Water Saturation Conundrum: Getting It Right Through the Recommended Best Practices in Dean-Stark Analysis

Decades of research and lab approaches have been conducted to gain better understanding of the factors controlling water saturation (Sw) from resistivity, porosity, electrical properties measurement and determining the true formation water salinity across the different reservoir units by minimizing mud filtrate contamination in the water samples captured downhole. This paper will highlight Dean Stark technique on the core samples to quantify water saturation in various oil and gas accumulations ranging from shallow water to deep water reservoirs. The key principle in Dean Stark analysis is to quantify the contamination from the water portion in the mud through appropriate tracing technique, retaining water volume in the plug as much as possible through effective preservation technique and correcting for brine saturation from water volume. Brine water inside the fresh core samples will be expelled out through high- speed centrifuge process where pore water salinity and formation water resistivity will be measured on the expelled fluid volume which are used to validate water saturation computation from Dean Stark. Comparison of Dean Stark results are done on the core samples acquired with and without tracer components, having variable rock quality index (RQI) and samples which are cut and preserved at different timing after the core barrel is at surface. Results from few case studies analyzed indicate that core plugs with good RQI and less clay content show more accurate Dean Stark results compared to low RQI core plugs while core plugs cut immediately at the rig site and properly preserved in wax seal also show more accurate Dean Stark results as opposed to plugs cut and analyzed later in the lab. Choice of tracer type used also impacts Sw computation where use of salt or chemical based tracer involves more assumptions and corrections to be done on Sw as opposed to using radioactive based tracer. Dean Stark analysis in both gas and oil reservoirs do not show any significant difference except for gas phase which involves more direct Sw computation due to high mobility. In conclusion, given the right procedures, right timing and right technique to retain, preserve, trace and extract the brine volume from the core samples, Dean Stark lab analysis will remain the source of ground truth for Sw computation both at the well and reservoir scale. Capturing the factors affecting analysis quality and quantifying the uncertainty in the measurement will further improve the robustness of this technique while appropriately triggering more innovation in this subject for years to come.