The geomechanics of gas recovery from coal seams

Abstract

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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