Productivity Difference Of CO₂ Rich Coalbed Methane Wells And Adsorption Thermodynamic Mechanism In The East Of Haishiwan Coalfield

Yaojie mining area is a typical CO₂rich mining area.Compared with other coal mining areas,the CO₂content in mine gas is high and changes greatly,and there are obvious differences in the genesis,composition and occurrence of coalbed methane.In order to explore the productivity characteristics and gas production mechanism of coalbed methane wells in this area,this paper takes Haishiwan minefield in Yaojie mining area as the research area,based on the geological data of coalbed methane in the area and the production data of 8 coalbed methane wells,the geological conditions and gas bearing distribution characteristics of coalbed methane in Haishiwan minefield were systematically studied.Using the numerical simulation software,the productivity of 8production wells was simulated according to the CO₂concentration of 20%,40%,60%and 80%.With the help of relevant testing methods,the thermal reasons for the difference of gas adsorption of coal to different compositions and its influence mechanism on coalbed methane well productivity were discussed.The research results further deepen the theory of coalbed methane adsorption,provide guidance for the practice of coalbed methane exploration and development in Haishiwan mine field,and provide reference for the development of coalbed methane in other mine fields in the mining area.The results show that:（1）The structure in the study area is developed,and the composition of coalbed methane is mainly CO₂.The distribution in the well field is characterized by high in the middle and east,low in the west.The distribution of CH₄is characterized by high in the south and low in the north,high in the west and low in the east.And affected by the fault,the closer it is to F₁₉fault,the higher the CO₂content.The gas content of coal seams in the study area is controlled by many factors,among which the thickness of coal and coal measure strata have a great influence on the gas content,and the buried depth and fault structure are the main factors affecting the gas composition.（2）The effects of temperature and pressure on the heat of adsorption and CH₄single-component gas adsorption showed an opposite pattern,showing that the higher the temperature,the smaller the heat of adsorption of coal on the gas,the longer the adsorption equilibrium time,and the smaller the CH₄adsorption;the higher the pressure,the larger the heat of adsorption,the reduced adsorption equilibrium time,and the larger the CH₄adsorption,indicating that high temperature is not conducive to gas adsorption.And both temperature and pressure showed a negative correlation on the time to reach the peak of heat of adsorption.（3）In adsorption of gas mixture with different components,the adsorption amount of Kiln Street coal to CH₄/CO₂gas mixture is always between pure CO₂and pure CH₄,and the adsorption amount of coal to gas mixture increases with the increase of CO₂proportion.That is,the higher the CO₂content in the coal seam,the stronger the coal’s adsorption capacity for the gas,and reaching the same desorption amount will require a larger pressure drop,which is not conducive to gas desorption.With the increase of pressure and decrease of temperature,both N₂and CO₂heat of adsorption increase.However,the increase of heat of adsorption of CO₂is stronger than that of N₂,which is the main reason why the coal body has a stronger ability to adsorb CO₂than N₂.（4）Production practice shows that as the distance from the extraction workings and the F₁₉fracture decreases,the gas wells take longer to see gas and stabilize production（such as D2 well）.The reason is that the CO₂content in the coal seam increases as the burial depth becomes shallower,and due to the higher heat of adsorption of CO₂and the stronger adsorption capacity than CH₄,the more difficult it is to desorb the coalbed gas,the longer the time to see gas,and the lower reservoir temperature,the lower gas desorption rate,so the stable gas production time increases while the gas production efficiency decreases.