Study On Gas Adsorption/Desorption Deformation And Seepage Characteristics Of Coal Under The Effect Of Water

In the context of"dual carbon",the development and utilization of coalbed me-thane can improve clean energy supply,reduce coal mine gas accidents and greenhouse gas emissions.In situ coal reservoirs are a three-phase coexistence system of gas,liquid,and solid,and the presence of water can reduce the adsorption capacity of coal to gas,change the adsorption and desorption deformation characteristics of coal to gas,and thus affect the seepage and migration law of gas in coal reservoirs.Therefore,mastering the adsorption and desorption deformation characteristics of coal and rock under the influence of water and the gas migration law is of great significance for the safe and efficient mining of coalbed methane.To this end,a combination of theoretical analysis,laboratory experiments,and numerical simulation was used to study the adsorption,desorption,and deformation characteristics of different gases in coal under the influ-ence of moisture.The influence of moisture on the gas migration law of coal and rock was analyzed,and the interaction of multiple factors on the gas extraction effect was explored.The main conclusions were as follows:（1）The Langmuir saturation adsorption capacity of coal to N₂ and CO₂ shows a decreasing trend with the increase of water content,which is related to the preferential adsorption of coal to water and the blockage and compression of coal pores caused by the water absorption and expansion deformation of coal.The desorption hysteresis ef-fect of coal on N₂ and CO₂ gradually weakens with the increase of water content.This is due to the preferential occupation of effective adsorption sites by water and the abil-ity of water to block some pore throats and narrow channels in the coal,thereby hin-dering gas entry into these pores.（2）Under the same equilibrium pressure conditions,the adsorption expansion de-formation of N₂ and CO₂ by coal rock decreases with the increase of water content.Under the same adsorption equilibrium pressure,the adsorption deformation caused by CO₂ adsorption on coal with different water contents is always greater than the expan-sion deformation caused by N₂ adsorption on coal.As the moisture content of the coal increases,the leading effect of desorption strain in the coal body gradually weakens.Analysis suggests that this is mainly caused by the blockage of some pore throats and narrow channels in the coal pores by preferentially adsorbed water,which hinders gas entry into these pores.（3）The analysis of error statistical indicators indicates that the empirical model for adsorption deformation has the highest fitting accuracy,and has better applicability for characterizing gas adsorption deformation of coal samples with different water con-tents.The presence of water will reduce the adsorption expansion deformation of coal and rock,which is related to two factors:the competition between water molecules and gas molecules for adsorption will occupy the effective adsorption sites of coal,and the water that is preferentially adsorbed will block some pores of coal.Two factors jointly inhibit the adsorption of gas by coal,reduce the surface free energy of coal,and lead to a reduction in volume expansion deformation caused by gas adsorption by coal under water containing conditions.（4）Based on the dual pore structure characteristics of coal and rock,combined with a coal and rock permeability model considering the influence of moisture,a multi field coupling model for coal and rock gas transport considering the influence of mois-ture was established.The COMSOL numerical simulation results show that with the increase of moisture content,the decrease in gas pressure in coal decreases while the relative gas content shows an increasing trend,indicating that the presence of moisture will weaken the gas extraction effect of coal.Based on the response surface method analysis,it is pointed out that the order of significance of the influence of coal and rock parameters on the effective extraction radius of boreholes is:water content>initial gas pressure>initial permeability.The response surface model analysis of the interaction of three factors indicates that the initial gas pressure of the coal body has a greater impact on the effective extraction radius of the borehole than the initial permeability of the coal rock.The influence of water content on the effective extraction radius of the bore-hole is greater than that of the initial gas pressure of the coal body.