Summary
Compensated neutron logging (CNLSM) uses a two-detector system that was developed to reduce borehole effects. The ratio of counting rates from the detectors provides the basic tool response from which a porosity index is obtained. Each detector in this system has a different vertical resolution because of its spacing. A new method of processing the counting rates has been developed to enhance the vertical resolution capabilities of the neutron porosity index by exploiting the better vertical resolution of the near detector. Because no additional or new measurements are required, data from older wells can easily be re-evaluated.
Results from the new method have been compared with microspherically focused logs (MicroSFLSM) and electromagnetic propagation logs (EPTSM). They show repeatable thin-bed resolution on the order of 1 ft [0.3 m] for data sampled at 6-in. [15-cm] intervals; the typical vertical resolution from ratio processing is approximately 2 ft [0.6 m]. The statistical precision of the high-resolution processing is superior to that of the standard ratio method.
An additional parameter, obtained with the new processing method, provides information about borehole effects. This parameter can be used for qualitative indications of gas when invasion is not deep and environmental effects are not large.
The new method has been applied successfully in carbonate and laminated sand formations. Studies show that thin beds can be detected in high-porosity formations where normal processing has significant statistical variations resulting from reduced counting rates.
