Compliance of drilling-rod hydro-fracturing in situ stress measurementsystem and its effects on measurements at great depth

摘要

The widely-accepted hydro-fracturing system in China mainland mainly consists of seven parts:rncontrol system for hydraulic fluid, high-pressure pump, and power supply system, data recordingrnsystem, straddle packers, pressure fluid hose and watertight drilling rods. This whole testing systemrncan be classified into two kinds: one shallow testing system for boreholes with depth of less than 100rnm, and one deep testing system for boreholes with depth of more than 100 m. Regarding therndrilling-rod hydro-fracturing measurement system, the elastic deformations of drilling rods,rnconnection high-pressure hoses and rock mass around testing interval have little effects on therncompliance of testing system, and the elastic deformation of packers and the compressibility ofrnfracturing fluid are two major factors. When testing interval is less than 100 m in depth, therncompliance of testing system is controlled by the elastic deformation of packers and therncompressibility of fracturing fluid; and when testing interval is greater than 100 m in depth, therncompliance of testing system is controlled by the compressibility of fracturing fluid. For the testing atrngreat depth, due to the large compliance of testing system, it is very difficult to determine thernreopening pressure P_(r0). In order to eliminate the negative effects from the compliance of testingrnsystem during measurement campaign in deep boreholes, it is recommended to adopt other methods torndetermine T_(fh), the tensile strength of rock mass around the testing interval so as to determine S_H, orrnutilize other techniques to estimate the maximum horizontal principal stress, S_H. Feasible principlesrnare proposed for selecting a pressurization pump according to the compliance of test system at arncertain depth and the estimated vertical stress. Generally, the lowest flow rate of a pressurizationrnpump should meet such a requirement that the maximum compressibility of fracturing fluid can bernsatisfied in a minute, that it to say, it is required to raise the borehole pressure to the estimated verticalrnstress in a minute. In the future research, the potential feasible way is to develop new down-wellrnpressure gauges and flow meters in order to completely eliminate the negative effects from therncompliance of testing system.