Changbao Jiang

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

A New Experimental Apparatus for Coal and Gas Outburst Simulation

70\,^{\circ }\hbox {C}

Fluid-solid-heat coupling triaxial servo percolation device for gas-contained coal

70∘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.

Geomechanical and flow properties of coal from loading axial stress and unloading confining pressure tests

Experimental coal cores were collected from a coalbed of Sihe colliery and Zhaozhuang colliery, Qinshui Basin, China. Their gas effective permeability was studied under effects of water content and effective stress. The experiments were mainly carried out on a self-made “Triaxial Stress Thermal–hydrological–mechanical Coal Gas Permeameter.” The results showed that when the temperatures of gas and coal were constant, a negative effect of either water content or effective stress was reported on the gas transportation, i.e., the gas effective permeability decreased with the increasing of water content under constant effective stress and it also decreased as the effective stress increased when the water content was constant. Under experimental conditions as in this study, the effects of water content and effective stress on the gas effective permeability was described by a linear–exponential equation, which presented that the gas effective permeability had a linear relationship with the water content and an exponential relationship with the effective stress. The permeation pores were defined as the primary places of transporting the coalbed gas. They were affected by water content and effective stress in different ways. The water content occupied the space of permeation pores, while the effective stress changed the shape of permeation pores. Consequently, the gas effective permeability was also affected by the two aspects.

Coal-rock hydraulic fracturing testing method under true triaxial state

Instantaneous coal and gas outbursts are dynamic underground mining phenomena in which coal and gas are suddenly and violently ejected from a coal face (Shepherd et al. 1981). Since March 22, 1843, when the first recorded coal and gas outburst happened in the Isaac mine, Zeroual coalfield, France (Lama and Bodziony 1998), experts have conducted extensive investigations on these outbursts. However, understanding and predicting such a disaster has achieved only limited progress in the intervening years (Chen 2011). Field data of coal and gas outbursts are rarely captured; thus, experts from different countries have worked on mathematical models or explored the mechanisms for coal and gas outbursts (Litwiniszyn 1985; He and Zhou 1994; Frid 1997; Beamish and Crosdale 1998; Valliappan and Zhang 1999; Li 2001; Cao et al. 2001, 2003; Alexeev et al. 2004; Xu et al. 2006; Aguado and Nicieza 2007; Wold et al. 2008; Zhang and Lowndes 2010; Chen 2011; Xue et al. 2011). For example, a porous rock is composed of a skeleton and pores. Its skeleton is made up of solid particles and its pores form a system of capillaries filled with gaseous substances. This model of a porous medium suggests a domino effect that leads to a catastrophic failure, or outburst, of the coal. The occurrence of an outburst depends on the local stress state, gas content, and physicomechanical properties of the coal. Such models or mechanisms lack sufficient experimental evidence. Coal and gas outbursts are so devastating that field trials are almost impossible. An alternative is to simulate the dynamic phenomenon of a coal and gas outburst in the laboratory with a specially designed apparatus. Some efforts have been made to develop an experimental apparatus for coal and gas outburst simulations (Meng et al. 1996; Guo et al. 2000; Cai 2004). However, their designs for the apparatus had some limitations, such as using small coal samples and simulating only horizontal coal seams. A key limitation is that all outburst caverns are manually opened. These limitations may significantly affect the experimental results. For instance, a smaller coal sample may have a greater boundary effect. Outburst accidents occur more frequently in dip coal seams, especially in rock cross-cut coal uncovering, which refers to the excavation process of rock cutting just before the exposure of the coal seam. Manually opening the outburst cavern may be too slow. This may reduce the intensity of coal and gas outbursts in the tests. In addition, the current apparatus cannot simulate the local concentration of stress in front of the coal face because it can only apply a uniformly distributed load on coal samples through a single loading plate. Finally, the current apparatus cannot simulate gas infiltration from the entire cross-section because the coal samples only have a single inflatable hole. & Changbao Jiang jcb@cqu.edu.cn

A Novel True Triaxial Apparatus to Study the Geomechanical and Fluid Flow Aspects of Energy Exploitations in Geological Formations

The deformation and fracture characteristics of shale in the Changning-Xingwen region were experimentally studied under triaxial cyclic loading with a controlled pore-water pressure. An RLW-2000M microcomputer-controlled coal-rock rheometer was used in the State key Laboratory of coal mine disaster dynamics and control in Chongqing University. These experimental results have indicated the following. (i) The shale softened after being saturated with water, while its failure strength decreased with the increase of axial strain. (ii) A complete cyclic loading–unloading process can be divided into four stages under the coupling action of axial cyclic loading and pore-water pressure; namely the slow or accelerated increasing of strain in the loading stage, and the slow or accelerated decreasing of strain in the unloading stage. (iii) The axial plastic deformation characteristics were similar when pore-water pressures were set to 2, 6 and 10 MPa. Nevertheless, the shale softened ostensibly and fatigue damage occurred during the circulation process when the pore-water pressure was set to 14 MPa. (iv) It has been observed that the mean strain and strain amplitude under axial cyclic are positively correlated with pore-water pressure, while the elastic modulus is negatively correlated with pore-water pressure. As the cycle progresses, the trends in these parameters vary, which indicates that the deformation and elastic characteristics of shale are controlled by pore-water pressure and cyclic loading conditions. (v) Evidenced via triaxial compression tests, it was predominantly shear failure that occurred in the shale specimens. In addition, axial cyclic loading caused the shale to generate complex secondary fractures, resulting in the specimens cracking along the bedding plane due to the effect of pore-water pressure. This study provides valuable insight into the understanding of the deformation and failure mechanisms of shale under complicated stress conditions.