In order to explore the influence of coal deformation caused by temperature and desorption on seepage characteristics in the process of heat injection mining of coalbed methane, the permeability test, thermal expansion, and constant temperature adsorption desorption of coal samples under different temperature and stress states were carried out using the high temperature multifunctional triaxial test system, and the influence of thermal expansion and desorption deformation effect on coal permeability in the process of temperature increase is studied. The results show that (1) with the increase of temperature, the sensitivity of coal thermal expansion deformation to temperature decreases gradually. The thermal expansion deformation makes the coal matrix expand, and the seepage channel is squeezed and the permeability decreases. (2) The effect of thermal expansion deformation is related to the porosity of coal. When the porosity of coal is high, the thermal expansion deformation reduces the permeability; on the contrary, the inward expansion of thermal expansion deformation is limited, and the effect on permeability is weakened. (3) The desorption of coal cause matrix shrinkage. The higher the desorption amount, the more obvious the shrinkage and the higher the permeability. Increasing temperature promotes desorption deformation of coal and increases permeability. (4) In the process of increasing temperature, the change of coal permeability is affected by thermal expansion deformation and desorption deformation. With the increase of temperature, when the influence of thermal expansion deformation on coal permeability is dominant, the permeability decreases gradually, and when desorption deformation is dominant on coal permeability, the permeability increases gradually. (5) With the increase of axial pressure, confining pressure, and pore pressure, the decrease of coal porosity is smaller. When the temperature increases, the temperature corresponding to the minimum permeability point is smaller. The research conclusion provides a basis for the technology of heat injection mining coalbed methane.
The non-uniform adsorption characteristics of coal strictly restrict the development and utilization of CBM, and the structure of coal is the fundamental reason for its non-uniform adsorption of methane. To investigate the fundamental causes of methane adsorption capacity differences in coal with various meso-structures, small coking coal and lean coal samples were subjected to isothermal adsorption, computed tomography (CT) scanning, scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) to obtain the correlation between the meso-structure of coal and its non-uniform adsorption characteristics. The reasons for the differences in methane adsorption capacity between the specimens were summarized from the perspectives of material and structural heterogeneities, meso-morphology and pore size distribution. The following conclusions could be drawn: (1) The coal samples with different heterogeneity degrees exhibit varying methane adsorption capacities. (2) Compared to the specific surface area, the mineral composition exerts a greater influence on the adsorption capacity of the coal samples. (3) The gas adsorption capacities of the coal samples with similar meso-morphology and pore size distribution characteristics are similar, while among the coal samples with similar pore size distributions but different meso-morphologies, the gas adsorption capacities significantly differ.
The slippage effect is an important phenomenon of coalbed methane (CBM) migration. As the exploitation of CBM, the slippage effect becomes more and more obvious. For exploring the evolution of the slippage effect as CBM extraction by heat injection, the thermal expansion test and the temperature increasing desorption test under different stress states, as well as the seepage test under variable temperature conditions, were conducted by operating the hightemperature multifunctional triaxial test system. The effect rules for temperature and pore pressure on the slippage effect were analyzed, and then the evolution mechanism of the slippage factor at the united action of pore pressure and temperature was comprehensively analyzed based on thermal expansion deformation and adsorption characteristics. The results showed that: first, the influence of pore pressure on the slippage effect is mainly caused by the molecular mean free path and effective stress when pore pressure is greater than 0.8 MPa in the range of 30−150 °C. With the pore pressure increase, the slippage effect decreases. Second, the temperature influence on the slippage effect is mostly induced by the variation of cracks in coal due to thermal expansion deformation created by temperature. With the increase in temperature, the slippage factor average increases first from 30 to 60 °C is 19 times and then decreases from 60 to 90 °C is 36%. Finally, at the combined influence of temperature and pore pressure, the slippage effect is mainly influenced by thermal expansion deformation, desorption deformation, and effective stress. At 30−60 °C, the slippage factor is principally affected by thermal expansion deformation. At 60−90 °C, the slippage factor is mostly influenced by desorption deformation and effective stress. The research results provide a foundation for the technology of CBM exploitation by heat injection.
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