Abstract

The addition of a photoelectric factor (P_e) 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, P_e 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, P_e was obtained only from the longer spaced of two density detectors. With the new SDL⁺ system, a separate P_e is measured from each detector. It has been determined from test formation and well data that the short-spaced detector P_e 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 P_e measurement. P_e measurements with different relative borehole sensitivities provide a mechanism for identifying and reducing erroneous P_e values in zones where mudcake or washout obscures the true photoelectric properties of the formation.

The SDL tool has high counting rates and therefore small statistical fluctuations, resulting in very good density and P_e 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 P_e values.

Test formation and well data are presented that illustrate the improved P_e response from the short-spaced detector, and the advantages of combining the two detector data. P_e determined from the uncollimated Compensated Spectral Natural Gamma tool, which is inherently borehole sensitive, is shown to compare more closely with the long-spaced P_e from the Spectral Density Log.