Abstract

Excessive development of methane and water pressures were believed to be a contributory cause of roof fall in a coal mine in the Eastern Transvaal Coalfield. Accordingly, a laboratory investigation of the primary permeability of the strata enclosing the coal seam concerned was undertaken. The principal aim of the investigation was to see if there was a relationship between the various facies of the coal bearing strata and changes in permeability. In this way it may be possible to determine likely zones of pressure build-up.

Permeability testing was carried out on drillholesamples using modified versions of the Ohle cell, the Bernaix cell and the Hoek triaxial cell. The use of the Hoek cell allowed testing to be carried out under different stress conditions. Nitrogen, methane and water were used as the permeating fluids. The liquid equivalent permeability of each sample was determined for the two gases. The permeability tended to decrease with increasing pressure when the two gases were used as the permeating fluid but not when water was used. In addition, the permeability was influenced by the grain size of the material tested and by the direction in which it was tested, that is, along or across the bedding. Hydraulic fracturing occurred in some specimens when tested at high water pressures in divergent flow in the Bernaix cell. An increase in hydrostatic pressure, confining pressure or axial pressure gave rise to a reduction in permeability in triaxial testing, the former giving rise to the greatest and the latter the least decrease. Staged loading showed that the greatest reduction in permeability occurred during initial loading. On cyclic loading the liquid equivalent permeability decreased with increasing applied stress and then increased with subsequent reduction in stress.

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