A Method for Estimating Fluid Saturations from Well Logs
Authors: Campbell Robert A. 
Published: December 2007

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.

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Gas and oil saturation are among the key values sought from a logging suite. The recent introduction of the "crossplot" interpretation methods and the sophistication of logging tools can allow better estimates for their values. Utilization of standard log data in the equations presented herein appear to result in better estimates of oil and/or gas saturations.

Introduction

Well logging is by far the most widely used method of evaluating potential reservoirs. Recent improvements in logging tools have resulted in very accurate and reliable data. However, we are interpreting this "new" data with the same methods developed years ago. This is not to say that the "old" methods are without merit, but rather the accuracy and sophistication of the present logging tools offer better interpretation methods.

The purpose of this paper is to develop an interpretation method that yields estimates of hydrocarbon saturations without utilizing complex computer programs. Water saturation, as calculated by conventional methods, is very sensitive to porosity, formation water. resistivity, and saturation exponents. Having calculated a value for water saturation, we may then only say that hydrocarbons occupy the remaining pore space. But this value does not indicate whether the hydrocarbon material is gas or oil nor the percentage of each.

Rodermund et al. were the first to present a method for estimating gas saturations. present a method for estimating gas saturations. However, the resulting value can only be considered as the minimum gas saturation and is only valid under ideal borehole conditions. The recent crossplot presentation of the compensated density log porosity with porosity from either the compensated neutron log or sidewall neutron log has allowed a new approach to practical log interpretation. Among other practical log interpretation. Among other things, it has supplied the well site engineer with a "quick look" method for evaluating potential pay sections and indicates lithology, potential pay sections and indicates lithology, as well as the presence of gas. However, the concepts introduced in this paper provide an interpretation method that can lead to better values for hydrocarbon saturations in sandstones.

THEORY

The Compensated Density Log, being a wall contact tool, is not affected greatly by borehole conditions except when extreme washout is present. Apparent porosity in a gas zone is present. Apparent porosity in a gas zone is always optimistic since the calculation is based on a formation fluid density (Df) ranging from 1.0 to 1.1 gm/cc. The density of gas, which is compressible, varies but is usually between 0.15 and 0.35 gm/cc. Thus, as gas saturation increases, the more apparent density log porosity diverges from true porosity (T).