Coupling Seepage Characteristics Of Loaded Coal Containing Gas And Water

The extraction and utilization of gas is of great significance to the prevention of gas disasters in coal mines,the realization of the efficient utilization of clean energy and protection of the atmospheric environment.Seepage characteristic of coal seam gas is the key factors determining the gas drainage effect.Besides,In the process of increasing permeability and eliminating outburst,external water will also be introduced into coal seam.The influence of water on the seepage characteristic of coal seam gas is obvious and there is a coupling relationship between coal,water and gas.Therefore,studying the coupled seepage characteristics of loaded coal,gas and water is not only helpful to understand the influence of water on gas flow in coal seam,but also has important guiding significance for gas drainage in water-containing coal seam.Through a series of experiments,the influence of water on the mechanical properties,gas adsorption and seepage process of coal was analysed systematically in this paper.A coal permeability model and a relative permeability model considering gas-water coupling effect were established.The two types of permeability models were applied to the gas-water two-phase seepage coupling model and the gas single-phase seepage coupling model respectively and the gas-liquid two-phase seepage model and the gas single-phase seepage model are constructed.Based on this,according to the actual situation of coal face,two types of coupling models were used to simulate the gas extraction process of high water saturation coal seam,residual water state coal seam and the whole gas drainage process of water containing coal seam.The flow law of gas and water in coal seam and the influencing factors are analysed.According to the simulation results,the improvement measures for gas drainage in water-bearing coal seam are put forward.Finally,the improved measures are verified on-site by using gas drainage measurement data of coal seam using hydraulic measures.In this paper,four types of coal samples used in experiments were divided into low rank bituminous coal,middle rank bituminous coal,high rank bituminous coal and anthracite.First,the microstructure characteristics of four coal samples with different metamorphic degrees were analysed.The liquid nitrogen adsorption experiment,mercury intrusion experiment and Fourier transform infrared spectroscopy experiment analysis have obtained the micro-pore structure of the coal and the evolution law of oxygen-containing functional groups with coal metamorphism,which lays a foundation for subsequent related experimental analysis.Then,from the perspective of the influence of moisture on gas and coal properties,the study was conducted on the effects of moisture on gas adsorption characteristics and on the effect of moisture on the mechanical properties of the coal.In terms of the effect of moisture on gas adsorption characteristics of coal,first,the amount of adsorbed water of four coal samples with different metamorphic degrees under the same temperature and different humidity conditions was measured,and the relationship between water absorption capacity and coal rank was analysed.Then,the isothermal adsorption lines of four types of coal under different conditions of moisture content were measured.The variation law of gas adsorption characteristics with moisture content and the influence mechanism of moisture on coal gas adsorption were analysed.The fitting of experimental data by three modified gas adsorption equations considering the influence of moisture was compared.The adsorption water of four types of coal samples with different degrees ofmetamorphism at the same temperature and humidity was measured and the relationship between the absorbability of coal body and the rank of coal was analysed.Then,the isothermal adsorption lines of four kinds of coal under different moisture content are measured,and the variation law of gas adsorption characteristics with water content is analysed,and the influence mechanism of moisture on gas adsorption is analysed.The experimental data were fitted by three modified equations considering the influence of moisture on gas adsorption capacity.Finally,the effects of micropore structure and oxygen functional group parameters on the adsorption of water and gas by coal body are analysed,and the reasons for the difference of moisture effect on gas adsorption of coal body with different degrees of metamorphism are discussed.Finally,the effects of micropore structure and oxygen functional group parameters on the adsorption of water and gas of coal were analysed,and the reasons for the difference of moisture effect on gas adsorption of coal with different degrees of metamorphism were discussed.In terms of the effect of moisture on the mechanical properties of coal,uniaxial compression experiments of raw coal and briquette samples with different moisture content have been carried out in this paper.It was found that,with the increase of moisture content,the stress-strain curves of raw coal samples and briquette samples show that the interval of compaction stage increases,the elastic phase decreases,and the yield stage is more obvious.The increase of moisture content makes the drop velocity of stress-strain curve of coal body slow down,and the multi-stage drop platform appears.The compressive strength and moisture content of the two kinds of coal samples both satisfy the negative linear function,the peak strain and moisture content both satisfy the positive linear function,and the elastic modulus and moisture content both satisfy the negative exponential function.Poisson's ratio and moisture content all satisfy the positive linear function.With the increase of water content,the failure forms of raw coal samples are shear failure,tensile combined failure,and briquette failure in turn:shear failure,tensile failure and tensile shear combined failure.There are two parts of damage caused by water:the initial damage caused by water and the superimposed damage caused by free water.The combined effect of the two kinds of damage intensifies the damage of loaded coal.On the basis of experiments,a statistical constitutive model of coal damage with different moisture content is derived in this paper.The model overcomes the problem that the stress-strain fitting curve deviates greatly from the test curve before the peak.Compared with previous models,it is more suitable to analyse uniaxial compression mechanics of coal under different moisture content.Then,according to the characteristics of gas and water flow in water-bearingcoal seam drainage process,the gas seepage flow in coal body is divided into two stages:the single-phase gas seepage stage affected by water absorption and the gas-water two-phase seepage stage affected by free water.Experimental and theoretical studies were carried out respectively.In order to study the relationship between water content?moisture content?and gas permeability of coal body,the gas seepage experiments of raw coal sample and briquette coal sample under different water content,effective stress and gas pressure combination are carried out in this paper.The results show that the permeability of raw coal samples and briquette coal samples with different moisture content decreases with the increase of effective stress,showing a negative exponential relationship.In the low effective stress region,the permeability decreases rapidly with the increase of effective stress,and decreases slowly with the increase of effective stress in the high effective stress range.Under the condition of constant axial pressure and confining pressure,the permeability of raw coal samples and briquette coal samples with different moisture content decreases first and then increases with the increase of gas pressure,and the relationship between permeability and gas pressure is in accordance with the quadratic polynomial relationship.Both raw coal and briquette samples with different moisture content have obvious klinkenberg effect.Under the same effective stress and gas pressure,the permeability of raw coal sample and briquette coal sample decreases with the increase of water content,and the permeability of raw coal sample is exponentially related to water content.The relationship between permeability and water content of briquette samples is linear.On the basis of the improved firewood stick model,the author considers the effect of water on the volume of crack,the influence of moisture on the gas adsorption of coal,the effect of moisture on the mechanical properties of coal and the influence of water absorption and expansion of coal.A permeability model considering the coupling of gas and water is derived for arbitrary boundaries,and the model is simplified for different boundary conditions.The seepage experimental data of Hongling raw coal sample and briquette coal sample with different moisture content were verified by using the simplified model.The results show that the theoretical calculation value is in good agreement with the experimental data,and the model has good applicability.In order to analyse the influence of free water on the percolation characteristics of coal body,the relative permeability of coal under different stress conditions and different metamorphic degree of coal under the same condition was measured by using the gas-water two-phase seepage experimental system.It is found that with the increase of effective stress,the concave degree of relative permeability curve increases,which indicates that the effective porosity of coal body decreases,the seepage channel decreases,and the difficulty of gas-driven water increases.Under the condition of higher effective stress,the gas does not pass completely through the coal fissure,and part of the gas passes through the pore of coal matrix.The gas permeability of coal sample is mainly determined by the fracture pore structure of coal body and the gas desorption ability of coal matrix,while the water permeability of coal sample is related to the hydrophobicity of coal sample.In order to analyse the influence of free water on the percolation characteristics of coal body,the relative permeability of coal under different stress conditions and different metamorphic degree of coal under the same condition was measured by using the gas-water two-phase seepage experimental system.It is found that with the increase of effective stress,the concave degree of relative permeability curve increases,which indicates that the effective porosity of coal body decreases,the seepage channel decreases,and the difficulty of gas-driven water increases.Under the condition of higher effective stress,the gas does not pass completely through the coal fissure,and part of the gas passes through the pore of coal matrix.The gas permeability of coal sample is mainly determined by the fracture pore structure of coal body and the gas desorption ability of coal matrix,while the water permeability of coal sample is related to the hydrophobicity of coal sample.In the gas-water two-phase seepage process,the water seepage of middle rank bituminous coal is dominant,while that of high-order bituminous coal and anthracite gas is dominant.With the increase of coal rank,the water saturation of coal isosmotic point decreases,the residual water saturation increases and the relative permeability increases.On the basis of two-phase seepage experiment,according to the relation between residual water saturation and porosity,the relative permeability model of gas-water coupling of coal body is established by combining the porosity equation under the gas-water coupling action obtained in this paper.The experimental data of coal relative permeability measured in this paper under different stress conditions are verified and the fitting results show that the model has good applicability.Finally,the gas permeability model and the relative permeability model considering the gas-water coupling are introduced into the gas seepage field equation and the water seepage/transport field equation.Combined with the stress field equation of water-bearing coal,the gas-liquid two-phase seepage model and the single-phase gas seepage model are constructed.According to the actual situation of1507 coal face in No.2₁ coal seam of Hongling coal mine,the process of gas extraction in high water saturation coal seam and residual water state coal seam and the whole process of gas extraction in water bearing coal seam are simulated,and the law of gas and water flow in coal seam is analyzed.The influencing factors and the law of water/gas production in two stages were analyzed.It is found that in the early stage of coal seam extraction with high water saturation,the borehole water production is larger,the coal body water saturation is greatly reduced,the adsorbed gas is desorbed,the initial gas pressure of the coal seam will rise briefly,and the borehole gas production will rise rapidly.And reach the peak.With the increase of extraction time,the water production decreased significantly,and the gas production decreased rapidly after the peak,then slowly.The moisture content of the coal body around the borehole in the residual water state is large in the early stage of extraction,which has a great influence on the gas extraction.With the increase of the extraction time,the moisture of the coal body evaporates and with the gas transport loss,the coal body around the borehole gradually approaches the dry state.Gas drainage is almost unaffected by water.In this paper,the variation of gas drainage flow in residual water state coal seam under the action of various control factors is analyzed.The results show that the elastic modulus,water evaporation coefficient,The maximum water adsorption strain,Langmuir gas adsorption volume and Langmuir limit gas adsorption strain are positively correlated with gas extraction.Poisson's ratio,the initial moisture content and the water influence coefficient of gas adsorption are negatively correlated with the gas extraction quantity and the Langmuir gas pressure.Through numerical analysis and field verification,it is found that hydraulic measures make external water intrude into coal seam.Due to the coupling action of coal body,gas and water,the initial permeability of coal bed decreases,and the negative pressure of gas drainage is increased in the early stage of gas extraction.In order to improve the permeability of coal body and improve the effect of gas extraction,the moisture of coal body around the borehole can be removed at the fastest time.