Wettability Index Determination by Nuclear Magnetic Resonance

Abstract

Knowing the wetting condition of a reservoir at an early stage is crucial for selecting optimum field development options. Paying insufficient attention to the wetting condition e.g. assuming water wet behavior) may result in incorrect STOIIP estimates, and in unexpected dynamic behavior, e.g. under water flooding.A novel method is presented to determine the in-situ wettability of rocks from 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 wettability index. It is based on a model for the microscopic distribution of the crude oil and the wetting state of the rock at any given overall saturation. The method requires an NMR measurement on a sample containing two reservoir fluids, i.e. brine and crude oil. Multi-acquisition schemes including diffusion effects make the interpretation more robust, but a normal NMR acquisition suffices and can be made with all available NMR tools (wireline and LWD).The new NMR based method has been extensively verified on core data against standard wettability tests. Application to NMR logs from several fields, and in particular carbonates, show continuous profiles of varying wettability, ranging from fairly oil-wet, through intermediate, to waterwet, all in line with the available core data.