Analysis of borehole temperature data in the Northeast German Basin: continuous logs versus bottom-hole temperatures

Temperature recordings from exploration boreholes for hydrocarbons and geothermal energy are evaluated to select the best-suitable data for determining formation temperature and heat flow in the Northeast German Basin. Repeated temperature logs measured in several boreholes allow a classification into equilibrium logs and perturbed logs. For the latter, a simple empirical correction is proposed based on mean annual ground-surface temperature, the estimated amount of temperature correction at total depth of log, and the depth-dependent cross-over point of temperature perturbation due to drilling. Analysis of bottom-hole temperatures (BHTs) showed that the uncorrected values deviate, on average, by as much as 22.2±10.2°C (1σ) from the formation temperature. BHTs corrected using the Horner-plot method are systematically too low by 1–2°C compared to values corrected using the exact solution to the full line-source equation. Both correction methods yield values that underestimate formation temperatures by as much as 9°C and 8°C, respectively. The large standard deviation (1σ) of about ±8°C indicates that a corrected BHT reflects (with 96% confidence, 2σ) formation temperature not better than ±16°C. However, using temperature logs in conjunction with the corrected BHTs allowed a prediction of formation temperatures with an error less than ±10°C at somewhat deeper depths than the log in several boreholes. The availability of continuous temperature logs as a prerequisite for a reduction of error of BHTs significantly reduced the quantity of useable corrected BHTs in the basin.