Coal Bed Pressure, Gas Content, Sorptional Capacity of Coal Bed, and the Langmuir Equation

The coal bed pressure (reservoir pressure) is shown to be the pressure of the gas within the sealed borehole cavity within the bed after equilibrium has been established between the desorption of methane from the bed and its resorption on account of the pressure that develops. It is established that the Langmuir equation cannot be used to assess the gas content of the coal bed if the saturation pressure in adsorption is replaced by the bed pressure, since these concepts are fundamentally different. In fact, their physical significance is different: the saturation pressure is created by an external source and drives methane into the bed, whereas the bed pressure is created within the bed as a result of methane desorption within a sealed volume containing gas-bearing coal. The Langmuir equation may be used to describe the methane sorption by coal up to a saturation pressure of 5 MPa, since the influence of the coal’s structural transformation and consequent swelling may be disregarded within that range. In addition, it is shown for a specific example that, instead of using the Langmuir equation, whose constants are determined theoretically, the isotherms of methane sorption by coal with moderate methane filling may be described by the Freundlich equation, whose constants are determined empirically.