BHT (Bottomhole Temperature): Wireline Log Temperature and Correction Methods

Key Takeaways

• BHT in wireline log petrophysics: Rw and Rmf temperature correction: The most direct and frequent application of BHT in WCSB well evaluation is the correction of formation water resistivity (Rw) and mud filtrate resistivity (Rmf) from surface laboratory temperature to BHT for use in Archie's water saturation equation (Sw = (aRw)/(φmRt))^(1/n)). The resistivity of an electrolyte solution decreases as temperature increases, approximately following the Arps equation: Rw(BHT) = Rw(Ts) × (Ts + 6.77) / (BHT + 6.77) for temperatures in °F (or equivalent conversion in °C). For example, if a WCSB Viking formation water sample measures Rw = 0.12 ohm-m at 20°C (laboratory temperature) and the log BHT is 48°C, the corrected Rw at BHT is: Rw(BHT) = 0.12 × (68 + 6.77) / (118.4 + 6.77) = 0.12 × 74.77 / 125.17 = 0.0717 ohm-m — a 40% reduction from the surface measurement. Using the uncorrected Rw = 0.12 in Archie's equation would overestimate Sw by a factor related to the resistivity ratio, potentially misclassifying a productive oil-bearing interval as water-bearing (because the apparent Rw is too high, calculated Sw is too high). This temperature correction is one of the first steps in any WCSB formation evaluation workflow, and its accuracy depends directly on the BHT value: using a BHT that is 15°C below BHST introduces a proportional error in all subsequent petrophysical calculations throughout the log interpretation.
• Multiple BHT readings and the thermal recovery curve: The thermal recovery of a wellbore after drilling fluid circulation follows a predictable curve: temperature rises rapidly in the first few hours (when the gradient between wellbore and formation temperature is large), then more slowly as the difference decreases, asymptotically approaching BHST over 24-72 hours for deep WCSB wells. By recording the BHT from multiple sequential log runs (run 1 soonest after circulation, runs 2-4 at progressively longer elapsed times), the thermal recovery curve can be established and extrapolated to BHST. The Lachenbruch and Brewer (1959) method and the Horner temperature correction are the two most commonly applied techniques for this extrapolation. Standard practice in WCSB exploration wells is to record the BHT for every log run in the well, noting the exact time since last circulation. When three or more BHT measurements at increasing TSLC (time since last circulation) are available, the Horner correction plot gives a reliable BHST estimate. When only a single BHT reading is available (common in development wells where time is limited), empirical correction factors (published by Lachenbruch and Brewer, or company-specific correction tables based on local geothermal gradient data) are applied to estimate BHST — with accuracy of ±5-10°C for shallow-intermediate wells and ±10-20°C for deep HPHT wells where the correction is largest. The AER encourages operators to record all BHT readings with exact timestamps and to use multi-run Horner corrections for HPHT wells to ensure accurate BHST determination for cement design and tool rating compliance.
• SP log interpretation and BHT correction for formation water salinity: The spontaneous potential (SP) log, one of the oldest and most fundamental wireline measurements, generates a static SP voltage proportional to the chemical potential difference between the mud filtrate and the formation water at the borehole face. The SP is converted to formation water resistivity (Rw) using: Rw = Rmf × 10^(SSP/(-61 + 0.133 × BHT)) (temperature in °F, simplified Doll equation). The BHT appears explicitly in the exponent of this conversion, meaning that an error of 10°F (5.6°C) in BHT propagates into a proportional error in the derived Rw, which then propagates through Archie's water saturation equation into the reserve estimate. For WCSB Cardium wells where SP log response is the primary Rw determination method in the absence of formation water samples, BHT accuracy directly controls the quality of the water saturation interpretation and the identification of productive versus water-bearing intervals in the Cardium sandstone. A BHT that is 15°C (27°F) too low versus BHST causes the SP-derived Rw to be overestimated by approximately 15-20%, which in turn causes Sw to be overestimated by 10-15% in the Archie model — enough to misclassify a marginal oil-bearing zone (Sw = 0.55) as unproductive (calculated Sw = 0.65, above the typical 0.60 cutoff for productive WCSB sandstones).
• Neutron-density crossplot temperature correction and environmental effects: Neutron porosity log response varies slightly with BHT due to the temperature dependence of the hydrogen index of water (the neutron log responds to hydrogen-containing materials, and water hydrogen index changes by approximately 0.5% per 10°C change in temperature). While this effect is secondary compared to BHT's role in Rw correction, it becomes significant in high-temperature Duvernay and Montney wells where BHT differences of 20-30°C from true BHST can cause measurable neutron porosity offsets. Density log response is essentially independent of temperature (bulk density is a nuclear measurement insensitive to thermal effects). The neutron-density crossplot used for lithology and porosity determination in WCSB formation evaluation is therefore subject to a small BHT-dependent offset in the neutron axis — typically corrected automatically by modern log processing software that reads BHT from the log header and applies the standard neutron temperature correction. Sonic log velocity response is weakly dependent on temperature through the temperature-dependence of elastic moduli in the formation fluid; this effect is negligible in most WCSB formations but may require consideration in quantitative seismic-to-well ties for WCSB Montney and Duvernay characterization studies using high-precision well-seismic calibration at deep HPHT depths.
• BHT in regional geothermal mapping and heat flow estimation: BHT data collected from wireline logs across hundreds or thousands of WCSB wells provides the raw dataset for regional geothermal maps — temperature versus depth plots averaged across large areas that define the geothermal gradient for different parts of the basin. These maps are used in play fairway analysis (identifying where source rocks have reached thermal maturity), in SAGD and geothermal energy resource assessment (identifying deep aquifers or formations at productive temperatures), and in carbon storage feasibility studies (reservoir temperature affects CO2 density and injection performance). The complication in regional geothermal mapping from BHT data is that raw BHT values underestimate BHST by variable amounts depending on depth, circulation time, and TSLC — which are often not precisely known from well files. Statistical correction methods (Pollack et al., Jessop et al.) apply depth- and region-specific correction factors to large BHT datasets to produce corrected temperature grids suitable for geothermal resource maps. The AER's WELLT database provides BHT data from over 400,000 Alberta wells, and the Canadian Geothermal Data Repository maintains Horner-corrected BHST estimates for a subset of key calibration wells to anchor the regional correction models. These geothermal maps are increasingly used by emerging geothermal energy companies evaluating WCSB deep aquifer and abandoned well geothermal resources as a complement to the Alberta oil and gas industry's primary use of the same temperature data.