In coals, in rocks associated with coal (sandstones), and in salt strata there is often gas at high pressure (methane, carbon dioxide).In mining these rocks catastrophic phenomena in the form of sudden bursts occur in which gas release from the coal (rock) is accompanied by breakage and transfer of considerable masses of coal and gas into the goal (tens or even thousands of tons of coal and sometimes many hundreds and thousands of cubic meters of gas).In order to anticipate sudden coal and gas outbursts and to understand processes causing them, it is necessary to know and determine correctly the initial condition of the gas in seams which are undisturbed by mining. There is extensive information in the literature on questions connected with measuring gas pressure in coal seams [1][2][3][4].Gas is contained in coal seams in the free state in pore space and in solution (sorbed) within the coal itself. At constant temperature the amount of sorbed gas depends on its pressure and with an increase in pressure it rapidly tends toward saturation.Gas content in coal is estimated as 15-20 kg/m 3, and it may reach 25-30 kg/m 3 (density of methane under normal conditions 0.72 kg/m3). A considerable part of this gas is in the sorbed condition. According to laboratory test data a typical sorption isotherm for gas (methane) by coal has the form [3] (Fig. i):where M is mass of gas in a unit volume of coal (the volume is calculated from the mass of coal taken for the test and its density) at pressure p; b is mass of gas in a unit volume of coal with p ~ | (typical value of b is 10-20 kg/mS); a is a constant having a dimension inverse to pressure [typical value of a -1 (MPa)-I]. The saturation pressure for sorption p depends on temperature and it is normally within the limits 3-4 MPa.Concerning the pressure of free gas contained in coal pores, there is as yet no single opinion about its amount and the structure of the pore space itself.In addition, there are instructions in [5] determining the process for measuring pressure and mass of gas in an intact coal seam, and many such measurements are carried out. The method adopted for measuring gas pressure is based on the assumption that in coal sesm~ still intact from mining there is permeability.There is a suggestion that coal pores are always joined to each other} and although the permeability is very small it always differs from zero. Measurements [3] give a value for the permeability in coal of the order of I0"~-10 -3 D (i D = i0 -~ cm2). This is very small compared with the permeability of gas producing strata (i0-I0 s D).A suggestion has been made that with the presence of initially stable filtration capacity coal seams, as for other rock strata exhibiting a stable filtration capacity, would be in communication with water-lmpregnated strata, so that gas pressure and strata would be hydrostatic.However, measured data by no means confirm this.If pores in an isolated coal seam communicate with each other, then the initial gas pressure should be the same throughout the seam. This a...

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For correct understanding of processes occurring in coal seams durin E their development the question of the filtration capacity of coal seams is of primary importance.Currently it is possible to assume t_hat there is general acceptance of the point of view in which a coal seam unbroken by mining exhibits stable, although very amall (of the order to i0 "~ mD), natural permeability. The main argument which is advanced in favor of this conclusion is experience of measuring Eas pressure in coal seams by means of boreholes. Shown in Fig. 1 is a diagram of such tests taken from [i]. A borehole crossing a seam is drilled from the worklng through the thickness of the surrounding rock alon E the normal to the seam (or at an angle). For pressure measurement the borehole is sealed so that in the section passing throuEh the coal seam there is an unsealed section (measurement chamber) which is connected to a manometer through a tube. In addition, there is a valve to open the measurement chamber to the atmosphere.In order to measure Eas pressure, after drillin E and sealing the borehole, the valve is closed. Gas from the seam enters the measurement chamber, the pressure gradually increases and reaches a steady value. This pressure is assumed to be gas pressure in the coal seam. If after the gas pressure steadies the valve is opened dlscharglnE the pressure in the measuring chamber to the atmosphere, and then the valve is closed again, after a certain time, the pressure steadies aEaln at almost the prevlous value. The procedure for openin E and closing the valve may be repeated several times, and each time the same pressure will be established. This apparently indicates that gas enters the borehole from an unllm~ted volume of seam.530

In connection with the depletion of large and avail able deposits of hydrocarbon raw materials, the devel opment of nontraditional sources acquires increas ingly larger topicality. Methane, which is contained in gas saturated coal seams, is a promising local energy source and high quality and environmentally pure energy carrier.The main problem of recovery of methane from coal seams is the low natural permeability of the coal massif [1]. To increase the permeability, a system of fil tration channels should be formed in it. Currently, the leader of recovery of gas from coal seams is the United States, where the technology developed for shale gas and shale oil and based on multiple hydraulic fractur ing of the seam is used for this purpose [2]. In addition, the methods of hydroimpulsive effect and shock wave effect, the method of formation of instantaneous repressions on the seam, etc., are known [3].All these methods of intensifying the inflow have been developed for oil and gas collectors and trans ferred to coal seams. However, there is a distinction in principle between them-both for the conditions in which the hydrocarbon feedstock is contained in the seam and for the deformation strength and filtration properties of seams. Currently, there is not even one effective technology developed specially for the recov ery of methane from coal seams and taking into account their specific features. This is also referred to the technology of multiple hydraulic fracturing the coal seams from horizontal wells. The gas from the seam, after hydraulic fracturing is performed, can enter the hydraulic fracturing crack only from the region where coal cracked under the effect of tangential stresses, and an artificial system of filtration channels is formed. Allowing for the high plasticity of coal, this zone is small, and the gas debit from the well will rapidly drop as it is "emptied" and will stop rather rapidly.A new approach to the creation of an effective recovery technology of gas from coal seams, which dif fers in principle from the technology of multiple hydraulic fracturing of the seams, is proposed. It is based on the idea of coal cracking and formation of induced permeability in it due to the use of the energy of gas arranged in coal.As far back as in the 1980s, Academician Khris tianovich put forward the statement that the gas in a coal seam is arranged not only in the adsorbed state but also in a free state in isolated pores and cracks and its pressure is close to the local rock pressure [4]. With nonuniform unloading of the rock pressure from the seam, this gas expands and forms a system of oriented cracks in the direction perpendicular to the direction of the smallest compression. Here, we should note that calculations show [5] that the pressure in cracks during their growth falls rather slowly. This is explained by the fact that along with an increase in the crack size, their flattening occurs simultaneously, due to which their volume increases insignificantly. Thus, a system of ori ented gas filled cracks is form...

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