Accuracy Of Porosity Determinations

Porosity values of reservoir rocks may be obtained directly by measurements on core samples, or may be calculated indirectly from data provided by various types of downhole logs. Calculation of porosity values by the indirect methods requires the use of certain factors which are characteristic of the formation and its continued fluids. These factors can be determined best by "calibration" of the log data with directly measured porosity data. The direct methods most commonly used for determining porosity by core analysis are presented. The advantages, limitations and errors of each are discussed and the methods are evaluated. In the usual porosity ranges, the acceptable methods for determining porosity are accurate to within ñ « of a porosity per cent. An understanding of the methods will assist the log analyst in evaluating the porosity data available for use in determining the factors to be used in log interpretations.