How does the Repeat Formation Test (RFT) work?

How does the Repeat Formation Test (RFT) work? What is its objective?

The repeat formation tester (RFT) tool was designed to measure formation pressure downhole via wireline. It measures pressure at specific points on the borehole wall. The diagram below shows a typical RFT tool. Formation pressure is measured by the formation sampler when it is extended from the tool to contact the formation. Fluid samples from the formation can also be taken with the tool.

The tool is located by correlation with other logs, usually a gamma ray. Once the formation sample probe is at the right depth, the tool is powered into sample mode with the seating pad pushed against the wellbore wall and the probe seated on the opposite wall.

The test begins by opening the mini chamber to the formation to test pressure and check on sufficient permeability to flow a sample. Once a positive result is verified there the operator makes a choice to obtain a fluid sample or move the tool. If there is a reason to obtain a fluid sample, the main sample chamber is opened and fluid either flows in or is pumped in from the formation. To move the tool the power sequence is reversed and the probe and seating pad are retracted. The tool is then moved to the next depth.