Abstract

A new full-spectrum gamma-gamma logging instrument, theZ-Densilog®, measures bulk density (pb) and thephotoelectric cross section (Pe) and represents a significant advancement overpresent instrumentation of a similar type. Instead of recording count rates inonly a few energy gates, the state-of-the-art downhole electronics transmitsthe entire 256-channel gamma ray energy spectra to the digital surface loggingunit. This facilitates sophisticated data analyses previously not obtainable bystandard techniques.

Novel, advanced data analysis methods minimize statistical errors, providereal-time compensation Ear variations in detector gain and crystal resolution, and a "four-dimensional" rib and spine model allow the concur cent but separatecorrection for mud cake density and photoelectric cross section effects. Thisresults in greatly improved bulk density and Per measurements.

The present paper reviews the basic concepts of the Z-Densilog and discussesrecent Canadian field experiences in clastic, carbonate, and evaporitesequences.

Basic Concepts

The Z-Densilog (ZDL) instrument model and the techniques for measuring Formation bulk density (Pb g/cc) and photoelectric cross section(Pe, barns/electron) through the use of full spectrum gamma-gamma cay logginghave been discussed previously (Minette et al., 1985; 1986). Basically theZ-Densilog measures and records the 256-channe1 spectra obtained from the longspace detector and the count rate from the discriminated short space detector.While the short space count rate is obtained utilizing a voltage discriminator, the long space spectrum is recorded using B high-speed (12-bit) analog-to-digital converter (ADC). A custom-built flask protects the electronics fromhigh borehole temperatures for prolonged periods of time.

Acquisition of data from these detectors along with gamma ray, compensatedneutron, and caliper data is controlled by a surface computer via the Dresser Atlas WTS (wireline telemetry system) which operates at 120 KB/s.

As previously discussed in detail by Minette et al. (1986), the Z-Densilog"full-spectrum" technique offers major advantages over the conventional"few-gate" methods presently in field use.

Energy Calibration

Employing two radioisotopes with different but Known energy levels, the long Space spectrum can be calibrated. While the external americium source exhibitsa peak energy of 60 keV, the energy level of the internal cesium source peak is660 keV. The cesium source is also used for both real-time recalibration of thespectrum and detector resolution calculations. Based on the measuredscintillation detector resolution data, the real-time compensation of theobserved spectrum allows for the determination of bulk density (Pb)and the photoelectric cross section (Pe), independent of detectorresolution. The phenomenological model by Minette (1983) describes thisinteraction of the initial 660 keV energy level gamma rays with the surroundingmaterial and its eventual measurement by the detector.

Photoelectric Cross Section

The photoelectric crass section (Pe, b/e) is a function of two parameters, theenergy (E) of the incident photon and the mean atomic number (Z). Decreasingenergy" (E) and higher atomic numbers (Z) significantly influences theph1toe Lectric cross section, such as Pe = f' v, Z,E-1.

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