CO2-enhanced coalbed methane recovery (CO2-ECBM) can significantly improve gas extraction efficiency while simultaneously inducing permeability reduction. However, the evolution on permeability during displacement is still indistinct. To examine the evolution of permeability and gas displacement efficiency, a suite of experiments is performed to measure the strain characteristics and gas flow rates of coal samples. A mixed gas permeability model is established, and the advantages of injecting pressure at different phases of displacement are discussed. The finding reveals that the coal exhibits higher deformability in the direction perpendicular to bedding planes. CO2 adsorption significantly reduces permeability. While the displacement process is taking place, the permeability is expressed in the increase and decrease phases, and finally, it displays a stable tendency. Under different pressures, the permeability decreases to 95.9%-98.2% of the initial permeability, respectively. The CH4 flow rate experiences decreases, increases, and sustained decline phases during displacement. A stepwise increase in gas injection pressure during different displacement stages enhances the gas displacement efficiency.
To elucidate the dynamic characteristics of in-situ methane deflagration in coalbed methane wellbores and its mechanisms for fracturing coal rock, this study first developed a simulation experimental system specifically designed for methane in-situ deflagration fracturing. This experimental system, which is capable of withstanding pressures up to 150 MPa and meanwhile applying axial and confining pressures of up to 50 MPa to rock cores, enables the coupled simulation on methane deflagration and rock core fracturing processes. With the aid of this experimental system, physical simulation experiments on in-situ methane deflagration fracturing were conducted, and the following findings were obtained. Methane deflagration loads in enclosed wellbores exhibit characteristics of multi-level pulsed oscillation. With the rise of initial gas pressure, the peak deflagration load increases approximately linearly, with the pressure amplification factor spanning from 23.14 to 31.10, and its peak loading rate grows exponentially. Accordingly, the fracture volume and fracture porosity augment. To be specific, when the initial gas pressure rises from 0.6 to 2.4 MPa, the fracture volume and fracture porosity augment by factors of 14.0 and 8.73, respectively. The fractal dimension of spatial distribution of fractures also increases with the rise of deflagration load, indicating that a higher deflagration load conduces to the development of a larger and more complex fracture network. Methane deflagration fracturing is characterized as a composite fracture mode that involves the impact of strong stress waves and the driving force of high-pressure fluids. The primary factors influencing damage to coal-rock include the high-stress impact in the initial stage of deflagration, the fluid pressure driving effect in the middle stage, and the thermal shock resulting from high temperatures in the later stage.
The heavy section ferritic ductile iron material is prone to graphite degeneracy in the matrix under the influence of cooling rate and Si content, which affects the mechanical properties of the material. In this paper, the Sb element is used to mitigate graphite degeneracy, and the effect of Sb on the microstructure and mechanical properties of heavy section ferritic ductile iron is analyzed in detail. After adding the Sb element, the enrichment of Sb around the graphite effectively inhibits the diffusion of C toward the periphery at the interface, thus making the graphite nodules more spherical. When the Sb content is 0.002
After injecting inert gas into the mine gob, the oxidation-susceptible gaseous environment surrounding coal transforms into an oxygen-deprived state, and then effectively suppressing coal spontaneous combustion. Employing a configurable gas delivery system with rapid source-switching capability alongside real-time electron paramagnetic resonance spectroscopy, this research quantitatively characterizes the dynamic behavior of free radicals during gaseous phase transitions from air-dominated to nitrogen-saturated conditions. Furthermore, the inversion calculation of radicals with different activities in coal was carried out. The results showed that the sudden change in the gas atmosphere significantly impacted the coal radical reaction. Following gaseous environment transition, both g factor and radical concentrations in coal displayed biphasic attenuation patterns - rapid initial reduction followed by gradual stabilization. The g factor and radical concentration variation increased with the temperature increase when the gas atmosphere changed. When the temperature dropped to room temperature, the g factor of the coal was not much different from the initial g factor of the raw coal. At the same time, the radical concentration was significantly higher than the initial radical concentration of the raw coal. The analysis showed that the active radical was rapidly annihilated during the cooling period after the gas atmosphere changed, and the stable radical was retained in the coal after cooling. Through inversion calculation, it was obtained that the concentrations of active radical and stable radical both conform to the exponential type function growth trend with the temperature rising, but the growth of active radical was significantly faster than that of stable radical, which proved that the active radical is mainly involved in the oxidation reaction.
