Summary and Introduction
Potential California reservoirs for secondary and enhanced oil recovery Potential California reservoirs for secondary and enhanced oil recovery (EOR) projects are typically of moderate to high porosity with formation waters ranging from fresh to brackish. Under these conditions, pulsed neutron devices, such as the continuous carbon/oxygen (C/O) log, are invaluable for monitoring sweep and displacement efficiency, locating fluid or gas breakthrough, and evaluating residual oil saturation (ROS). In west Texas the C/O log assists in the re-evaluation of partially depleted oil reserves and exploration for bypassed oil in old wells in mature producing areas. Field case examples, including results of subsequent production tests in the Yates, Seven Rivers, Queen, Grayburg, San Andres, Glorieta, and Clearfork formations are presented.
Pulsed Neutron Logging Pulsed Neutron Logging For more than 15 years, pulsed neutron logging devices that use several different gating systems to measure the macroscopic cross section ( ) for thermal neutron capture in a borehole environment have been successful in distinguishing between high-salinity formation waters and hydrocarbons behind the casing, both qualitatively and quantitatively. Hence, they can be used to monitor time-related variations in water, oil, and gas saturations in reservoirs under primary or EOR schemes. The resulting information is invaluable input in reservoir engineering and workover projects. projects. While such neutron lifetime and thermal decay time, devices have their optimal range of applications in saline, high-porosity reservoirs, they cannot be used to evaluate and/or to monitor hydrocarbon saturations where the macroscopic cross section of formation water is not significantly different from that of the hydrocarbons, or in cases where salinity is constantly changing and unknown (fresh water or steamfloods, etc.). The continuous C/O logging instrument is a pulsed neutron device that measures the energy and intensity of gamma rays resulting from neutron irradiation of the formation. This information then can be related to the relative abundance of particular elements in the formation (such as carbon, oxygen, silicon, and calcium). Ratios formed from the relative abundance of these elements are particularly useful in identifying formation lithology, porosity, and fluid particularly useful in identifying formation lithology, porosity, and fluid content. In particular, a ratio of the number of carbon and oxygen gamma rays produced by inelastic scattering of fast neutrons will identify and measure the hydrocarbon potential of the formation, along with other measurements that determine the lithology and porosity of the borehole environment. Since the C/O ratio is derived from the inelastic scattering of fast neutrons from carbon and oxygen and as such is insensitive to the chlorine content of borehole and/or formation waters, the C/O log can be used in areas where other pulsed neutron logs are not applicable. Introduced on a commercial basis in 1976, the continuous C/O logging system has been very successful in delineating and quantifying oil reserves in many wells throughout the world under a wide variety of reservoir conditions.
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