The Compensated Z-Densilog instrument is a new full spectrum gamma-gamma logging instrument, measuring the density and photoelectric cross section (Pe) of the formation. This instrument represents an advancement over present instruments in that the observed gamma spectrum is sent to the surface as a full 256-channel spectrum instead of count rates in a few energy gates. This advancement is made possible by the use of high speed digital electronics which are protected from the high temperatures observed downhole by custom-built flasks. In this paper, the new advanced electronics will be considered in some detail. This consideration will include a comparison of full spectrum data acquisition with the multiple discriminator window technique, which is the foundation of the "few gate" method. Both of these methods will be critiqued based upon electronic stability and the information obtained. The full spectrum provided by high speed electronics facilitates a much more sophisticated data analysis. This data analysis, based on a phenomenological model of the instrument (Minette, 1984), includes real time compensation for changes in detector gain and crystal resolution. It also includes a "four-dimensional" rib-spine plot, which separates the compensation for mudcake density from the compensation for the Pe of the formation and the mudcake. Also included in the software are real time error minimization and real time decision-making techniques. These techniques maximize the sensitivity of the instrument to the density and Pe of the formation and minimize error due to statistical fluctuations. Gain and resolution compensation completes the data analysis. This compensation virtually eliminates density and Pe errors caused by changing gain and resolution. The application of the full spectrum technique will be illustrated in several test well examples. These examples include a sample of the Compensated Z-Densilog presentation (including the new data confidence curve, STAB), the repeatability of the Compensated Z-Densilog instrument, and comparison of the density obtained with the Compensated Z-Densilog instrument with density measurements made by a borehole gravitometer (BHG) and a Densilog instrument. In this manner, the advantages and accuracy of full spectrum density logging will be demonstrated.
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The Application Of Full Spectrum Gamma-Gamma Techniques To Density/Photoelectric Cross Section Logging
Daniel C. Minette;
Daniel C. Minette
Dresser Atlas, Dresser Industries, Inc.
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Bernie G. Hubner;
Bernie G. Hubner
Dresser Atlas, Dresser Industries, Inc.
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J.C. Koudelka;
J.C. Koudelka
Dresser Atlas, Dresser Industries, Inc.
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Mathew Schmidt
Mathew Schmidt
Dresser Atlas, Dresser Industries, Inc.
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Paper presented at the SPWLA 27th Annual Logging Symposium, Houston, Texas, June 1986.
Paper Number:
SPWLA-1986-DDD
Published:
June 09 1986
Citation
Minette, Daniel C., Hubner, Bernie G., Koudelka, J.C., and Mathew Schmidt. "The Application Of Full Spectrum Gamma-Gamma Techniques To Density/Photoelectric Cross Section Logging." Paper presented at the SPWLA 27th Annual Logging Symposium, Houston, Texas, June 1986.
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