Downhole viscosity measurement platform using tuning fork oscillators

The viscosity of subsurface hydrocarbons is commonly determined by sampling produced fluids at the surface. However, bringing hydrocarbons to surface temperature and pressure can irreversibly alter their viscosity. There are wireline tools that make in-situ measurements, but these require long, heavy cables, specialized equipment and large service crews, making the process logistically challenging and expensive. In this paper we describe a novel sensor system for downhole measurements of viscosity, density, and other fluid properties. The sensing capabilities are realized by the use of a commercially available piezoelectric tuning fork oscillator, which probes the fluid through the interaction of its prongs with the boundary layer of fluid around it. The miniature footprint of the device allows us to integrate it into a small-scale untethered tool that can be easily deployed for well logging. We show our implementation and laboratory characterization of the tuning fork. To gauge its capabilities under harsh environmental conditions, the sensor was studied while immersed in fluids representative of those found downhole and, separately, under conditions of high pressure and temperature. The tool integration and measurement schemes are also described. This system is a first step toward the development of an inexpensive, integrated, and miniaturized sensing platform for the in-situ characterization of reservoir fluids.