爆燃压裂中饱和脆性岩石细观损伤机制及其对宏观破坏的控制规律

To know the rule of dynamic damage of saturated brittle rock during the deflagration fracturing process is the basis to insure the fracturing effect and the gas oil field safety. Experiments indicate that under the strong dynamic load, the mesoscopic cracks would be extended within the saturated rock. And the lag of the free fluid would occur instantaneous in the crack, which could stimulate the cohesive force on the crack surface and increase the strength of the rock. While, most of the current theories are based on the static condition, only considering the fluid splitting weakening to the rock strength. Therefore, this project will focus on the scientific problems, which include the dynamic balance issue between the leaked fluid and the crack space, the dynamic transformation issue between the splitting and cohesion caused by the free fluid within meso cracks. Firstly, based on the test and the analysis of the mesoscopic structure in the various types of reservoir rocks, the rock breaking experiments under the displacement impact and the fluid leakage pulse will be carried out in sequence, employing the Hopkinson and the rock dynamic damage simulation experimental devices. Accordingly, the mesomechanics functionary mechanism of the saturated fluid to the rock dynamic strength will be revealed. And the evolution law will be clarified, which involves the dynamic seepage – mesoscopic fracturing – macroscopic failure process under the high speed impact of shaped charge fluid. Meanwhile, the coupled dynamic constitutive model considering the balance of splitting and cohesive forces generated by the free fluid within meso-cracks will be constructed, using the evaluation of the dynamic balance between the fluid seepage velocity and the change rate of the free volume within the cracks. On this basis, the brittle failure simulation technology of the saturated rock based on the meso-macro mechanics will be achieved using the FEM simulation technology of the fluid-solid coupling dynamic seepage. The research would improve the simulation accuracy of the blasting fracturing process. In addition, regarding other related dynamic response issues of saturated brittle rocks, such as earthquakes and mining, this research could also provide a certain theoretical support.

明确爆燃压裂中储层岩石动态破坏规律，是确保压裂效果和井身安全的基础。实验表明，饱和岩石在强动载下，其细观裂纹内自由流体会激发对裂纹面的粘结力，增加岩石强度。但现有理论多从静态加载条件出发，仅考虑流体对岩石强度的劈裂弱化作用。由此，本课题将针对爆燃压裂过程中，聚能流体泄入与裂纹空间的动态平衡、细观裂纹内自由流体劈裂与粘结的动态转换等科学问题，基于对各类储层脆性岩石细观构型的测试分析，借助霍普金森压杆和岩石动态冲击损伤实验装置，开展位移载荷冲击和流体泄入性冲击破岩实验，揭示饱和流体对岩石动态强度的细观力学作用机制，阐明聚能流体高速冲击下岩石动态渗流-细观断裂-宏观破坏的演化规律，建立可考虑流体泄入-劈裂-粘结的动态转换与耦合的力学本构，构筑爆燃冲击破岩过程的数值仿真技术。研究成果将有效提高对爆燃压裂过程的模拟精度，也可为地震、开矿等饱和岩石动力学问题提供一定理论支撑。

多级脉冲爆燃诱导水力压裂技术，是解决常规大型水力压裂上述问题的有效途径。但是，对于上述深层、超深层、页岩、碳酸盐等特种油气藏，爆燃压裂技术的核心矛盾在于火药量的精确设计。而突破这一焦点的关键在于，明确爆燃强动载下各类储层饱和岩石的动态破坏响应机制。基于此，本项目将致力于揭示聚能流体泄入性冲击下饱和脆性岩石破坏的动态响应机制，剖析其主控因素和影响规律。本项目主要研究爆燃压裂聚能流体冲击下，饱和脆性岩石动态破坏的细观力学作用机制，解决饱和岩石细观裂纹内自由流体对岩石动态强度控制的基础理论问题，结果表明：岩石的动强度跟冲击加载速率有明显的相关性，在在低加载速率下饱和岩石破裂强度小于干燥岩石破裂强度，在高加载速率下饱和岩石破裂强度大于干燥岩石破裂强度；流体的泄入对岩石的动态强度以及岩石的破裂形态有着重要的影响，主要是液体在岩石接触面发生的“楔入效应”以及液体在岩石内部的受力作用导致的；饱和岩石动态损伤模拟研究结果表明：井周破碎区半径随岩石泊松比、储层饱和压力、流体粘度、地应力水平和压力峰值的增加而增加，随岩石弹性模量和地应力差的增加而减小，权重分析结果表明岩石弹性模量是影响破碎区半径大小的主控因素；地应力差是影响损伤裂缝数量的主控因素；影响地损伤裂缝长度的主控因素为地应力水平大小；载荷加载速率存在最优施工范围；基于权重分析方法构建了破碎区半径、裂缝条数和裂缝长的归一化预测公式，可用于施工前后动态压裂致裂效果预测。