Advances in Pulsed Neutron Well Logging Technology Enable Efficient Reservoir Surveillance in an Indonesian Offshore Oilfield

Abstract Carbon‒oxygen (C/O) logging is an essential pulsed neutron measurement for determining water saturation if the salinity of the formation water is low or mixed. The disadvantage of C/O logging is that the logging speed is slow and requires multiple logging passes, resulting in high operating costs and possibly postponing production. We developed a new pulsed neutron well logging tool with three improved features, allowing pulsed neutron logging (including C/O logging and pulsed neutron capture (PNC) logging) at faster speeds. First, the next-generation multidetector pulsed neutron tool features lanthanum bromide (LaBr3) scintillation gamma-ray detectors. The density of the detector material is higher than that of the previous-generation detector crystal. The resolution of the spectral data was therefore enhanced. The second improvement is the development of a pulsed neutron generator (PNG). Compared with the legacy PNG, the fast neutron output of the new tool is twice as high. Finally, a new digital electronics system allows fast gamma-ray processing with significantly reduced dead time. We deployed the new pulsed neutron tool in an offshore oil and gas-producing well. The operator required formation surveillance in one of the wells that produced a high water content. The objective of logging was to determine the remaining hydrocarbon volumes of the producing reservoir and identify potential candidates for perforation. Since neither a workover rig nor a barge was available, the well logging operation had to be conducted using a modular logging unit during daylight hours. These operational constraints require efficient and fast data acquisition. We acquired key measurements for formation evaluation, such as ratios of gamma-ray count rates sensitive to the hydrogen index, formation sigma, C/O ratios, and pulsed neutron porosity. Data acquisition was three times faster than that with the legacy tool. In addition, compared with that of the legacy tool, the operating time of the new tool saved two days of daylight operation (or 50% of the operating time). The post-acquisition data analysis workflow incorporates measured data and forward modeling of tool responses using the Monte Carlo simulation method. The monitoring analysis revealed that the perforated reservoir contained a high volume of water. This observation was consistent with the high water cut. In addition, gas-bearing zones were identified as potential perforation intervals. Furthermore, the operator identified and optimized the candidate infill drilling wells on the basis of the pulsed neutron analysis results. These data acquisition and analysis techniques facilitate decision-making in reservoir management and development.