四川盆地海相页岩水蒸气吸附特征及其主控因素

Characteristics and main controlling factors of water vapor adsorption in marine shale： A case study of the Lower Silurian Longmaxi shales in southern Sichuan Basin

张砚 惠栋 张鉴 张德良 蒋睿

Yan Zhang Dong Hui Jian Zhang Deliang Zhang Rui Jiang

页岩普遍含水，深入认识页岩孔隙系统中的水分赋存机制对页岩气高效开发具有重要意义。以四川盆地川南地区下志留统龙马溪组页岩为研究对象，采用重量法真空水蒸气吸附仪获取水蒸气等温吸附-脱附曲线，结合页岩组分及孔隙结构参数，定量化研究页岩孔隙中水分的赋存特征，剖析影响水分吸附行为的主控因素。结果表明：页岩水蒸气等温吸附曲线呈现单层吸附、多层吸附和毛细凝聚3个典型阶段，水蒸气回滞环可划分为中-低相对压力阶段“上凸型”和全相对压力阶段“扁平型”，粘土矿物层间水滞留是中-低相对压力阶段回滞环形成的主要原因，采用GAB和FHH模型可有效描述页岩水蒸气吸附曲线形态特征；页岩吸附水量与粘土矿物含量呈显著正相关趋势，粘土矿物强亲水特性是影响水蒸气吸附量的主要因素，而水蒸气吸附量与碳酸盐矿物间存在负相关性，碳酸盐矿物含量高不利于水分赋存；对于富有机质页岩，水蒸气吸附能力与总有机碳含量（）相关性较强，无机矿物与有机质组分共同控制着水分赋存行为，干酪根有机孔可为水分吸附提供赋存空间，且样品比表面积和孔容越大，水分吸附能力越强，但是对于较低的粘土质页岩，粘土矿物与水分间的强物化作用是水分能够大量赋存的关键。

All shales contain a certain amount of water， and in-depth understanding of the occurring mechanism of water in shale pore system is of great significance to the efficient development of shale gas. Typical shale samples taken from the Lower Silurian Longmaxi Formation in the southern Sichuan Basin are applied to quantitatively study the occurrence characteristics of water in shale pores and analyze the main factors controlling water adsorption within shales. The study is carried out by means of the isotherm adsorption and desorption curves of vacuum water vapor generated by gravimetric adsorption instrument， coupled with parameters of shale compositions and pore structures. The results indicate that the isothermal curves of water vapor adsorption for the shale samples present three typical stages： single-layer adsorption， multi-layer adsorption and capillary condensation. The water vapor hysteresis loop can be divided into two stages of the “convex type” in medium-low relative pressure stage and the “flat type” in the whole relative pressure stage. The formation of the hysteresis loop in medium-low relative pressure zone mainly lies in reluctant interlayer dehydration of clay minerals. GAB and FHH models can effectively describe the characteristics of the adsorption curves. Furthermore， there is a significant positive correlation between the clay mineral content and the water adsorption amount， and it is believed that the strong hydrophilic property of clay minerals is the main factor controlling the amount of water vapor adsorption. While a negative correlation between carbonate mineral content and the water adsorption amount is observed. High carbonate mineral content is not conducive to the occurrence of water. For organic-rich shales， the adsorption capacity of water vapor has a strong correlation with the content. Inorganic minerals and organic matter components jointly control the occurrence behavior of water molecules. The organic pores of kerogen can provide storage space for the adsorption of water molecules， and the larger the specific surface area and pore volume of the sample， the stronger the water adsorption capacity. However， for the clayey shale with low content， the strong physicochemical interaction between clay minerals and water is key to large occurrence of water.

10.11743/ogg20220612