The five detector pulsed neutron (FDPN) tool is one of the most advanced tools in cased-hole reservoir evaluation. The evolution of multi-detector pulsed neutron tools (MDPN) created a turning point in the methodology to properly handle the different challenges within petrophysical cased-hole evaluation either in conventional or unconventional formations. This study demonstrates the applications of the FDPN tool and the methodology to handle the challenges in the multi-barrier well bore systems.
The FDPN tool was used in two double-barrier well bore field cases in addition to one case with a triple casing system. Each case was unique in terms of objectives and challenges: Case 1 (13.375 inch, 9.625 inch and 7 inch casings) aimed to estimate the presence of gas in the annulus between the two casings and the formation gas behind the double-barrier wellbore system, while Case 2 (7 inch liner and 4.5 inch ICD) required to perform a three-phase calculation. In Case 3 (13.5 inch, 10.75 inch and 7.625 inch casings) diffusion-corrected sigma was recorded to be able to determine the water saturation in a triple casing system. In all these case studies, Monte Carlo simulations (MCNP) were performed to characterize tool response and to determine if the data to be recorded will achieve the objectives of these jobs.
In Case 1, the FDPN tool helped to establish the presence of gas in the annulus between casings and determined the gas saturation profile in the formation using the difference between the normalized capture and inelastic ratios. This case was unique since other MDPN tools were not able to quantify the volume of gas in the formation due to their lower gas sensitivity. In Case 2, the objectives were successfully achieved. Also, it was possible to differentiate between the formation gas and the gas pockets between the 7 inch liner and the 4.5 inch ICDs using the comparison between inelastic and capture measurements. In Case 3, a diffusion-corrected sigma was measured and identified formation water movement in the zone of interest which helped to make a decision about drilling of the offset well.
The FDPN tool provides a higher spacing between the detectors and neutron generator which gives a better quantification of hydrocarbons especially for large casing and borehole sizes. Furthermore, the spacing between the long and proxy detectors is increasing the gas measurement sensitivity compared to other MDPN tools. Also, the 1.69 inch pulsed neutron logging tool was successfully run for first time in Kuwait to record diffusion corrected Sigma over a triple casing interval.