FIELD EXAMPLES WITH A SLIM LWD DENSITY/NEUTRON INSTRUMENT CONTAINING A CALIFORNIUM-252 NEUTRON SOURCE AND THREE NEUTRON DETECTORS

摘要

A new nuclear logging-while-drilling (LWD) instrument for use in boreholes as small as 5? inches is discussed. It includes a standard Cesium-137 source for the density measurement and a Californium-252 neutron source for the porosity measurement. This neutron source provides a neutron output comparable to a conventional Americium-Beryllium (Am-Be) source but with three orders of magnitude lower radioactivity. The physical size of the Cf-252 source is relatively small, allowing it to be securely mounted in the slim tool without requiring unfortunate design tradeoffs. Substantially improved porosity sensitivity is obtained with the Cf-252 source because the average neutron energy is about half that of Am-Be. An accurate Monte Carlo model was developed for the neutron measurements. Neutron porosity sensitivities and statistical variations were computed for both types of sources. Three thermal neutron porosities are obtained from three detectors with different spacings from the source. After correcting for environmental effects, these porosities agree closely in clean formations. Residual separations between the three corrected porosities in shales are consistent with the separations expected from Monte Carlo simulation results. Examples illustrate shale effects and show the utility of the additional detector. A new neutron processing method is discussed with significantly smaller standoff and holesize effects compared to the traditional method. It is called the “water subtraction” method because the counting rates in water are subtracted from the individual detector counting rates. Field examples compare neutron porosities and borehole corrections between the new and traditional methods. Two identical ultrasonic standoff transducers are located in-line with the nuclear sensors. Each transducer contains piezo-composite elements that operate in pitch-catch mode. Standoff measurements from the two transducers are used to calculate independent caliper measurements and to form independent sets of standoff-weighted neutron and density data, resulting in improved reliability and quality control. Features of the transducer design which enable the transducers to operate over a wide range of standoff conditions are discussed. This instrument was designed without a density sleeve, making it “driller-friendly” and less likely to get stuck. Although density corrections become severe for large standoff values, standoff weighting automatically selects data with the smallest standoff and consequently mitigates the need for a sleeve. Field examples illustrate that the density measurements are accurate.