Experimental Ivestigation of the Temperature and Pore Pressure Effect on Permeability of Lignite Under the In Situ Condition

The worldwide primary demand of energy will keep rising over the coming decades. It becomes essential and desirable to exploit lignite, 23 % reservation of the whole coal energy, by a suitable and effective technique. However, the current technology limitations of the exploiting and utilizing of lignite have caused low exploiting rate and high pollution situation. Owing to the development of the in situ gasification techniques, the lignite has become more attractive in the energy field than ever. The permeability of coal is a crucial factor in determining the tars (liquid phases) and gases production during the in situ gasification process. And the transport properties of the coal will in turn affect the thermal and chemical-mechanical reactions. Here, in this work, the permeability of lignite has been tested from room temperature (25 ∘\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{\circ }$$\end{document}C) up to as high as 650 ∘\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{\circ }$$\end{document}C through a triaxial rock permeability testing system under different pore pressures. A remarkable decrease of permeability can be observed during the whole temperature zone (25–650 ∘\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{\circ }$$\end{document}C) as the pore pressure increasing. The permeability curve versus temperature up to 650 ∘\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{\circ }$$\end{document}C has been divided into four stages based on the three peak values of the permeability. The competition between the strength of the frame structure with the shrinkage due to pyrolysis determines the significant fluctuation of the permeability versus temperature. Furthermore, the combined effect of temperature and pressure shows that the valleys and peaks in stage II will shift to higher temperature zone, while the valleys and peaks stay in the same temperature place in stage III and IV. The current investigation results of the permeability property of lignite will provide essential and valuable information for the exploiting and utilizing of lignite, especially by the in situ pyrolysis technique.