A Dual Porosity CNL Logging System

Abstract

ABSTRACT 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 (Σ) in the formation is small. Corrections are required to the measured 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 Σ, 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 Σ 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 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 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 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.