Pulsed neutron spectroscopy (PNS) is a well-established technology for characterizing reservoir saturation through cased hole, using either sigma (Σ) or carbon/oxygen (C/O) ratio measurements. However, the current technologies struggle to deliver reliable answers in complex completions. Tubing and casing, with varying tubing and annulus fluids, or dual tubing completions with changing annulus and tubing fluids represent cases in which it becomes difficult to identify fluid contacts in the formation and calculate remaining saturations.

A new-generation slim pulsed neutron logging tool has been developed to deliver reliable answers in conditions that challenged existing technologies. It introduces the new petrophysical measurement, fast neutron cross section. This measurement is highly sensitive to variations in gas volume and insensitive to variations in water volume, independent of neutron porosity and formation Σ properties. It provides high-resolution spectroscopy with a much-improved accuracy and precision of all elements measured, including the key element for oil saturation, carbon. The carbon measurement is used conventionally for C/O, and it is used directly to derive total organic carbon (TOC), which is then converted to oil saturation.

This tool delivers the self-compensated Σ and neutron porosity measurements in a wide range of conditions, including complex completions and varying amount of gas in the wellbore or annulus.

The field test results in this paper demonstrate the performance of this new tool in a few wells from Malaysia. All present some complex completions, from single tubing inside 7-in. casing and 8.5-in. hole to dual 3.5-in. tubing in 95/8-in. casing and 12 1/4-in. hole. Additional challenges include gas-filled annulus, multizone completion with sliding side doors (SSD) and wire-wrapped screens (WWS), and environments in which there are no water sands for C/O measurement calibration.

The logging objectives include determining theoil/water contact (OWC) and the gas/water contact (GWC),quantifying the current saturation, confirming the source of water for water shutoff determination and anticipated gain, and verifying sand-filled annulus. A back-to-back comparison with the previous technology was also run in the first well, allowing a directcomparison of the measurements from the new and the existing tools in the same conditions.