As oil fields mature, progressively more effort is being devoted to diagnosing production and reservoir problems and to finding cost-effective solutions for those problems. One of the key inputs for any reservoir management scheme is the timely monitoring of behind-pipe fluid saturation profiles across the field. In the past, this was conventionally accomplished using tools and methods requiring known, non-varying and sufficiently high formation water salinities. In the cases of water flooding with fresh water, mixed water, and/or water of unknown salinity, these methods become useless. In such cases, cased-hole saturation monitoring may only be achieved using tools and methods independent of water salinity such as the carbon-oxygen (C/O) method.
Using the C/O techniques to compute water saturation offers many advantages over conventional techniques that depend on formation water salinity. These techniques relate directly to the volumes of oil and water in the formation, and the conversion of C/O ratio to oil saturation is based on a large database acquired in a wide range of formations and well bore environments. Recent enhancements to spectral processing and job execution have resulted in dramatic improvements in the effectiveness of C/O logs. These enhancements include improved elemental standards and the development of a full spectrum calibration to provide better tool-to-tool accuracy and precision in a wider range of porosities even in the presence of gas. The enhancements also include operational improvements such as combination with production logging tools and appropriate "time-stepping" of shut-in and flowing surveys.
Several Gulf of Suez fields have undergone water flooding with non-native waters and have thus presented formidable challenges to traditional cased hole saturation monitoring techniques. More recently, operators in the area have started employing enhanced C/O techniques with great success. Enhanced C/O methods are now routinely used to effectively detect unswept hydrocarbons and to track fluid contact movements in water-flooded reservoirs. This paper discusses several case studies from the Gulf of Suez that clearly demonstrate the efficacy and economic benefits of the enhanced C/O methods.
Cased Hole Saturation Monitoring Methods
Saturation monitoring through casing is generally carried out in one or both of two ways: pulsed neutron capture (PNC), which measures the decay of thermal neutron populations, and gamma ray spectroscopy, which determines the relative amounts of carbon and oxygen in the formation. 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 yield measurements of formation and borehole capture cross-sections. The capture cross section of the formation, often called Sigma, 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). Because chlorine has a large neutron capture cross section, the PNC technique provides good results in areas with highly saline formation waters.
