Experimental Study On Seepage Characteristics Of Gaseous/Supercritical CO₂ And CH₄ Mixture In Coal

Coalbed methane reserves are huge in China,and the coalbed methane industry has good development potential.However,the permeability of most coal seams in China is relatively low,and the CH₄ stored in the coal is difficult to desorb,resulting in unsatisfactory coal seam gas production efficiency.Injecting CO₂ into the coal seam is considered to be one of the most effective ways to solve this problem.This technology not only can effectively increase the output rate of CH₄,but also can carry out CO₂ geological storage and reduce the greenhouse effect.In addition,the high temperature and high pressure environment of the deep coal seam makes the injected CO₂ extremely easy to reach the supercritical state.In the process of gaseous or supercritical CO₂flooding CH₄?CO₂-ECBM?,the permeability of the two has an important influence on the adsorption of CO₂,the desorption and production of CH₄.To this end,the seepage test of gaseous/supercritical CO₂ and CH₄ mixed gas in coal is carried out,and the variation of coal permeability and effective permeability of CH₄ and CO₂ during the seepage process is studied.In the process of permeation experiment,the gas composition and content of the injection end are arranged in advance according to the experimental purpose and the scheme;the gas composition and content in the gas produced at the outlet end are regularly and accurately measured.In addition,considering that natural coal seams generally have a certain moisture content,this paper also conducted a permeability test of CO₂/CH₄ under certain moisture content conditions.The main conclusions are as follows:?1?During the mixed infiltration process of gaseous or supercritical CO₂and CH₄,it was found that under the two kinds of mixed mode flooding,in the initial stage of the experiment,the CH₄ volume fraction in the produced mixed gas was larger than the injected CH₄ volume fraction,which reflected the replacement effect of CO₂ on CH₄;with the increase of time,the volume fraction of CH₄ produced gradually decreases under the condition that the volume fraction of CH₄ in the injected mixed gas remains relatively stable,but it is always higher or not lower than that of CH₄ volume fraction in the injected mixed gas.Among them,when the supercritical CO₂ and CH₄ are mixed and permeated,the volume fraction of CH₄ in the produced gas is more than 2%higher than the volume fraction of CH₄ in the injected mixture.It is characterized by a stronger replacement of the CH₄ effect by supercritical CO₂.?2?When He,gaseous CO₂ and CH₄ are mixed in different volume fractions,the permeability of all the above-mentioned osmotic medium in the coal decreases first and then increases slowly with the increase of the inlet pressure.Under the same injection pressure conditions,He permeability is the largest,CH₄ is the second,and CO₂ is the smallest.When CO₂ and CH₄ are mixed and infiltrated,the mixed gas permeability decreases with the increase of CO₂ volume fraction in the mixed gas.For the bound coal-water specimens,the permeability of the different component gases?He,CH₄,CO₂?is reduced by an order of magnitude compared to the no water conditions.?3?The permeability of supercritical CO₂ and CH₄ mixed in coal is always smaller than that when CH₄ is simply injected.And as the volume fraction of supercritical CO₂ in the mixed gas increases,the permeability of the coal gradually decreases.?4?Compared with CH₄ and gaseous CO₂,supercritical CO₂ are mixed with CH₄.On the one hand,the viscosity is larger and the compressibility is smaller,which leads to the weakening of the gas Klinberg effect.On the other hand,supercritical CO₂ may cause greater swelling deformation in the coal due to a large amount of adsorption,which makes the coal seepage channel become narrower.Under the combined effect of viscosity and swelling deformation,the effective permeability of CH₄ during supercritical CO₂ and CH₄ mixed percolation is always less than the effective permeability of CH₄ when the same concentration of gaseous CO₂ and CH₄ are mixed.?5?When gaseous or supercritical CO₂ is mixed with CH₄,the effective permeability of CH₄ increases with the increase of CH₄ volume fraction.When CO₂ and CH₄ are mixed and infiltrated,the effective permeability of CO₂decreases with the decrease of CO₂ volume fraction.When supercritical CO₂ and CH₄ are mixed and infiltrated,the effective permeability of CO₂ remains basically unchanged with the decrease of CO₂ volume fraction.?6?Single component CH₄,CO₂ and gaseous CO₂ mixed with CH₄:The relationship between the axial strain produced by coal and time can be approximated by Langmiur relationship;the coal deformation and inlet pressure caused by the adsorption swelling of different volume fractions of CH₄/CO₂mixed gas obey the Langmiur equation.The maximum axial swelling strain?a?of the coal during CO₂ penetration is the largest,reaching 0.00569,and with CO₂ volume fraction in the mixed gas increases,the value of a gradually increases.