Pulsed Neutron Application in Monitoring Formation Gas Movement and Reservoir Depletion Surveillance in a Mature Niger Delta Field

Abstract Pulsed neutron logging (PNL) is a known acceptable technique for oil reservoir surveillance, especially in mature fields where reservoir depletion needs continuous monitoring. In cased wells, multi-detector pulsed neutron logging systems provide good investigation and accuracy in determining fluid saturation changes, particularly in freshwater environments were traditional sigma logging techniques face challenges due to similar readings in water and oil. PNL service operates by emitting high-energy neutrons that propagate into the nearby environment, and then, loose their energy through interactions with the wellbore (casing, completion sections and borehole fluids) and formation (lithology and fluids) environment. Multiple detectors, at precise distances from the neutron source, capture these gamma-ray emissions which enable detailed analysis of the formation fluid. In freshwater environments, the lack of contrast between oil and water readings, due to low chlorine presence, complicates fluid identification using the traditional sigma measurement. In this case, multi-detector pulsed neutron systems utilize advanced gamma-ray inelastic scattering measurements, which differentiates between oil and water by focusing on elements such as carbon and oxygen presence despite of the salinity amount in the formation fluids. This approach was used in a brown field in the Niger Delta which has been in production for over sixty years. Some of the reservoirs logged were completed in the well selected. In these wells, the hydrocarbon initial saturation profile was mostly uniform and there was a marked movement in hydrocarbon/water contacts from initial conditions. This was not the case in some other reservoirs logged; these showed a depletion as a result of production from nearby wells. These showed slightly more complex depletion patterns. Generally, the contacts seen in the reservoirs are not uniform across the field; this is due to intra reservoir faults presence. The use of specific Monte Carlo N-Particle (MCNP) models, based on completion and formation fluid properties input data, are critical and fundamental to predict an emulated tool response to hydrocarbon presence, whose outcomes are then integrated into the field data analysis workflow. A depth-based triangulation interaction between the theoretical water, gas and oil responses as a function of the effective porosity range, is obtained integrating two main PNL measurements and relative saturation analysis, one specifically sensitive to gas versus liquid phases and the second sensitive to oil versus water. The application of this triangulation method is compared with actual measured processed data and computes a three-phase corrected saturation profile. Potential gas presence or lighter oil condition, imply the necessity to characterize gas measurement. High-quality data was obtained through four passes of C/O and two passes of highly sensitive gas data (PNC3D) measurements. The heightened sensitivity to gas is attributed to the third gamma-ray detector employed. Two key measurements, RIN13 and RATO13, respectively the short and third detector ratio from the inelastic and capture measurements are utilized in computing gas saturation. This paper presents a case of a time lapse logging in a reservoir with a long substantial monitoring time gap. PNL data was acquired in 2006 the first time and then in 2023 where some interesting phenomenon in the gas cap movement were noticed. There is also the effect of aquifer pressure drive on the fluid movement, since these reservoirs are known to have very good aquifer support.