Mechanism Of Gas Transport In Water-intrusion Coal Seam And Its Application

The hydrogeological conditions of coal mine are very complex in China,and most of the coal seams are located in the water-rich coal strata.However,it is a serious problem that formation water intrudes into the boehole in mine gas prevention and control.For the measuring-pressure borehole,the formation water in borehole can lead to the faliure of test.For the extraction borehole,especially for the long-distanece downward borehole,it is difficult to predict the gas extraciton quantity because the water in borehole and coal seam changed the gas migration mechanism.In this paper,a series of study methods including theoretical analysis,laboratory experiments,numerical simulation and field test were used to improve the success rate of measuring pressure and to predict the gas extraciton.First,the evolution law of pore structure in the water-intruion coal was studied.Second,the dissolution law of gas in aqueous solution was analyzed.Then,the mechanism of gas diffusion in water and water-saturated coal was reveled.Moreover,the mathmatic model of gas transport in water-intrusion coal seam was constructed and the mechanism of gas transport in water-intrusion coal coal was investigated.Finally,the gas extraction quantity was predicted accurately and a method for measuring pressure based on gas solubility were proposed.The influence of high-pressure water intrusion on the pore structure of coal with different ranks were simulated in laboratory.The evoluaiton law of mineral and pore structure in coal before and after water intrusion were qualitatively and quantitatively analyzed by using XRD,low-temperature N₂ adsorption and CO₂ adsorption experiments.Results show that,water intrusion has different effects on different minerals in coal,but generally reduces minerals in coal,especially for the clay minerals and calcite.Water intrusion could increase the pore volume and specific surface area of coal,especially for the pore volume and specific surface area in micropore.The main material components in different mine aqueous solutions were determined experimentally.Furthermore,different types of aqueous solutions were configured based on the test results.By using the self-designed gas solubility experimental system,the law of gas dissolution in different temperatures,pressures,mineral concentrations and mine aqueous solutions were investigated.Results show that,the cations in the mine water are primarily K⁺and Na⁺,and the anions are primarily Cl^-and HCO₃^-.In addition,the mine water contains a certain amount of organic matter.In the test range,the gas solubility increases with the rising pressure,decreases with the increase in temperature and salinity.The gas solubility in the mine aqueous solution is significantly higher than that in the distilled water,and the gas solubility has a significant linear relationship with the equilibrium pressure.It can be inferred that the organic matter in the mine water is an important reason to improve the gas solubility in the mine water by comparing with the distilled water.Combined with the experimental results,the effects of various factors on gas solubility were explained by gap filling theory,hydration theory and soluble organic matter solubilization.The diffusion and migration laws of gas in different pressures,temperatures,mineral concentrations and mine aqueous solutions were investigated.Results show that,the diffusion coefficient of gas in aqueous solution increases with the increase in pressure and temperature,and it increases first and then decreases with the rising mineral concentration.The soluble organic matter in the mine aqueous solution could greatly improve the gas diffusion coefficient.A model for effective diffusion coefficient of gas in water-saturated coal was established.The effective diffusion coefficients of gas in different-rank water-saturated coal with different pressures were tested.The influence of pore structure on the effective diffusion coefficient of gas was investigated by NMR experiment.Results show that,compared with the traditional radial effective diffusion coefficient model,our model is more suitable for calculating the effective diffusion coefficient of gas in porous media with strong adsorption capacity such as coal.As the pressure and the coal rank increase,the effective diffusion coefficient of gas in the water-saturated coal gradually increases.With the rising coal rank,the pore size of coal gradually changes from microporous/mesoporous to microporous,and the pore connectivity of coal is reduced,resulting in a decrease in the effective diffusion coefficient of gas.Finally,the influence mechanism of gas pressure,pore structure and adsorption effect on the effective diffusion coefficient of gas was analyzed.A mathematical model of gas migration in water-intrusion borehole for measuring pressure was established,and it considered the coal deformation,solution convection induced by density gradient and solute migration.Then,this model was validated by field data.The effects of diffusion coefficient,water-intrusion radius and solution convection on the equilibrium time of gas in borehole water were studied numerically.Furthermore,a method for measuring gas pressure based on gas solubility was proposed,and the relevant pressure measuring equipment was developed.The equipment was used to measure the gas pressure of the No.10 coal seam of Taoyuan coal mine and the No.5 coal seam of Xinhu coal mine.The research results could provide a new idea and a method for measuring gas pressure in water-intrusion borehole.A mathematical model of gas migration in water-intrusion borehole for gas extraction was established.This model is divided two parts:gas seepage in original coal seam and gas diffusion in water-saturated coal seam.Henry's law was used to transform the seepage flow driven by the pressure difference into the diffusion migration driven by the density difference.The influences of diffusion coefficient,permeability,Henry's constant and water-intrusion radius on gas migration were investigated by numerical simulations.Finally,the gas extraction quantity of the water-intrusion borehole in the 726 working face of Qingdong coal mine was accurately predicted by this model.The research results can provide theoretical guidance and engineering experience for the gas prevention and control of mine.This paper contains 59 figures,25 tables,and 248 references.