Methane Sorption Hysteresis And Its Mechanism Of Tectonically Deformed Coal Samples From The Yueliangtian Coal Mine,Guizhou Province

Because of the unique properties and methane adsorption/desorption characteristics of tectonically deformed coals(TDC),the locations with TDCs are gas and coal outburst prone;at the same time,sorption hysteresis of methane will increase the outburst risk.Therefore,methane sorption hysteresis behavior and its mechanism of TDC have theoretical and practical significance for preventing coal and gas outbursts.However,there are few reports on methane sorption hysteresis behavior in TDCs.Theories of molecular dynamics,coal petrography,tectonic geology and gas geology were applied,and TDC samples were extracted from the same coal seam from the Yueliangtian coal mine(Guizhou Province,southwest China).Deformation characteristics of TDCs were studied through hand specimens and microscope observations.Pore structure and the development of closed pores were analyzed combined with fluid injection techniques and fractal theory.The low-field nuclear magnetic resonance of methane adsorption/desorption generated sorption hysteresis index values for methane,and sorption hysteresis behavior was quantitatively evaluated.The energy variation in methane sorption hysteresis behavior was revealed by adsorption potential and surface free energy.Finally,the correlations between the sorption hysteresis index and various factors were carried out.The following results were obtained.(1)The TDC samples can be classified into cataclastic,fragmented,and mylonitic coal.There are slight disparities in sample properties,indicating that the main difference between TDC samples is the tectonic deformation.As the degree of tectonic deformation increases,the pore specific surface area of micropore,ultramicropore and supermicropore increase,while those of mesopore and macropore vary in a wave-like manner.The micropore fractal dimension of strongly deformed coals is larger than that of primary and weakly deformed coals.(2)Strongly deformed coals have more closed pores compared with weakly deformed coals.Closed pores may form from heterogeneous shrinkage,extrusion/shear stress,and debris plugging.In addition,the homogenous shrinkage of the coal matrix might break the walls of closed pores,generate new open pores,and adsorb/desorb methane again,which may cause sorption hysteresis.(3)Methane sorption hysteresis with different degrees appears in TDC samples.The methane responsible for sorption hysteresis is the adsorbed methane rather than the free methane.It is the adsorbed methane rather than the free methane trapped in the pores and caused methane sorption hysteresis.The methane sorption hysteresis index of TDC ranges from 0.41 to 0.73.With the enhancement of tectonic deformation,the hysteresis index decreases in cataclastic coals and increases in fragmented and mylonitic coals.(4)The adsorption potential,surface free energy reduction,and rate in the desorption process are larger than those in the adsorption process,while the Langmuir pressure and Langmuir volume in the desorption process are both smaller than those in the adsorption process.The above proves that coal matrix expands and shrinks after methane’s adsorption and desorption,manifesting as coal matrix expansion.Then the pores(especially the micropores)between coal matrix are compressed,and pore diameter decreases.As a result,the superposition of the micropore adsorption potential raises,the adsorption potential increases,and the dispersion force enhances.(5)Methane sorption hysteresis of TDC is mainly affected by tectonic deformation,micropore fractal dimension,adsorption potential,and closed pores.While the correlation of other factors(coal properties,pore specific surface area,pore volume,the maximum adsorption pressure,and surface free energy)is almost negligible.(6)The sorption hysteresis discussed here is not sufficient to be explained with experimental error,dissolution of methane into residual moisture,or absorption of methane into coal matrix.The author believes that the direct cause of sorption hysteresis is that the expansion of the coal matrix in the early stage of desorption causes the pore diameter to be smaller than the molecular dynamic diameter of methane,which closes the channels of methane desorption and migration,the shrinkage of the coal matrix in the later stage of desorption causes the empty closed micropores to be converted into open micropores and adsorb methane again.Compared with primary and weakly deformed coals,strongly deformed coals have lower Langmuir pressure and strength,larger micropore fractal dimension and adsorption potential,and more closed micropores.Therefore,strongly deformed coals have greater enhancement on coal matrix shrinkage and adsorption potential after methane adsorption/desorption,showing more evident methane sorption hysteresis behavior.There are 77 figures,8 tables and 180 references in this dissertation.