Using the Continuous NMR Fluid Properties Scan to Optimize Sampling with Wireline Formation Testers

Peter Weinheber, Chanh Cao Minh, Adriaan Gisolf, and Wich Wichers
Schlumberger, Luanda, Angola

One of the most important objectives of fluid sampling using wireline formation testers is to ensure that representative samples of the different fluids encountered in the wellbore are obtained. Usually the wireline/LWD petrophysical logs will guide the sample acquisition program. This typically means that resistivity and porosity logs are used to infer basic fluid types, calipers and acoustics are used to verify that the borehole is suitable for sampling and nuclear and NMR logs are used to gauge if permeability is sufficient for a sample to be taken. However most of these logs are not able to capture subtle variations in the hydrocarbon column to allow the operator to ensure that all representative fluids are sampled. The most important a-priori information, a continuous fluids type and property log, is still not widely used in the industry.

Modern NMR logging tools can deliver a continuous fluid log of oil versus gas versus water versus OBM. Additionally, within the hydrocarbon column the NMR measurements can be used to provide a continuous log of gas-oil ratio and oil viscosity. With this information acquired before or after the sampling operation it is easier to ensure that a full suite of representative samples are acquired and that we do not indulge in needless over sampling.

Here, we show through examples how continuous NMR fluid scan logs can help to:
- select and optimize pretest and sample points
- identify compositional gradients
- identify fluid differences that highlight potential reservoir compartmentalization
- confirm gas cap existence
- fill the gaps of missing fluid information in zones where formation testers cannot sample (thin beds, poor borehole, tight formations etc.)

We conclude that the addition of the continuous NMR fluid log to existing methods is beneficial to WFT operations in reservoirs with complex fluids distributions.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery