页岩黏土孔隙含水饱和度分布及其对甲烷吸附的影响

考虑储层原始含水特征,甲烷在页岩的吸附特征属于气液固三相复杂作用结果,水分在很大程度上影响页岩吸附能力,将成为制约页岩气资源量评估可靠性的主要原因之一.鉴于页岩水分主要分布于黏土等无机矿物孔隙内部,分析了甲烷-水膜-页岩黏土三相作用特征,结果表明:甲烷在干燥黏土表面吸附满足气固界面Langmuir吸附特征,在黏土水膜表面吸附满足气液界面Gibbs吸附特征,在气液固三相作用下满足"气固"与"气液"界面混合吸附特征;同时研究还发现:不同尺度孔隙内含水饱和度分布特征存在差异,部分小孔隙可以被水分充满,而大孔隙仅吸附一定厚度水膜.因此,水分对甲烷吸附能力的影响主要表现为两个方面:小孔隙被水分阻塞而失去吸附能力;大孔隙表面水膜改变甲烷吸附特征(气固界面吸附转变为气液界面吸附),以黏土样品为例,两者综合效应可以致使甲烷吸附能力降低约90%.从微观角度揭示了水分对页岩吸附能力的影响机理,将为建立合理评价页岩吸附气含量的方法奠定理论基础.

EFFECT OF WATER DISTRIBUTION ON METHANE ADSORPTION CAPACITY IN SHALE CLAY

Methane adsorption in shale is the result of gas-liquid-solid interaction when considering water saturation in actual condition. And the moisture (water saturation) which significantly influences methane adsorption capacity will likely make shale gas resources misestimated. In this paper, we analyze the interaction characteristics between methane, water film and clay base on adsorption theory, and results reveal that: (1) methane adsorption on clay (dry) could be described by gas-solid interface Langmuir adsorption equation;(2) methane adsorption on water film could be described
by gas-liquid interface Gibbs adsorption equation;(3) gas-liquid-solid interaction could be described by ’gas-solid’ and ’gas-liquid’ integrated equation. Meanwhile, we find that water saturation distribution is significantly effected by pore size, and micropores could be filled with water in certain condition while macropore only bound by water film. Therefore, the influence of moisture on methane adsorption is mainly for two aspects: (1) micropores which blocked by water are invalid for methane adsorption;(2) macropores bounded by water film change the interaction characteristics for methane adsorption (from gas-solid interaction to the gas-liquid interaction), and the overall effect could decrease the adsorption capacity by 90%in our study. Our present work reveals mechanism of moisture effect on the shale absorption capacity and lays the foundations of evaluating the adsorbed gas in shale gas reservoir more accurately.