Technical Paper: Focused Sampling of Reservoir Fluids Achieves Undetectable Levels of Contamination

Society: SPE
Paper Number: 101084
Presentation Date: 2008
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A new generation of sampling technology is introduced that allows a wireline formation tester (WFT) to sample reservoir fluids in open hole with levels of filtrate contamination that are, in many cases, below measurable limits. Also, the time required on station to clean up before sampling is significantly reduced in comparison to conventional sampling methods.

Formation-fluid sampling has always been adversely affected by mud-filtrate contamination, which introduces errors into the laboratory measurements of fluid properties and requires analytical methods to back-calculate the measured properties to approximate the uncontaminated reservoir fluid. The ability to secure a totally clean sample of formation fluid at reservoir conditions is a significant advance that provides accurate fluid information for characterization of the reservoir, flow assurance, facility design, production strategies, and defining reserves.

The application of this new focused sampling technology is presented in four case studies from wells drilled on the Norwegian continental shelf. A wide range of formation fluids and permeabilities are examined, in both oil-based and water-based drilling fluids. Results from focused sampling are compared directly with conventional sampling in the same reservoir zones. This study also gives insight into the cleanup dynamics of invaded filtrate and explores the different factors that affect performance of the focused sampling technique.

An important consequence of achieving negligible contamination is the ability to accurately measure fluid properties in-situ. Reduced cleanup time allows for efficient reservoir fluid profiling, whereby multiple zones can be scanned sequentially in real time to quantify the fluid properties at a much higher resolution than traditional sampling methods. Downhole fluid analysis (DFA) can thus provide an additional source of information in the process of revealing complex reservoir architectures.

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