| Methane and CO2 related disasters are significant problems during coal mining. For the drainage and emission of coal bed gas, due to the gradient of gas pressure, the gas in fractures of coal seams flows to free faces(such as mining and tunneling working face or gas drainage borehole), then the gas concentration in coal fractures is decreased and gas desorbs from coal matrix. The characteristic of adsorption determines the occupation of gas in coals, while it is the characteristic of desorption determines the volume of gas flows from coal matrix during the change of gas pressure. If the adsorption and desorption are not reversible, the adsorption isotherms which has been used for a long time should not be used to predict the desorption process of coal bed gas. Numerous scholars investigated the adsorption and desorption characteristics of methane and CO2, the sorption hysteresis were observed in most of the experiments. Analyzing the past researches, it can be found that specific and systematic studies are rare, and some key points, such as the influencing factors of sorption hysteresis, the mechanism of sorption hysteresis and the influence of hysteresis on gas permeability property of gas in coal seams cannot be found. In order to understand the above difficulties, the main research contents and achievements is shown as following:The indices for evaluating sorption hysteresis used in other areas, such as soil, rock and polymer are reviewed, and the disadvantages are analysed. We propose a novel evaluating method, which chooses the best representing sorption model and uses am improved hysteresis index(IHI). 102 groups of experimental data from 12 scholars are restored. The IHI of methane, subcritical and supercritical CO2 are calculated, and the basic impression are analysed. The hysteresis indices used in other areas are not fitted for evaluating sorption hysteresis of coal seam gas, the index based on the Freundlich exponent relies on a specific isotherm model(the Freundlich equation) which is not commonly used for fitting gas sorption isotherms on coal. Indices based on slope and equilibrium concentration in the solid phase describe the hysteresis degree at different single points rather than the whole isotherm, thus small experimental errors can seriously affect the result. The advantage of the improved index is that it can reflect the hysteresis tendency from the hypothetical fully reversible state to the irreversible state. The index is 0 for the completely reversible state, and approaches 100% as the process tends to complete irreversibility. Since the proposed IHI is calculated from area rather than a single point, a small change in the desorption curve does not greatly affect the calculation result, the IHI has a strong applicability.By analyzing the evaluated results of historical experiment results, the influences of experiment methods, moisture of coal samples, gas species, maximum sorption pressure and coal properties on IHI are discussed. Based on this, using a high accurate indirect gravity sorption test setup, the adsorption and desorption isotherms of methane and CO2 are conducted on coal samples with different sizes, at different maximum pressure and experimental temperature. Results show the sorption hysteresis of methane and CO2 exist generally. The increase of maximum pressure, coal sample size, the decrease of temperature and the existence of removable water boost the sorption hysteresis. At the same conditions, the sorption hysteresis of CO2 is larger than that of methane. For the geo-storage of CO2, enhancing the injection pressure can increase the hysteresis degree, which will enhance the long-term stability of the sequestrated CO2. Current required sorption equilibrium time are insufficient, and it should be prolonged to guarantee the credibility and applicability of experimental results. The desorption isotherm, rather than the adsorption isotherm, should be testes to predict the emission and drainage of coal seam gas. During the test of desorption isotherm, the maximum pressure should be chose according to the in-situ pressure of coal seam gas.The action process of different sorption mechanisms for gas sorption in porous media, such as monolayer adsorption, multilayer adsorption and micropore filling adsorption are analysed. Combining the previous explanation of coal seam gas sorption hysteresis, a pore throat deformation hypothesis for coal seam gas sorption hysteresis is proposed. To verify the hypothesis, a whole pressure range 77 K nitrogen sorption experiment is performed, and the adsorption and desorption isotherms of coals from Qianqiuer mine and Changcun mine are testes. Considering the defect of BET method, a new analytic method is proposed, in which the surface areas of micropore and mesopore are calculated separately, and the distribution of sub-micropore is analysed. Coal samples from Qianqiuer mine and Changcun mine are disposed by adsorbing different pressure(2MPa, 4MPa and 6MPa) of methane and CO2, then the samples are tested by Fourier transformed infrared experiment to examine whether chemisorption is included in the high pressure sorption process. It is believed that coal contains different sizes, shapes and connectivities of pore structures. The gas molecules of coal seam gas need to pass through some pore throat of which size is similar to the dynamic diameter of gas molecules. The sorption-induced swelling leads to some inner-swelling of pore throat, which constricts the migration of gas molecules during desorption, and induces the occurrence of sorption hysteresis. The direct reason is the constriction of gas migration due to the deformation of pore throat, and the primary reason is the coal deformation due to micropore filling adsorption. The results of 77 K nitrogen sorption indicates a large proportion of sub-micropore with pore radius less than 1.7 nm, and the results of Fourier transformed infrared show the sorption between coal seam gas and coal is pure physical sorption, these conclusions provided parts of theoretical foundations for the proposed hypothesis.The deviation between traditional adsorption models and the represented desorption data is analysed, and the reason is discussed. By adding a residual gas content term, the improved sorption model is established, and the applicability of traditional and improved models are analysed and compared. Taking the improved Langmuir model as instance, the physical significances of terms in the improved model is discussed. Based on the desorption model, a improved absolute permeability model, and relative permeability model involving sorption hysteresis and Klinkenberg effect are proposed. Results show the improved desorption model consider the actual desorption property of coal seam gas, thus can describe the desorption process more accurately. The calculated result indicates all the correlations of improved desorption model are better than 0.99, which are satisfied for application. The calculated permeability using improved permeability model is between two results calculated from original permeability models, indicating the improved permeability model can correct the deviation of original model, and reflects the flow mechanism of coal seam gas accurately.Using QTS-2 and TPTA permeability test apparatus, anthracite from Changcun mine and bituminous coal from Bulli seam, the permeability of helium, methane and CO2 under different confining pressure are measured during pressurization and depressurization. The influence of sorption hysteresis on gas permeability in coal is analysed. Results show the permeability of methane and CO2 during depressurization is lower than the corresponding permeability during pressurization, while the permeability of helium keeps constant, indicating the sorption hysteresis impacts permeability by adsorption induced swelling and desorption induced shrinkage. Comparing with the permeability under 4MPa of confining stress, the permeability differences under 8MPa is smaller, which indicates the influence of sorption hysteresis is affected by effective stress. |
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