DETERMINATION OF THERMAL-CONDUCTIVITY FOR DEEP BOREHOLES

Two methods for thermal conductivity determinations on rock cores and fragments were tested on a suite of samples from the Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland (KTB) superdeep drill hole in Germany. They were also compared with estimates of thermal conductivity using the mineral composition of the rock and physical well logs and with in situ thermal conductivity measurements. Laboratory methods provide reasonably precise determinations of the thermal conductivities of both solid core (+/-5%) and drill cuttings (+/-10%) at room temperature and pressure. The most common methods presently used for crystalline rocks are the steady state ''divided-bar'' (DB) technique and the transient ''half-space'' line source (LS). Sample preparation and measurement times are comparable for the DB and LS, with sample preparation being more time consuming on average. For isotropic rocks there is little to choose from between the two methods, which both give reliable values of conductivity in the vertical direction. The LS is easier to set up and use in field laboratory situations, which renders it the preferred method for field reconnaissance. The gneissic crystalline rocks penetrated by the KTB boreholes typically have anisotropy of the order of 10-20%. The DB provides unambiguous values of conductivity in a given direction, so its use is preferable for obtaining both principal conductivities and the vertical component. Anisotropy can be estimated using LS measurements in many different directions, but the potential for large random errors is much greater than with the more straightforward DB approach. For deep research wells the difficulties of extrapolating laboratory results to in situ conditions (particularly temperature) present additional obstacles to determining heat flow. Laboratory measurements of water-saturated samples under in situ conditions, combined with in situ measurements and judicious use of calculations based on mineralogy and well log derived physical properties, can aid in the accurate characterization of thermal conductivity in deep wells. The application of different methods helped to link variations of heat flow with depth in the KTB hole to the anisotropy of thermal conductivity or thermal refraction and thus allowed the calculation of background heat flux in this geologically complex area.