which indicated the fractures of overburden strata developed in an upward direction. The fracture development causes were explained by numerical simulation, which showed that stress increase exceeded the strength of coal and rock strata, and fractures were formed and expanded. Both experiments and numerical simulation results showed the two sides and the top of fracture areas provided channels and spaces for gas migration and reservoir, respectively. In addition, the breaking angle of overburden strata and the height of fracture areas were analyzed quantitatively. Through microseismic monitoring at the mining site, the fracture scales and ranges of overburden strata were verified by the energy and frequency of microseismic events, which were consistent with the support of maximum resistance. The position of drainage boreholes was considered based on the results of overburden strata fracture evolution. Our study is aimed at promoting coal mining in safety and improving gas drainage with a sustainable approach.