Single phase sampling apparatus and method

Abstract

A single phase sampling apparatus and method for retrieving a formation fluid sample at or above the bubble point of the sample. The apparatus utilizes a gas charge contained between a sample piston and a charging piston to maintain a formation sample at the desired pressure. The charging piston utilizes the hydrostatic pressure present at the depth of the desired formation sample to compress and therefore increase the gas charge to the appropriate pressure necessary to maintain the formation sample at the desired pressure. The utilization of hydrostatic pressure to increase the pressure of the gas charge allows the use of a low pressure gas charging system to prepare the apparatus prior to sampling, thereby increasing the safety and ease of use of the device.

1 . A method for downhole fluid sample collection comprising: 
 inserting pressurized gas in a space between a first piston and a second piston of a tube having a first end and a second end, and the first piston and the second piston located within said tube;    collecting a formation fluid sample in the space between the first end of the tube and said first piston, whereby the pressure exerted on the formation fluid through said first piston by the pressurized gas is equal to or greater than the hydrostatic pressure in the borehole.    
   
   
       2 . The method of  claim 1 , wherein the step of inserting pressurized gas in the space between said first and second pistons, comprises (i) attaching a pressurized gas source to the second piston and (ii) forcing pressurized gas through said second piston into the space between said first and second pistons.  
   
   
       3 . The method of  claim 1 , wherein the step of inserting pressurized gas in the space between said first and second piston is preceded by forcing said first and second pistons together to purge any existing gas from the space between said first and second pistons.  
   
   
       4 . The method of  claim 3 , therein the step of forcing said first and second pistons together comprises filling the space between the second piston and the second end of the tube with a borehole mud at hydrostatic pressure.  
   
   
       5 . A method for downhole fluid sample collection comprising: 
 inserting pressurized gas in a space between a first piston and a second piston of a tube having a first end and a second end, and the first piston and the second piston located within said tube;    collecting a formation fluid sample in the space between the first end of the tube and said first piston, at pressure substantially higher than the hydrostatic pressure in the borehole.    
   
   
       6 . The method of  claim 5 , wherein the step of inserting pressurized gas in the space between said first and second pistons, comprises (i) attaching a pressurized gas source to the second piston and (ii) forcing pressurized gas through said second piston into the space between said first and second pistons.  
   
   
       7 . The method of  claim 5 , wherein the step of inserting pressurized gas in the space between said first and second piston is preceded by forcing said first and second pistons together to purge any existing gas from the space between said first and second pistons.  
   
   
       8 . The method of  claim 7 , therein the step of forcing said first and second pistons together comprises filling the space between the second piston and the second end of the tube with a borehole mud at hydrostatic pressure.  
   
   
       9 . A method for extracting a single phase fluid sample from a wellbore formation and maintaining the sample in a single phase, comprising: 
 forcing a sample piston and a charging piston together within a formation fluid sample bottle to purge any existing gas from a pressurized gas chamber;    attaching a pressurized gas source to said charging piston;    forcing pressurized gas through said charging piston into said pressurized gas chamber;    removing said pressurized gas source from said charging piston;    inserting said formation fluid sample bottle into a formation evaluation tool;    lowering said formation evaluation tool into a wellbore;    exposing said charging piston to the wellbore mud at hydrostatic pressure;    pumping formation fluid into said single phase collection apparatus, wherein the pressure excreted on the formation fluid through said sample piston by the pressurized gas and the wellbore mud is greater than the hydrostatic pressure in the wellbore;    raising said formation evaluation tool; and    removing said single phase collection apparatus from said formation evaluation tool.    
   
   
       10 . The method of  claim 8 , further comprising increasing pressure of the formation fluid sample in the single phase collection apparatus by increasing pressure exerted by the charging piston on the pressurized gas.