Ad-/desorption Equilibrium Of Coal Reservior And Its Alterations After Super-critical CO₂ Exposure

Coalbed methane?CBM?was conventionally deemed as a dangerous factor threatening underground mining safety,which has also been recognized as an unconventional natural gas with priority of less carbon emission comparing to fossil fuel.On the other hand,the world is committing to carbon emission cuts against climate change,where the captured CO₂ can be alternatively sequestrated into deep unminable coal seams which can simultaneously enhance CBM recovery?CO₂-ECBM?.The main mechanism of CO₂-ECBM is the competition adsorption between CH₄ and CO₂ molecules,whereby the adsorbed CH₄ molecules are displaced by CO₂ molecules which have a greater affinity with coal matrix.Meanwhile,the composition,organic structure and pore structure of coal reserivors are key factors of CH₄ and CO₂adsorption capacity.Therefore,the study of coal composition and structural characteristics as well as its ad-/desorption capacities of CH₄ and CO₂ have great significance for evaluation of the potential of CBM production and CO₂ sequestration.It has been acknowledged that complex fluid-solid interactions do exist between super-critical CO₂ and coal matrix,which may alter physical and chemical structure of coal reservior,and further influence both CBM recovery efficiency and CO₂sequestration capacity.Therefore,it is essential to study the alteration of ad-/desorption characteristics of coal reservoir by super-critical CO₂ exposure.Four coals of different rank were studied in this research.The compositions of the coal samples were first analyzed by ultimate and proximate analyzes.Then,Fourier Transform infrared spectroscopy?FT-IR?analysis was conducted to characterize coal surface properties.Combining mercury intrusion,physisorption method with carbon dioxide?CO₂?at 273 K and nitrogen?N₂?at 77 K were used to quantify a broad pore size distribution of the coal samples.CH₄ and CO₂ ad-/desorption isotherms at 288 K,308 K,and 328 K and water vapor adsorption isotherms at 298 K were established using the accurate gravimetric method.A specially designed super-critical CO₂-coal reactor was used for super-critical CO₂ exposure experiments?T=40?,P=16 MPa?.The surface properties and pore structure of coal samples before and after super-critical CO₂ exposure were analyzed.The main conclusions are as follows:?1?With the increase of coal rank,the fixed carbon content increased,while the volatile matter content decreased.Also,density of the functional groups on coal surface decreased with maturity.In addition,the determined micropore specific surface area?SSA?and volume showed an“U”shaped function in terms of coal rank,while the mesopore SSA and volume decrease with the increase of coal rank.The macropore volume is mainly affected by deformation of the coal seams under tectonic stress which have no obvious relationship with coal maturity.Cracks are dominant in low-ranked coal,cylindrical and wedge-shaped pores developed in middle-ranked coals while slit-shaped pores dominant in high-ranked coal reserviors.?2?The adsorption isotherms of CH₄ and CO₂ for different coal reserviors can be well fitted by Langmuir equation.The monolayer adsorption capacity of CH₄ is determined by micropore SSA,while that of CO₂ is determined by conbined micropore SSA and the density of surface functional groups.The equilibrium selectivity of CO₂over CH₄ decreased with the increase of temperature,following an“U”shaped function of fixed carbon.The water vapor adsorption isotherms of different coal ranks are type III and can generally be described by Dent equation.The water vapor uptake capacity decreased with the increase of coal rank,which is mainly determined by the density of surface functional groups.?3?The adsorption/desorption hysteresis can be observed in both CH₄ and CO₂isotherms of different coal rankes under low pressure.Also,the mass of coal samples could not reach to their initial value under vaccum,indicating residual amount of sorptive gas exists in coal samples.In order to describe the adsorption/desorption hysteresis quantitatively,the hysteresis index?HI?and residual index?RI?were introduced in this study.The results indicate that both the HI and RI decaeased with the increase of temperature.In addition,the HI and RI of CO₂ were smaller than that of CH₄ under the same conditions.The adsorption isotherms were then decomposed into simultaneously running physical adsorption and absorption branches based on the assumption that the former is totally reversible and the latter completely irreversible.The physical adsorption branches can be well described by both Langmuir equation and Dubinin-Radushkevich?D-R?model.The absorption,which represents the hysteresis portion,decreased with temperature and follow an U-shaped function of coal rank.?4?After exposing to super-critical CO₂,species of the functional groups on coal surface kept constant,while the density of oxygen-containing functional group decreased.In addition,The SSA and pore volume decreased in mesopore but increased in micropore pore.The surface properties and pore structure alteration in reservoir led to the following effects on the ad-/desorption characteristics of CH₄ and CO₂:?1?The q_m increased for CH₄ but decreased for CO₂;?2?The equilibrium adsorption selectivity of CO₂ to CH₄ increased,which is benefical for the displacement of CH₄ by CO₂;?3?The hysteresis index for both CH₄ and CO₂ become weaker,The HI and RI decreased;?4?The q_m and V₀ of physisorption decreased for CH₄ but increase for CO₂.The absorption capacity of CH₄ and CO₂ decreased.