Probabilistic High Resolution Permeability Estimation Integrating Dielectric Dispersion Log Measurements With Mud Filtrate Invasion Modelling
Authors: Pirrone Marco , Cossa Alessandro , Galli Maria Teresa 
Published: May 2016

A novel probabilistic methodology for the estimation of the formation permeability, based on the integration of dielectric dispersion log measurements and near wellbore invasion modelling, is proposed.

A new-generation, high-resolution dielectric dispersion wireline log provides a reliable estimation of the main petrophysical parameters including porosity and water salinity. Due to a shallow depth of investigation, the tool measurements are strongly affected by the presence of mud filtrate in the flushed zone. In particular, dielectric-derived salinity can take advantage of a salinity contrast between water based mud and formation water showing a depth-by-depth characteristic salinity profile.

In turn, the salinity profile within the invaded zone is function of rock-fluid interactions, petrophysical properties and, in particular, of permeability. The novelty of the approach is to numerically simulate the mud invasion phenomenon and match the salinity profile provided by the dielectric dispersion modelling in order to estimate the formation permeability. In the proposed methodology the match of the salinity profile is obtained by integrating the near wellbore simulator with a probabilistic Bayesian algorithm.

The approach is presented by means of a study carried out on a cm-scale laminated gas-bearing reservoir in the Mediterranean offshore area. A complete set of wireline logs including dielectric dispersion measurements have been acquired and routine and special core analyses performed. The methodology provided a continuous quantitative estimation of permeability in the entire cored interval with the associated uncertainty. The match with core measurements proved very accurate even in this complex laminated formation.

In general, the new approach can be used to predict permeability in un-cored and un-tested wells, and, given the high vertical resolution of the dielectric tool, it is one of the few methods suitable for thin-layered reservoirs.

Introduction

Quantitative permeability prediction is probably the most challenging issue in reservoir characterization and, at the same time, it is one of the most desired targets. According to conventional procedures, the estimation of rock permeability can be obtained either by laboratory analysis (at the plug centimetre scale) or through the interpretation of a well test or data from wireline formation testers, representative of the portion of the reservoir investigated during the test. However, cores and tests are not always available and, in general, the measurements are expensive and suffer from sampling and scale representativeness.

In this respect, big efforts have been deployed to obtain a reliable method for predicting permeability from well log data. With such indirect measurements, permeability can be inferred from a different property (e.g. porosity, nuclear magnetic resonance) and then extrapolated to un-cored/un-tested intervals and wells using models and assumption. However, as the models are not exact, the uncertainty attached to the results is high. The uncertainty is even higher in thin layered reservoirs characterized by bed thicknesses well below the resolution capabilities of standard logging tools. Nevertheless, since these reservoirs represent major gas reservoirs in the Mediterranean Sea, a proper characterization is mandatory (Chelini et al. 2009, Pirrone et al. 2011a, 2011b, 2014).