水化作用下页岩微观孔隙结构的动态表征——以四川盆地长宁地区龙马溪组页岩为例

为了研究真实压裂环境下水化作用对页岩孔隙结构的影响，选取四川盆地长宁地区下志留统龙马溪组页岩样品，在90 ℃储层温度下开展了页岩压裂液自吸及水化实验；采用扫描电镜、低温N2吸附、高压压汞、CT扫描等实验手段，对比了水化0 d、5 d、10 d、20 d时页岩样品颗粒形态、孔径、比表面积等孔隙结构参数的宏观演变过程，并且对页岩孔隙结构变化的原因进行了剖析；开展单黏土矿物（蒙脱石、伊利石）水化实验，对比单黏土矿物的水化特征，进而从机理上研究了水化对页岩微观结构的影响。研究结果表明：①黏土矿物水化可以促进页岩层理面间微裂缝的产生，由于水化诱导裂缝尺度较小，分布较为密集，微观上能局部相互连通，从而对页岩储层物性有明显的改善作用；②随水化时间延长，微裂缝由延伸扩展到趋于闭合，孔隙体积先增大后减小，并在水化5 d时达到最大值；③黏土矿物水化膨胀相对于水化应力变化的滞后性是导致页岩微观结构变化的主要原因，并且伊利石的水化膨胀体积小于蒙脱石；④无机阳离子可以抑制黏土矿物水化，K+、Na+、Ca2+的抑制效果依次变差。结论认为，水化作用可以提高页岩储层的渗透率，研究区龙马溪组页岩储层压裂后的合理焖井时间推荐为5 d，而对于蒙脱石含量较高的页岩气储层则可以适当延长焖井时间。

Dynamic characterization of microscopic pore structures of shale under the effect of hydration: A case study of Longmaxi Formation shale in the Changning area of the Sichuan Basin

In order to study the effects of hydration on shale pore structure in a real fracturing environment, this paper selected the samples of Lower Silurian Longmaxi Formation shale in the Changning area of the Sichuan Basin to carry out imbibition and hydration experiments of fracturing fluid under the reservoir temperature of 90 ℃. Then, the macroscopic evolution process of pore structure parameters (e.g. morphology, size and specific surface area) of shale sample particles after hydration of 0 d, 5 d, 10 d and 20 d were compared by means of scanning electron microscopy, low-temperature N2 adsorption, high-pressure mercury injection, CT scanning and other experiments. In addition, the reasons for the change of shale pore structure were analyzed. Finally, hydration experiments were carried out on single clay mineral (montmorillonite and illite) to compare the hydration characteristics of different clay minerals. And based on this, the influencing mechanism of hydration on shale microstructure was studied. And the following research results obtained. First, clay mineral hydration can promote the generation of micro-fractures between shale bedding planes. Hydration induced fractures are of small scale and dense distribution, so they can be locally interconnected in the microscopic level to improve the physical properties of shale reservoirs significantly. Second, as hydration goes on, micro-fractures approach to closure from propagation, and the pore volume increases first and then decreases. And it reaches the maximum value after hydration of 5 days. Third, the hysteresis of clay mineral hydration swelling relative to hydration stress change is the main reason for the change of shale microstructures, and the hydration swelling volume of illite is smaller than that of montmorillonite. Fourth, inorganic cations can inhibit clay mineral hydration, and the inhibition effects of K+, Na+ and Ca2+ become worse in turn. In conclusion, hydration can increase the permeability of shale reservoirs, and the reasonable soak time after the fracturing of Longmaxi Formation shale reservoirs in the study area is 5 days. As for the shale gas reservoirs with higher montmorillonite contents, however, it is recommended to extend the soak time appropriately.