The two-detector CNL* neutron porosity logging system has been commercially available for about 10 years. It provides an accurate log when the amount of elements with high thermal neutron capture cross section (E) in the formation is small. Corrections are required to the measured porosity when this is not the case. In order to porosity when this is not the case. In order to take advantage of the better measurement statistics obtainable with thermal neutron detectors and the insensitivity of epithermal neutron detectors to the presence of elements with large, a new CNL logging tool incorporating two thermal and two epithermal neutron detectors has been designed.
Two separate porosity measurements are thus obtained—one from each pair of detectors. In "clean" formations the measured porosity values agree. However, in formations containing high elements (such as shales), the porosity measured by the epithermal detectors will read lower and agree more closely with the density-derived porosity. The epithermal measurement thus porosity. The epithermal measurement thus provides better gas detection in shaly gas sands. provides better gas detection in shaly gas sands. A comparison of the two porosity measurements can provide an indication of the clay content of the provide an indication of the clay content of the formation.
A data processing technique, utilizing the individual detector counting rates rather than their ratios, is used to derive the porosity values. This technique provides considerable improvement with regard to correction for borehole effects. An adaptive filtering scheme has been used to minimize the statistical uncertainty of the porosity measurement. This filter allows quick response to borehole environment changes while reducing statistical variation in slowly varying zones.
Log examples are shown which illustrate the shale effects on both the thermal and epithermal porosity measurements. Also presented is an porosity measurements. Also presented is an example showing the improvements due to the use of the new data processing technique. The advantage of the epithermal measurement for gas detection can be seen in a representative example.
For the last decade, the two-detector CNL* neutron porosity logging system has provided a good measurement of formation porosity. The two-detector technique has offered improved immunity to environmental effects when compared to single- detector measurement techniques. However, because the measurement of porosity is based on the detection of thermal neutrons, the presence of thermal neutron absorbing elements in the formation sometimes complicates the interpretation of the results. Such elements are commonly associated with clay, but are not the same elements responsible for the natural radioactivity of clay. Gamma Ray, SP, and electrical measurements can be used as indicators of clay, but do not provide a direct determination of the amount of thermal neutron absorbing elements present. Offsetting this complication, a porosity measurement based on the detection of thermal neutrons has good statistical precision, allowing the source-to-detector spacing to be set to reduce the borehole environmental effects.
Epithermal neutron detection offers relative immunity to the effects of thermal neutron absorbing elements, but at reduced detector counting rates. To prevent a resulting poorer statistical precision, epithermal detectors must be spaced closer to the neutron source. This closer spacing increases the magnitude of some environmental effects when the ratio of detector count rates is used for the porosity determination.