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

Abstract

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 (1 ) 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 (1 ) of about 8 C indicates that a corrected BHT reflects (with 96% confidence, 2 ) 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.

The 3D conductive thermal field of the North Alpine Foreland Basin: influence of the deep structure and the adjacent European Alps

“…As illustrated in Fig. 6, 12 sites are showing temperature differences in a range of ±10 K, a value as high as the standard deviation of bottom hole temperatures and temperature logs (Hermanrud et al 1990;Förster 2001;Noack et al 2010;Agemar et al 2012Agemar et al , 2014a. At further 5 locations the model results show temperature difference of more than +10 K in the area of the described negative thermal anomaly southeast of Munich, which is less cold in the model than observed.…”

The origin of deep geothermal anomalies in the German Molasse Basin: results from 3D numerical models of coupled fluid flow and heat transport

“…The remaining average temperature misfit between the measured data (GeotIS, StM-WIT 2010) and the model predictions of ±10 K lies in the range of the standard deviation of bottom hole temperatures and temperature logs (Hermanrud et al 1990;Förster 2001;Noack et al 2010;Agemar et al 2012Agemar et al , 2014. Moreover, Agemar et al (2012) recommend a careful treatment of the deeper temperature estimates of GeotIS (StMWIT 2010), since the data base decreased with depth and hence possible local thermal effects may not be considered in the temperature interpolation.…”

Heat flow and lithospheric thermal regime in the Northeast German Basin

“…Temperatures from continuous logs are available at 50-m intervals (Förster, 2001), except in the GrSk 3/90 borehole, where data with a recording interval of 0.15 m (Hurter, 2002) are used. Temperature gradient versus depth plots helped to select suitable intervals for heat flow calculation.…”

“…Logs not entirely recovered from drilling perturbation (labelled 'D' in Table 1) are corrected Heat flow and lithospheric thermal regime in the NEGB - using the empirical approach of Förster (2001) due to the lack of data on drilling and circulation times.…”

“…During temperature recovery, temperatures above the COP decrease towards thermal equilibrium and below COP they increase, respectively. For the NEGB, the COP can be determined from the empirical relation (Förster, 2001):…”