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 Tie fbohrprogramm der Bundesrepublik Deutschland (KTB) superdeep drill hol e in Germany. They were also compared with estimates of thermal conduc tivity using the mineral composition of the rock and physical well log s and with in situ thermal conductivity measurements. Laboratory metho ds provide reasonably precise determinations of the thermal conductivi ties of both solid core (+/-5%) and drill cuttings (+/-10%) at room te mperature and pressure. The most common methods presently used for cry stalline 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 prepa ration being more time consuming on average. For isotropic rocks there is little to choose from between the two methods, which both give rel iable values of conductivity in the vertical direction. The LS is easi er to set up and use in field laboratory situations, which renders it the preferred method for field reconnaissance. The gneissic crystallin e rocks penetrated by the KTB boreholes typically have anisotropy of t he order of 10-20%. The DB provides unambiguous values of conductivity in a given direction, so its use is preferable for obtaining both pri ncipal conductivities and the vertical component. Anisotropy can be es timated using LS measurements in many different directions, but the po tential for large random errors is much greater than with the more str aightforward DB approach. For deep research wells the difficulties of extrapolating laboratory results to in situ conditions (particularly t emperature) present additional obstacles to determining heat flow. Lab oratory measurements of water-saturated samples under in situ conditio ns, combined with in situ measurements and judicious use of calculatio ns based on mineralogy and well log derived physical properties, can a id in the accurate characterization of thermal conductivity in deep we lls. The application of different methods helped to link variations of heat flow with depth in the KTB hole to the anisotropy of thermal con ductivity or thermal refraction and thus allowed the calculation of ba ckground heat flux in this geologically complex area.