Determining Asphaltene Onset

Abstract

Methods and downhole tools for operation within in a wellbore that extends into a subterranean formation. The operation includes simultaneously causing a change in a first parameter of fluid drawn into the downhole tool from the formation and determining a change in a second parameter of the fluid relative to the change in the first parameter. A third parameter of the fluid is determined based on the first and second parameter changes.

What is claimed is: 
     
         1 . A method comprising:
 obtaining a sample of fluid from a subterranean formation;   then reducing a first parameter of the sample while measuring a second parameter of the sample, wherein the first parameter is pressure or temperature; and   determining asphaltene onset point of the sample based on the second parameter measurements.   
     
     
         2 . The method of  claim 1  wherein:
 obtaining the sample comprises drawing the sample into a portion of a downhole tool positioned within a wellbore extending into the subterranean formation; 
 the downhole tool portion is at least a portion of a flowline and/or a sample chamber of the downhole tool; and 
 reducing the first parameter comprises conveying the downhole tool uphole within the wellbore while the downhole tool portion remains in fluid communication with the wellbore. 
 
     
     
         3 . The method of  claim 1  wherein:
 obtaining the sample comprises drawing the sample into a portion of a downhole tool positioned within a wellbore extending into the subterranean formation; and 
 the method further comprises, before reducing the first parameter, fluidly isolating the sample within a flowline and/or a sample chamber of the downhole tool. 
 
     
     
         4 . The method of  claim 3  wherein reducing the first parameter comprises conveying the downhole tool uphole within the wellbore, thereby exposing the downhole tool to wellbore fluid of decreasing temperature and/or pressure within the wellbore. 
     
     
         5 . The method of  claim 3  wherein reducing the first parameter comprises operating a pump of the downhole tool to withdraw some of the isolated sample. 
     
     
         6 . A method comprising:
 operating a downhole tool within in a wellbore that extends into a subterranean formation, wherein operating the downhole tool comprises:
 simultaneously:
 causing a change in a first parameter of fluid drawn into the downhole tool from the formation; and 
 determining a change in a second parameter of the fluid relative to the change in the first parameter; and 
 
 determining a third parameter of the fluid based on the first and second parameter changes. 
   
     
     
         7 . The method of  claim 6  wherein causing the change in the first parameter of the fluid is one of:
 causing the fluid to decrease in pressure; 
 causing the fluid to decrease in temperature; or 
 causing the fluid to decrease in pressure and temperature. 
 
     
     
         8 . The method of  claim 7  wherein causing the change in the first parameter of the fluid comprises exposing the fluid to the wellbore as the downhole tool is conveyed uphole within the wellbore. 
     
     
         9 . The method of  claim 7  wherein causing the change in the first parameter of the fluid comprises conveying the downhole tool uphole within the wellbore while the fluid is fluidly isolated within the downhole tool. 
     
     
         10 . The method of  claim 7  wherein causing the change in the first parameter of the fluid comprises operating a pump of the downhole tool. 
     
     
         11 . The method of  claim 10  wherein:
 the fluid is fluidly isolated within the downhole tool; and 
 operating the pump decreases the first parameter. 
 
     
     
         12 . The method of  claim 6  wherein the third parameter is a parameter of a change in phase of the fluid. 
     
     
         13 . The method of  claim 12  wherein the phase change is from a fluid to a fluid-solid mixture. 
     
     
         14 . The method of  claim 13  wherein the third parameter is asphaltene onset point. 
     
     
         15 . The method of  claim 12  wherein the phase change is from a liquid to a liquid-gas mixture. 
     
     
         16 . The method of  claim 12  wherein the phase change is from a gas to a gas-liquid mixture. 
     
     
         17 . The method of  claim 6  wherein the second parameter is optical density of the fluid. 
     
     
         18 . The method of  claim 6  wherein the second parameter is fluorescence intensity of the fluid. 
     
     
         19 . The method of  claim 6  wherein the second parameter is an asphaltene onset indicator based on fluorescence of the fluid. 
     
     
         20 . A method comprising:
 assessing an accuracy of a previously determined value of a phase-change parameter of a fluid drawn into a downhole tool from a subterranean formation, wherein the accuracy assessment comprises:
 at each of a plurality of different wavelengths, determining a difference between a maximum measured optical density of the fluid and another measured optical density of the fluid that corresponds to the previously determined value of the phase-change parameter; and 
 determining whether the determined differences increase monotonically relative to decreasing values of the wavelengths.