Assessment of the permeability of Vryheid formation sediments.
Venter, Bernardus Jacobus.
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Permeability is that physical property of a porous medium that controls the flow of fluids through that medium. The flow of methane and water may be induced by the excavation of a mine opening in methane-bearing strata. Methane flow into a mine opening constitutes one of the biggest hazards in the coal mining industry. It is poisonous to humans and can ignite at concentrations as low as 5 % per volume and create explosions in the presence of coal dust from mining. If the flow of methane and/or water into the mine opening becomes blocked by an impervious layer, excessive pressures may develop, particularly in the roof strata of the mined seam, which can lead to roof falls. In order to characterize the flow of methane and water into and around the openings in a mine, that was plagued by roof falls suspected of being the result of excessive fluid pressure build-up, a large scale laboratory investigation of the permeability of the roof sediments of the working coal seam in the area was undertaken. The permeability was measured under atmospheric conditions by means of a modified Ohle permeameter, and under triaxial conditions with the aid of a modified Hoek cell. The permeability of the sediments towards methane and water was measured. Nitrogen was used as a control because it is much less reactive than methane towards the sediments used in this project. It was found that the permeability decreases with increasing gas pressure, in the case of gas being the permeating fluid, and increased with increasing water pressure, in the case of water being the permeating fluid. In some instances anomalous plots of permeability versus reciprocal mean gas pressure were obtained. These were attributed to the effects of methane adsorption or the Klinkenberg effect, and a possible method to determine which of the two processes is dominant is discussed. To characterize the flow in the roof strata of the coal seam being mined, the permeability was correlated to fades type. The different fades types were numbered from 1 to 14 with increasing grain size for ease of correlation. Due to the variable nature of the sediments, even in a fades type, no single permeability could be obtained for a fades type. Instead permeability ranges were obtained for each fades type. The definition of the lower and upper limits for each range were found to be dependant on the number of tests done on samples for that fades type. Nonetheless a relationship of increasing permeability with increasing grain size was found in the coarser grained fades (facies type 8 and higher). For the fIner grained fades types the permeability was found to decrease with increase in grain size. A graph could be constructed for use in predicting possible hazardous zones by identifying the fades type and then reading the permeability range that can be expected off the graph. Due to the variable nature of the sediments, the graph is, at this time, only applicable to the areas where the samples were obtained. A permeability prediction graph for all localities would be an ideal but is beyond the scope of this project. Such a graph, and the methods discussed have a wide range of applications in the coal mining and methane gas exploitation industries.