Modeling Mud-Filtrate Invasion Effects on Resistivity Logs to Estimate Permeability of Vuggy and Fractured Carbonate Formations

Abstract

Abstract We develop and validate a new method to diagnose and estimate secondary porosity and absolute permeability of fractured and vuggy carbonate formations based on the numerical simulation of the process of mud-filtrate invasion. The method includes the geological characterization of core laboratory data and their integration with well logs and fluid production measurements. We apply the new method to the interpretation of data acquired in a carbonate reservoir in the Barinas-Apure Basin in southwest Venezuela. The latter reservoir behaves as a triple-porosity petrophysical system which exhibits inter-crystalline, intra-crystalline, moldic, vuggy (connected and non-connected) and fractured porosity, all embedded in a tight matrix. Rock-core data and wellbore resistivity images indicate that vugs are the mayor component of secondary porosity while fractures and interconnected vugs account for most of the permeability. The initial phase of our interpretation method consists of integrating core measurements with conventional and non-conventional well logs to calculate static and dynamic petrophysical properties via standard carbonate evaluation procedures. Starting with the calculated petrophysical properties, we simulate the process of invasion with both water- and oil-base muds. Resulting spatial distributions of water saturation and salt concentration in the near-borehole region give rise to spatial distributions of electrical resistivity which are used to numerically simulate laterolog and induction apparent resistivity logs. If the input values of porosity and permeability are not correct, the simulation of mud-filtrate invasion will result in a poor match of resistivity logs. In such cases, we update porosity and permeability until securing a good match between measurements and simulations. This procedure was tested on several key wells with and without core measurements, wellbore resistivity images, and well-testing measurements. We conclusively find that our final estimates of porosity and permeability are in good agreement with the properties of the global petrophysical system. Differences between porosity and permeability before and after simulation of the process of invasion are reliable indicators of presence and influence of vugs and/or fractures in the displacement of hydrocarbons by mud filtrate.