Gas emission under coal and gas outbursts

Abstract

The paper considers the conditions of methane hydrate existence in coal seams. The authors estimate volumes of gas emission in a fractured coal layer upon a drop in gas pressure. The authors also give the assessment of outburst hazard of coal seam edge zone.The analysis of coal and gas outbursts observed across the Kuznetsk Coal Basin from 1975 through 1982 show that the gas emission under outbursting significantly exceed the total free and desorbed gases, with taking into account natural gas content in the analyzed mines. We have previously proved that only 38.5% gas ("free" gas) can be extracted from a seam, whereas 61.5% gas is connected with the inner surfaces of coal pores [1]. The large forces between gas molecules and coal surface can result from the high inhomogeneity of polarizing electric field, or a special form of the gas molecular bond with pore water in the pore space. These compounds are clathrates, that is, the "pore water-methane-coal matrix" system can exist as the gas hydrates, given specific thermodynamic conditions. The present paper considers the possible physical conditions for gas hydrates in coal seams. The formulation of the problem lies in calculating the volume of gas emitted during gas hydrate dissociation and estimating the gas contribution to the outburst hazard.In the 1970th, a variety of authors suggested that gas hydrates in coal seams or host rocks can be formed under specific conditions, such as the presence of mixture of hydrocarbons in coal seams, lower temperature in a seam (below 20°С), and increased gas pressure (higher than 6 MPa) [2].In order to explain the effect of gas hydrates on the outburst hazard in coal seams, consider the "methane-water" phase diagram in the pressure-temperature coordinates (Fig. 1). The area of hydrates is limited to the area above the curve. For current mining depths (500-1000 m) in the eastern part of Russia, the pressure is over 5-25 MPa and the temperature is above 7-8°С, which agrees with the diagram in Fig. 1. Fig. 1. The experimental phase curve of the water-methane system [3].

Gas Hydrates of Coal Layers as a Methane Source in the Atmosphere and Mine Working

“…The method 13 C of a nuclear magnetic resonance spectroscopy obtained the data reflecting changes in structure of coals of various extent of metamorphism [13,14]. Several functional groups are allocated, we will consider only those which have electrical dipole moment: CH 3 Comparing the radius of molecules of water and cross sectional dimension of a time, it was received that in micropores (cross sectional dimension up to 0,6 nanometers) there can be only two molecules of water that corresponds to one layer; in the transitional time (cross sectional dimension of 1 -10 nanometer) of 1 -18 layers of molecules of water; in macropores (cross sectional dimension of 10 -100 nanometers) of 1 -180 layers of molecules of water. Considering distribution of a time, for coals of different brands the percent of moisture which cannot share in formation of gas hydrates is calculated.…”

“… crystallohydrate or just gas hydrate of a methane; fragile crystal connections which are usually unstable also exist in a coal matrix under particular thermo dynamic conditions [3]. Fig.…”

Identification of mine water inrush using laser-induced fluorescence spectroscopy combined with one-dimensional convolutional neural network

“…There are ve common types of disasters in coal mines: gas, 1 water, 2 re, 3 coal dust 4 and roof; 5 water disaster has now become the second largest disaster in coal mines. 6 The coal mine site not only needs to provide the early warning of water inrush, 7 but also needs to correctly identify the type of water source the rst time aer the occurrence of water inrush disaster 8 .…”

The research of coal seam gas pressure and initial gas emission characteristics of borehole

“…24 In those fractures and pores, gas desorbs fast and emits quickly into the environment, which further affects the measurement of gas pressure. [26][27][28] The gas emission is not representative and uncorrelated with primitive gas pressure in short time later. 25 In drilling process, with a rapid increase in stress and instantaneous change in gas pressure around the surface of borehole, the desorbed gas emits into borehole constantly.…”

“…However, due to its short duration, this fact is usually neglected by researchers. [26][27][28] The gas emission is not representative and uncorrelated with primitive gas pressure in short time later. Therefore, only the feature of initial gas emission can be an efficient indicator to character the primitive gas pressure in coal seams with the consideration of other parameters.…”