Improvements in Downhole Fluid Identification by Combining High Resolution Fluid Density Sensor Measurements and a New Processing Method: Cases from a Saudi Aramco Field

Abstract Accurate chemical and physical properties of hydrocarbon and formation water are required for efficient reservoir and production management. During exploration and field development, a sample of formation fluids is frequently required. One method to obtain reservoir fluid samples is with a pumpout wireline formation tester (PWFT) in which the objective of sampling is to collect a representative fluid sample with minimum rig time. Sampling mixed phases or immiscible fluids has been a long standing challenge for fluid identification using PWFTs. The mixing of fluids that are flowing inside the tool increases sensor noise, making the interpretation of fluid type and amount of contamination difficult, if not impossible. Consequently, these noisy sensor readings are often ignored and attributed to fluid transitions during of sampling. This erratic behavior has been observed with most sensors, including resistivity and optical sensors. With the introduction of a new high resolution fluid density sensor and improved data handling techniques, it is now possible to identify these mixed fluid flow regimes of hydrocarbon and water more precisely. The new fluid density sensor monitors the frequency change of a vibrating tube that is filled with the fluid sample quickly and reliably. The high accuracy of this technique enables the determination of additional fluid properties, such as density, water salinity, and fluid compressibility. Furthermore, the new processing methods provide a clearer understanding of the flow behavior, enabling more accurate estimates of fluid contamination. Data from the tool can also be used to observe changes in fluid properties over depth intervals and to aid in the identification of fluid interfaces and compartments. Introduction The traditional method for fluid identification before the advent of PWFT with downhole fluid sensors used gradient analysis of pressure surveys. Although gradient analysis of PWFT pressure surveys is a well established method to evaluate in-situ fluid density, this method presents accuracy challenges. In addition to the accuracy of the pressure measurement, the depth accuracy must be considered and, when the producing layers are less than 20 ft, accurate gradients may not be possible (Phelps et al., 1984; Kabir and Pop, 2006). Also, when a layer contains more than one fluid type (i.e., oil and water transitions), capillary effects can influence the pressures and gradients. Another main factor that limits the accuracy of gradient analysis is formations with low permeabilities or mobilities (i.e., < 10 mD/cP).