New and More Accurate Method for the Direct Measurements of Earth Temperature Gradients in Deep Boreholes

摘要

A technique has been developed for the direct determination of bottomhole equilibrium rock temperatures during economically acceptable interruptions in drilling operations: 12 to 24 hrs, depending on depth and rock type. This technique was developed during the drilling of GT-2, the second deep exploratory test hole in the Los Alamos Scientific Laboratory's Dry Hot Rock Geothermal Energy Project. The major innovative aspect of this technique is a new method of analyzing bottomhole thermal recovery data, enabling equilibrium rock temperatures to be determined from measurements of substantially shorter term than previously possible. Assuming for an arbitrary time interval, short in comparison with the time required for temperature recovery to be complete, that the rate of temperature relaxation depends only on the difference between the borehole temperature and the undisturbed rock temperature, we can write for the time interval, t, (theta/sub infinity/ - theta(t))/(theta/sub 8/ - theta/sub 0/) = e/sup -ct/. A linear dependence of c on theta/sub infinity/ enables determinations of equilibrium rock temperature. Relaxation temperatures observed at any specific depth can be treated in this manner giving the same rock temperatures predicted by theory. However, sufficient data can be collected in hours rather than months or years, if direct bottomhole temperature measurements are used.Using this technique, the equilibrium rock temperature at the GT-2 terminal depth of 2928 m is 197 exp 0 C, with a measured temperature gradient of 60 exp 0 C/km over the bottom 0.8 km.