Integrating Core Measurements and NMR Logs in Complex Lithology

Abstract

Abstract Standard analysis of NMR logs, based on the use of a unique T2 cut-off to discriminate free and bound fluid and estimate irreducible water saturation and permeability, gives erroneous results in complex lithologies. In the example of a mixed platform presented, the main difficulty came from the strong variations between clastics, dolomite and limestones due to the heterogeneity of the porous network. In the proposed method, illustrated by a field case, a single T2 cut-off (derived from lab NMR) for both sandstone and dolomite was found and gave good results whereas a more detailed porous network description was necessary to handle limestones. A complete set of lab measurements was used to characterize the porous network and classify carbonates following their relaxation behavior: The results obtained with this method based on core data integration are presented and validated by irreducible water saturation from centrifugation and conventional permeability measurements. Full understanding of the porous system based an extensive core analysis is necessary to link the continuous log response and the dynamic parameters of the reservoir. Small changes in mineralogy affect significantly production performance. Introduction A complex geology requires an extensive coring program to understand better the rock properties and link their petrophysical behaviors (irreducible water saturation, permeability) to laboratory measurements. A simple description of core is not sufficient to integrate crucial information like mineralogy and pore geometry. Nuclear Magnetic Resonance (NMR) logs can provide the main petrophysical properties such as porosity, irreducible water and permeability but needs a careful calibration on core NMR. In this work, it is shown that applying directly a model from core is not enough. A further understanding of rocks is necessary to enforce a model applicability. A key to this understanding is an objective confrontation between mineralogy, porous network characterization (qualitatively and/or quantitatively) and dynamic data such as capillary pressure. After presenting the geological and petrophysical context, a complete review of lab measurements is done and the NMR log intrepretation model is explained, illustrated by various figures that compare core and log data. Finally some conclusions are drawn and some recommendations are expressed concerning the benefits and the dangers of using NMR in complex lithology.