Modern Carbon/Oxygen Logging: Hydrocarbon- Saturation-Determination Techniques
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 90339, "Modern Carbon/Oxygen Logging Methodologies: Comparing Hydrocarbon-Saturation-Determination Techniques," by Ramsin Y. Eyvazzadeh, Oscar Kelder, A.A. Hajari, Shouxiang Ma, and Abdallah M. Behair, SPE, Saudi Aramco, prepared for the 2004 SPE Annual Technical Conference and Exhibition, Houston, 26-29 September.
Recent advances in carbon/oxygen (C/O) logging technology have provided measurements that have become more quantitative in reservoir-monitoring projects, especially in mixed-salinity environments. Although innovations have led to major improvements, fluid-saturation profiles provided by different service companies often vary. These variations are caused by differences in tool design and interpretation method. A study was conducted in a large Middle Eastern carbonate field to evaluate and compare accuracy and precision of different types of pulsed-neutron-spectroscopy (PNS) tools in a controlled environment.
Introduction
An important reservoir-management factor is accurate determination of water saturation. Accuracy is critical in time-based measurements used in tracking reservoir depletion, enhanced recovery, workover strategies, and injection-water breakthroughs. This parameter is even more sensitive in large fields, where small variations in water-saturation values can translate into large hydrocarbon-volume differences.
The carbonate field in this study was discovered in the 1950s, and an enhanced-oil-recovery project was initiated in the 1970s. Since then, several types of water, with varying salinities, have been injected. This salinity variation complicates the saturation computations that rely on accurate measurements of formation-water resistivity (i.e., resistivity-tool data). Resistivity measurements are used in empirical equations to calculate fluid saturations on the basis of known formation-water resistivity. In formations that contain fresh water or unknown water salinity, these equations produce answers with large uncertainties. These limitations contributed to development of the PNS tools. The tools use neutron generators to produce high-energy neutrons that collide with different types of elements in the formation. These collisions produce gamma ray signatures that are used to measure the relative abundance of elements. Two of the most important elements are carbon and oxygen. The abundance of these elements is used directly to calculate hydrocarbon volume.
C/O logging was introduced in the 1960s. The early technology was cumbersome, difficult to use, and, often, provided unreliable answers. In the 1990s, several new-generation tools were introduced by three major logging companies: Schlumberger, Halliburton, and Baker Atlas. These new tools have high-output neutron generators with better detectors and characterizations that provide more-accurate answers. Even though these tools operate on the same principles, the service companies use different methods, resulting in nonunique solutions.
This study examined these differences in two wells. In the first well, saturation data from nuclear-magnetic-resonance (NMR) logs were compared with C/O-log and laboratory core-measured saturation data to determine the applicability of this technology in this field. In the second well, resistivity and capture data were used as references to compare data from three different types of PNS tools.