A new approach to the borehole temperature relaxation method

Abstract

The borehole temperature relaxation method is used to determine in situ thermal properties of rock by a thermal disturbance in an uncased borehole. The conventional application of this method is based on the assumption that the heat flow through the borehole wall is constant in time and only a function of depth during the disturbance phase. The new approach described in this paper offers a solution to the problem which does not refer directly to the thermal history during the disturbance phase. It is only based on temperature measurements during the relaxation phase when the disturbance fades away by conduction. The initial temperature disturbance T0 in the borehole, the undisturbed temperature Tf of the rock, and the ratio (p2c2)/(p1c1) of the thermal capacities of the rock and the borehole fluid are determined for given values of the corresponding thermal diffusivities x1 and x2 using a Marquardt inversion scheme. Also, formal errors of the determined parameters are derived. A practical application to the data of a controlled experiment yields extremely low values for the in situ conductivity of the rock. This unexpected result is probably a consequence of convective motions in the borehole during the experiment. Copyright © 1990, Wiley Blackwell. All rights reserved