水压致裂应力测量系统柔性分析及其对深孔测量的影响

Compliance of Drilling-rod System for Hydro-fracturing in Situ Stress Measurement and Its Effect on Measurements at Great Depth

在中国大陆范围内广泛使用的钻杆式水压致裂测试系统由6个部分组成,分别为压力流体控制系统、高压水泵、动力系统、数据记录系统、跨接式封隔器和钻杆系统。该套测试系统又可以分为两类,一种是针对孔深<100 m的浅孔测试系统,一种是针对孔深>100 m的深孔测试系统。对于钻杆式水压致裂系统,钻杆、连接管线以及钻孔变形对系统柔性影响均极微小,封隔器变形和压裂液压缩性是主要影响因素。当测试深度小于100 m时,测试系统柔性主要受封隔器变形和系统压裂液压缩性控制;而当测试深度大于100 m时,测试系统柔性主要受系统压裂液压缩性控制。对于深孔测试,钻杆式水压致裂测试系统柔性过大,会影响重张压力Pr0的准确确定。为了消除系统柔性对深孔测试的影响,可以通过其他方法来确定实验段岩体抗拉强度Tfh,进而确定最大水平主应力SH,或者通过其他方法来直接估算SH。在未来的研究工作中,开发井下传感器和井下流量计将是从根本上消除测试系统柔性影响的一条可靠途径。

The widely-accepted hydro-fracturing system in China mainland consists of six parts,i.e.control system for hydraulic fluid,high-pressure pump,power supply system,data recording system,straddle packers,and watertight drilling rods.This whole testing system can be divided into two kinds： one is the shallow testing system for boreholes with depth less than 100 m,the other is the deep testing system for boreholes with depth more than 100 m.As for the drilling-rod hydro-fracturing measurement system,the elastic deformations of drilling rods,connection high-pressure hoses and rock mass around testing interval have little effects on the compliance of testing system,and the elastic deformation of packers and the compressibility of fracturing fluids are two major factors.When testing interval is less than 100 m in depth,the compliance of testing system is controlled by the elastic deformation of packers and the compressibility of fracturing fluid,and when testing interval is greater than 100 m in depth,the compliance of testing system is controlled by the compressibility of fracturing fluids.For the testing at great depth,due to the large compliance of testing system,it is very difficult to determine the reopening pressure Pr0.In order to eliminate the negative effects from the compliance of testing system during the measurement campaign in deep boreholes,it is recommended to adopt other methods to determine Tfh,the tensile strength of rock mass around the testing interval so as to determine the maximum horizontal principal stress SH,or utilize other techniques to estimate SH.In the future research,the potential feasible way is to develop new down-well pressure gauges and flow meters in order to completely eliminate the negative effects from the compliance of testing system.