甲烷在煤的微孔隙喉道通过性及其对解吸的影响机理

煤是天然的多孔介质,其内部含有大量破碎煤块与多种类型的微孔隙结构,甲烷在煤的微孔隙喉道通过性是影响甲烷解吸效率的重要原因之一。基于单孔喉模型,分别对孔喉直径、孔喉几何形态与甲烷分子在孔喉附近的Leonard-Jones势的关系进行了分析;基于两能态模型,得到了孔喉势阱/势垒的几何临界尺寸及变化规律,研究发现,当R/r_0(孔喉半径/甲烷分子直径)0.89时,孔喉平面位置对甲烷分子存在势垒,孔喉平面两侧存在势阱;当R/r_00.89时,孔喉平面仅存在势阱,势阱深度随着孔喉增大逐渐降低。基于玻尔兹曼能量分布定律与麦克斯韦分子速率分布原理,推导出了温度、压力、孔喉直径对微孔隙吸附/解吸甲烷过程中的孔喉通过性影响的定量关系,分析表明温度、压力差、孔喉直径是影响甲烷通过孔喉的主要因素。温度越高,压力差越大,孔喉直径越大,孔隙甲烷的通过性越好,反之则通过性越差。基于上述理论分析,建立了煤的非均匀孔喉结构模型,通过数值模拟方法进行了含孔喉微孔隙吸附/解吸甲烷规律研究。研究证明,煤中孔喉势阱对甲烷分子运移的阻滞作用,是引起煤层甲烷吸附/解吸速率下降与甲烷解吸滞后现象,导致煤层气开采期限内解吸率低下的重要原因之一;微孔隙孔喉越小,其影响越明显。研究结果对于煤层气开采效率评价及煤层原位致裂增透改性强化煤层气开采提供借鉴。

Accessibility of methane at micro-pore passage and its effect on the methane desorption in coal

Coal is a natural porous medium with a large number of fractured coal blocks and various types of pore. The accessibility of methane at micro-pore passage has effect on the methane desorption in coal. In this paper,the blocking mechanism of potential well and barrier at micro-pore passage to methane migration is analyzed,the inhomogeneous micro-pore passage model of coal is established,the methane adsorption and desorption rate as well as methane desorption hysteresis characteristics in coal are studied. It is found that when R / r0(pore passage radius/ methane diameter) is less than 0. 89,there is a potential barrier at the pore passage plane,and there are potential well on both sides of the pore passage plane. When R / r0 is greater than 0. 89,there is only a potential well at the pore passage plane,and the depth of the potential well decreases gradually with the increase of the pore passage radius. The temperature,the pressure difference and the pore passage diameter are the main factors affecting methane passing through the pore passage. By numerical experiment results,it is found that the blocking effect of micro-pore passage potential well and barrier is the fundamental reason for the slow process of methane desorption and the failure reach to 100% of desorption rate. The blocking effect of micro-pore passage on methane migration in coal micro-pores results to methane desorption hysteresis under different pressures. In the process of coalbed methane exploitation,the methane desorption rate and methane desorption proportion should be effectively improved by changing the distribution characteristics of micro-pore passage in coal through fracturing. The research results have important theoretical guiding significance for the evaluation of CBM recovery efficiency and the enhancement of CBM recovery by coal seam fracturing and permeability enhancement.