Experimental Study Of Both The Binary Gas Adsorption And The Seepage On Coal In ECBM

Coalbed methane is one of the major hazard in underground coal mine,and it is also one of the most excepted energy in the list of new energy.The gas injected for enhanced coalbed methane recovery techique(ECBM),not only has a positive effect on methane drainage in both the underground borehole engineering and surface drilling well engineering,but also has invaluable potential for both the ouburst control management and the CO₂ geo-sequstration enginnering.This paper focus on the process analysis of the ECBM techique,refines the changing characteristics of the outflow CH₄composition,finishes the methodology on the experimental investigation for displacement effect of injected gas in ECBM by considering the time factor during diffusion process.Based on the research data,the relative relationships between the displacing effect and stripping effect of gas injected during ECBM process was revealed,and the change of gas flow capacity on coal during ECBM test was analysed.The migration of the stress-state in coal caused by the injection of pressurized CO₂ was discussed through theoretical analysis and stress-strain testing data.By a comprehensive study on the above research,the optimized of both the CH₄ production and CO₂ sequstrated strata's stability was analysed.The main conclusions are as follows.(1)Though the adsorption/desorption on pore surface in coal is a transient behaviour,the diffusion beviour on coal matrix has a strong time effect.Under the influence of diffusion,the adsorption of injected gas and the desorption of CH₄ on coal during ECBM is also time-dependent behaviour.Based on the changing characteristics of outflow CH₄ composition in ECBM experiment,CO₂ displacement adsorption on CH₄ saturated coal considering the time factor is conducted by referring to the conventional binary gas adsorption experiment,and the change in the adsorbed-state on coal during ECBM process has been obtained.By comparing with the EX-Langmuir theorestical curve,it is found that the adsorb-state curve in test is behind the theorestical curve,indicating that the time effect in difusion behaviour actually has a strong influence on gas migration in ECBM.(2)There are two basic sub-effects in the dispalcing effect of CO₂ to CH₄;the first is the free desorption due to CH₄ pressure(partial pressure/concentration)grasient,the second is the driven-desorption due to the decreasing effect of CO₂ on CH₄ adsorbed ability.According to CO₂ displacement adsorption experiment on CH₄ saturated coal,it is found that the pressure gradient effect presents an Extreme-Peak trend,and the CO₂enhanced effect on CH₄ desorption presents an exponential decay trend during test,and the CO₂ displacing effect shows a Double-Peak trend that decrease first,increase then,and decrease again,under the combined effect of two sub-effects.The exponetial decay trend also can be used to decribe the stripping effect in test.With the further study it is found that both stripping and displacing effect are all controlled by CH₄ composition in seepage fracture,but the stripping effect is more sensitive.Hence,stripping effect is the main effect in the early stage in ECBM and displacing effect is in the later stage.(3)The coal permeability in N₂-ECBM test has been improved,and in CO₂-ECBM test is damage obviously.However,the permeability data indicates that the coal permeability do not increase or decrease monotonously during test,but presents a complex trend.The coal permeability in both two tests shows a trend of rise,pseudo-steady,drop slowly,and steady.According to the theoretical analysis and experimental data,the flow state on coal can be divided into three stages:CH₄ free seepage,ECBM on unsteady-flow,ECBM on steady-flow.Meanwhile,three adsorbed-state simultaneously change with the flow state:CH₄ desorption(shrinkage deformation);injected gas adsorption/CH₄ desorption(pseudo-steady),injected gas adsorption(swelling deformation),which can be the convincing explanation for the complex trend of permeability in test.(4)The pressurized injection of CO₂ fluid into strata has a strong impact on stress-state in objective reservoir.The CO₂ fluid pressure,the swelling deformation and the deterioration of mechanical properties due to CO₂ adsorption,will increase the instability trendency of CO₂ sequsetration reservoir.The testing data of coal volumtric strain suggests that the adsorption strain caused by 2 MPa CH₄/CO₂ mixture can be equivalent to 4 MPa surface force on coal,indicating that the stability of strata cannot be ignored.According to the experimental data,the adsorbed ability of coal to CO₂decreases exponentially,which means the CO₂ sequestrated are gradually uneconemical in test.The simulated results of the soak process in test prove that the CH₄ concertration can be improved remarkable.Given that the soak can also relieve the accumulated elastic energy in strata,the CH₄ production concentration and CO₂ sequestrated strata's stability may be improved by a right time of soak technology.The paper contains 51 figures,13 tables and 151 references.