Petrophysical evaluation using the geometric factor theory and comparison with archie model

Abstract

The Archie model is empirical because it is based on curve-fitting methods, while the Geometric Factor Theory (GFT) is a physics-based porosity-conductivity model. Both these are used to compute water saturation (Sw) in simple Archie rocks (clean sandstones). Studies considering Archie's work demonstrated that Archie equation was not applicable to carbonates and shaly sands, and all the scientifically incorrect shaly sand equations were counterproductive modifications of the Archie equation. Nevertheless, instead of replacing Archie equation with some more meaningful equation, researchers tried to improve and modify Archie equation so that it can work in other reservoirs (carbonates and shaly sands) where it was not even applicable. In this paper, the Archie's and Geometric Factor Theory (GFT) equations were applied to the same data sets (well log and core), and the results were compared qualitatively and quantitatively. Outcomes from these approaches are closely similar in some wells; however, they differ in others by 2–3 porosity units. The wells where results differ from the core-derived results, n (saturation exponent) is modified to account for the differences. The log-based Sw values of 0.27 (Well-A), 0.24 (Well-B), 0.38 (Well-F) and 0.45 (Well-G) against core driven 0.27 (Well-A), 0.24 (Well-B), 0.36 (Well-F) and 0.46 (Well-G) show that the GFT driven estimates provide a good match with the core data than the Archie. Additionally, the cross-plots of GFT provide peculiar information about the pore geometry and diagenesis of rocks. It is, therefore, recommended to supplant Archie model with the Geometric Factor Theory model because the latter gives better results and is a physics-based porosity-conductivity model that also has a physical interpretation.