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.
Introduction
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.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
2 February 2005
- Revised manuscript received:
5 January 2006
- Manuscript approved:
13 January 2006
- Version of record:
20 April 2006
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