The Influence of Drying and Stress on the Permeability of Texas Lignite

Tests have been carried out to quantify the variation in permeability of Texas lignite with drying and with applied stress. It has been shown that the matrix permeability of lignite may be increased from effectively zero to the order of 10 darcies by removing about 20 percent by weight of water. In addition, an increase of confining pressure after drying will reduce the permeability, but only by about one order of magnitude. Drying of the matrix thus may produce matrix permeabilities of the same order as the undried field fracture permeability. The permeability increase of the matrix is initially greater parallel to the bedding than perpendicular, but after further drying the two orientations show similar final permeabilities. This drying effect could have a significant influence on the operation of an in-situ gasification process by increasing the transmissivity and injectivity of the producing seam. Drying of the seam could occur by the flow of unsaturated gas and will be enhanced by combustion.