Wettability-Index Determination by Nuclear Magnetic Resonance
- Wim J. Looyestijn (Shell Intl. E&P BV) | Jan Hofman (Shell Intl. E&P BV)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- April 2006
- Document Type
- Journal Paper
- 146 - 153
- 2006. Society of Petroleum Engineers
- 1.6 Drilling Operations, 5.2.1 Phase Behavior and PVT Measurements, 4.3.4 Scale, 5.6.2 Core Analysis, 1.11 Drilling Fluids and Materials, 5.1.5 Geologic Modeling, 1.12.2 Logging While Drilling, 5.8.7 Carbonate Reservoir, 5.5.2 Core Analysis, 4.1.5 Processing Equipment, 1.6.7 Geosteering / Reservoir Navigation, 5.2 Reservoir Fluid Dynamics, 5.1 Reservoir Characterisation, 5.6.1 Open hole/cased hole log analysis, 5.4.1 Waterflooding
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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.
Importance of Wettability Determination. Wettability relates to the relative attraction of the rock to either water or oil and, thus, has a strong impact on the dynamic properties of the rock. Wettability ranges from pure water-wet (through intermediate-wet, or neutral) to oil-wet. Sandstone reservoirs have a tendency toward being water-wet to neutral, whereas carbonates are often neutral to oil-wet. However, there are too many exceptions to make reliable assumptions. Moreover, the wettability is likely to vary over the reservoir, and possibly also over time as a result of changing saturations during production. In current practice, wettability is poorly known; if identified at all, it is determined on a few core samples, and variation in 3D is hardly known.
The purpose of the NMR wettability research is to take a first step toward alleviating these shortcomings by developing the results of recent work into a practical tool for use in reservoir studies. Wettability is rated as one of the critical uncertainties in many fields, particularly the Middle East carbonate fields. The ability to obtain wettability information at an early stage of field development is a significant improvement over current practices.
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