An in situ thermo-hydraulic experiment in a saturated granite II: Analysis and parameter estimation

This paper is Part II of a series of two papers concerned with an in situ Thermo-Hydraulic Experiment carried out at the Underground Research Laboratory of Atomic Energy of Canada Limited. The Thermo-Hydraulic Experiment was designed to determine the thermoporoelastic parameters that control the magnitude of the pore pressure induced by thermal loading. The experimental set-up involved a heater installed in a sub-horizontal borehole drilled from an underground gallery, and piezometers and thermistors located at different distances from the heater in auxiliary boreholes drilled from an adjacent gallery. Several water injection and heater tests were conducted during this experiment. Part II focuses on the interpretation of the experimental data to estimate some thermoporoelastic constants of the Lac du Bonnet Granite. The interpretation is based on matching the pore pressure and temperature data with theoretical predictions based on singular thermoporoelastic solutions. To estimate the parameters, the distances from the center of the test zone to all piezometers and thermistors have to be known. However, sufficiently accurate distances for back-analyses of the experiment could not be obtained due to measurement uncertainties. A two-step scheme is described in this paper to minimize the uncertainty in the distances. In a first step, parameters lumping distance and material constants were estimated by matching a theoretical prediction to experimental data. In a second step, the material constants were extracted from the lumped coefficients using a least square technique involving the tentative distances deduced from the borehole survey.