Abstract

Laboratory ultrasonic velocity measurements of different types of coal demonstrate that their dynamic elastic properties depend on coal rank and applied effective pressure. In spite of the growing interest in coal beds as targets for methane production, the high abundance in sedimentary sequences and the strong influence that they have on seismic response, little data are available on the acoustic properties of coal. Velocities were measured in core plugs parallel and perpendicular to lamination surfaces as a function of confining pressure up to 40MPa in loading and unloading cycles. P- and S-wave velocities and dry bulk and dry shear moduli increase as coal rank increases. Thus, bituminous coal and cannel show lower velocities and moduli than higher ranked coals such as semianthracite and anthracite. The VP-VS relationship for dry samples is linear and covers a relatively wide range of effective pressures and coal ranks. However, there is a pressure dependence on the elastic properties of coal for confining pressures below 5MPa. This pressure sensitivity is related to the presence of microcracks. Finally, the data show that coal has an intrinsic anisotropy at confining pressures above 5MPa, the closing pressure for most of the microcracks. This intrinsic anisotropy at high pressures might be due to fine lamination and preferred orientation of the macerals.

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