A triple porosity model for petrophysical analysis of naturally fractured reservoirs

Abstract

The analysis of vuggy and fractured reservoirs has been an area of significant interest in the past few years. Several researchers have studied the characterization of these reservoirs using dual porosity models and have looked for means of estimating values of the dual porosity exponent m for use in calculations of water saturation. There are instances where the reservoir is composed mainly of matrix, fractures and non-connected vugs. In these cases a triple porosity model appears more suitable for petrophysical evaluation of the reservoir. A new technique is presented for these types of reservoirs that is shown to hold for all combinations of matrix, fracture, and non-connected vug porosities. At low porosities, the fractures dominate and the m values of the composite system tend to be smaller than the porosity exponent of the matrix (mb). As the total porosity increases, however, the effect of the non-connected vugs becomes more important and m of the triple porosity system can become larger than mb. To the best of our knowledge a solution to the problem associated with the triple porosity exponent has not been addressed previously in the petrophysical literature. This research is inspired by the availability of modern magnetic resonance, micro-resistivity and sonic image tools that permit reasonable characterization of complex reservoirs. The use of the triple porosity model is illustrated with an example.