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Al-Ghamdi, Ali (Geoscience Department, University of Calgary) | Chen, Bo (Yangtze University) | Behmanesh, Hamid (Petroleum University of Technology) | Qanbari, Farhad (Petroleum University of Technology) | Aguilera, Roberto (Schulich School of Engineering, University of Calgary)
Abstract Many 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.
Copyright 1988 Society of Petroleum Engineers This manuscript was provided to the Society of Petroleum Engineers for distribution and possible publication in an SPE journal. The material is subject to correction by the author(s). Permission to copy is restricted to an abstract of not more than 300 words. Abstract Porosity exponent m is a characteristic parameter for detecting naturally fractured reservoirs by being smaller than the porosity exponent mb of the matrix depending on the degree of fracturing. This is not a new concept and has been analyzed by R.Aguilera previously.
Compared with conventional reservoirs, complex reservoirs often have more variety of pore types and more complex pore shape. For example, carbonate rocks can have a variety of pore types, such as moldic, vuggy, interparticle, intraparticle and crack. Therefore the essential of rock physics modelling of complex reservoirs is to characterize the complicated pore structure, that is to say, the rock physics model can contain a variety of pore types. This paper selected the critical porosity model as the study object, by combining with Kuster-Toksöz equations, established the relationship between the critical porosity of rocks and the pore structure (i.e. the pore aspect ratio) of rocks, proposed a new critical porosity model for multiple-porosity rock which can contain various pore types and can be used to model complex reservoirs.
The elastic properties of the rock depend on the pore structure significantly. Most of rocks usually have two or even more than two different pore types, such as pore, crack, cavity, etc., whose complex pore system makes the relationship between the velocity and porosity of the rock highly scattered (Sayers, 2008). Therefore, it needs to establish a multiple-porosity rock physical model for characterizing the elastic modulus of porous rock varying with porosity accurately.
Effective medium theory is often used to study the elastic properties of porous rock, such as Kuster-Toksöz theory. Kuster and Toksöz (1974) derived expressions for bulk and shear moduli of multiple-porosity rock by using wave scattering theory, in which the effects of elasticity, volume content and pore shape of inclusions are taken into account. Also there are several empirical models to calculate the dry-frame bulk and shear moduli. Nur (1992) proposed the concept of the critical porosity, by which established a linear relationship between the bulk and shear moduli of the rock matrix and the dry frame, and the critical porosity value dependent on the rock type.