Coal and gas outburst disasters in coal seams are becoming more serious as coal mines extend deeper underground in China. Damage evolution has affected gas flow in coal seam greatly, which also controls gas permeation and gas extraction results, and finally, it has tremendous influence on prevention and control of gas disaster accidents. In this paper, the cracking process of coal under compressive stress condition and the permeability variation during the whole process is experimentally studied with the aid of 3D acoustic emission monitoring system. The experimental results suggest that the coal failure and degradation of mechanical properties are essentially related to the propagation and coalescence of induced cracks. Using a damage tensor defined by the direction-dependent crack density, the damage evolution during the whole loading process is studied. It is revealed that the damage evolution is mainly initiated with the appearance of microcracks and accelerated in the post-peak region. During the post-peak region, a similar increase tendency of damage and permeability is noticed, and therefore, a post-peak permeability model is proposed considering the influence of damage evolution on the permeability variation in the post-peak region. Finally, we analyze the gas outburst hazard with coal mining. There exists a transition zone around the vertical stress concentration, and energetic failure may result in this transition zone.

Effect of damage evolution of coal on permeability variation and analysis of gas outburst hazard with coal mining

The evolution and formation mechanisms of closed pores in coal

A fully coupled electromagnetic-thermal-mechanical model for coalbed methane extraction with microwave heating

A thermally sensitive permeability model for coal-gas interactions including thermal fracturing and volatilization

Complex thermal coal-gas interactions in heat injection enhanced CBM recovery

Direct observation of coal-gas interactions under thermal and mechanical loadings

Gas migration in coal bed is a multiple-physical process, of which not only includes gas desorption/diffusion through coal matrix and gas Darcy flow through the cleat system, but also results in deformation of solid coal Especially for enhanced coal bed methane (ECBM) and CO2 capture and sequestration (CCS), gas injection is mainly controlled by the gas diffusivity in the coal matrix and coal permeability Although the relevant coal permeability models have been frequently developed, how the dual-porosity system of coal affects gas adsorption/diffusion is still poorly understood In this paper, a series of experiments were carried out in order to investigate deformation evolution of intact coal subjected to hydrostatic pressure of different gases (including pure H2, N2 and CO2) under isotherm injection In the testing process, the coal strain and injected gas pressure were measured simultaneously The results show that the pressure of non-adsorptive helium remained unchanged throughout the isothermal injection process, in which the volumetric strain of the coal shrinked firstly and maintained unchanged at lower isobaric pressure With the injected pressure increasing, the coal volume underwent a transition from shrinking to recovery (still less than initial volume of the coal) In contrast, N2 injection caused the coal to shrink firstly and then recover with decreasing gas pressure The recovery volume was larger than the initial volume due to adsorption-induced swelling For the case of CO2 injection, although the stronger adsorption effect could result in swelling of the solid coal, the presence of higher gas pressure appears to contribute the swelling coal to shrink These results indicate that the evolution of coal deformation is time dependent throughout the migration of injected gas From the mechanical characteristics of poroelastical materials, distribution of pore pressure within the coal is to vary with the gas injection, during which the pore pressure in the cleats will rapidly increase, in contrast, the pore pressure in the matrix will hysteretically elevate Such a difference on changes of pore pressure between the cleats and the matrix will contribute to the shrinkage of the matrix as a result of initially greater effective stress Besides, both gas-adsorption-induced swelling and decreasing effective stress also control the coal deformation transition This work gives us an insight into investigation on influence of effective stress on coal-gas interaction

Deformation transition of intact coal induced by gas injection

The invention discloses a rock core clamp holder capable of simulating complex migration process of formation fluid and a using method of the rock core clamp holder. The clamp holder is a closed combined body formed by an axial compression plunger, a clamp holder barrel body and a pressure-resistant hose embedded in a rock core, wherein the tested rock core is processed to be of a hollow cylindrical shape, the pressure-resistant hose penetrates into a pore passage inside the rock core, the axial compression plunger is arranged at two ends of the rock core, three metering pumps are respectively arranged to be communicated with the axial compression plunger to apply the maximum principal stress to the rock core, are communicated with an inner annular pressure cavity of the clamp holder barrel to apply intermediate principal stress to the rock core, and are communicated with the pressure-resistant hose to apply the minimum principal stress to the inner pore of the rock core; one metering pump is additionally arranged to be communicated with the pore passage deviating from the axis of the plunger to inject the fluid into the rock core; the pressure and the flow rate of pressure transmitting fluid in the liquid chambers are controlled to realize the functions of controlling the stress and the deformation of the rock core. The rock core clamp holder provided by the invention can realize gas/liquid seepage of pores of the rock core under a triaxial stress state and also can meet the rock core deformation and the permeability test during stress relieving.

Rock core clamp holder capable of simulating complex migration process of formation fluid and using method of rock core clamp holder

Clarifying the creep behaviors of hydrate-bearing sediment (HBS) under long-term loading is crucial for evaluating reservoir stability during hydrate exploitation. Figuring out a way of characterizing deformation behaviors and their geophysical responses to HBS is the basis for modeling creep behaviors. In this study, we propose a novel device to test time-dependent deformation and the ultrasonic response of HBS under high-pressure and low-temperature. The experimental device consists of a high-pressure chamber, an axial-load control system, a confining pressure system, a pore pressure system, a back-pressure system, and a data collection system. This testing assembly allows temperature regulation and independent control of four pressures, e.g., confining pressure, pore pressure, back pressure, and axial loading. Columned artificial HBS samples, with a diameter of 39 mm and a height of 120 mm, can be synthesized in this device. Afterward, in situ creep experiments can be achieved by applying stable confining pressure and axial load, together with geophysical signals acquisition. During loading, the stress-strain relationships and ultrasonic data can be obtained simultaneously. Through analyzing the stress-strain relationship and ultrasonic data, the macroscopical failure and microcosmical creep deformation law of the samples can be figured out. Preliminary experiments verified the applicability of the device. The method provides some significance for field observation of reservoir failure via geophysical techniques during hydrate exploitation.

Bin Gong

Papers

Direct observation of coal-gas interactions under thermal and mechanical loadings

Deformation transition of intact coal induced by gas injection

Rock core clamp holder capable of simulating complex migration process of formation fluid and using method of rock core clamp holder

Integrating test device and method for creep failure and ultrasonic response of methane hydrate-bearing sediments.

Coupling Model of Submarine Deformation Response Prediction during Methane Hydrate Exploitation