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

The influence of fracture geometry variation on non-Darcy flow in fractures under confining stresses

Correlational fractal characterisation of stress and acoustic emission during coal and rock failure under multilevel dynamic loading

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.

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

Three-dimensional fractal distribution of the number of rock-mass fracture surfaces and its simulation technology

Halite and thenardite,which are the main constitutes in bedded salt rock deposits,are chosen as the experimental samples. The uniaxial compression behaviors of the samples are recorded and analyzed with loading strain rates increasing from 10-5 s-1 to 10-3 s-1. It is found that the ultimate strength and the elastic modulus of the two lithologies hardly change with loading strain rate in the test scale. The uniaxial compressive strength of thenardite is larger than that of halite when they are loaded with the same strain rate. Opposite to the trend of the elastic modulus,the Poisson′s ratios of the two lithologies decrease with strain rate for the halite and thenardite specimens. Similarly,the strains of the test specimens at the point of peak strength decrease with loading strain rate. A logarithmic relationship between deformation modulus and loading strain rate is also established with the test results. During the experiments,it is found that all the halite specimens are broken in the type of brittle tensile fracture around the specimen column face,while single shear failure along an inclined plane in the specimens is the characteristic of thenardite breakage. The damage characteristics of the halite and thenardite hardly change with the loading strain rate. Stress ratio at the point of the onset of volumetric increase to the ultimate compressive strength of the tested specimens is also calculated and analyzed;and an average value of 87.3%–91.0% of the ratio is recorded,which is obviously larger than those of general rocks. The large ratio demonstrates that salt rocks possess large deformation capacity before volumetric increase occurs during compression. It is concluded that a salt cavern wall of strain rate between 10-5–10-3 s-1 during salt cavern storage operations can guarantee the safety and stability of the cavern,while other operational demands are satisfied.

Test study of strain rate effects on mechanical performances of salt rock

Carbon dioxide is one of the main greenhouse gasesGeological disposal of carbon dioxide in coalbeds is regarded as an economical approach because of the mutual benefit of greenhouse gas sequestration and recovery of coalbed methane by replacementIn the past,most of works were focused on either theoretical analyses and numerical simulations,or experiments of adsorption and desorption of different gases on coal grains in size of millimeters or the lessIn this paper,the experimental study results of carbon dioxide storage and methane replacement in coal specimens of large size(100 mm×100 mm×200 mm) are presented,which realistically simulates the process of the greenhouse gas sequestration and methane replacement in coalbedIn the experiment,the permeability of coal specimen is measured firstly with methane and carbon dioxide respectively;and it is found that the permeability of coal is different for the two gasesThe permeability for carbon dioxide is larger than that for methane two magnitudes at least under the testing conditionThere exists a negative exponent relationship between the permeability and applied body stress on the specimenUnder the simulated stress condition in the experiment,1747 to 2800 units of carbon dioxide can be stored in per unit of coal;and the replacement ratio of carbon dioxide to methane is as large as 703 to 1391The processes of injection,adsorption and desorption,replacement,and output of gases can proceed smoothly under the given pressures;the percentage of methane in the production gas can be amounted to 20% to 50% at the early stage and still can be maintained at the level of 10% to 16% even at the last stage during the experiment processIt is concluded that the effects of carbon dioxide storage and methane replacement are determined by several factors including injection replacement pressure,injection amount and velocity,methane content in the coalbed,and the development of pores and fissures in the coalbed,etcDuring the process of carbon dioxide injection and gas replacement,coal body swelling can be found with comprehensive effects of gas adsorption and desorption,and the deformation of coal frameworkThe research is expected to provide certain illuminations for practice of carbon dioxide disposal in coalbed or methane replacement recovery by carbon dioxide in the future

Experimental study of coalbeds methane replacement by carbon dioxide

The hydraulic fracture in rock salt is a complicated solid fluid and mass transfer coupling process. Through theoretical analysis, a solid fluid and mass transfer coupling mathematical model of hydraulic fracture in rock salt is established in this work, and numerical simulations are carried out with the model. The simulation results indicate that rock salt cracks in the typical way of wing-crack (or tensile crack) during the fracture, and the relation of fracture aperture (w) with expanding distance (x) and fracture time (t) is w=(0.0034+0.0006t)e(0.0007+0.0018t)x . Furthermore, it has been found that both the water pressure in the crack and the expanding velocity of the crack decrease gradually as a result of the influence of salt dissolving during fracturing. These numerical simulations well illustrate the process of hydraulic fracture in rock salt and are significantly meaningful in engineering practice.

Liang Weiguo

Papers

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

Three-dimensional fractal distribution of the number of rock-mass fracture surfaces and its simulation technology

Test study of strain rate effects on mechanical performances of salt rock

Experimental study of coalbeds methane replacement by carbon dioxide

A mathematical model for solid liquid and mass transfer coupling and numerical simulation for hydraulic fracture in rock salt