Depth-time correction of petroleum bottom-hole temperatures in the Po Plain, Italy

We have analyzed a wide set of time-temperature data from petroleum wells in two areas of the Po Plain, Italy, and propose empirical temperature corrections for mud circulation. Formation thermal parameters and the temperature dependence of thermal conductivity are taken into account. Analyses show that formation equilibrium temperatures computed with the Horner method compare well with those obtained by the Cooper and Jones method, which gives, on average, temperatures lower by only 2 degrees C for shut-in times less than 10 hours. The corrected temperatures compared with temperatures measured during drillstem tests show that the proposed corrections are relatively accurate. The two data sets give coherent results, and the inferred average geothermal gradient is 21.5 mK/m in the Apenninic buried arc area and 25.2 mK/m in the South Piedmont Basin-Pedealpine homocline area. The Horner slope data as a function of depth are then fitted with a second-order polynomial, and depth-time-correction equations are calibrated for the two areas. Finally, we propose an empirical function for the estimate of the mud-circulation time, which is often unavailable.