Speaker
Description
Thermal excitation of coal seams by high-temperature steam is a highly promising technology to increase gas production. Among them, steam permeability is a key parameter characterizing the injection capability of thermal fluids. However, the seepage law of steam in coal and its evolution mechanism are still unknown. In order to solve the above problems, experiments were carried out to determine the permeability and thermal strain of high-temperature steam in coal, and the evolution law of the permeability of high-temperature steam and thermal strains of coal was obtained. The experiments found that during the process of high-temperature steam injection into coal, with the extension of the injection heat time, the liquid-measured permeability shows an intermittent oscillation law. With the increase of steam temperature, the peak values of the oscillation decrease, the periods are shortened, and the oscillation is more intense. During the heat injection process, the radial and volumetric strains of the coal show 2~3 expansion stages. When the steam temperatures are low, the axial strains are com-pressive, and when the temperatures are high, the axial strains turn to expansion. The theories of Kelvin capillary condensation, plug flow and thermal stress were used to investigate the mechanisms of condensation phase change, oscillatory seepage of steam and thermal strains change of coal, respectively. The study shows that the equilibrium pressure of steam in the micro-pores of the coal is less than the saturated vapor pressure in the large space, and the smaller the pore diameter, the lower the pressure required for steam condensation, and the easier steam condense. The gas-liquid plug flow induced by steam in the coal is the main mechanism causing intermit-tent oscillation of permeability. In addition, the high-temperature steam has superimposed inward and outward expansion influence effects on the permeability of the coal, leading to a decrease in the permeability of large pores and an increase in that of small pores in the matrix. During the steam injection process, the rapid expansion strains in the early stage are mainly controlled by the pore pressure, and the slow expansion strain in the middle and late stages by the thermal strain. The results provide factual basis and theoretical reference for the practice and numerical simulation of steam thermal recovery gas.
| Country | China |
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