Optimized Formation Evaluation with Simultaneous Acquisition ofSigma-Based Fluid Saturation and Through-Casing Mineralogy - ACase-Study from Jurásico Units, México
Authors: Alarcon Nora , Rodriguez Luis , Mendez Freddy , Arenas John , Julián Gonzalez Morales 
Published: October 2017

Abstract
In current market conditions, performing logging operations as efficiently as practically feasible has become crucial to reduce the overall cost of formation evaluation. One of the most important formation evaluation parameters is hydrocarbon saturation. At the same time, it is imperative to obtain detailed lithological and mineralogical characteristics of the formation for optimal reservoir characterization. Acquiring all the data required for performing complete reservoir characterization from separate runs may become a time-consuming and costly activity.
Typically, in openhole conditions, resistivity tools are used for determining the hydrocarbon saturation of the formation. However, when the wells are cased, conventional resistivity tools cannot be used due to the metal present in the casing. Consequently, for through-casing formation evaluation, pulsed neutron logging tools have been used for several yearsto determine hydrocarbon saturation and perform reservoir monitoring. In a high-salinity formation-water reservoir, the thermal neutron capture cross section of the formation, sigma, can be used for estimating hydrocarbon saturation. Historically, an independent deployment of a slimhole pulsed neutron logging tool was required to measure the sigma of the formation. Although the results can be very accurate, this approach is not cost-effective during the current period of oil-market downturn.
Natural and pulsed neutron-induced spectroscopy measurements have been used in openhole conditions for more than a decade to determine lithology and mineralogy of the formation. In recent past, mineralogical pulsed neutron spectroscopy instruments have been characterized as tools capable of generating similar results via measurements through casing. When a mineralogical pulsed-neutron spectroscopy logging tool is used to obtain the elemental weight fractions to determine lithology and mineralogy, sigma formation parameters can also be extracted from the same measurements to provide hydrocarbon saturation. In highly saline environments, for example, chlorine, due to its high sigma response, causesa significant difference between saline-water-bearing zone and the hydrocarbon-bearing zone. Simultaneous acquisition of detailed lithology and mineralogy, along with sigma, helps in considerably reducing the logging time and completing the entire formation evaluation operation very efficiently. In addition, the acquisition capability through casing reduces health, safety &environment (HS&E) hazards.
This paper demonstrates the above-mentioned methodology being applied successfully in high-salinity carbonate reservoirs in Mexico. The customer deployed the mineralogical pulsed-neutron spectroscopy tool for obtaining a detailed characterization of formation lithology and mineralogy. The same data set was used to calculate the sigma-based hydrocarbon saturation. Results showed a very high degree of agreement with conventional openhole hydrocarbon saturation analysis. These results provided the client with timely and accurate information for completion decisions. In addition, the paper shows the mineralogical results in open and cased-hole environments to illustrate the accuracy of the measurements and the proposed methodology.