Ward E. Schultz

Field Examples of Ultrasonically-Enhanced Density, Neutron-Porosity, and Caliper Logs Obtained While Drilling

Density, Pe, and neutron-porosity measurements made while drilling are extremely sensitive to the distance between the sensors and the borehole wall (standoff). To minimize these effects, an ultrasonic transducer is positioned collinearly with the nuclear sensors. Rapid pulsing of the transducer provides frequent measurements of the standoff. These measurements are used to average the nuclear data in a manner that emphasizes the smallest standoff data. This greatly improves the quality of the density, Pe, and neutron-porosity logs. Additionally, the standoff measurements are used to correct the neutron log for residual standoff effects. Measurements from the standoff transducer are also combined with those from two other ultrasonic transducers to obtain a high-resolution caliper log. Besides providing useful borehole information, the caliper readings are used to correct the nuclear measurements for hole-size effects. This new technology is illustrated with log examples from a variety of wells. One of the examples illustrates the effectiveness of an enhanced-vertical-resolution processing technique in identifying thin beds.

A borehole corrected pulsed neutron well logging system

Abstract An improved well logging method has been developed for more accurately measuring the neutron capture cross section (Σ) of earth formations. A measurement of Σ with depth (well log) is useful in estimating hydrocarbon saturation of saline reservoirs. Gamma rays are detected following each burst of neutrons emitted by a pulsed 14 MeV neutron generator. An iterative procedure was developed to obtain borehole and formation exponential components from the measured data. This yields a formation Σ with reduced borehole dependence, particularly in high Σ formations. Accuracy of the two-exponential fitting procedure was verified by applying it to simulated data. An extensive set of test formation measurements further verified the accuracy of the two-exponential model and indicated only a small dependence on borehole conditions in the estimation of formation Σ.