Enhanced C/O Logging as an Effective Cased Hole Saturation Monitoring Solution Case Histories from the Gulf of Suez

Abstract

Abstract As oil fields mature, progressively more effort must be devoted to diagnosing production and reservoir problems and to finding cost-effective solutions for those problems. One of the key inputs to any reservoir management scheme is the timely monitoring of behind-pipe fluid saturation profiles across the field. Using the C/O ratio to compute water saturation offers many advantages over conventional techniques that depend on formation water salinity. The C/O ratio relates directly to the volumes of oil and water in the formation, and conversion of C/O ratio to oil saturation is based on a very large database acquired using laboratory formations with a wide range of wellbore environments. Recent enhancements in spectral processing techniques for pulsed neutron spectroscopy tools have improved accuracy and precision of measured carbon-oxygen (C/O) ratios. These include improved elemental standards and the development of a full spectrum calibration to provide better accuracy and precision in a wider range of porosities and in the presence of gas. Many of the Gulf of Suez fields are characterized by edge water drive mechanisms or are undergoing water flooding with different salinity water than the formation water. These conditions have traditionally presented formidable challenges to cased hole saturation monitoring, often resulting in highly uncertain results. This paper highlights several case histories from the Gulf of Suez which demonstrate how Enhanced C/O methods were used to effectively detect unswept hydrocarbons and track gas/oil/water contact movements in the reservoirs. Cased Hole Saturation Monitoring Methods Saturation monitoring through casing is generally carried out in one of two ways. One way, pulsed neutron capture, measures the decay of thermal neutron populations, and the other, inelastic gamma ray spectroscopy, determines the relative amounts of carbon and oxygen in the formation. Pulsed neutron capture (PNC) tools first became available to the petroleum industry in 1968 to measure the thermal decay time of neutrons bombarded into the formation. Fast neutrons (exiting the minitron with an energy level of around 14 Mev) are slowed down to thermal energy (0.025 eV) by multiple collisions with formation nuclei. Thermal neutrons are susceptible to capture by formation nuclei, and the resulting nucleus becomes excited and emits a characteristic gamma ray. The thermal neutron population around the tool can therefore be analyzed to give formation and borehole sigma measurements. Because chlorine has a large neutron capture cross section, the PNC technique provides good results in areas with highly saline formation waters. Sigma, the capture cross section of the formation, is determined by analyzing the approximately exponential decline of the gamma ray count rate with time as the neutrons are captured by the surrounding materials (neutron capture) and as they diffuse farther away (neutron diffusion). Sigma is inferred from this observed decline in the gamma ray count rate versus time. In addition to the neutron capture, two key environmental effects, diffusion and borehole contamination, contribute to the observed decline or decay rate and need to be carefully characterized in order to determine the correct Sigma throughout the wide range of operating conditions typically encountered in the oilfield. These effects are controlled by such parameters as borehole size, casing size, casing weight, borehole fluid salinity, porosity, and lithology.