Modern Carbon/Oxygen Logging Methodologies: Comparing Hydrocarbon Saturation Determination Techniques

Abstract

Abstract Recent advances in Carbon/Oxygen (C/O) logging technology have provided measurements that have become more quantitatively robust in reservoir monitoring projects, especially in mixed salinity environments. Enhancements in tool design, characterization and interpretation algorithms have contributed to increases in precision and accuracy of fluid saturation measurements. Even though these innovations have led to major improvements, fluid saturation profiles provided by different service companies are often varied. The variations are due to differences in tool designs and interpretation methodologies and thus, a unique solution is not always provided. The inconsistencies in answers are generally not understood due to characterization complexities of the formation and the borehole environment including cements, casing, fluid types, etc. As a consequence, 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. Several open-hole wells were chosen and logged with the latest types of tools under identical conditions with well defined environmental parameters, thus enabling accurate correlations between tool responses and fluid saturations. The accuracy of the results was then evaluated based on a comparison to core, production logs, pulsed neutron sigma, resistivity and fluid sample data. This paper presents the results of the study and illustrates how differences in service company tool designs, characterizations and interpretational methodologies affect fluid saturation computations in this field. In addition, recommendations are provided on job planning and utilization of this technology in large scale reservoir monitoring projects. Introduction One of the most important factors in managing reservoirs is an accurate determination of water saturation. The accuracy of this value is very critical in time based measurements that are 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 calculations can translate into large volumes of hydrocarbons. The carbonate field in this study was discovered in 1950's and an enhanced oil recovery project was initiated in 1970's. Since then, several types of waters with varying amount of salinities have been injected. This salinity variation has caused complexities in the saturation computations that rely on accurate measurements of formation water resistivity, i.e. resistivity tools. Resistivity measurements are used in empirical equations to calculate fluid saturations based on known formation water resistivity. In formations that contain fresh waters and unknown water salinities, these equations produce answers with large uncertainties. These limitations have contributed to the development of the Pulsed Neutron Spectroscopy (PNS) tools. The tools use neutron generator produces high energy neutron that collide with different types of elements that are present in the formation. These collisions produce gamma ray signature values that are used to measure the relative abundances of elements. Two of the most important elements are carbon and oxygen. The abundances of these elements are directly used to calculate volume of hydrocarbons. C/O logging, introduced in the 1960s, has been commercially available since the 1970s. The earlier technology was cumbersome, difficult to use and often provided unreliable answers. In the 1990s several new generation of tools were introduced by the three major logging companies; Schlumberger, Halliburton and Baker Atlas. In comparison to the older tools, these new tools have high output neutron generators with better detectors and characterizations that provide more accurate and precise answers. Even though these tools operate on the same principles, service companies have taken different avenues of technological developments and methodologies, which have resulted in non unique solutions among the vendors.