黏土矿物吸附水蒸气特征及对孔隙分布的影响

摘要

页岩原始储层普遍具有一定的含水饱和度,水分的赋存方式很大程度上影响页岩孔径分布特征、吸附能力和产气机制.选用页岩常见黏土矿物蒙脱石、高岭石和伊利石,开展不同湿度环境下的水蒸气吸附试验和低温氮气吸附-脱附试验,研究水分在黏土矿物上的吸附行为,揭示纳米孔隙中水的赋存方式和分布特征,分析含水饱和度对孔隙分布的影响.结果表明:GAB模型能够很好地描述黏土矿物水蒸气吸附曲线,随着湿度的增加,水蒸气的吸附机制存在单层-多层-凝聚的转变;黏土矿物中水分的赋存特征随孔隙尺度而变化,并进一步对孔隙分布产生影响,以蒙脱石为例,当含水饱和度接近50%时,水分以吸附水(束缚水膜)形式存在于大孔隙中,以凝聚水的形式存在于小于5 nm的孔隙中并导致该尺度孔隙从孔径分布曲线上消失,同时造成微小孔隙(孔径小于10 nm)贡献的比表面积下降幅度超过80%,而干燥情况下明显夸大了微小孔隙对页岩气的吸附及流动的作用.%Water is ubiquitous within shale gas reservoirs,and its distribution can have a remarkable influence on pore size distribution,gas adsorption capacity and production mechanisms. In this study, montmorillonite, kaolinite and illite, as common clay minerals in shale reservoirs,were selected to carry out water vapor adsorption experiments. Meanwhile,N2ad-sorption/desorption experiments were also performed on the clay samples that were equilibrated in different relative humidity (RH) environment before the adsorption experiments. The characteristics of water adsorption were investigated,and its effect on pore size distribution of clay minerals was studied. The results indicate that the Guggenheim-Anderson-de-Boer (GAB) model can provide a good fit for the water adsorption data,and there is an adsorption mechanism transformation depending on the humidity,which can be called as"monolayer-multilayer-condensation". It also reveals that the distribution characteris-tics of water are different within different size of pores, which can cause a"disappearance"of fine pores in measured pore size distribution and decrease the specific surface area. In the case of montmorillonite, when water saturation approaches to 50%,water can exist in larger pores in the form of water films, and capillary water can appear in small pores, which can make the small pores(the diameter is less than 5 nm)"disappear"in the pore size distribution and reduce over 80% of the surface area contributed by the pores within 10nm. Therefore,the experiments conducted at dry conditions may overestimate the contribution of fine pores in clay minerals on shale gas content and flow capability.