Improved characterization of formation fluids in downhole sampling tools is an industry development goal. In-situ viscosity (η) and fluid density (ρ) measurements are important to improve estimates of formation permeability, to monitor downhole sample cleanup, and analyze downhole fluid compositions such as gas/oil ratio. Dynamic viscosity in combination with other fluid parameters such as fluid density, sound speed, refractive index, absorption spectroscopy and thermal conductivity can provide a comprehensive characterization of the sample fluid. Estimating the formation permeability is critical for predicting the reservoir's production potential. Mobility measurements performed on the formation using various downhole sampling tools can be used to calculate the permeability of the formation when accurate in-situ viscosity of the formation fluids is known.
Sample fluid varies during cleanup from mud-filtrate at the start of the drawdown process via filtrate-contaminated formation fluid toward the final clean formation sample. The sample fluid may be any combination of various molecular weight hydrocarbons, brine, oil- or water-based mud filtrate and gases. The fluids also can be conductive, and they can have partially non-Newtonian properties and a viscosity typically in the range of 0.5 to 4 cP (mPa*s) or even up to 40 cP in heavy oils. Fluid density can range from 0.2 up to 1.5 g/cc.
A sensor applicable in a downhole formation sample and analysis tool must fulfill the demanding requirements of the measurement task with the huge dynamic range and an accuracy of better than 10%. The sensor must also be capable of measuring in temperatures up to 175°C and pressures exceeding 25 kpsi.
This paper presents a new sensor that can manage all the aforementioned requirements. Using the concept of a mechanical oscillator that is interrogated for resonance-frequency and damping, a sensor was designed that is highly accurate and provides sufficient robustness against temperature, pressure and drilling vibrations. The accuracy for measuring viscosity is 0.1 cP for fluids less than 1 cP and 10% for all viscosities outside the range. The density accuracy is better than 0.01 g/cc. The sensor is applicable for wireline and logging-while-drilling (LWD) tools.
This paper also presents the high-temperature and high-pressure qualification tests. Results of viscosity and fluid density lab measurements conducted with the new sensor are shown using a variety of calibration fluids that represent downhole fluids as they appear in a formation sampling tool.