Complex carbonates with a triple porosity system represent significant challenges for saturation evaluation and permeability estimation. The nature of the pore space, where intergranular and intercrystalline connected pores coexist with isolated vuggs and fractures, creates quite a complicated passage for both fluid flow and electric current. Therefore, one will expect significant variations in the cementation exponent and these variations will have direct influence on permeability changes.
These changes in the flow passage are more closely related to porosity components (intergranual-connected, vuggy-disconnected and fractured-shortest path connected) then to total porosity (Watfa et al. 1987). Thus, proper evaluation of permeability and saturation in complex carbonates requires knowledge of the porosity components. Watfa's solution works for both intergranular-vuggy and intergranular-fracture systems. On the other hand, the same system can be resolved through rock-physics templates using total porosity and shear modulus as the inputs (Vorobiev et al 2013). The technique exploits the Kuster-Toksoz or Differential Effective Media solution for vugs and fractures; and the friable sands grain contact model for intergranular connected pores. In order to estimate all three porosity components we have to anchor one of the components and there from resolve either Watfa's equations or the elastic model system for the other two components. Alternatively, we can resolve both systems simultaneously for all three independent components.
In the paper the combined solution for the shear modulus and cementation exponent in the triple porosity system has been tested both on well data from a Brazil offshore reservoir and core data from a reservoir in the Middle-East. Watfa solution has been corrected to the triple porosity system conditions, in order to combine it with rock-physics templates. Therefore fracture vuggy and intergranular porosity components have been derived. This data in combination with NMR driven or directly core measured irreducible water saturation have been used to estimate permeability. Permeability has been separately estimated for the fracture and vuggy-intergranular parts of the reservoir. For the vuggy-intergranular system the Chen-Jacoby modified Coates equation has been employed. Discussion on the technique of combining fracture and intergranular-vuggy permeability profiles into one is given. Permeability estimated this way on the core samples demonstrates good correlation with direct core permeability measurements.
Finally, discussion on the model selection (Kuster-Toksoz versus DEM), potential sources of errors and technique limitations, as well as necessary logging program and data acquisition recommendations are given.
