Advancements in Uncased-Hole, Wireline Formation-Tester Techniques
Authors: Schultz A.L. , Bell W.T. , Urbanosky H.J. 
Published: December 2007

Improvements in wireline formation testing have been incorporated into a tool with multiple-set capabilities. The toot permits pretesting of the formation for permeable regions and checking of packer seal integrity, before sampling. Two fluid samples can be obtained on each trip and any number of pressure recordings can be taken during the same trip.

Introduction
The wireline formation-tester (FT) technique was introduced to provide confirmation of formation-fluid type, indications of productivity, and formation pressures. Various improvements have been made in the pressures. Various improvements have been made in the technique and interpretation methods have been developed for best use of the information from the recovered fluid samples and the pressure recordings.
While the technique has been successful locally, it has not reached its full potential, basically because of the long rig time required with existing testers for multiple-zone testing. Once the tester was set in the well, it could not be repositioned at another level in the zone of interest. Consequently, any test failure caused by a tool setting in an impervious streak or by a packer-seal failure resulted in an extra trip in the well. packer-seal failure resulted in an extra trip in the well.Performance in many unconsolidated sands was not acceptable with these older tools. Techniques to combat the flow of sand into the tester were never completely successful; this sand flow caused undermining of the packer seal with subsequent mud-sample recovery. packer seal with subsequent mud-sample recovery. These factors combined to produce an over-all success ratio of about 70 percent for all formations and about 35 percent for unconsolidated sands. percent for unconsolidated sands.Also, toot redressing required between runs was extensive. This added to the over-all operating time unless additional tools were available at the well.
Another limitation of existing FT tools was the insufficient accuracy of the recorded pressures (in the range of 2 to 3 percent). This, combined with the single-test-per-trip capability, often discouraged the use of these tools for recording several pressure measurements in a well.
In summary, major limitations of these older tools were their inability to be repositioned, their single-test capacity, and the lack of a reliable means for testing the integrity of the seal before attempting a sample.
To overcome these limitations, a new formation tester has been developed.
Principal Features of the New Tester Principal Features of the New Tester The new tester has several distinguishing features as compared with the older tools. Several successive tool settings are possible without bringing the equipment out of the hole. Combined with this is a "pretest" capability that permits the operating engineer to ascertain, before attempting to take a sample, whether the packer is sealing properly and, if so, whether fluid flow is adequate to properly and, if so, whether fluid flow is adequate to obtain a sample in a reasonable period of time.
Thus, if the tool is set and the packer seal fails, or if the indications are that the tool - is set in an impervious streak, the tool is simply retracted and moved to another position in the formation. If both seal and flow indications during pretest are satisfactory, a sample is taken.
Two separate sample chambers make it possible to obtain two samples on a single trip into the well.
JPT
P. 1331