Formation temperature estimation by inversion of borehole measurements

Abstract

We describe a new numerical method that uses inverse methods to model thermal stabilization of a borehole after drilling mud circulation has stopped. The following five geophysical parameters can be estimated from the method: (1) true formation temperature [Formula: see text] (2) mud temperature [Formula: see text] at the time the mud circulation stops; (3) thermal invasion distance (R) into the formation before the formation is at the true formation temperature[Formula: see text]; (4) formation thermal conductivity (K) perpendicular to the borehole; and (5) efficiency factor (F) for heating mud in the borehole after mud circulation has stopped. Crucial input data for the model are the temperature measurements with shut‐in time taken at a fixed depth, more than two measurements being required, and the mud temperature at the surface at the time circulation stops. Other input data include the radius of the borehole, and the densities and specific heats of the drilling mud and of the formation on which the temperature measurements are made. Applications of the new inverse procedure to both synthetic data and field data show that the true formation temperature in many cases can be estimated precisely (to within about 0.4 percent); that the mud temperature can be estimated with acceptable accuracy (5 percent or so); while the thermal conductivity (K), the thermal invasion distance (R), and the efficiency factor (F) can be roughly estimated, provided high‐quality data are available.