Inhibition And Promotion Mechanism Of Coal Gas Adsorption/Desorption On Electrostatic Field

China's coalbed methane reservoirs are characterized by high storage and low permeability.In order to study the inhibition and promotion mechanism of effects the electrostatic field has on adsorption/desorption of coal samples,and exploring the anti-reflection technology of coal seam,the experimental platform of coal gas adsorption/desorption in electrostatic field was set up,and the isotherm of gas adsorption,gas desorption capacity and desorption speed were measured under different electrostatic field strengths.The dominant field strength of coal adsorption and desorption was researched,and the internal relations between the adsorption,desorption and electrostatic field were determined,and the affecting mechanism of electrostatic field on coal samples was explored from three aspects: electric polarization,thermodynamics and pore crack.The research indicates:(1)The gas adsorption capacity of coal sample is closely related to the electrostatic field voltage.The experimental results show that the adsorption of coal sample in electrostatic field is consistent with Langmuir adsorption,and the gas adsorption capacity is affected by electric field strength: the adsorption capacity is the largest when the electric field strength is 60kV/m,and it takes the second place without electric field.When the electric field strength is 90kV/m,the adsorption capacity is smaller comparing with that without any electric field,and when the electric field intensity is 30kV/m,the adsorption capacity is the smallest;(2)The dominant field strength of the coal gas adsorption is 60kV/m.Calculate the Langmuir limit adsorption capacity and adsorption heat of different electrostatic field voltages,and the results show that with the increase of electric field strength,the adsorption capacity of coal samples to gas increases first and then decreases,which if in the form of parabola.When the electric field strength is 60kV/m,the adsorption capacity of coal samples to gas is the largest;(3)The dominant field strength of the coal sample gas desorption is 60kV/m.Comparing gas desorption rates under different electrostatic fields which has the following rules: the desorption rate under electric field strength of 60kV/m is larger than that under electric field strength of 90kV/m,and the desorption rate at 30kV/m is only lower than that of 90kV/m.The desorption rate is the smallest when the electric field is not applied.The desorption capacity under different electric fields also shows the same law.At the same time,the desorption rate decays faster in the first 40 s of the desorption process,and then tends to be stable;(4)The electrostatic field can increase the gas desorption rate under the dominant field strength.In condition of dominant field strength,only when the electrostatic field is applied in the desorption process,the gas desorption speed is greater than that when the electrostatic field is not applied.The desorption speed of gas when the electrostatic field is only applied in the adsorption phase,is obviously higher than that when the electrostatic field is only applied in the desorption phase.The desorption rate is the highest when the electrostatic field is applied throughout the adsorption desorption.Therefore,the electrostatic field further increases the gas desorption rate based on the enhanced gas adsorption capacity;(5)The promoting and inhibiting mechanism of gas adsorption under electrostatic field is revealed.With the increase of electric field intensity,the gas adsorption capacity tends to decrease first and then increase in a parabolic form.The mechanism is analyzed as: when the electric field intensity is small,the polar groups on the surface of the coal matrix will turn to polarize,resulting in the decrease of the "adsorption potential" corresponding to the adsorbate molecules,thus reducing the adsorption capacity of coal to gas;with the increase of electric field intensity,the polarization effect has enhanced,and the attraction of coal and gas molecules has increased,so that the adsorption capacity of coal to gas molecules enhances and the adsorption amount increases.When the electric field intensity is over large,the electronegativity of the coal increases greatly,resulting in the decrease of the heat of coal gas adsorption,the shallowness of the molecular adsorption potential well of coal gas,and the decrease of the gas adsorption amount;(6)The promotion and inhibition mechanism of gas desorption under the action of electrostatic field is revealed.The electrostatic field can promote the desorption process of gas in the coal sample,and its mechanism is analyzed as follows: the electrostatic field enhances the gas adsorption ability of coal samples,improves the gas pressure gradient during the coal sample desorption,and increases the power of the gas absorption.On the other hand,the electrostatic field enhances the connectivity of empty cracks in the coal sample and reduces the migration resistance of gas inside the coal sample.The combined action of the two has promoted the gas desorption process.