脉动与静力水压组合驱动下水力裂缝网络深入扩展机理研究

It has been recognized that the pulse loading during hydraulic fracture can increase the fracture network width and improve the fracture network connection, which has an advantage in improving the permeability of the formation. However, the deficiency of using pulse hydraulic fracturing lies in that it is difficult to obtain deep fracture network, while the static hydraulic fracturing is characterized by a small number but long hydraulic fractures. If the fracturing is combined with a certain form of the pulse and static hydraulic fracturing, can both the fracture network width and depth be increased by the combination of these two methods? Therefore, this project intends to study the mechanisms and methodologies using the combined static and pulse hydraulic fracturing to increase hydraulic fracturing network area by means of triaxial hydraulic fracturing experiments, theoretical analysis, and numerical simulation. The propagation direction in the vicinity of natural fractures or new static loading hydraulic fractures will be studied, which may reveal the role of the existing fracture on the pulse hydraulic fracture propagation direction. The pulse hydraulic fracture propagation behavior and conditions will be studied when the pulse hydraulic fracture interacts with natural fractures or new static loading hydraulic fractures. The combined hydraulic fracture propagation modes will be explored to establish the pulsing hydraulic fracture propagation law in the new static hydraulic fracturing area. The numerical module suitable for the combination hydraulic fracturing will be developed. The optimal fracturing mode and mechanisms for increasing hydraulic fracture network area will be concluded.

脉动水压使微小裂隙得到充分扩展和沟通，形成较好裂隙网络，压裂区域宽度大，对提高岩体渗透率有优势，但其不足是不易深入压裂，而静力水压压裂特点是水力裂缝数量少、深度大。那若对脉动加载与静力水压以一定形式组合压裂，能否使水力裂缝网络在区域宽的优势上，再增大压裂深度？因此本项目拟以三轴水力压裂实验、理论分析及数值计算为手段，研究组合驱动增大水力裂缝网络扩展区域的机理及实现方式。拟研究脉动水力裂缝在天然裂隙或静力水压新造裂隙附近的扩展方向，揭示既有裂隙对脉动水力裂缝扩展方向的诱导规律及机理；拟研究脉动水力裂缝扩展接触到天然裂隙或静力水压新造裂隙后的扩展形态，建立脉动水力裂缝接触到既有裂隙后的扩展准则及实现条件；拟研究复杂组合水压驱动下水力裂缝扩展模式，建立脉动水力裂缝在静力水压新造裂缝区域的扩展模型，开发适于组合驱动水力裂缝扩展的计算模块，得出增大水力裂缝网络扩展区域的最优加载模式与机理。

在石油、页岩气等能源开采中，静压水压致裂技术存在裂隙形式单一的缺点，脉冲水压致裂技术可以产生裂隙网络，但有加载时间长的不足。针对既有加载方式的缺陷，本项目提出组合水压加载方式，先对岩体进行一定时间的脉冲水压预加载，在岩体内部累积疲劳损伤，劣化岩体，后进行静压加载至试块失效，旨在形成裂隙网络的同时提高加载效率。首先研究了静压水力裂缝扩展特征，分析了静压压裂下水力裂缝起裂和扩展过程中的应力状态及裂缝扩展路径，揭示天然裂缝及洞室对水力裂缝起裂及扩展的影响机理，然后研究了脉冲压裂下水力裂缝扩展特征，对比分析了静压和脉冲压裂下水力裂缝扩展规律，建立了脉冲压裂下水力裂缝穿透既有裂缝判别准则。然后依据静压和脉冲压裂特点提出了组合水压加载方式，探究组合水压加载下水力裂纹扩展规律及缝网压裂效果。研究成果可以为石油、页岩气开采所需的水压致裂提供基础理论支持与数值模拟手段。