Estimation of the in-situ thermal resistance of a borehole using the Distributed Temperature Sensing (DTS) technique and the Temperature Recovery Method (TRM)

Abstract

The temperature recovery method (TRM) is used to acquire in situ thermal properties, i.e. the thermal conductivity and diffusivity of the bedrock and the thermal resistance of the borehole wall. For a reliable determination of the thermal resistance of the wall, at least six temperature measurements are required within the first hour of recovery after generation of a temperature disturbance. To achieve this goal, a temperature recovery experiment was carried out in collaboration with the Geoforschungszentrum (GFZ, German Research Centre for Geosciences), Potsdam, using their distributed temperature sensing (DTS) equipment. Nine temperature profiles were recorded with this device in the first hour of recovery in the 265 m deep borehole Moosengrund in the Black Forest, southwest Germany. The temperature data were evaluated using an inversion algorithm based on a Bayesian approach. This algorithm provides an optimum value and an error estimate, and an index of the gain of information obtained from the observed data (IGID) for each parameter. A comparison between the results from conventionally logged data and DTS data shows that the DTS data yield an increase of the IGID and a decrease of estimated errors for appropriate a priori data sets. For thermal resistance this is primarily a consequence of the enhanced number of measurements shortly after shut-in, provided by the DTS equipment.