煤层甲烷吸附相密度、吸附模型、吸附机理的再认识

Recognition of adsorption phase density,adsorption model,and adsorption mechanism of CH_(4) in coal

煤中甲烷吸附相密度的确定是准确测量煤中甲烷吸附量、评价煤中煤层气资源潜力的关键,也是认清煤中甲烷吸附控制机理、模型表征的基础。针对两淮煤田碎裂煤样(R_(o,max)约0.7%)进行了不同温度的高压等温吸附解吸实验,结合压汞和低温液氮对煤的孔隙结构定量表征,分析了煤中甲烷的高压吸附解吸特征,基于Gibbs过剩吸附量测量原理和截距法拟合分析了甲烷的吸附相密度、吸附相体积、理论吸附量与温度的关系,利用单分子层吸附理论的Langmuir模型、多分子层吸附理论的BET模型和微孔充填理论为基础的DA模型及其优化模型进行了拟合分析,基于截距法预测的甲烷吸附相密度和理论吸附量建立了基于不同吸附理论的混合模型,初步探讨了煤中甲烷的赋存及其控制机理。结果表明:甲烷过剩吸附量在吸附阶段和解吸阶段均会达到峰值拐点出现下降,过剩吸附量峰值对应的平衡压力与煤样的实验温度和煤样中孔隙结构有关;甲烷吸附相密度随温度升高而下降,两者呈幂函数关系,甲烷吸附相密度受温度和煤中孔隙结构控制,温度越高,孔隙越不发育甲烷吸附相密度越低。Langmuir模型、BET模型、DA模型及其校正模型对煤中甲烷过剩吸附量拟合表明:未经校正的吸附理论模型仅适用于低压阶段(低于10 MPa)煤中甲烷过剩吸附量的拟合,针对全压力段煤中甲烷过剩吸附量,未经校正的吸附理论模型和经过校正的多分子层吸附理论模型均不适合,采用甲烷吸附相密度优化校正的Langmuir模型、DA模型拟合煤中甲烷过剩吸附量时存在一定的偏差,能用于煤中甲烷过剩吸附量的拟合。建立的基于单分子层吸附和微孔充填吸附理论的混合模型拟合结果表明随着温度的升高,煤中甲烷赋存形式由单分子层吸附为主逐渐转变为以微孔充填为主,赋存方式的转变与温度、煤中孔隙的发育有关。

The determination of CH_(4) adsorption phase density in coal is crucial for accurately measuring methane adsorption capacity in coal,evaluating the potential of coalbed methane resources,and understanding the control mechanisms and model characterization of methane adsorption in coal.High-pressure isothermal adsorption-desorption experiments were conducted at different temperatures on some fractured coal samples(Ro,max about 0.7%)from the Huainan-Huaibei coalfield.The pore structure of coal samples were quantitatively characterized by mercury injection and low-temperature liquid nitrogen adsorption.The high-pressure adsorption-desorption characteristics of methane in coal were analyzed,and the relationship between CH_(4) adsorption phase density,adsorption phase volume,theoretical adsorption capacity,and temperature was analyzed based on the Gibbs excess adsorption measurement principle and the intercept method.The Langmuir model based on the monolayer adsorption theory,the BET model based on multimolecular layer adsorption theory,and the DA model based on the microporous filling theory and its optimized model were used for fitting and analysis.A mixed model of different adsorption theories was established based on the predicted CH_(4) adsorption phase density and theoretical adsorption capacity obtained from the intercept method.The occurrence and control mechanism of methane in coal were preliminarily explored.The results show that the CH_(4) excess adsorption reaches the peak and decreases at the inflection point in both the adsorption and desorption stages.The equilibrium pressure corresponding to the peak value of the excess adsorption capacity is related to the experimental temperature and the pore structure of the coal sample.The CH_(4) adsorption phase density decreases with increasing temperature and the two are in a power function relationship.The CH_(4) adsorption phase density is controlled by the temperature and pore structure of the coal,with higher the temperature and developed pores leading to,lower CH_(4) adsorption phase density.The fitting of the Langmuir model,BET model,DA model,and their calibration models for CH_(4) excess adsorption in coal shows that the uncalibrated adsorption theory models are only suitable for fitting the excess adsorption capacity of methane in coal at the pressures below 10 MPa.For the entire pressure stage,the uncalibrated adsorption theory model and the calibrated BET model are not suitable.However,the Langmuir model and the DA model calibrated based on CH_(4) adsorption phase density optimization have certain deviations when fitting the CH_(4) excess adsorption capacity in coal,but they could still be used for fitting.The mixed model based on the monolayer adsorption and microporous filling adsorption theory shows that as the temperature increases,the occurrence form of CH_(4) in coal gradually changes from monolayer adsorption to mainly microporous filling,and the transformation of the occurrence form is related to the temperature and pore development in coal.