Research On The Influence Of Cyclical Microwave Treatment On Coal Structure And Methane Adsorption In Coal

China’s coal seams generally have the characteristics of low porosity,poor permeability and strong adsorption to methane,which seriously restricts the efficient development and utilization of coalbed methane in China.The enhanced extraction with reservoir stimulation as a main means has been studied by numerous researchers.However,although the common hydraulic methods as well as the emerging physical,chemical,electronical and acoustic methods have been proven productive,some shortcomings,such as aggravated water locking damages,complex operations,low efficiency and eco-unfriendliness,still existed.Considering the unique instantaneity,strong penetration,selectivity,controllability,and the joint action of"thermal effect"and"nonthermal effect"of microwave irradiation,cyclical microwave treatment will affect the structures of coals and the adsorption performances of gas in coals.In this paper,the long flame coal（No.6 up coal seams of Suancigou mine）,coking coal（No.5 coal seams of Shaqu No.2 mine）in Ordos Basin and anthracite（No.15 coal of No.2 well of Sihe mine）in Qinshui Basin were selected for cyclical microwave treatment.The response laws of coal pore and fracture structures,surface characteristics and molecular structures under the action of cyclical microwave treatment were systematically studied.The influence mechanism of cyclical microwave treatment on the structural characteristics of coals and the adsorption performance of methane in coals was revealed.The main work and achievements are as follows:（1）The pore volume and specific surface area of micropores,small pores and macropores of three coal samples before and after cyclical microwave treatment were tested and characterized by low-temperature nitrogen adsorption and mercury intrusion respectively.The correlation between the number of microwave treatment cycles and the complexities of pore and fracture structures of coal samples were quantified based on the fractal theory of porous media.The results showed that with the increase of microwave treatment cycles,the total pore volume of three kinds of coal samples increased,and the total specific surface area decreased.Specifically,the volume of micropores decreased while that of small pores and macropores increased.The specific surface area of micropores and macropores decreased while that of small pores increased.The changing mechanism was revealed that the cyclical microwave treatment promoted the development,expansion,and connection of pores and fractures,and some micropores were expanded into small pores,so that small pores increased and the connectivity of macropores was enhanced.The pore fracture network model of coal samples before and after cyclical microwave treatment was constructed combined with micro CT scanning and visual three-dimensional reconstruction technology.The morphological characteristics and connectivity of micron-scale pores and fractures of coal samples before and after microwave circulation were quantitatively analyzed.The influence mechanism was that under the cyclical microwave,the number and volume of the equivalent ones of connected pores increased,and the number and radius of equivalent throats increased.The heterogeneous distribution of pores and fractures of coal samples correspond to the fractal feature.The distribution irregularity of micropores,small pores,as well as one part of macropores（diameter 50 nm-20μm）decreased,while the irregularity of the other part of macropores（diameter greater than 20μm）increased.The density of micron pores and fractures increased.（2）The changes of methane adsorption capacity,methane desorption rate,methane diffusion coefficient,as well as surface groups,microstructure parameters and surface energy of three coal samples before and after cyclical microwave treatment,were measured and characterized by high-pressure gas adsorption/desorption rate analyzer,Fourier transform infrared spectrometer,and contact angle meter.The results showed that with the increase of the microwave treatment cycles,the methane saturated adsorption capacity of long flame coal,coking coal,and anthracite decreased exponentially.The methane desorption rate and diffusion coefficient increased logarithmically.The cyclical microwave treatment destroyed the self-association hydrogen bond of coals,reducing the oxygen-containing functional group.Some alkane side chains,such as methyl or methylene group,broke,fell off and were released in gas form.Some minerals decomposed.The aromatic carbon fraction,aromaticity and aromatic ring polycondensation increased,and the length and branch of alkane side chains reduced.The absorption performance of methane by coal is jointly affected by pores and surface.The influence of cyclical microwave treatment on low rank coal is obviously higher than that of medium and high rank coal.It is because there are many alkane side chains and oxygen-containg functional groups of low-rank coal and higher content of irreducibled water and bound water,which have significant response to microwave,considerably reducing micropores and surface energy.（3）The atomic number ratio,the molecular carbon skeleton structures,the form of heteroatoms,and microcrystalline parameters of aromatic lamellae of three natural coal samples and three coal samples after cyclical microwave treatment were tested and characterized by elemental analysis,¹³C NMR,XPS,and XRD.The results showed that compared with natural coal samples,after cyclical microwave treatment,the content of oxygen and hydrogen in long flame coal,coking coal,and anthracite coal samples decreased,the ratio of aromatic bridge carbon to peripheral carbon increased.The interplanar spacing of the aromatic layer decreased,while the stacking thickness,ductility,and the average number of stacking layers increased.Specifically,for the macromolecular structure of long flame coal,one anthracene ring was added,one naphthalene ring,three carboxyl groups,and one hydroxyl group were reduced.For coking coal,one anthracene ring was added,one naphthalene ring,one carboxyl group,and two hydroxyl groups were reduced.For anthracite,a phenanthrene ring was added,a naphthalene ring,a hydroxyl group,and an ester group were reduced.The average molecular formulas of the three coals changed from C₂₃₅H₁₈₅O₃₅N₃S,C213H114O16N2S,and C220H82O13N2S2 to C222H161O28N3S,C211H106O12N2S,and C₂₁₀H₇₅O₁₀N₂S₂ respectively.In general,cyclical microwave treatment improved the degree of aromatic polycondensation of three coals samples,reduced the content of methyl,methylene,methine,hydroxyl,carboxyl,and other groups,resulting in the increase of aromatic structure and the decrease of alkane side chains and oxygen-containing functional groups.The differences of molecular orbital,electrostatic potential,bond order,and bond length of coal molecular fragment model under microwave treatment and traditional heating were compared and analyzed using the quantum chemical calculation method.The mechanism of microwave"nonthermal effect"was revealed as under the action of microwave electric field,the electron migration capacity of coal molecular fragments was enhanced,the energy gap difference of Frontier orbit was reduced,the dipole moment was increased,the electrophilic/nucleophilic reactivity was enhanced,and the covalent bond strength was weakened.（4）The plane model and three-dimensional models of three kinds of coal macromolecules before and after microwave treatment were constructed after modification and optimization using Marvin Sketch/CNMR Predictor,g NMR,and Materials Studio software.The correctness of coal molecular models was verified by analyzing and comparing the element content of the models,simulated ¹³C NMR spectrums,and methane adsorption parameters with the experimental data.The variation characteristics of methane adsorption capacity,isosteric heat of adsorption,and interaction energy,radial distribution function,methane diffusion coefficient,and methane diffusion activation energy before and after microwave treatment were calculated by grand canonical Monte Carlo and molecular dynamics simulation.The results showed that after cyclical microwave treatment,the adsorption capacity of methane molecule by three coal samples was reduced,the self-diffusion coefficient of methane molecule increased,and the transfer diffusion coefficient increased.The effect of cyclical microwave treatment decreased with the increase of coal rank.The molecular mechanism of microwave weakening the methane adsorption capacity in coals was further revealed from the perspective of heat,energy,binding strength,and diffusion activation energy that the isosteric heat of adsorption,interaction energy,binding strength between coal molecules and methane molecules as well as the diffusion activation energy of methane molecules decreased.（5）The coalbed methane extraction system enhanced by cyclical microwave treatment was designed based on the deep study on how cyclical microwave treatment changes the pore and fracture structures,surface properties and molecular structure of coals.Meanwhile,the enhanced extraction effect was simulated and analyzed using Comsol,under the stratigraphic conditions of No.15 coal of No.2 well of Sihe mine.The results showed that from the macro perspective,the conventional extraction caused little change of temperature of coal seam,while the extraction enhanced by cyclical microwave treatment increased the temperature of coal seam（the average temperature rose by 6.08℃after 360 days）,and the closer to the microwave emitter,the faster the temperature rose.Compared to the conventional extraction,the microwave enhancement made the extraction rate of coalbed gas faster.Therefore,the gas well productivity was greatly improved.After 360 days,cumulative production increased by34.25%.