Detecting Restricted Diffusion Effect when Identifying Hydrocarbons with NMR Logs

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractAdvanced nuclear magnetic resonance (NMR) methods for fluids identification and reservoir evaluation were developed recently. These techniques, focused on identifying and characterizing fluids in the reservoirs, were mostly applied in clastic and carbonate reservoirs. In such environments, fluid characterization with NMR logs is relatively simple when the fluid responds to standard models. However, exceptions to NMR models such as restricted diffusion produce variations in model behavior and cause erroneous answers. Experience shows that restricted diffusion occurs mostly in carbonates with water trapped in small pores and in short T2(ms) domains (bound water) in clastic formations. This effect reduces water diffusibility and produces NMR signals occurring in oil diffusion domains, with the consequent incorrect computation of liquid hydrocarbons.The purpose of this study is the characterization of the restricted diffusion effect from rock samples measured in the laboratory with an NMR logging tool, as well as from field examples in diverse lithologies. It is expected that such analysis will help in reservoir petrophysical characterization and will avoid misinterpretations in fluid typing, when performing fluids identification based on NMR principles.Since restricted diffusion phenomena have been observed in medium to long transverse relaxation times T2(ms), above T2 cutoff for free fluids (in moderate to high porosity environments), some possible explanations are proposed to understand such phenomena and the implications on reservoir evaluation both in exploratory and production wells.