Field Examples of Ultrasonically-Enhanced Density, Neutron-Porosity, and Caliper Logs Obtained While Drilling
- Ward E. Schultz (Halliburton Energy Services) | Gordon L. Moake (Halliburton Energy Services) | Charles E. Jackson (Halliburton Energy Services)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- June 1998
- Document Type
- Journal Paper
- 252 - 260
- 1998. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 2.4.3 Sand/Solids Control, 5.6.1 Open hole/cased hole log analysis, 1.12.2 Logging While Drilling, 1.6 Drilling Operations, 4.3.4 Scale, 4.1.5 Processing Equipment, 1.5.1 Surveying and survey programs
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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.
Density and neutron porosity logging-while-drilling (LWD) measurements have been available for several years. However, accuracy often has been inferior to that obtained by wireline tools. The lower accuracy is primarily caused by rapid variations in sensor standoff from the borehole wall due to tool rotation. Although full-gauge density sleeves reduce standoff, they hinder drilling; and standoff still occurs when the borehole is enlarged.
Refs. 1-3 describe statistical and orientational techniques for emphasizing nuclear data with small standoff. This paper illustrates another technique that directly measures standoff with an ultrasonic transducer. Weighted averages are used to emphasize nuclear data having the smallest standoff. This technique also utilizes a caliper measurement that is obtained from three ultrasonic transducers located at 1200 intervals around the tool.
Field examples include a comparison of unweighted and weighted data processing, as well as a comparison of standoff-weighted results to wireline. Another example shows LWD logs obtained in a 9.875-in. borehole with a tool designed for 8.5-in. bits. Directionality of the nuclear readings also is illustrated. An example with heavy mud illustrates the value of two new density-correction curves. The final example shows logs with enhanced vertical resolution.
Fig. 1 shows two sizes of the Density-Neutron-Standoff-Caliper (DNSC) tool, The smaller size was designed for drilling with an 8.5-in. bit and the larger with a 12.25-in. bit. Both sizes measure density, photoelectric factor (Pe), neutron porosity, tool standoff, and borehole diameter (caliper). Data is processed and stored in nonvolatile tool memory. When a mud-pulser section is included, selected results are transmitted to the surface for real-time display. The DNSC tool also can be combined with resistivity and acoustic LWD-devices.
Standoff Weighting. Nuclear data is acquired in 0.02-second intervals. During each interval, standoff of the nuclear sensors from the borehole wall is measured by an ultrasonic transducer using the pulse-echo method. Weighted averages are calculated from hundreds of nuclear data samples (Fig. 2), which typically correspond to 10 seconds. As shown in Fig. 3, the weight factors decrease exponentially with standoff, and range from 65,535 down to 1. This method yields nuclear counting rates characteristic of the smallest standoffs encountered during the averaging period.
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