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The present invention provides a downhole method and apparatus using a flexural mechanical resonator, for example, a tuning fork to provide real-time direct measurements and estimates of the viscosity, density and dielectric constant of formation fluid or filtrate in a hydrocarbon producing well. Th

The present invention provides a downhole method and apparatus using a flexural mechanical resonator, for example, a tuning fork to provide real-time direct measurements and estimates of the viscosity, density and dielectric constant of formation fluid or filtrate in a hydrocarbon producing well. The present invention additionally provides a method and apparatus for monitoring cleanup from a leveling off of viscosity or density over time, measuring or estimating bubble point for formation fluid, measuring or estimating dew point for formation fluid, and determining the onset of asphaltene precipitation. The present invention also provides for intercalibration of plural pressure gauges used to determine a pressure differential downhole. A hard or inorganic coating is placed on the flexural mechanical resonator (such as a tuning fork) to reduce the effects of abrasion from sand particles suspended in the flowing fluid in which the flexural mechanical resonator is immersed.

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What is claimed is: 1. An apparatus for estimating a property of a downhole fluid comprising: a flexural piezoelectric resonator disposed in the downhole fluid, wherein the resonator is coated to reduce effects of abrasion from sand particles suspended in the downhole fluid; a controller that actua

What is claimed is: 1. An apparatus for estimating a property of a downhole fluid comprising: a flexural piezoelectric resonator disposed in the downhole fluid, wherein the resonator is coated to reduce effects of abrasion from sand particles suspended in the downhole fluid; a controller that actuates the a flexural piezoelectric resonator at a frequency; a monitor that measures electrical impedance versus the frequency of the a flexural piezoelectric resonator; and a processor that estimates the property of the downhole fluid from the measured electrical impedance. 2. The apparatus of claim 1, wherein the resonator is coated with a hard coating. 3. The apparatus of claim 1, wherein the resonator is coated to control the electrical conductivity at a surface of the resonator. 4. The apparatus of claim 2, wherein the coating comprises VitriSeal inorganic silicate. 5. The apparatus of claim 2, wherein the coating comprises titanium nitride. 6. The apparatus of claim 2, wherein the coating comprises Praxair Coatings. 7. The apparatus of claim 2, wherein the coating comprises an EverShield water-borne ceramic coating. 8. A method for estimating a property of a downhole fluid comprising: coating a flexural piezoelectric resonator to reduce effects of abrasion from sand particles suspended in the downhole fluid; disposing the flexural piezoelectric resonator in the downhole fluid; directly moving the fluid by actuating the flexural piezoelectric resonator; measuring an electrical impedance versus frequency of the flexural piezoelectric resonator; and estimating the property of the downhole fluid from the measured electrical impedance. 9. The method of claim 8, wherein the coating is a hard coating. 10. The method of claim 8, wherein the coating controls the electrical conductivity at a surface of the resonator. 11. The method of claim 9, wherein the coating comprises an EverShield water-borne ceramic coating. 12. The method of claim 9, wherein the coating comprises titanium nitride. 13. The method of claim 9, wherein the coating comprises Praxair Coatings. 14. The method of claim 9, wherein the coating comprises VitriSeal inorganic silicate. 15. A downhole tool for estimating a property of a downhole fluid comprising: a flexural piezoelectric resonator associated with the downhole tool and disposed in the downhole fluid, wherein the resonator is coated to reduce effects of abrasion from sand particles suspended in the downhole fluid; a controller that actuates the flexural piezoelectric resonator at a frequency; a monitor that measures electrical impedance versus the frequency of the flexural piezoelectric resonator; and a processor that estimates the property of the downhole fluid from the measured electrical impedance. 16. The downhole tool of claim 15, wherein the resonator is coated with a hard coating. 17. The downhole tool of claim 15, wherein the resonator is coated to control the electrical conductivity at a surface of the resonator. 18. The downhole tool of claim 16, wherein the coating comprises VitriSeal inorganic silicate. 19. The downhole tool of claim 16, wherein the coating comprises titanium nitride. 20. The downhole tool of claim 16, wherein the coating comprises at least one of the set consisting of Praxair Coatings and an EverShield water-borne ceramic coating.