Measurement of gas pressure in coal seams

Abstract

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...

Gas emission into the excavated space during the mining of coal seams

“…Figure 2 shows the graph of variation of p(x,t) in the filtration region (0, l,) at time T = 600min (curve 1). Here we also display the graph (curve 2) of variation of p(x, l) obtained by another method in [2,3]. It can be seen from Fig.…”

“…Curve 1 corresponds to the solution of the boundary-value problem (1), (3), curve (2) to the case when the linearized filtration equation is considered instead of Eq. (1).…”

Study of methods of hydrocarbon recovery enhancement from the depleted oil fields

“…Among the methods of oil recovery enhancement which are characterized with the most effective processes rather than the traditional water flooding, we can be name the http://dx.doi.org/10.7494/drill.2013.30.1.127 method of oil recovery with micellar solvents, carbon dioxide, steam, by means of fireflooding in situ combustion (Chatzis et al, 1988;Safonov & Almaev, 1997;Khristianovich & Kovalenko, 1988;Surguchev et al, 1991;Surguchev, 1985;Yurkiv, 1994;Zhdanov, 2001).…”

The geomechanics of gas recovery from coal seams

“…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.…”