Using methods of the scanning electron microscopy, Raman scattering of light(RS), and electron paramagnetic resonance (EPR), consistent research of the local structure and magnetic features of different types of raw coal samples from Donetsk basin is carried out. It is established that the ratio of the main peak intensities of RS spectrum D and G is inversely related to the volatile substance amount Vdaf in the coal samples. The study of the kinetic behavior of the EPR line width in hydrogen, oxygen, and methane sorption-desorption processes in each coal sample helped determine that the diffusion coefficient value for hydrogen in coal at room temperature is equal to DН = (2 ÷ 7) × 10−5 cm2/s. It is demonstrated that the oxygen diffusion occurs with time according to two different exponential laws with diffusion coefficients DO,1 = 5 × 10−6 cm2/s and DO,2 = 5.5 × 10−7 cm2/s, respectively. The smaller coefficient corresponds to the diffusion caused by the hopping process. Finally, it is established that the anthracite is a unique type of coal which does not possess the ability “to conserve” the significant EPR line width after oxygen pumping out from the samples.

Experimental results of deformation and fracture of coals under true triaxial compression at various stress states as defined by the Nadai-Lode parameter obtained using the original true triaxial loading (TTAL) apparatus developed at the Institute for Physics of Mining Processes NAS Ukraine are presented. In particular, alteration in the mechanical performance of coals in different stress states, effect of the stress state on methane emission kinetics from coals at various temperature regimes and effect of moisture content on the mechanical performance of coals in different stress states are discussed.

Kinetics of gas desorption from a material with a developed nano- and mesostructure is studied by the example of methane in coal. It is supposed that the solute gas passes from coal fragments to capillaries by solid-state diffusion and then proceeds to the outer volume by filtration. Desorption is found to occur in three stages. Estimations of the duration of these stages are given. Comparing the sorption experiments with the theory developed here is the suggested way to evaluate the size of the fragments comprising the system, as well as the closed porosity and gas solubility in the studied porous material.

Институт физики горных процессов НАН Украины

Experimental results on rock deformation and fracture under true triaxial compression have revealed a misfit between strain state and stress state, strain state varying from generalized compression to generalized shear at σ3 ≠ 0. This misfit can lead to data misinterpretation during the stress field reconstruction after unloading. Fracture of rock specimens under true triaxial compression occurs by a combined longitudinal/transverse shear and produces the highest dilatancy. An increase in the hydrostatic pressure level diminishes limiting values of shear strains and suppresses the dilatancy effect. A maximum of dilatancy coincides with a maximum of fresh surface area formed during the fracture of the rock. The generalized cleavage of rocks becomes energetically disadvantageous in a true triaxial compressive stress field. Some sandstone becomes more brittle under true triaxial compression (σ2 ≠ 0) at low values of the minimal stress component (σ3) due to high initial porosity and dilatancy.

We study the desorption of methane from a coal-bed. A model taking into account both methane diffusion in coal – blocks and its filtration through the system of open pores and cracks is developed. Methane pressure in the coal-bed is found for an arbitrary instant of time. Dependency of the rate of methane release upon the block size, open and closed porosity, viscosity, solubility, bed pressure and temperature is established. We derive the effective coefficient of diffusion of methane in blocks containing closed pores filled with gaseous methane. It is shown that at a hindered diffusion methane is distinctly divided into the “quick” and the “slow” one.

Methane desorption from a coal seam is theoretically investigated using a model including both the diffusion of methane in coal lumps and its filtration through net-shaped pores and cracks. The methane density distribution along the seam at an arbitrary time instant is found. Explicit dependences of the amount of the methane escaped from the seam on the lump size, open and closed porosity, viscosity and solubility of methane, and pressure and temperature in the seam are determined. An effective diffusion coefficient in lumps containing methane-filled closed pores is found. In the case of hindered diffusion, the methane can be subdivided into the "fast" and "slow" fractions.

Low temperature phase transitions in water and methane occurring in fossil coals were studied experimentally using Nuclear Magnetic Resonance (NMR) techniques. Contributions of constituent fluids into narrow line of 1H NMR wide line spectrum were analyzed.

The dynamics of hydrogen release from metals to the closed volume at a fixed temperature is studied. The surface lag-time of hydrogen depending on both the reaction of association of hydrogen atoms into a molecule and the presence of impurity barriers at the surface is taken into account by means of introduced coefficient of boundary permeability. It has been shown that the time dependence of hydrogen pressure is characterized by three stages. These are the linear stage, the square-root stage and the approaching of equilibrium according to the exponential law. The model allows to reveal the predominant mechanism influencing hydrogen release on the base of the form of the kinetic curve.

Two generations of true triaxial loading (TTAL) apparatuses are presented. First generation apparatuses were intended primarily for true stress state imitation in rock or mineral specimens. Advanced second-generation installations are designed to provide precise measurements in any stress and simulation of rock outburst at sudden relief of one sample face. Both TTAL apparatuses can apply pressure up to 250 MPa corresponding to earth depth about 10,000 m independently along each of three axes. Experimental results are given on effect of absorbed water on ultimate state in coal as well as adsorbed methane influence on simulated coal outbursts.

NMR measurements have revealed that methane can exist in coal samples in the state of solid solution rather than only adsorbed gas, opening new ways to prevention of gas dynamic accidents in underground coal mines and true estimation of coalbed methane resources.Understanding molecular structure of coal constituents and forms of methane occurrence in coal is the only way of extracting safely either coal or methane. We had studied nuclear magnetic resonance lines in various coals at room or low temperatures and have found that there exist three species of methane molecules differing in molecular mobility. Based on estimated diffusion parameters, these species were attributed to free methane, adsorbed methane, and solid solution of methane in crystalline coal substance. While first two phases are well known and can be analyzed by many different techniques, the last one hardly can be studied by methods other than NMR, resulting in inadequate estimations of methane resources. (C) 2004 Elsevier Ltd. All rights reserved.

It is established experimentally using 1H NMR wide line spectroscopy that methane can exist in coals not only in open or closed porosity and fracture systems but also in solid solutions in coal substance, in particular, under methane pressure 2 MPa or higher. Methane dissolved in coal minerals reversibly modifies their lattice parameters as determined from X-ray diffraction analysis. Co-existence of these methane forms in fossil coals causes multi-step desorption kinetics. It is shown experimentally that the long-term latent methane desorption is effected mainly by closed porosity, which in turn is determined by coal rank.

The kinetics of gas molecule diffusion is considered in a solid with pores. The results obtained describe the effects of porosity and gas solubility on the gas pressure alteration in surrounding closed volume. Estimations have been made for the case of methane absorption by coal substance.

The kinetics of gas molecule diffusion is considered in a solid with pores. The results obtained describe the effects of porosity and gas solubility on the gas pressure alteration in surrounding closed volume. Estimations have been made for the case of methane absorption by coal substance.

The presence of closed porosity in fossil coals is shown. The technique of definition of closed porosity is developed. Volumes of closed pores for coals of the various sorts are determined experimentally. It is established that the contribution of closed pores in total porosity in most cases exceeds 60%. The tendency of closed pore volume to increase in outburst-prone coals is found out.

A need for the experimental estimation for the mechanical properties of fossils with the depth arises due to the trend of mining at the deeper levels. However, the experimental studies on the mechanical properties of rock, and especially the coal impose a great problem. These problems are due to the great variability of rock properties and the necessity to reproduce the loading conditions close to the mining ones, i.e. the conditions of three-axial nonequicomponent loading as well. The variations in mechanical properties of coal with lying depth were determined for depths from 800 to 3000 m with the use of an unique plant for the three-axis nonequicomponent compression (USSR patent No 1285340). This plant provides the independent loads along three axes in a closed chamber. It is found that the increase in lying depth leads to an enhancement of the rigidity and ultimate strength of coal. Also there arises a trend towards the more viscous nature of destruction. Some features of the variations of mechanical properties of coal with depth appeared to be due to some physical acid chemical processes taking place when a certain level of spherical tensor of stress is reached.