An approach to defining tortuosity and cementation factor in carbonate reservoir rocks

Tortuosity and cementation factor are two critical parameters that significantly affect estimates of reservoir properties. Tortuosity factor can be used to estimate permeability using the Carman-Kozeny equation and is an important parameter for formation resistivity factor calculation using a modified version of Archie's formula. It is also used to predict water saturation of reservoir rocks. Tortuosity as an input parameter in Biot's equation can be used to estimate velocity dispersion. In this work, based on the Generalized Archie Equation Curve Fitting (GAECF), tortuosity and cementation factor are determined for selected intervals in a carbonate reservoir. Formation resistivity factor (FRF) analysis was carried out on both core samples and on well log data. The analyses were carried out separately on porous and recognized tight intervals. The intervals selected were fracture-free and include both matrix porosity and somewhat vuggy porosity. FRF analysis on well log data indicates that the carbonate intervals examined follow the GAECF well. However, unreasonable values for the tortuosity parameter, 'a', and cementation factor, W, were obtained. To overcome this, the average minimum apparent tortuosity value was calculated for these intervals. Approximate values for tortuosity factor from both core samples and well log data lie close to each other (for porous intervals, zones S1, S7 and S11). This indicates that the results obtained from FRF analysis on well log data can be effectively and reliably used as an alternative to FRF studies on core samples. (C) 2007 Elsevier B.V. All rights reserved.