An Improved Triple Porosity Model for Evaluation of Naturally Fractured Reservoirs

Abstract

AbstractMany naturally fractured reservoirs are composed of matrix, fractures and non-touching vugs (it can also be any other type of non- connected porosity that can occur, for example, in intragranular, moldic and/or fenestral porosity). An improved triple porosity model is presented that takes into account these different types of porosities. The model can be used continuously throughout a reservoir with segments composed of only matrix porosity, or only matrix-fractures, or only fractures-vugs, or the complete triple porosity system.The model improves a previous triple porosity algorithm by handling rigorously the scale associated with each, matrix, fractures and vugs. This permits determining more realistic values of the cementation or porosity exponent, m, for the composite system and consequently improved values of water saturation and reserves evaluations. The values of m for the triple porosity reservoir can be smaller, equal to, or larger than the porosity exponent of only the matrix blocks, mb, depending on the relative contribution of the vugs and fractures to the total porosity system.It is concluded that not taking into account the contribution of matrix, fractures and vugs in the petrophysical evaluation of triple porosity systems can lead to significant errors in the determination of m, and consequently the calculation of water saturation, hydrocarbons in place, recoveries, and ultimately poor economic evaluations, either too pessimistic or too optimistic. This is illustrated with a couple of examples from Middle East carbonates.