Use of a model of a fissured porous medium for solving problems of liquid and gas filtration in a coal seam

Abstract

To solve the problems involved in degassing and wetting a coal seam, we must consider it as a filtering medium. Most of the reports in this field disregard the characteristic feature of a coal seam, namely that it is a medium with markedly differentiated porosity and fissuring, which may greatly affect the character of gas and liquid transport.It is usually assumed that a coal seam as a filtering medium may be represented by a modification of classical spherical packing. The particles of such a medium are assumed to be fairly small, so that, allowing for desorption, movement of gas in the seam is regarded as an equilibrium process. Attempts have been made to analyze nonequilibrium desorption, but these were based on the assumption that all the particles of the medium and therefore the filtering pores are of the same size. Such a model is correct only for markedly crumpled coal.Bearing in mind the usual model, the movement of liquid injected into a coal seam is represented as filtration with a mobile boundary under rigid or elastic conditions, or as filtration into an unbounded seam under elastic conditions. In both cases all the filtering pores in the filtration region are understood to be filled with liquid. A uniform increase in the moisture content must therefore occur in the zone of the seam being wetted.Investigations have established that a coal seam contains several systems of relatively large fissures, which split it into blocks, penetrated by small fissures which in turn divide them into smaller blocks, etc. Transfer of liquid and gas in a coal seam takes place via the system of largest fissures. The other fissures and pores serve as channels for the movement of liquids and gases within a block and as channels for ,.xchange between the blocks and the large fissures. In certain cases a crumpled band may serve as a channel of predominant transfer of liquid ar~ zas if its permeability is much greater than that of the remainder of the seam [1].This pattern of transfer of liquid and gas in a coal seam is confirmed, for example, by the following: the decrease in the rate of degassing of a seam by boreholes, drilled across the bedding, as the seam thickness increases; the different types of gas feed channels; the nonuniformity of distribution of the moisture content in the vicinity of wetting boreholes; and the decrease in the mean moisture content with increasing distance from the borehole. The specific structure of the porosity of a coal seam may also explain the effect of vacuum on the degassing rate. A detailed mathematical model of filtration in such a medium, taking account of sorption, capillary transfer, and diffusion, is apparently fairly complex and cumbersome.As a first approximation to the characteristics of a coal seam as a filtering medium, we may use the picture of a fissured porous medium developed in [2,3]. By fissured porous medium we mean a medium which can be represented as a system of porous blocks, separated by a monodispersed system of fissures. Intense exchange of liquid (gas) may occur ...