A New Algorithm for Foot-By-Foot Permeability and Irreducible water Saturation Estimation in Iraqi Limestone Reservoirs

Abstract

This paper discusses the methodology developed for petrophysical modeling and its application to the main limestone formations in the Mid-region of Iraq. The systematic approach presented in this study evaluates not only combination of porosity, permeability and irreducible water saturation at each depth, but also free-water level and capillary pressure curve, such that water-cut of flow tests is closely reproduced in numerical simulation. It is one of the major achievements in this study. The algorithm matched to several sets of core samples: 2,903 porosity, 2,767 permeability, and 95 Irreducible water saturation samples that give the algorithm the robustness for future properties prediction. To develop the prediction Algorithm, it was employed multiple regression analysis (MRA) which mathematically requires independence between explanatory variables. Since possible explanatory variables (porosity, permeability and facies) indicate interdependency, it was investigated when necessary to transform the variables into independent axes. It was found that logarithm of permeability has correlations positive to porosity and FZI and negative to irreducible water saturation. Therefore, foot-by-foot evaluation shall meet the following conditions, (Sw) along wells shall be explained by single free-water level (FWL) for each pressure compartment and Water-cut observed on production test shall also be closely simulated with the FWL, For modeling purposes, Sw is split into irreducible water saturation (Swir) and transitional (mobile) water saturation (Swm) as water-cut is a function of only the latter.