页岩储层压裂液渗吸与离子扩散相互作用机理研究

Understanding the special flow-back characteristics of shale gas well (the flow-back efficiency is generally lower than 20%, and the salinity of flow-back fluid is generally higher than 15%) is significant for the evaluation of reservoir characteristics and fracture network complexity. The study finds out that fracturing fluid imbibition and salt ions diffusion are the main mechanism for the low flow-back efficiency and high salinity flow-back water respectively. However, the simultaneous migration of imbibed water and saline ions in micro-nano pores is not understood well. In this study, the correlativity among various physical parameters is analyzed and the control parameters of imbibition and ion diffusion are determined by using dimensional analysis method. Combined with MRI facility, the simultaneous experiment apparatus of fracturing fluid imbibition and ion diffusion is set up to explore the macro-micro phenomena, characteristic parameters, control factors and the interrelationships. Based on the theory of heat and mass transfer in porous media, the imbibition-diffusion model is simplified and solved by combining with the dimensional analysis and simultaneous experiment. The mathematic model is used to analyze the synchronous change of water saturation and salinity profile, illustrating the imbibition-diffusion mechanism and the interactions. This research provides theoretical foundation for special flow-back dynamic characteristic and fracture network complexity evaluation of shale gas well.

理解页岩气井特殊的返排动态特征（返排率普遍低于20％，返出液盐度往往高于15%）对评价储层特征和压裂缝网复杂度具有重要意义。研究发现，压裂液渗吸、储层盐离子扩散是导致页岩气井具有低返排率和高盐度返排液特征的主要机理。然而，对于页岩微纳米孔隙中同步发生、相互影响的渗吸水、盐离子运移微观机理尚不清楚。本项目将开展如下工作：①通过量纲分析法研究各物理参量之间的关系，确定压裂液渗吸与离子扩散的控制参数；②建立压裂液渗吸和离子扩散同步实验装置，结合核磁共振成像设备，研究宏微观现象、特征参数、主控因素以及各因素间的关系；③以多孔介质传质传热理论为基础，建立压裂液渗吸－离子扩散模型，结合量纲分析和同步实验结果对模型进行简化、求解，并分析含水饱和度和盐度剖面同步变化规律，以期揭示压裂液渗吸与离子扩散机制及相互作用机理。本项目的研究对理解页气井特殊的返排动态特征和评价缝网复杂度具有重要意义。

理解页岩气井特殊的返排动态特征（返排率普遍低于20％，返出液盐度往往高于15%）对评价储层特征和压裂缝网复杂度具有重要意义。研究发现，压裂液渗吸、储层盐离子扩散是导致页岩气井具有低返排率和高盐度返排液特征的主要机理。然而，对于页岩微纳米孔隙中同步发生、相互影响的渗吸水、盐离子运移微观机理尚不清楚。本项目通过量纲分析方法获得了各物理参量之间的关系，确定了压裂液渗吸与离子扩散的主控因素：粘土矿物含量、渗透率、孔隙度、初始含水饱和度、表面张力、润湿角和暴露面积等；建立了室内压裂液渗吸-离子扩散同步实验装置及测试方法，定量研究压裂液渗吸和离子扩散特征参数之间的相关关系，发现压裂液渗吸和离子扩散是同步发生的，渗吸前缘会引起离子溶解、扩散进入压裂液中；然而，两者不是同步结束的，离子扩散的特征速度远低于压裂液渗吸，导致离子扩散过程持续更长的时间；建立了基质孔隙压裂液渗吸-离子扩散模型，分析了含水饱和度与盐浓度剖面同步变化规律，发现两者都与时间的平方根呈线性相关关系，主控因素基本一致。本基金主要围绕页岩储层压裂液渗吸-离子扩散机理开展研究，提出的三量纲参数返排率预测方法，在理论上实现了页岩气井返排率的定量预测，具有一定的应用前景。