水压致裂起裂压力的细观离散元模拟及试验研究

水压致裂起裂压力的预测对于油气开采、地应力测量、水工结构物抗裂设计等具有重要的意义。采用颗粒离散元结合域-管道渗流模型的流固耦合非连续数值模拟方法，基于扩展前端法生成的含规则形状钻孔的颗粒体模型，对水压致裂的细观起裂过程和起裂压力大小进行了定量模拟。结果表明，在消除了颗粒体中钻孔形状不规则性的基础上，钻孔壁的接触力链分布与理论解较为一致，拟合的离散元起裂压力公式也与理论解较为接近。进一步地，从颗粒材料受挤压时产生局部张拉力的角度解释了起裂压力拟合公式与理论解之间的差别。最后，设计了含预制钻孔的抗渗砂浆试块制备方法，对不同主应力组合下的起裂压力大小进行了真三轴室内试验，验证了离散元模拟结果的可靠性。

Discrete element simulation and experimental study on the initiation pressure of hydraulic fracturing

Predicting the initiation pressure of hydraulic fracturing is of great significance for oil and gas exploitation, in-situ stress measurement and anti-cracking design of hydraulic structures. In this paper, the discontinuous numerical simulation method of particle discrete element combined with domain-pipe flow model is applied considering the fluid-solid coupling. Based on the front-advancing method, the granular model with regular-shaped borehole is generated to analyze the meso cracking process and initiation pressure of hydraulic fracturing. According to the simulated results, the distribution of contact force chain on borehole wall is consistent with the theoretical solution on the basis of eliminating the irregularity of borehole shape. The fitted initiation pressure formula by DEM is also close to the theoretical solution. Furthermore, the difference between the fitting formula of initiation pressure and the theoretical solution is explained from the perspective of the local tensile force generated when the granular material is compressed. Finally, a method for preparing impermeable mortar samples with prefabricated boreholes is developed. The initiation pressure under different combinations of principal stresses is quantitatively studied based on the true triaxial laboratory tests, which verifies the reliability of the discrete element simulation results.