Characterizing Natural Gamma Ray Tools without the API Calibration Formation
Speaker: Dr. Gordon Moake
Dr.Gordon Moake is
a chief scientific advisor for Sensor Physics in the Halliburton Drilling and
Evaluation division. Currently, his primary focus is the development of LWD
nuclear tools, although he sometimes works on other projects. Before joining
Halliburton in 1984, Moake worked for four years at Baker Tubular, developing
electromagnetic flaw detectors. Moake obtained BS degrees in math and in
physics from the University of Wisconsin, and MS and PhD degrees in physics
from Purdue University. He is a member of SPWLA and holds 30 US patents related
to the oil and gas industry. Moake has authored or co-authored 20 conference
papers and six journal papers in the oil and gas industry.
The natural gamma ray API formation maintained by the University of Houston (UH formation) defines the API unit to which natural gamma ray tools are calibrated. Unfortunately, the narrow borehole of the UH formation cannot accommodate logging-while-drilling (LWD) tools, and planned expansion of the university will soon make the formation unavailable. This talk illustrates how the UH formation can be replaced with a combination of computer modeling and a single calibration point. The effectiveness of the method is illustrated with a wireline tool and an LWD tool. Besides describing the new calibration method, the talk provides a foundation for understanding natural gamma measurements.
The new calibration method defines a formation to be used with computer models (digital API formation) that emulates the UH formation. However, unlike the UH formation, the digital API formation has an uncased borehole. With modeling, it is easy to vary the size of the borehole to match the tool size being calibrated. To account for imperfections in the tool model, the model is calibrated by comparing its predictions to physical-tool measurements in a large tank of potassium chloride brine. Tool sensitivity is computed by dividing the calibrated count rate computed for the digital API formation by the API value assigned to the formation.
Designing the digital API formation began by developing a computer model that emulates the UH formation. The count rates computed with this model for a wireline tool matched the corresponding measured count rate to within 1%. Like the UH formation, the source of the digital API formation contains a combination of potassium, thorium, and uranium. The relative combination of these elements was determined so that the spectrum of photons on the surface of a wireline detector in the digital API formation is the same as in the UH formation. The absolute concentrations were defined so that a wireline tool would have the same count rate in both formations.
This method is shown to match the sensitivity of a wireline tool that was calibrated in the UH formation to within 1%. The method was also used to compute the sensitivity of an LWD tool, which compares favorably to the measured sensitivity determined with granite blocks. Using this method, the UH formation can be safely discarded. In addition, better agreement between wireline and LWD logs can be obtained because they are all calibrated in the same formation and in their natural logging positions. Details of the digital API formation are disclosed.
There are two identical sessions, please register for only ONE session which fits your schedule.
Morning - Tuesday, January 30th, 8am – 9am US Central Time;
Evening - Wednesday, January 31st 8pm – 9pm US Central Time.
This event is free for current members only. Login to your account to register. New Members are welcome by creating a profile and becoming active in the Society.