In deepwater Nigeria, accurate fluid characterization is often challenging, and results are sometimes unexpected. Several cases of isolated sand bodies containing both water and oil within the main oil column have been documented. In other examples, variation from light oil to gas within some sands is often very subtle and commonly undetected using conventional formation pressure gradient analyses. Both cases tend to increase the level of complexity of the reservoir and make accurate fluid characterization increasingly difficult. In an expensive deepwater exploration or appraisal scenario, accurate fluid characterization is critical for efficient and effective planning of reservoir and field development.

In deepwater Nigeria wells, accurate and comprehensive characterization of fluid types can be made if nuclear magnetic resonance (NMR and NMR fluid typing is applied before formation pressure data acquisition. The NMR fluid-typing method exploits the fact that the molecular diffusion coefficients of brine, oil, and gas molecules typically have values that are well separated from one another. Thus, the diffusion attenuation of a suite of measured NMR signals contains sufficient information to allow differentiation of brine, oil, and gas. NMR fluid typing brings new insight into formation fluid characterization and helps to optimize formation pressure and sampling acquisition.

NMR fluid-typing data were acquired in two deepwater (Nigeria wells. In one case study, an average formation pressure gradient analysis could not accurately identify water-bearing sands in the well while the NMR fluid typing technique unambiguously identified the presence of water. Fluid sampling in the same interval post-NMR fluid typing has shown that the formation is indeed water bearing. The second case was used to determine whether the main sand of interest contains light oil or condensate or gas. Fluid typing suggested the presence of volatile oil with a high gas/oil ratio, while pressure gradient analysis indicated only the presence of gas. Fluid sampling in the same interval has shown that this particular formation is indeed a gas condensate reservoir.

The ability to ascertain both reservoir quality and fluid type information with NMR is emerging as a very valuable technology in deepwater Nigeria developments. By using the NMR fluid-typing technique, uncertainty in these developments has been significantly reduced. Without this technique, the interpretation of fluid types in the examples presented here would have been totally different. This, in turn, has directly impacted decisions and conclusions made about the field that will help in future geological modeling and depletion planning.

This content is only available via PDF.
You can access this article if you purchase or spend a download.