Study On The Influence Mechanism Of Moisture On The Replacement Methane In Coal By Gas Injection

It has been widely recognized that coalbed gas injection can improve the flow capacity of coalbed gas.However,coalbed gas injection replacement experiments are mostly conducted for dry coal samples,and the influence of water in the coal seam during gas injection has not been considered.Especially in the process of gas injection to replace coalbed gas,the influence mechanism of water on coalbed gas injection replacement is unclear.In this paper,by means of experimental research,theoretical analysis,digital simulation and other means,the following conclusions are drawn from the above research:pore structure characteristics of water bearing coal samples,adsorption characteristics of water bearing coal samples for different gases,differences in the impact of water on methane in replacement coal under high-pressure injection and isobaric diffusion conditions,evolution characteristics and engineering prediction of methane replacement by gas injection in water bearing coal seams（1）The pore structure of coal samples and the adsorption characteristics of coal to water vapor were tested and analyzed by low-temperature liquid nitrogen adsorption and water vapor adsorption experiments respectively,and the adsorption forms of water in coal pores were calculated and analyzed based on DLVO theory.The adsorption isotherms of water molecules on the test coal samples are mainly of type II（approximately"S"type）.Under the action of van der Waals force,electrostatic force and structural force,water molecules exist on the surface of coal pores in the form of water film;When the humidity increases to the critical humidity,the film thickness reaches the critical thickness,leading to capillary condensation.（2）Competitive adsorption test for adsorbing single component gas（CH₄,CO₂and N₂）and mixed gas（CH₄/CO₂,CH₄/N₂ and CO₂/N₂）of coal water samples.With the increase of water content of coal sample,the adsorption constant a decreases linearly with the increase of water content,and the adsorption constant b decreases first and then increases slowly with the increase of water content.The adsorption capacity of CO₂ on coal surface is relatively high（compared with CH₄ and N₂）,which is the comprehensive performance of CO₂ affinity and accessibility.The SLD model shows that the three single component gases have different accessible surface areas in the test coal samples,revealing the differences in adsorption properties and relative affinity of the three gases on coal;In addition,with the increase of water content,the accessible surface area corresponding to N₂/CH₄/CO₂ gradually decreases,and its adsorption capacity decreases,revealing the mechanism of water’s influence on single component gas.（3）In the isobaric diffusion test,with the increase of water content in coal,the replacement amount of CH₄ gradually decreases,and water inhibits the replacement of CH₄ by gas injection;At the same water content,with the increase of adsorption equilibrium pressure,the replacement amount of CH₄ increases continuously,and the pressure plays a role in promoting gas injection replacement.At the same initial CH₄adsorption equilibrium pressure,with the increase of coal moisture content,the adsorption equilibrium pressure after CO₂ isobaric replacement increases approximately linearly,while the adsorption equilibrium pressure after N₂ isobaric replacement decreases approximately linearly.For coal seams with low water content,CO₂replacement can be preferred,which is conducive to reducing the gas pressure in coal seams;At the same water content,the smaller the initial adsorption equilibrium pressure,the better the displacement effect.（4）In the high-pressure injection test,the ratio of the amount of replacement CH₄with CO₂/N₂ for water bearing coal samples and that of replacement CH₄ with CO₂/N₂for dry coal samples decreases with the increase of water content;Moisture inhibits the replacement of CH₄ in coal by N₂ to a greater extent than that by CO₂.When CO₂ is injected,displacement occurs not only in large cavity adsorption sites,but also in small cavity adsorption sites,while when N₂ is injected,displacement occurs only in large cavity adsorption sites.（5）The smaller the initial adsorption equilibrium pressure of high-pressure injection is,the better the displacement effect will be.When the initial adsorption equilibrium pressure is 1.0MPa,1.3MPa and 2.0MPa,the displacement effect of CO₂injection is only 5-10 times that of N₂ injection;When the initial adsorption equilibrium pressure is low（0.5MPa,0.75MPa）,the displacement effect of CO₂ injection is far better than that of N₂ injection.At 0.5MPa,the displacement effect of CO₂ injection is 30～50times that of N₂ injection;At 0.75 MPa,the displacement effect of CO₂ injection is 13～20times that of N₂ injection.（6）Based on the dynamic model of gas injection replacement of water bearing coal samples,a theoretical model of gas injection replacement of water bearing coal seams was established,and the dynamic evolution characteristics of CO₂/N₂ injection replacement of methane in water bearing coal seams were analyzed numerically.When the coal body is injected with CO₂,the CO₂ rapidly flows into the coal body and diffuses in the coal body pores.The volume fraction of CO₂ increases rapidly and the volume fraction of CH₄ decreases rapidly.With the increase of the time for injecting CO₂ to replace CH₄ in the coal body,the CO₂ injected into the coal body gradually penetrates into each position of the coal body,and the volume fraction of CO₂/CH₄ in the coal body changes with the same rule.（7）With the increase of coal seam water content,the penetration time of CO₂injection increases sharply,and the penetration time of CO₂ injection is longer than that of N₂ injection;When the water in the coal seam is mainly adsorbed water,the penetration time of CO₂/N₂ injection increases linearly with the water content,while when there is pore water in the coal seam,the penetration time of CO₂/N₂ injection increases sharply with the water content,that is,the water greatly hinders the migration of injected CO₂/N₂in the coal seam,thereby greatly prolonging the penetration time of CO₂/N₂ injection.（8）The numerical simulation of enhanced gas drainage by coal seam gas injection shows that the daily gas production of coal seam gas only by using the drainage hole is the minimum;In the process of replacing CH₄ in coal by N₂ injection in water bearing coal seams,the daily gas production of CH₄ pumped increases significantly,and the peak of gas production occurs earliest,and the daily gas production is also the largest,but the peak of gas production lasts for a short time;In the process of replacing CH₄in coal by CO₂ injection in water bearing coal seams,the effect on the increase of methane production is long,and the peak of gas production comes late,but the duration is the longest.There are117 figures,47 tables and 249 references in this thesis.