Accurate New Solutions for Fluid Sample Contamination Quantification, With Special Focus on Water Sampling in Water-Base Mud

Abstract

Abstract The role of formation testers has evolved significantly over the past decades. Early versions of this tool focused solely on obtaining pressure measurements and fluid samples. With advances in technology, the ability to measure fluid properties downhole at reservoir pressure and temperature has grown continuously. Today, formation tester downhole fluid analysis (DFA) and pressure measurements are routinely used to characterize fluids and address reservoir connectivity concerns in real time. However, quantitative use of fluid property measurements requires the user to measure—and correct for, as required—fluid contaminants caused by invaded mud filtrates. New measurements are combined with new algorithms to yield a harmonized contamination quantification solution that is applicable to all fluids and mud types. The quantification of drilling fluid content is at its most challenging when the formation fluids and drilling mud filtrate are fully miscible. Water-base mud (WBM) filtrate contamination measurement in formation water samples is particularly challenging because formation water and WBM filtrate exhibit very similar near-infrared optical spectroscopy signatures and are therefore difficult to differentiate. Native and invaded fluid typically do have measurably different live fluid density or resistivity. However, resistivity measurements should only be used if a temperature correction is applied. The general workflow for real-time hydrocarbon contamination quantification in oil-base mud (OBM) has been applied to water sampling in WBM, using live fluid density and resistivity measurements. First, a temperature correction is applied to the resistivity measurement and the data are converted to conductivity. Next, density- and temperature-corrected conductivity data are crossplotted and a novel linear relation is generated. The independently determined filtrate and formation water properties (endpoints) should fall on the extrapolated linear relation because this relation is intrinsic to the fluid mixture. Lastly, the fluid mixture measurements and endpoints are entered into mixing rules to calculate contamination. With this new method for quantifying water sample contamination, there is now a complete solution that is applicable to all fluid combinations. Several case studies of water sampling in WBM show the effectiveness of the new mud filtrate contamination quantification method.