Study On Gas Displacement With Gaseous Phase And Two Phase Synergy Mechanism Within Coalbed

Gas disaster is the major hazard to the underground mining work.With the increase of mining depth and intensity of coal mine,the management of gas disaster is becoming more and more urgent.Gas displacement using gaseous phase plays an important role in the gas control,and the domestic and foreign scholars have carried on the thorough research and proposed some theoretical models.However,it is necessary to study the mechanism of gas desorption and seepage in gas displacement.In this paper,the effect of temperature and pressure on the gas displacement with carbon dioxide was studied,and the seepage and desorption experiments of strongly adsorbed gas in coal were carried out.The important role of pore deformation in gas seepage and desorption was revealed.The theoretical model of coal permeability was also constructed.Carbon dioxide can effectively displace gas from coal,but the high deformation of matrix leads to the decrease of coal permeability.The amount of gas injection would be reduced,and a lot of carbon dioxide would be adsorbed.In order to reduce the content of mixed gas in coal and improve the control of gas disaster,gas displacement using gaseous and liquid was proposed to absorb the residual gas by injected alkaline water after carbon dioxide injection.This method significantly improves the water content of coal seam after water injection.A large amount of water is injected into the pores to replace the gas,which improves the displacement capacity of gas.At the same time,the "fragmentation" effect of the minerals in the alkaline solution resulted in the development of the pores,which further improved the gas displacement in coal.The effect of temperature and pressure on the gas displacement using carbon dioxide in coal bed was studied.The increase of pressure and temperature is helpful to weakly improve the displacement efficiency.The result shows that increasing the injection pressure simply can easily lead to the increast of cost.It is concluded that the suitable displacement pressure is twice the original reservoir pressure.The increase of temperature is helpful to improve the displacement efficiency,However,it is difficult to improve the temperature of underground coal seam.Coal sample permeability testing device was set up,the control mechanism of stress,gas pressure and Klinkenberg effect in the evolution of coal permeability was analyzed by experiments.With the increase of pore pressure,adsorbed gas permeability decreases first and then increases with the matrix deformation and Klinkenberg effects at the constant stress.With the increase,of stress The coal permeability decreases exponentially at the constant pore pressure.The coal permeability decreased exponentially at the constant effective stress,but the coal permeability using weakly absorbed gas became smooth and permeability suing strongly absorbed decreased until the coal swelling become stable.The results show that the carbon dioxide injection increases coal swelling and reduces the permeability,which will reduce the carbon dioxide injection.Desorption experiments of methane and carbon dioxide from coal samples were carried out using isothermal adsorption device,desorption hysteresis of gas was analyzed by changing the initial desorption pore pressure.The hysteresis effect of methane is higher than that of carbon dioxide,and the hysteresis effect of gas in high rank coal was weaker than tat in other ranks coal.At the same time,the adsorption capacity of coal is greater at the larger initial pore pressure.The results of coal pore testing show that the three kinds of coal samples expand after the carbon dioxide adsorption,aperture of coal pore reduces.The “pore deformation” can only explain the desorption hysteresis of gas in some coal sample.The “pore deformation” is the main reason and “solid solution” methane is the secondary factor,the desorption hysteresis of gas occurred combing both of two reasons.The permeability and porosity models of coal were established with the stress strain and sorption strain,and the models were verified.The results show that the weight of matrix deformation in permeability and porosity is extremely significant.The fractal theory is used to analyze the relationship between the fractal characteristics of coal porosity and evolution laws of absolute permeability.The theoretical value is close to the experimental data at the low pore pressure,but the experimental value is lower than theoretical value with the Klinkenberg effect.It can be used to predict the coal permeability when the carbon dioxide was injected.The width of fracture changes with the stress strain and sorption strain effects,the variation of Klinkenberg coefficient with stress and pore pressure changes was discussed.A permeability model with variation Klinkenberg effect was established,and it was verified using experimental data.Carbon dioxide can effectively displace the adsorbed methane in coal.But the obvious strain of coal body after carbon dioxide injection can easily lead to the decline of coal permeability,then caused the carbon dioxide injection reducing.At the same time,a large amount of mixed gas would be adsorbed in coal and influences the coal mining.Aiming at the problem of increasing the content of mixed gas in coal after carbon dioxide injection,the method which is injecting alkaline solution after carbon dioxide injection was proposed.The water injection is improved and the residual carbon dioxide is dissolved by alkaline solution.Water injection can also enhance the displacement of gas in the coal seam.Self absorption ability of coal to alkaline solution in gas-water-coal system is improved after carbon dioxide injection,especially in the non-penetrating fracture.The problem that a large amount of carbon dioxide was adsorbed was solved.The effect of gas displacement using gaseous and liquid was analyzed by the coal permeability test device.Compared with the coal seam water injection,the water content of coal body is improved by 54.8% after alkaline solution injection.The gas outburst prevention by water injection is improved,and the gas displacement is enhanced with the increase of water content.The corrosion mechanism of alkaline solution on the physical and chemical properties of coal was analyzed by means of low temperature nitrogen adsorption,scanning electron microscopy and infrared spectroscopy.The minerals of coal body would be broken after alkaline solution eroding,resulting in the development of coal pores and increasing the coal porosity.The results show that the method which is injecting alkaline solution after carbon dioxide injection could reduce the gas content of coal.The water content of coal is increased,and then the gas displacement is enhanced.At the same time,the gas disaster controlling is improved with the alkaline solution eroding.