Influence of the interval length of hydraulic packer systems on thermally-induced pore pressure measurements in rock

Abstract

During the past several decades, hydraulic packer systems have generally been used to measure the thermally-induced pore pressures in many in-situ experiments in nuclear waste management programs. The measured value using hydraulic packer system is a combined value of the pore pressures in the rock along the packer interval, and not the pore pressure at the middle point. The suitability of using the packer system measured value to represent the thermally-induced pore pressure at the middle point of the packer interval needs to be assessed. The accuracy of using hydraulic packer systems is examined in this paper using a coupled thermal-hydraulic-mechanical (THM) model for fully saturated conditions developed using the finite element program COMSOL. The results indicate that there could be large errors in the measured values, with the size of the error dependent on the size of the packer interval, the variation of pore pressure gradient along that interval and the orientation of the packer measurement borehole. To improve the accuracy of the measurement using packer systems, the variation in pore pressure gradient along the packer interval should be small; or if a substantial gradient variation is expected, the length of the packer interval should be made as small as possible. A packer interval with a length less than 0.5 m is suggested. If a packer system has been installed with a large interval length in a location where a large pore gradient variation may exist, calibration of any associated numerical model using this measurement should incorporate the packer measurement system. The numerical results of this coupled THM model are in good agreement with the integration of a point heat source theoretical solution throughout the heater line.