Publications

The addition of a photoelectric factor (Pe) measurement to density logging has proven to be quite useful in lithology identification. However, because of limited depth of investigation and strong sensitivity to elements with large atomic number, Pe measurements have been adversely affected by borehole rugosity, mudcake, washout, and barite in the drilling mud. A new Spectral Density Log has been developed that provides a reduction in borehole sensitivity for lithology determination.Previously, Pe was obtained only from the longer spaced of two density detectors. With the new SDL+ system, a separate Pe is measured from each detector. It has been determined from test formation and well data that the short-spaced detector Pe is less sensitive to mudcake. This is at least in part caused by the much tighter gamma ray collimation of the short-spaced detector, coupled with the very limited penetration of the photoelectrically sensitive low energy gamma rays. Conversely, the larger collimator opening of the long-spaced detector increases the mudcake effect on the relatively shallow Pe measurement. Pe measurements with different relative borehole sensitivities provide a mechanism for identifying and reducing erroneous Pe values in zones where mudcake or washout obscures the true photoelectric properties of the formation.The SDL tool has hgh counting rates and therefore small statistical fluctuations, resulting in very good density and Pe repeatability. Log quality curves are displayed that monitor gain control of both detectors. Important power supply voltages within the sonde are continuously sampled and any anomalies are reported. The SDL system operates over a new digital interactive telemetry system having increased data accuracy and capacity. These factors also serve to enhance the credibility of the observed Pe values.Test formation and well data are presented that illustrate the improved Pe response from the short-spaced detector, and the advantages of combining the two detector data. Pe determined from the uncollirnated Compensated Spectral Natural Gamma tool, which is inherently borehole sensitive, is shown to compare more closely with the longspaced Pe from the Spectral Density Log.