Wettability-Index Determination by Nuclear Magnetic Resonance

Summary 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 oil-in-place estimates and in unexpected dynamic behavior (e.g., under-waterflooding). A novel method is presented to determine the wettability of rocks from nuclear-magnetic-resonance (NMR) data. The method is based on the additional nuclear relaxation that fluids experience when in direct contact with the rock surface. Reduction of oil relaxation time away from its bulk value is generally known as a qualitative wettability indicator, assuming external factors to be negligible and/or invariant from one experiment to another. Through detailed modeling of the NMR response, this concept has been developed further 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). Multiacquisition schemes including diffusion effects make the interpretation more robust, but a normal NMR acquisition suffices as can be made with all available NMR tools (wireline and while-drilling). The new NMR-based method has been verified extensively on core data against standard wettability tests. Application to NMR logs is in progress.