AAPG Meeting, Dallas, Texas, April 2004
SMU Geothermal Lab Poster on Bottom Hole Temperature (BHT)
Poster as a
Poster as a
BLACKWELL, DAVID, and
MARIA RICHARDS, SMU Dept of Geological Sciences,
Dallas, TX 75275-0395, 214-768-2745, email@example.com
The AAPG Geothermal Survey of North
America (GSNA) was a large effort carried out in the early 1970’s that
culminated in the production of a massive data base (over 20,000 Bottom Hole
Temperature (BHT) points from over 10,000 wells in the US, Canada, and
Mexico). The publication of continent scale maps by the USGS resulted from
the project based on the collated data (Kehle, 1970, DeFord and Kehle, 1976).
AAPG made the data base available on a
CD-ROM in 1994. The data set has received some use, but that use has been
limited, in part because of the uncertain error associated with the raw
measurements and the lack of calibration comparisons to accurate equilibrium
temperature measurements. There were several corrections proposed, but only
two simple ones were finally applied, one for Louisiana and another for west
Texas (the description of these in the final project is ambiguously stated).
Harrison et al.
(1983) based their calibration of BHT data to measured temperatures in
Oklahoma, using mostly drill stem temperature (DST) measurements. SMU also had
success comparing BHT data corrected using their equation to thermal
reconstructions for the Anadarko Basin in Oklahoma, based on heat flow and
conductivity modeling (Gallardo and Blackwell, 1999). So in this
study of the GSNA data base we started from that correction. The exact
BHT Correction Equation used, from Harrison et al. (1983), is:
Tcf = -16.51213476 + 0.01826842109*Z - 2.344936959E-006*Z2
values are subtracted or added to the original BHT values. Z is the depth in
meters. This equation was only calibrated to a depth of about 3 km. The
equation is similar to ones originally proposed by Kehle et al. (1970), but
not actually used in the final GSNA project.
We calibrated the Midcontinent and
Gulf Coast portion of the GSNA data base for
the 0.6 to 3km (4 km in the Gulf of Mexico region) depth range, using accurate
equilibrium temperature measurements in over 30 wells, (Figure 1), that SMU
collected over many years (see for examples Blackwell and Steele (1989) and
Blackwell et al. (1999)) and a few logs that others have published. Limiting
the GSNA data to well measurements within 50 km (0.5°) of each equilibrium log
we had, resulted on average 50 BHT measurements for each area comparison.
These were considered our “test” case. We applied the Harrison et al. (1983)
correction to these BHT values. After application of that correction there
was still a bias related to depth/gradient/temperature difference, so we
applied a secondary correction that was a function of the BHT well gradient.
In general the higher the gradient the more negative the difference between
the corrected BHT and the measured temperature. A gradient of 26 °C/km was
the zero (no correction or crossover point for gradients). Using the final (2nd)
correction, the test BHT gradient data set had a standard deviation (SD) of
±2°C/km as opposed to a SD of ±5°C/km for the first (Harrison) correction.
For a well in the 2-3 km depth range the smaller SD corresponds to a gradient
variation of only ±1-3°C/km, or about 5 to 15% of the measured value in a
normal heat flow environment.
The shape of the
Harrison correction is such that it is negative above about 1000 m and
positive below. Because measured bottom hole temperatures at shallower depths
are noisier- due to the effects of a smaller temperature difference, less
regularity of the drilling effect (winter versus summer drilling), and some
small uncertainty in the surface temperature, the corrected gradients for
these also tend to be very noisy. Therefore we initially considered only
BHT’s below 600 m depth. When applying the secondary correction based on
gradient, it was applied generally below 750 m. The differences were
systematic below the 0 correction crossover point of the Harrison curve.
significant correlation found was between gradient and the temperature
correction error (Figure 2). The mean temperature differences between the
measured (equilibrium wells) and the two calculated corrected temperatures are
shown in Figure 2. These represent about 20 sites in the Midcontinent and
Gulf Coast with depth ranges of 2500 ±500 m. The differences are plotted as a
function of gradient. In ½ of the sites the error on the 2nd corrected
temperatures is less than 5°C and averages near 0. Thus it is the best
correlation between the error and the gradient. There are clearly anomalies.
The Lackland well in Texas is visibly colder than the surrounding BHT’s. Yet
the log is linear and there is no obvious effect of cold downflow in the
well. Several of the deeper wells are geopressured, suggesting that the
recovery may have been faster than in normally pressured wells. On the other
hand several wells with high gradients have very large errors. As a result
the tendency for using the BHT data is to miss the highest and lowest
gradients in an area.
correction was applied to all of the AAPG BHT data in the 1000 to 3000 m depth
range as a function of gradient based on the least squares line shown in
Figure 2 below. This corrected gradient was used with thermal conductivity
estimates to obtain heat flow estimates for Western Great Plains and Gulf of
Mexico (including Mexico) data shown below in Figure 1 (Blackwell
and Richards, 2004a).
from below 3 km is still not established. The correction should decrease with
depth below 3 to 4 km, but there are not enough data to easily quantify the form of
Association of Petroleum Geologists, CSDE, COSUNA, and Geothermal Survey
Blackwell, D.D. and
Maria Richards, Calibration of the AAPG Geothermal Survey of North America BHT
Data Base, AAPG Annual Meeting, Dallas, TX, Poster session, paper 87616,
2004a. (see poster above)
Blackwell, D. D., and M. Richards, 2004 Geothermal Map of North America, AAPG,
scale 1:6,500,000, 2004b.
Blackwell, D.D., and J.L. Steele, Thermal conductivity of sedimentary
rock-measurement and significance, pp 13-36, in Thermal History of Sedimentary
Basins: Methods and Case Histories, ed. N.D. Naeser, and T.H. McCulloh,
Springer-Verlag, New York, 320 pp., 1989.
Blackwell, D. D., G. R. Beardsmore, R. K. Nishimori, and M. J. McMullen, Jr.,
High Resolution temperature logs in a petroleum setting, examples and
applications, p. 1-34, in Geothermics in Basin Analysis, ed. D. Merriam and A.
Forster, Plenum Press, New York, 241 pp., 1999.
DeFord, R.K., and Kehle, R.O., Chairmen,
Geothermal gradient map of North America: American Association of Petroleum
Geologist and U.S. Geol. Survey, scale 1:5,000,000, 1976.
Gallardo, J. and D.D. Blackwell, Thermal
structure of the Anadarko Basin, Oklahoma, Bull. Amer. Assoc. Petrol. Geol.,
83, 333-361, 1999.
K.V. Luza, M.L Prater, and P.K. Chueng, Geothermal resource assessment of
Oklahoma, Special Publication 83-1, Oklahoma Geological Survey, 1983.
Kehle, R.O., R.J. Schoppel, and R.K. DeFord, The
AAPG geothermal survey of North America, U.N. Symposium on the Development and
Utilization of Geothermal Resources, Pisa 1970, Geothermics Special Issue
2(1), 358-368, 1970.
Kehle, R.O., Geothermal survey of North America,
1972 Annual Progress Report for the AAPG, 23 p., 1973.
Figure 1 Location of
equilibrium logs compared to AAPG GSNA data points.
Figure 2. BHT error as a function of the gradient in the well. The data
with only the Harrison et al (1983) correction are shown as the dark diamonds
(Series 1). The linear (Series 1) least squares fit equation is shown as the
black line. The change in value from the 2nd calculated
temperature (Series 2) is shown in pink. Based on the least squares line
(Series 2 red line) the data shows no gradient bias.
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