Studies on Hydraulic Fracturing Stress Measurement Assisted by Water Jet Boreholes Slotting

进行了水射流钻孔开槽辅助水力压裂应力测量的室内试验和现场试验。在室内试验中，用磨料水射流在大尺寸砂浆试件上钻出多对深孔纵槽，通过平板千斤顶和加载架对试件施加定型压力。对缝隙段进行了水力压裂试验，并在缝隙延长面上产生裂缝。测试结果表明，测得的应力略大于施加的应力。现场测试中，在地表以下约700米处的坚硬岩石中，既有垂直于钻孔的纵槽，也有垂直于钻孔的圆形槽。进行了水力压裂试验，证实在纵向槽延长面和圆形槽延长面上分别产生了纵向裂缝和横向裂缝。然后测量重新打开的压力。试验结果表明，测得的最大主应力和最小主应力与用同一孔测得的双裂纹法测得的主应力基本一致。基于无限大固体中孔口段加压裂缝扩展的边界元方法，进行了数值模拟。计算结果表明，裂缝扩展曲线的转折点随着主应力比的增大和裂缝增加引起的流体压降速率的减小而远离裂隙尖端。

Both laboratory tests and in-situ tests for hydraulic fracturing stress measurement assisted by water jet borehole slotting were carried out.In the laboratory tests, Pairs of deep longitudinal slots were made by abrasive water jet from the inside of boreholes which were drilled in large scale mortar specimens.Confining pressures were applied to the specimens by flatjacks and a loading frame. Hydraulic fracturing tests were carried out in the sections including the slots and the fractures on the plane of prolongation of the slots were produced. Shut-in pressures of the fractures were then measured.It was shown in the tests that the stresses detected were slightly greater than the stress applied.In the in-situ tests, both the longitudinal slots and the circular slots perpendicular to the boreholes were made in the hard rock at the site which is about 700 meters below the surface. Hydraulic fracturing tests were carried out and it was confirmed that the longitudinal and transverse fractures on the plane of prolongation of the longitudinal slots and the circular slots, were produced, respectively. Re-opening pressures were then measured. The test results show that the maximum and the minimum principal stresses detected were nearly equal to the stresses which were measured using the same borehole by the double fracture method.Numerical simulation based on the boundary element method of fracture extension by internal pressurization at the borehole section in an infinite solid was also carried out. The computed results shows that the turning point of fracture propagation curve movesfarther away from the slit tip with an increase of principal stress ratio and decrease of fluid pressure drop rate induced by the crack increment.