High-Temperature Saturation Logging For Reservoir Monitoring – Applications & Operational Best Practices

Abstract

In recent times, cased-hole pulsed neutron (PN) saturation logging has played a much important role in the formation evaluation of challenging wells with difficult conditions such as extreme wellbore temperature and incomplete or inconclusive open-hole data acquisition. This paper illustrates the diverse applications of high-temperature saturation logging for reservoir monitoring and discusses the best practices from both operation and interpretation perspectives for fluid identification and saturation computation.

High temperature (HT) Multi detector Pulsed-Neutron (MDPN) tool, equipped with a flask, was deployed in high-temperature steamflood-assisted gravity drainage (SAGD) wells. Sigma or advanced gas mode were logged for steam growth monitoring, while Carbon / Oxygen (C/O) logging was conducted to compute the oil saturation. Additionally, HT MDPN also proved effective in ultra-deep gas wells and wells with inconclusive open-hole data, with some covering logging intervals exceeding 2000m and bottomhole temperature (BHT) up to 230degC using advanced gas and C/O mode. Real-time monitoring of internal tool temperature was essential, with cooling protocols, including tool rotation upon reaching the surface, implemented to maintain operational integrity. Multiple runs in each pass were carried out to ensure full coverage of the logging interval.

Time-lapse PN logging in HT SAGD wells enabled consistent monitoring of steam chamber growth over the years through in-house computed thermal neutron porosity (TPHI) and diffusion-free formation sigma measurements. In the case of HPHT ultra-deep well, despite extreme BHT, successful data acquisition was achieved using flasked MDPN tool without major issues. By employing multiple runs from two tools per pass and adhering to tool cooling plan, temperature limitations were effectively mitigated, ensuring uninterrupted operation. Advanced gas mode Monte-Carlo N-Particle (MCNP) modeling, customized for specific borehole configuration and fluid type, was simulated and applied to process acquired raw data and quantify saturation. Furthermore, in situations where no available or inconclusive open-hole data are present, cased-hole inelastic measurement from C/O and capture measurements such as ratios curves from Sigma and advanced gas mode were used to emulate reservoir properties such as porosity for further evaluation. Good correlation between the emulated porosity with offset well open-hole porosity further improved the confidence of subsequent C/O oil saturation analysis.

In summary, the meticulous pre-job planning, and execution of the HT MPDN tool enables comprehensive reservoir evaluation in a variety of challenging conditions. The integration between the time and energy spectra-based logging and MCNP modeling significantly enhanced the accuracy of saturation results. These advancements facilitate more informed decision-making and support the planning of production optimization strategies based on the MDPN formation evaluation outcomes.