AN  INTEGRATED PETROLEUM  EVALUATION OF NORTHEASTERN  NEVADA


Introduction Evaluation Prospects


 

 

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BOTTOMHOLE THERMAL ANALYSIS

The temperatures recorded by logging companies while logging exploration oil and gas wells can provide very useful information about the present subsurface thermal regime. By reviewing and carefully editing this data, Western Cordillera has derived a regional data base of the relative current thermal gradient(s). This data base is tabulated in Appendix VII and is sorted by township and range location.

The bottomhole thermal data was derived from log headers of wells drilled for oil and gas in the evaluation area. Examination of these bottomhole thermal data has yielded 535 useful data points from 214 wells within the evaluation area. The temperature values are assumed to be approximate, and accurate to within several degrees. Relative thermal gradients have been derived for each well and are plotted by temperature versus depth, as gradient histograms, and are contoured on Overlay XIV.

The procedure of compiling recorded well log temperatures points out the difference between logging companies and company representatives that pay particular attention to minor yet important details on a regular basis, and those who settle for inconsistent quality control. The values recorded by the logging engineer in the log header are assumed to be correct for the ground elevation, KB elevation, temperature and the time since mud circulation stopped. Since the thermal gradient for the well has been derived from these four values, the quality control of these observations has a direct bearing on the quality of the data set.

The quality of the temperature values chosen from the logs often varies for a given log suite. In many cases a multi-run log suite of E-log, sonic, and neutron density logs may all show the same temperature values. The temperature on the first log run in this suite is probably a valid piece of data since a thermometer is used and the value is recorded in the log header. The logs run later often show the same temperature as the initial logging run, since common practice is to simply duplicate the initial temperature reading rather than running a thermometer on the other tools. The result is inaccurate and misleading temperature values in log headers, since logs run several hours later as the hole reaches temperature equilibrium should show increasing temperature for a given depth.

The relationship of the recorded temperature to the time the mud circulation has stopped in the well is also extremely important. Various factors affect the temperature recorded by the logging tools. The lithology and drilling rate control the amount of heat due to friction that is induced in the immediate proximity of the well bore. Circulation rates and the surface temperature of the drilling fluid control the cooling effects in the well bore as well. The overall balance of the system around the well bore tends to be cooler than the formation as a result of the circulation of the colder drilling fluid.

Unfortunately, in older logs the time since circulation has not been recorded. An estimate has been made from annotations of the amount of rig time used for the entire log run. Where it has not been recorded in recently drilled wells, it is assumed that the resistivity log was the first log recorded and that it was recorded within five hours after circulation stopped. In these cases 0 hours has been entered in the data table for the time since circulation in Appendix IX.

Log temperature values that were recorded within 5 hours of circulation are reported in the upper display of the temperature versus depth plots in Appendix VII. An exception was made for the deeper wells in Huntington Valley and the Adobe Range, where a cut off of 10 hours was chosen. The lower display in these plots contain all temperature and corresponding depth points within a given area. The first temperature versus depth plot shows all data for the entire study area.

Ideally, all the observed log temperatures should be recalibrated to an equilibrium based upon multiple data points over extended periods of time, rock thermal conductivity and thermal heat flow. Several companies have run thermal logs after their normal log runs in an attempt to allow the system to go back to thermal equilibrium. During this analysis, selected points from these curves have been plotted against the values for earlier log runs in the same well and other wells in the immediate area. First order approximations of the true thermal gradient for the particular area or valley can be made from these plots. Unfortunately, only 6 temperature logs are available for the entire study area.


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Last modified: 09/12/06