Experimental Research On CO₂ Enhanced Coalbed Methane Recovery

With the rapid growth in demand for natural gas resources,coalbed methane with CH₄ as the main component is considered as a potential solution for natural gas supply.The geological conditions of coal mines in our country are complex,with extremely low coal seam permeability.Under low pressure,it is difficult to desorb the methane adsorbed in the coal matrix using conventional drainage and pressure reduction methods,and the CH₄ recovery is low.In order to solve this problem,scholars have proposed many methods to improve the recovery of coalbed methane.Among them,CO₂enhanced coalbed methane(CO₂-ECBM)can not only improve coalbed methane recovery,but also promote carbon neutralization by sealing CO₂ in unmineable coal seams in terms of environmental protection.This study carried out the CH₄/CO₂ mixed gas isotherm adsorption test,the seepage test of CH₄ and CO₂ in the raw coal under different stress environments,and the CO₂ fracturing raw coal increased permeability test,revealing the mechanism of CO₂ enhanced efficient exploitation of coalbed methane.The main research results obtained are as follows:(1)The three-component static high-pressure adsorption system was used to carry out the isotherm adsorption test of CH₄,CO₂ and different ratios of CH₄/CO₂ mixed gas at 30?,revealing the characteristics of coal competitive adsorption of CH₄ and CO₂.The fitting effect of the extended Langmuir model and the BET multi-component gas adsorption model on the adsorption capacity of CH₄/CO₂ mixed gas was compared and analyzed.It was concluded that the BET multi-component gas adsorption model was more suitable for describing the characteristics of coal adsorption of CH₄/CO₂ mixed gas.(2)The self-developed CO₂ fracturing coal rock test system was used to carry out the CH₄ and CO₂ seepage test in coal rock under different stress environments,and it was concluded that the adsorption expansion strain caused by coal rock adsorption gas would reduce the coal rock permeability,and at the same time,the change of stress environment would change the gas seepage law in the coal and rock.In a low-stress environment,the permeability changes in a V shape with the gas pressure.In a high-stress environment,the permeability changes with a negative exponential function as the gas pressure increases.(3)The simplified initiation pressure model is used to predict the theoretical initiation pressure of CO₂ fracturing coal,and it is concluded that the initiation pressure increases with the increase of the buried depth of the reservoir.Through CT scanning of the internal fracture expansion of the sample before and after CO₂ fracturing,combined with the CO₂ fracturing coal acoustic emission signal,it is concluded that the crack propagates along the original coal fracture direction,and the acoustic emission signal was very obvious during the crack initiation and sample rupture stage.After fracturing,the permeability of coal and rock increased by 1.78 to10.29 times.