Research On Evolution Characteristics Of Gas Seepage In Low Metamorphic Grade Coal Under The Combined Effects Of Gas And Water

China is rich in coalbed methane reserves of low rank coal and has important development value.Water and gas coexist in the in-situ coal reservoir.Water will affect the internal pore structure of coal,and the co-adsorption with gas will lead to the deformation of coal matrix,thereby affecting the gas seepage characteristics of coal.Especially low rank coal often has high internal water content,so it is of great significance to master the gas seepage characteristics of low rank coal affected by water for mine gas disaster prevention and gas drainage.To this end,theoretical analysis,laboratory experiments,numerical simulation and field engineering are combined to carry out the research on the water adsorption behavior of coal and the evolution of coal adsorption deformation and permeability under the influence of water,and then analyze the influence of water on the gas migration law of coal.The main conclusions are as follows:（1）The analysis of water vapor adsorption models shows that the water adsorption mechanism transforms from monolayer,multilayer to condensation successively;Water can significantly reduce the specific surface area and pore volume of coal pores,especially the effect of adsorbed water in coal on micropores（<10 nm）.When the moisture content was lower than 4.74%,the fractal dimension D₁ of coal pores decreased significantly and D₂ tended to be stable;with the increase of water content,D₁ decreased slowly while D₂ decreased significantly.（2）Based on the adsorption thermodynamics theory,the deformation model of coal adsorption gas was constructed.The nitrogen adsorption test results of water-bearing coal samples show that water will weaken the swelling deformation ability of coal to adsorb gas.The main reason is that water is easier to adsorb on the surface of coal,resulting in the decrease of gas adsorption capacity,and plugging effect on the micro-pore system,weakening the gas adsorption effect of coal,thereby resulting in the decrease of swelling deformation ability of water-bearing coal to adsorb gas.（3）Gas seepage test of coal results show that,the existence of water improves the effective stress sensitivity coefficient of coal sample,that is,under the same effective stress change,the permeability of water-bearing coal sample changes more greatly.Secondly,the matrix adsorption expansion deformation caused by water and gas in coal will narrow the fracture seepage channel;at the same time,water adheres to the surface of pores and fissures,resulting in the closure of seepage channels,which leads to a significant decline in the seepage capacity of gas in water-bearing coal samples.（4）Based on the Dual-porosity coal,the Dual-permeability model of coal under the influence of moisture and the mathematical model of multi physical field gas migration are constructed.Under the same initial gas pressure,with the increase of water content,the deformation of moisture absorption and expansion of matrix increases,resulting in the narrow seepage channel,the pressure difference between matrix and fracture and the decrease of initial permeability of coal,which significantly affects the flow and migration ability of gas in coal.At the same time,the desorption shrinkage ability of matrix is weakened under the influence of water,resulting in the decrease of permeability recovery near the extraction point and the delay of recovery inflection point.Under the same extraction time,with the increase of water content,the gas extraction flow and cumulative extraction volume in coal show a downward trend.