A Model-Based Interpretation Methodology for Evaluating Carbonate Reservoirs

Abstract

Abstract Carbonate rocks are diverse, and their pore space complex. Petrophysical evaluation of carbonates with techniques developed for siliciclastic environments often fall short. In this paper, we present a new methodology that combines diverse texture and facies sensitive measurements. The method parametrizes the pore structure in terms of a multiporosity system of fractures, vugs, inter- and intra-granular porosities, and evaluates the parameters from the measurements. The process is based on physics and relies minimally on empirical correlations. The first step is to separate the fracture from the matrix using borehole images and then quantify mineral volumes and porosity through nuclear magnetic resonance (NMR) and/or nuclear logs with new mineral standards. Next, we classify the lithology with a self-organizing-map (SOM) algorithm using a vector of log-inputs. Following this, the vug fraction is estimated using shear modulus data and borehole image analysis. Finally, we separate the inter- and intragranular components using NMR data. This analysis includes molecular diffusion between different pore types. The NMR processing scheme also depends on lithology. The porosity components and the relevant length scales enable us to predict resistivity and permeability using an effective medium theory in an internally consistent manner. Our interpretation methodology was validated successfully with core/log data from the Middle East. The facies ranged from mudstone to grainstone.