构造煤中煤层气扩散-渗流特征及其机理

煤层气产出一般要经过解吸、扩散和渗流三个阶段,而煤层气在变形较强的构造煤中的扩散过程不同于在原生结构煤或变形较弱的煤体中的扩散。外界压力的变化只是构造煤吸附与解吸整个过程的一种外在因素,构造煤的变形和结构变化以及吸附势场的转换才是构造煤吸附与解吸的内在因素,是导致解吸过程不可逆性的根本原因。当构造煤体与CH4等多元气体间的吸附平衡状态遭到破坏时,变形较强的构造煤在降压后会产生解吸滞后现象;而变形较弱的煤,分子结构中的气体会很快解吸,第一阶段是气体解吸作用,第二阶段是游离气体从微孔向较大孔隙扩散的过程,气体扩散速率主要由第二阶段决定。构造煤气体扩散机理主要是由孔隙形状、大小、连通性和多元气体性质和状态所决定的。韧性变形煤的微孔隙比较发达,所以韧性变形煤以Knudsen扩散为主,脆性变形煤的中、大孔隙所占比例较大,而且脆性变形煤的孔隙之间具有很好的连通性,所以脆性变形煤以Fick型扩散为主,脆-韧性变形煤以及接近脆-韧性变形煤的脆性变形煤和韧性变形煤均以过渡型扩散为主。在试井渗透率比较中,一定变形程度的脆性变形煤>韧性变形煤,脆性变形煤中以过渡孔为主,其余为微孔,测不出亚微孔和极微孔,脆性变形还增加了各孔隙之间的相互连通性。韧性变形煤中过渡孔比表面积所占比例下降,微孔和亚微孔增高,扩散主要发生在微孔和过渡孔中,所以韧性变形煤的试井渗透率低于脆性变形煤的试井渗透率。

Diffusion-osmosis characteristics of coalbed methane in tectonically deformed coals and their mechanism

There are three stages for the production of coalbed methane:desorption,diffusion and osmosis.The diffusion process of methane and multi-component gases in the deformed coals is not consistent with that in primary coals.The change of external pressure is only an external factor in the whole process of adsorptiondesorption in tectonically deformed coals; the essential factors of the process are the deformation,the change of structure and the adsorption potential.Those factors are the root causes which induce the irreversible process of desorption.When the balance of coal and multi-gas like CH4 has been destroyed,the tectonically deformed coals that deformed stronger shows a phenomenon of hysteresis of desorption after the pressure reduced,and the gas in weakly deformed coal will desorb quickly.The process of desorption has two phases:one is the gas desorption and the other is the process of free gas from micropores diffusing to the bigger pores; the latter one determines the gas diffusion rate.The gas diffusion mechanism of tectonically deformed coals was determined by pore shape,size,connectivity and the properties and condition of multi-gas.Ductile deformation coal has many micro-pores and its diffusion mode is the Knudsen diffusion.Brittle deformation coal has many bigger pores and the connectivity of pores is much better,so the diffusion mode is the Fick diffusion.The diffusion mode of brittle-ductile deformation coal is the transitional diffusion.The well test osmosis of brittle deformation coal is higher than that of ductile deformation coal and the well test osmosis of primary structure coal is the lowest.There are many mesopores in brittle deformation coal,and the others are micropores; sub-micropores and uhra-micropores cannot be detected.The brittle deformation also can increase the connectivity of pores.Ductile deformation coal has a small amount of mesopores,and the amounts of micropores and sub-micropores increase fast; the process of diffusion and osmosis happened in mesoporea and micropores,so the well test osmosis of ductile deformation coal was lower than that of brittle deformation coal.