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Correcting Bottom Hole Temperature Data

By Jeff Corrigan

Log-header bottom hole temperature (BHT) data are notoriously unreliable. However, due to the general lack of better quality data, BHT data must often be used to constrain the subsurface thermal regime. This memo presents an analysis of 983 bottom hole temperature (BHT) and associated equilibrium temperature estimate (Teq) pairs. Based on this analysis, methods for correcting BHT data are recommended using information typically available with BHT data (the BHT measurement, time since circulation, and/or depth).

The recommended BHT correction method depends on the type of information available.

A Horner correction is recommended if three of more self-consistent BHTs from a given depth are available. For data in this study, the Teq estimate uncertainty (1 sigma) using the Horner correction is ±14 °F (±8 °C).

Time since circulation correction: If time-since-circulation information is available, but the data are unsuitable for a Horner correction, a correction method based only on the time-since-circulation information has been developed. Teq estimate uncertainties (1 sigma) are on the order of ±10-20 °F at a post-circulation time of 10 hours, decreasing to ±5-10 °F at a post-circulation time of 30 hours.

Last Resort Correction: If only BHTs are available with no time-since-circulation information, simply adding ±33 °F (±18 °C)is actually one of the best correction methods. The expected Teq uncertainty is ±17 °F (±9 °C). Averaging multiple corrected BHT data from a single depth using either of the latter two methods improves the predicted equilibrium temperature estimate.

BHT data should only be used as a last resort given the large uncertainties associated with correcting these data. More reliable types of temperature data (i.e., temperature measurements from electronic pressure/temperature gauges used on modern DST/PT equipment from permeable intervals, production log temperature measurements from shut-in production wells) should be sought to constrain the subsurface thermal regime whenever possible.