Reduction Of Standoff Effects On Lwd Density And Neutron Measurements

In wireline logging, standoff effects on density, photoelectric factor (Pe), and neutron measurements are reduced by forcing the density pad and neutron assembly against the borehole wall. However, this technique is not practical for logging-while-drilling devices. Consequently, a processing method has been developed that utilizes a tool-standoff measurement to emphasize small-standoff data. The standoff is measured with an ultrasonic transducer that is located on the same side of the tool as the nuclear sources and detectors. The nuclear counts are acquired in 20-millisecond intervals. The counts are immediately multiplied by a weighting factor that is appropriate for the standoff measured during the same time interval. The weight is large when there is no standoff, and it decreases rapidly with standoff. After a predefined time interval (e.g., 10 seconds), the weighted averages of the nuclear data are normalized and stored downhole in nonvolatile memory. The data is also processed down-hole to transmit real-time results, which include density, Pc, and neutron values, all with reduced standoff effects. Although standoff weighting greatly improves the accuracy of the nuclear measurements, it also increases their statistical uncertainties. These uncertainties are strongly affected by the drilling rate of penetration, as well. In order to monitor the overall statistical quality of the measurements, uncertainty curves are calculated for the nuclear measurements. In general, the increase in the statistical uncertainties of the measurements due to weighting is small compared to the reduction in systematic error that is achieved. After the data has been weighted, it is processed similar to wireline data, although several enhancements are made to the neutron porosity. The weighted standoff is used to dynamically correct the neutron log. Furthermore, measurements made with the standoff transducer and two other ultrasonic transducers, located at 120-degree intervals around the tool, are combined to make a caliper measurement while drilling. The caliper is used to apply borehole-size corrections to the nuclear measurements. As additional quality monitors, a neutron-porosity correction, density and Fe corrections are computed. The weighting technique significantly reduces these corrections, and consequently reduces potential systematic errors. Simulations based on test-formation data are used to compare this technique to unweighted, statistical, and four-quadrant processing. In addition, log examples illustrate the effectiveness of this new technique.