Investigation of temperature effect on permeability of naturally fractured black coal for carbon dioxide movement: An experimental and numerical study

Understanding the combined effect of stress, pore pressure and temperature on methane permeability is crucial to hazards detection and mitigation in deep coal mining. It is well known that mine temperature increases with mining depth and methane permeability decreases correspondingly. Methane extraction before coal mining lowers mine temperature and thus enhances permeability near working faces. On the other hand, coal seams can reach yield deformation more easily and even induce stress sharp drop or failure in higher temperature environments. The combined effect of stress, pore pressure and temperature might easily trigger a rapid enhancement of permeability near working faces or even coal and gas outbursts. Therefore, understanding this combined effect on methane permeability is a key issue to hazard detection and mitigation. So far, this combined effect has not been well understood in either experimental measurements or numerical simulations. This paper investigated the methane permeability of six gas-containing coal samples in a complete stress–strain process through our thermal-hydro-mechanical (THM) coupled test apparatus. In these tests, coal specimens were taken from the anthracite coals of Sihe colliery and Zhaozhuang colliery within the southeast Qinshui Basin in Shanxi province of China. A self-made ‘THM coupled with triaxial servo-controlled seepage apparatus for containing-gas coal’ was developed for these tests. The evolution of methane permeability in a complete stress–strain process was continuously measured under constant differential gas pressure, constant confining pressure and three constant temperatures of 30, 50 and \(70\,^{\circ }\hbox {C}\). These experimental results revealed that: (1) Higher temperature had lower compressive strength and lower limit strain, thus coal seams more easily failed. (2) The evolution of methane permeability of coal heavily depended on stress–strain stages. The permeability decreased with the increase of deviatoric stress at the initial compaction and elastic deformation stages, while it increased with the increase of deviatoric stress at the stages of yield deformation, stress sharp drop and residual stress. (3) Temperature effect on the permeability of coal varied with deformation stages, too. This effect was significant before yield deformation, where higher temperature caused lower permeability, but not important after yield deformation, at which the development of coal cracks became dominant. These observations and measurements are helpful for the design of hazards detection and mitigation measures during coal mining process.

https://link.springer.com/content/pdf/10.1007%2Fs11242-013-0202-6.pdf

Combined Effect of Stress, Pore Pressure and Temperature on Methane Permeability in Anthracite Coal: An Experimental Study

Coal and rock mass constitute a type of porous medium. This study investigated the influence of temperature impact on the mechanical properties and acoustic emission (AE) characteristics of coal. A mechanism analysis was performed from the perspective of microstructure. The results show that the temperature impact causes the development of pores and cracks in the coal, which reduces the strength of coal. The elastic modulus of coal generally decreases with increasing temperature gradient. AE parameters increase with the increase in the load and reach the maximum value at the peak stress. AE parameters and cumulative parameters decrease with increasing temperature gradient. Not only does temperature impact change the fracture structure of the coal surface, but also the internal fracture structure of the coal is significantly affected. After temperature impact, the cracks expand and new cracks are initiated, and the fracture volume of the coal increases. Temperature impact causes the volume and specific surface area of small pores and meso-pores in coal to increase, and promotes the opening of the necking pores within the coal. The impact causes macro-pores to penetrate through to form cracks, which increases the transport of coal gas and significantly improves the permeability of coal. The thermal stress generated by coal under temperature impact is greater than its tensile strength, which promotes the cracking of coal, along with the initiation, widening, extension, and expansion of crack networks, which significantly change the fracture structure of coal. The research results lay a certain theoretical and experimental foundation for further study of mechanical properties of coal affected by liquid nitrogen.

Mechanical and Acoustic Emission Characteristics of Coal at Temperature Impact

Structural changes in coal at elevated temperature pertinent to underground coal gasification: A review

Investigation of gas pressure and temperature effects on the permeability and steady-state time of chinese anthracite coal: An experimental study

Transport in porous media with chemical and thermal effects is a common phenomenon; it is also a complicated scientific problem with applications in the field of mining engineering. In situ pyrolysis for coal gas generation is just such a problem, involving material and structural changes in the coal and surrounding rocks, with massive thermal and chemical effects. The transport properties of the coal are substantially changed, which in turn affect the thermal and chemico-mechanical reactions. A series of laboratory experiments on pore structure and permeability changes during gas coal pyrolysis were carried; the experimental procedure and results are described and analyzed in this study. The pore volume and permeability of tested specimens experienced modest changes during the heating process from 20 to 300°C, but when heated from 300 to 400°C, large pores in the specimens greatly increased and the overall porosity reached 23% at 400°C, which is larger than the percolation threshold value of the rock mass with pores and cracks. The permeability of the specimens increased exponentially with temperature, evidencing the massive structural changes that took place in the specimens during the pyrolysis process. In the high temperature range from 400 to 600°C, fewer changes in the specific surface area of microscopic and small pores in the coal took place, but the pore volume and porosity increased linearly with temperature.

Qu Fang

Papers

Experimental Investigation on Correlation Between Permeability Variation and Pore Structure During Coal Pyrolysis