Dr. Steve Henderson, Associate Professor of Practice, Texas Tech University
“Abnormalities” in Log Headers – Why it Matters
“The best well-logging technology that money can buy
is only as good as the person who operates it.”
Well logs provide some of the most important data in the life of a well; however, in the modern age of processing huge volumes of petrophysical data, it is easy to lose sight of the importance of rudimentary data recorded in the log header. Bottom hole temperature (BHT) and mud filtrate resistivity (Rmf) are critical parameters in formation evaluation. Unfortunately, the accuracy of these values may never be questioned.
Estimates of temperature gradient calculated from BHT values in log headers vary dramatically, often within the same field. When these log-derived gradients are used to estimate formation temperature, it is common to see different temperatures at the same depths in adjacent wells. Estimates of ratio water saturation, movable hydrocarbon index and resistivity-derived porosity may show a reservoir to have excellent economic potential. However, those estimates are strongly dependent upon a value of Rmf, so accuracy in its measurement is a must.
Many “abnormalities” observed in log headers are not the result of technical issues, but instead are the products of human error. By following standard operating procedures, human error can be eliminated…but only if those procedures are followed. Variation in temperature gradient estimates results from the method the logging engineer used to measure or estimate BHT. Similarly, error in Rmf values can result from the procedure used to obtain the value.
Highlighted in this talk are case examples where accuracy of BHT and Rmf values can make a difference in formation evaluation. Methods are provided for detecting “abnormalities” in the log header data.