1.  Field of the Invention
The present technology relates to oil and gas production.  In particular, the present technology relates to
2.  Description of the Related Art
Fluid sampling is commonly conducted in reservoirs to test reservoir fluid.  In mature wells, for example, the chemical and physical parameters of fluid within a reservoir can change over time, and sampling and testing of the fluid can help to optimize recovery strategies from the wells.  Examples of reservoir properties that can be analyzed include density, formation volume factor, viscosity, interfacial tension, gas/oil ratio, and/or compressibility. 
The sampling and testing of downhole fluid properties, however, presents engineers with a number of technical challenges.  For example, engineers must select the correct zone for sampling, connect to a reservoir, minimize contamination of the samples, obtain sufficient amounts of samples to be tested, and transport samples to the surface and laboratory facilities.  To address such challenges, different techniques for sampling and testing reservoir fluids have been developed. 
For example, in a well that penetrates a staked reservoir, fluid samples can be taken before the well is cased and cemented.  At this early stage, taking samples from the well is simplified by the fact that drilling mud in the well provides hydrostatic pressure on the borehole wall to prevent the uncontrolled production of reservoir fluids.  A tool having a pair of packers can be lowered into the well to a desired location, and the packers can expand to isolate a portion of the well between the packers from the hydrostatic pressure of the drilling mud.  With the packers in place, samples from the reservoir can be withdrawn from the formation, after which the tool and samples are drawn to the surface.  Later, the well can be cased and cemented, and may be used an a cased hole observation well during the production life of the reservoir.  This technique of testing reservoir fluid is disadvantageous, however, because it is limited to the period of time before a well is cased and cemented. 
Another technique for sampling and testing reservoir fluids is to utilize a tool that penetrates the casing of an already-cased well.  Tools for carrying out this technique can be lowered into the cased well, and can perforate the casing at a desired location to access the formation and withdraw samples.  Such a technique is problematic, however, because casing integrity must be restored after perforation, which can be a difficult, dangerous, and costly endeavor.