A vibrating wire (VW) viscometer is introduced for the in-situ measurement of formation fluid viscosity. This is the first time this simple and robust technology has been applied downhole to measure formation fluid viscosity with a wireline formation tester in-situ. Extensive laboratory tests have demonstrated the efficacy of the VW viscometer over a wide range of pressures and temperatures, with tests performed with a large variety of live fluids under flowing or static conditions. Field examples described here were performed in water and oil zones, both in wells drilled with oil-based-mud (OBM) and water-based-mud (WBM).
Knowledge of formation fluid characteristics, including viscosity, is important for reservoir characterization. A decision as to the economic viability of a well can depend upon fluid mobility, and, by consequence, its viscosity. Fluid profiling vertically or horizontally provides information on compartments, compositional gradients, thin beds, transition zones, and zonal connectivity. Better understanding of the reservoir using in-situ viscosity measurements can illustrate the origin of compositional gradient, whether it originates from gravity segregation, thermal diffusion, incomplete equilibrium migration, asphaltene precipitation, or biodegradation. The ability to perform in-situ viscosity measurements decreases the need for an extensive sampling campaign and costly and time-consuming pressure/volume/temperature (PVT) analysis. It helps the operator make real-time decisions for perforation zones and side track drilling.
In this article we briefly describe the theory of the VW viscometer and its suitability for downhole measurements. Laboratory results obtained with the sensor at different conditions of pressure and temperature and over a wide range of viscosity are presented. Field results are presented that were obtained during sampling and/or downhole fluid analysis stations for various fluid and mud types, summarized in the examples below:
Oil viscosity in OBM: We present a case of advanced focused sampling (for very low contamination levels) and fluid analysis used to characterize the formation fluid and reservoir, in northern Kuwait. Viscosity measurements were obtained with a high accuracy and compared very well with two other sensors (DV-Rod sensors for in-situ density) that could be used for viscosity measurements in this environment.
Oil viscosity in WBM: The presence of immiscible fluids presents an extra challenge for sensors as the wetting phase may impede proper clean-up of sensor surfaces, thereby biasing the measurement. However, the VW viscometer, of extremely small cross-section, is able to quickly shed itself of debris, mud, and filtrate. In a first example, within 1 hour of sampling the viscosity measurements stabilized to that expected for oil in this reservoir. In the second example with more viscous oil, the measured viscosity of the emulsified fluid decreased as its water content decreased, in agreement with expectations.
Water viscosity in WBM: We discuss formation water viscosity measurements using the VW viscometer. Such measurements allow one to understand the relative mobility of the water and hydrocarbon phases as well as to discriminate between formation and filtrate waters. Downhole viscosity measurements are presented for formation water that are in agreement with their theoretical values at similar conditions.