Method of Acquiring Viscosity of A Downhole Fluid

Abstract

Methods of acquiring viscosity of a downhole fluid in a wellbore penetrating a subterranean formation is provided. The downhole fluid is measurable by a vibrating wire viscometer positionable in the wellbore. The method involves acquiring a signal of the downhole fluid from the viscometer, generating an initial estimate of the viscosity parameter based on measured viscosity signal parameters and by selectively adjusting the viscosity signal, and generating final estimates of the viscosity parameters by performing Kalman filtering on the initial estimates.

What is claimed is: 
     
         1 . A method of acquiring viscosity of a downhole fluid in a wellbore penetrating a subterranean formation, the downhole fluid measurable by a vibrating wire viscometer positionable in the wellbore, the method comprising:
 acquiring a signal of the downhole fluid from the viscometer;   generating an initial estimate of the viscosity parameter based on measured viscosity signal parameters and by selectively adjusting the viscosity signal; and   generating final estimates of the viscosity parameters by performing Kalman filtering on the initial estimates.   
     
     
         2 . The method of  claim 1 , wherein the acquiring comprises passing a voltage through a wire of the viscometer. 
     
     
         3 . The method of  claim 1 , wherein generating initial estimates comprises:
 obtaining a digitized signal;   measuring noise amplitude and time interval of the viscosity signal above a noise level;   obtaining an analog signal;   measuring an initial voltage;   obtaining frequency by counting the number of positive or negative peaks in the time interval and dividing the number of peaks by the time interval; and   obtaining the damping factor.   
     
     
         4 . The method of  claim 1 , wherein the generating initial estimates comprises:
 preprocessing the viscosity signal; and   determining initial values of states of the viscosity signal.   
     
     
         5 . The method of  claim 4 , wherein the preprocessing comprises:
 determining a noise variance of a baseline of the viscosity signal;   removing an offset of the viscosity signal; and   removing a part of the viscosity signal below the noise variance.   
     
     
         6 . The method of  claim 4 , wherein the determining initial values comprises estimating a coarse resonance frequency, a course damping rate, and a coarse value of an initial amplitude. 
     
     
         7 . The method of  claim 1 , wherein the generating final estimates comprises performing Kalman filtering of the initial estimates. 
     
     
         8 . The method of  claim 7 , wherein the performing comprises initializing the Kalman filter and performing the Kalman filtering. 
     
     
         9 . The method of  claim 1 , wherein the Kalman filter is one of an extended Kalman filter and an unscented Kalman filter. 
     
     
         10 . The method of  claim 1 , further comprising validating the estimated viscosity parameters by comparing the estimated viscosity parameters with viscosity parameters estimated using signal modulation. 
     
     
         11 . The method of  claim 1 , wherein the viscosity parameters comprise noise, voltage, frequency, damping factor, time interval, amplitude, number of signal peaks, and signal offset. 
     
     
         12 . A method of acquiring viscosity of a downhole fluid in a wellbore penetrating a subterranean formation, the downhole fluid measurable by a vibrating wire viscometer positionable in the wellbore, the method comprising:
 acquiring a signal of the downhole fluid from the viscometer by passing a voltage through a wire of the viscometer;   generating an initial estimate of the viscosity parameter based on measured viscosity signal parameters and by selectively adjusting the viscosity signal;   generating final estimates of the viscosity parameters by performing Kalman filtering on the initial estimates; and   validating the estimated viscosity parameters.   
     
     
         13 . The method of  claim 12 , wherein the acquiring comprises applying a shot sine or cosine wave electric voltage on the edges of a wire of the viscometer to induce an electric current, and generate an electromagnetic force on the wire. 
     
     
         14 . The method of  claim 12 , wherein the generating the initial estimate comprises passing a signals from a constant initial voltage, a oscillating damping signal, a digital peak counter, and a voltage meter to a central processing unit. 
     
     
         15 . The method of  claim 15 , wherein the generating the initial estimate further comprises passing signals back from the central processing unit to the digital peak counter and the voltage meter. 
     
     
         16 . The method of  claim 16 , wherein the generating the initial estimate further comprises generating a number of peaks at the central processing unit by passing the oscillating damping signal from a high pass filter for filtering and from the high pass filter to a differential analog circuit, and on to a digital peak counter. 
     
     
         17 . The method of  claim 17 , wherein the generating the initial estimate further comprises generating a voltage at the central processing unit by passing the signal from the high pass filter to the low pass filter, and on to a logarithmic analog circuit and a voltage meter. 
     
     
         18 . The method of  claim 12 , wherein the validating comprises comparing the estimated viscosity parameters with viscosity parameters estimated using signal modulation. 
     
     
         19 . The method of  claim 12 , wherein the validating comprises estimating using signal modulation, the estimating comprising:
 modulating the viscosity signal;   filtering the modulated signal;   determining frequency and phase of the modulated signal; and   determining decrement and amplitude of the modulated signal.   
     
     
         20 . A method of acquiring viscosity of a downhole fluid in a wellbore penetrating a subterranean formation, the downhole fluid measurable by a vibrating wire viscometer positionable in the wellbore, the method comprising:
 acquiring a signal of the downhole fluid from the viscometer by passing a voltage through a wire of the viscometer;   generating an initial estimate of the viscosity parameter based on measured viscosity signal parameters and by selectively adjusting the viscosity signal;   generating final estimates of the viscosity parameters by performing Kalman filtering on the initial estimates;   validating the estimated viscosity parameters; and   selectively adjusting the generating based on the validating.