| In this paper,a self-developed microwave irradiation and desorption experiment system was used to study the desorption changes of shale under different microwave irradiation power(on-line group)and time(off-line group)conditions.Different desorption models were used to fit the desorption curves and the fitting effects of different desorption models were analyzed.The effects of dielectric properties,sample saturation,microwave irradiation power and time on microwave heating were investigated.Nuclear magnetic resonance(NMR)method was used to reveal the microstructure development of shale samples with different saturation under microwave irradiation,and the effect of microwave irradiation on shale microstructure was explained from the micro scale.(1)According to the curve of the relationship between the desorption amount and time in the on-line group,the desorption rate is high at the initial stage and the desorption amount increases rapidly in the natural desorption stage of shale.The desorption rate then decreases rapidly and stops gradually after a few minutes.After the microwave heating device is turned on,the gas desorption volume increases rapidly and the desorption speed at the initial stage is much higher than that at the later stage.Subsequently,the desorption rate of shale gradually decreased,and the increase of desorption volume gradually decreased.(2)Different desorption models were used to fit the results of the on-line group,and it was found that the permeability model had the best fitting effect on the shale desorption curve in the microwave irradiation section of the on-line group,followed by the empirical formula,and the diffusion model was the worst.The fitting effect of the three fitting models on the natural desorption curve of shale samples in the on-line group is similar,and the fitting effect of the diffusion model is slightly better than that of the empirical formula and the permeability model.For the off-line group fitting,it can be found that the empirical formula and the permeability model have good fitting results,while the diffusion model has poor fitting results.(3)By studying the relationship between the temperature of microwave heated shale powder and the dielectric constant,it was found that the higher the dielectric constant was,the faster the heating rate would be,and the temperature would eventually reach the highest.At the same time,the content of pyrite and siderite is positively correlated with the dielectric constant.The different mineral components and contents further affect the sample's ability to absorb microwave energy by influencing the dielectric constant of the sample,which is finally reflected in the temperature difference of the sample.(4)Different microwave power was used to heat the shale samples,and it was found that the heating effect in the low microwave power was poor and not suitable for microwave heating,indicating that there was a threshold effect of microwave power when the samples were heated by microwave.When the power was lower than the threshold,the heating rate of the samples would be very slow.When the power is greater than the threshold value,the heating rate of the sample will increase greatly and the sample will be heated rapidly.(5)According to the NMR results of shale samples,the pore size distribution of shale samples before and after microwave treatment was analyzed.It was found that after microwave treatment,the number of mesopores and macropores increased significantly,while the number of micropores decreased slightly.With the increase of the initial saturation of shale samples,the NMR permeability of shale samples increases gradually after microwave irradiation.When the saturation of shale sample is low,there are fewer pores and fissures generated by microwave heating,so the NMR permeability of shale sample increases less.With the increasing of shale sample saturation,the NMR permeability increases greatly.This paper contains 26 figures,16 tables and 98 references. |
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