Study on the Deformation and Permeability of Raw Coal With Variable Confining Pressure Unloading Rate

In this paper, the gas-containing coal thermal fluid–solid coupling three-axis servo seepage experiment device is used to conduct experiments on the deformation and permeability characteristics of raw coal with variable confining pressure unloading rate. It is proposed that the axial strain change rate ε1′\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\varepsilon _{1}^{{\prime }}$$\end{document}, radial strain change rate ε3′\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\varepsilon _{3}^{{\prime }}$$\end{document} and permeability change rate k′\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$k^{{\prime }}$$\end{document} of specimens under unit stress state are used to analyze the sensitivity of axial pressure and confining pressure to deformation and permeability characteristics of specimens during loading and unloading. The influence of unloading rate of variable confining pressure on the axial strain, radial strain and permeability of each sample is analyzed by the change rate of axial strain, radial strain and permeability of each sample in unit time. The change rate of the axial strain ε1″\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\varepsilon _{1}^{{{\prime \prime }}}$$\end{document}, the change rate of the radial strain ε3″\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\varepsilon _{3}^{{{\prime \prime }}}$$\end{document} and the change rate of the permeability k″\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$k^{{{\prime \prime }}}$$\end{document} of each sample per unit time are used to analyze the influence of the variable confining pressure unloading rate on the axial strain, radial strain and permeability of each sample characteristic. At the beginning of the test, the axial strain rate of each specimen is larger than that of the permeability, while the radial strain rate is smaller. In the process of loading axial stress, axial stress has a great influence on the axial strain. At the initial stage of unloading confining pressure, the confining pressure has a great influence on the permeability of the sample, but has little influence on the axial strain. With the increase of axial stress level and unloading rate of confining pressure, the change rates of axial strain, radial strain and permeability increase gradually.