Prediction of In-Situ Cleaved Coal Permeability
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Prediction of In-Situ Cleaved Coal Permeability T. V. Shilovaa*, L. A. Rybalkina, and A. V. Yablokova a
Chinakal Institute of Mining, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630091 Russia *e-mail: [email protected] Received April 6, 2020 Revised April 9, 2020 Accepted April 10, 2020 Abstract—The procedure is developed for permeability prediction in deep-level fractured coal. Coal permeability and microstructure are experimentally studied on samples taken from Tikhov Mine. The predictive relationships between the occurrence depth and permeability of coal in parallel to master and side cleavage are obtained for the Nikitinsky, Tambovsky and Tarsminsky fields of the Leninsk geologicaleconomic region in Kuzbass. Keywords: Coal, cleavage, permeability, anisotropy, stress state, coal seam, occurrence depth. DOI: 10.1134/S1062739120026686
INTRODUCTION
Coal permeability needs to be predicted in designing preliminary degassing systems, for methane production from the day surface by borehole technologies. Models of uniaxial strain state of a coal seam are used to describe the changes in permeability with depth. The inhomogeneity of its properties is usually neglected, and it is believed that permeability depends either on the average stress of rocks, or on compression in the direction orthogonal to the dominant fracturing system. It is known that the matrix of most coals is divided by fractures of endogenous and exogenous cleavage into microblocks, whose sizes along the bedding are several millimeters. Coal seam capability to gas recovery is determined mainly by the conductivity of fractures and depends on their density, opening, connectivity, orientation and length [1, 2]. Endogenous fracturing caused by a change in the material composition of coals during their genesis is represented by mutually orthogonal master and side cleavage systems, as well as weakly expressed layer-by-layer ruptures (Fig. 1). The master system includes well-developed fractures up to several dozens of centimeters long. Between these fractures across the strike are short ruptures of side cleavage. Exogenous cleavage is caused by tectonic compression of rocks in the arches of folded geological formations and zones of disjunctive disturbances, where it complements the endogenous cleavage with several rupture systems, which cut the seam at an angle of about 45° to the bedding [4]. The permeability of fractures is 0.1–1000 µD, coal microblocks—units of microdarcy or nanodarcy [5, 6]. As a result, the time of gas diffusion from the matrix into the fractures is much shorter than the time of filtration through them to the boreholes [7]. Coal cleavage ensures a significant anisotropy of its permeability. A
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