Patent ID: 6892138

Claim:
A method of determining the viscosity of a hydrocarbon reservoir fluid that is present in a formation layer traversed by a borehole, said method comprises the steps of: a) selecting a location in the formation layer; b) lowering in the borehole to the location a tool that comprises a central conduit having an inlet, means for displacing fluids through the central conduit, and an optical fluid analyzer; c) making an exclusive fluid communication between the formation and the inlet of the central conduit; d) obtaining a spectrum of the optical density; e) calculating a first factor that is the maximum optical density in a predetermined short-wavelength range multiplied with the length of the short-wavelength range, calculating a second factor which is the integral over the same short-wavelength range of the spectrum, subtracting the second factor from the first factor to obtain a hydrocarbon oil property; and f) obtaining the magnitude of the in-situ viscosity from the oil property using a relation that had been obtained by fitting a curve through previously obtained data points comprising the measured magnitude of the actual viscosity as a function of the oil property.