L. A. Jacobson, R. Beals, D. F. Wyatt, Jr. and A. Hrametz
Halliburton Logging Services, Inc., Houston, TX
Abstract
In cased holes, oil saturation estimation in formations containing water that is fresh or of unknown salinity relies on the carbon/oxygen measurement. Unfortunately, the dynamic range of the measurement is small; thus, the measurement requires very high statistical precision for reasonable uncertainty in the oil saturation estimation. A new accelerator based pulsed spectral gamma-ray logging tool is introduced which employs a high-density scintillation detector. This new scintillator provides good dynamic range and better statistical precision than older systems. The PSG° tool is 3.375 in diameter and 33ft long, incorporates downhole pulse-height analysis and storage, and digital telemetry, A dewar provides l2hr of thermal protection for the detector at maximum ambient temperature of 150°C. Four 256-channel spectra are recorded: one during the neutron burst records inelastic events, a second and third between bursts record the capture events and a fourth during a long pause records the background events. The neutron burst rate is 10kHz.
Extensive laboratory measurements were used to characterize the cased-hole response of the tool to porosity, lithology, fluid type and borehole conditions. A C/O ratio constructed from windows positioned across the primary carbon and oxygen inelastic lines shows a dynamic range of 18% at 35pu in a cased l0in borehole. The C/O ratio is shown to be relatively unaffected by casing size and cement thickness. The dynamic range decreases linearly with increasing borehole size between 6in and l4in. Similar behavior is observed for an inelastic lithology ratio (Ca/Si). A properly scaled overlay of these two curves provides a good quick-look method for locating oil zones.
Capture data provide further estimators of lithology, shaliness, porosity and salinity. Several log quality indicators that monitor hardware (spectral gain stabilization and offset) and software functions (curve statistical precision) are available in real time. Logging examples obtained at a logging speed of 5 ft/min illustrate the improved continuous logging capability possible through the use of the bismuth germanate scintillator. The PSG tool can log at least twice as fast as previous C/O tools and still maintain comparable statistical performance. Logging at slower speeds will give statistical performance exceeding that of previous tools.