Wettability Index Determination from NMR Logs

Abstract

A novel method is presented to determine the in-situ wettability of rocks from nuclear magnetic resonance (NMR) logs. The method is based on the fact that fluids experience additional relaxation when in direct contact with the rock surface. A reduction of the oil relaxation time away from its bulk value is generally known as a qualitative wettability indicator. Through detailed modeling of the NMR response, this concept has been further developed to provide a quantitative index. The new NMR based method has extensively been verified on core data against standard wettability tests such as Amott and USBM. The method is now being applied to NMR logs, allowing the wetting condition to be determined in-situ along the entire reservoir interval. The input is a standard T 2 acquisition as can be measured with all currently existing NMR logging tools; more stable results are obtained when using diffusion weighted acquisitions. The method has been tested on data from various fields with different types of rock, crude and wetting conditions.