Abstract
Pulsed neutron spectral carbon/oxygen (C/O) logging is a widely used shallow measurement for time lapse reservoir saturation monitoring. Because of its shallow depth of investigation, its measurement is affected by several factors among which the near wellbore logging environment is the most important. For example, hole condition, cement quality and fluids in the wellbore have drastic effects on C/O measurement. Some of these effects, such as those due to oil holdup in the wellbore, can be corrected for in the data post-processing. Others might have a permanent imprint on the data, which cannot be removed. It is therefore extremely important to fully understand the effects of the different physical parameters within the wellbore vicinity that might be affecting the measurements.
An extensive study was conducted in a field where reservoir saturation monitoring logs encountered discrepancies in water saturation between open hole and C/O logs, where the latter showed changes in saturation not supported by production history. It was found that one of the main reasons behind this discrepancy is the effect of mud filtrate invasion on the shallow C/O measurement, which varies as a function of timeand rock type characteristics. Other factors, such as wellbore fluid reinvasion effect across zones of perforation, also play a role.
In this paper, it is demonstrated with field examples the effect of mud filtrate on C/O logs and define criteria used to diagnose this phenomenon. It is also highlighted other factors affecting C/O measurement such as wellbore fluid reinvasion and cement condition. Last, we present recommendations and best practices to conduct a more representative reservoir saturation monitoring.
