H. D. Scott, C. Stoner, B. A. Roscoe, R. E. Plasek, and R. A. Adolph
Schlumberger
ABSTRACT
Oil saturation behind casing can be determined by irradiating the formation with fast neutrons and measuring the resulting gamma ray spectrum. The gamma ray spectrum produced by inelastic neutron reactions yields information on the formation carbon and oxygen content and therefore on the oil saturation. This technique is particularly useful in cased wells when the formation water salinity is very low or unknown, conditions that prevent the effective use of the traditional formation thermal neutron capture cross-section measurement.
The Reservoir Saturation Tool (RST) is designed to make the carbon/oxygen measurement under conditions formerly unsuitable for this measurement. The tool’s smaller diameter makes measurements possible below tubing, and the use of dual gamma ray detectors allows compensation for the borehole fluid composition. This combination allows flowing wells to be logged and provides a formation oil saturation measurement under production conditions. A byproduct is the simultaneous determination of the borehole oil/water fraction, or holdup, under flowing conditions.
The 2 1/2-in,-diameter RST tool has successfully measured carbon/oxygen data in several North America, Middle East, and North Sea wells. The novel hardware design includes the first borehole use of fast, high-density gadolinium oxyorthosilicate (GSO) scintillation gamma ray detectors. The tool has proved reliable for extended logging times of more than 30 hours at temperatures to 135°C, which are close to the tool rating of 150°C.
A dual-detector interpretation model has been established using sets of laboratory measurements. Tool performance in known borehole fluids, in terms of precision and logging time, is comparable to that obtainable with the larger 3 5/8-in.-diameter Gamma Ray Spectrometry Tool (GST). Formation oil saturation values computed from flowing and shut-in passes in the same well are in good agreement.