不同岩性地层水力压裂裂缝扩展规律的模拟实验

Simulation experiment on prolongation law of hydraulic fracture for different lithologic formations

利用真三轴模拟压裂实验系统对玄武岩、巨砾岩、泥灰岩岩心进行了水力压裂裂缝起裂及裂缝扩展模拟实验,得到了压后裂缝几何形态和压裂过程中压力随时间的变化规律。研究结果表明,玄武岩中天然裂缝发育程度较低,抗拉强度较高,裂缝起裂会导致明显的压降,压后能够形成比较理想的双翼缝;巨砾岩中天然裂缝较为发育,裂缝起裂不会导致明显的压降,高排量压裂后形成的裂缝为多组复杂裂缝,裂缝扩展摩阻很大,裂缝延伸压力几乎与破裂压力相当;泥灰岩抗拉强度较低,部分发育有天然裂缝,破裂压力较低,裂缝起裂后延伸压力与最小水平地应力相当。

Simulation experiments on the initiation and prolongation mechanisms of hydraulic fractures were conducted by a true triaxial test system with basalt, boulderstone and muddy limestone, and the fracture geometry and the variation law of pressure with time were obtained. The results show that the basalt has few natural fractures and strong tensile strength, the initiation of fractures results in obvious pressure drop, and the desirable double wing fractures are finally formed. Boulderstone is found to have lots of natural fractures and indistinct pressure drop originated from the fracture initiation. The fractures fracturing at high delivery rates are multiple sets of complex ones. The friction resistance of fractures prolongation is larger, and the value of prolongation pressure is equal to the fracture pressure. Muddy limestone shows lower tensile strength and partially developed natural fractures. The fracture pressure is relatively lower, and the prolongation pressure is almost equal to the minimal horizontal in-situ stress in muddy limestone.