This clip shows the basic operating mode of the MDT when taking...

This clip shows the basic operating mode of the MDT when taking pressures.

1. Probe is extended from tool and pushed against the borehole wall. Seal is facilitated by the large black doughnut shaped packer
2. Fluid flows into flowline and into a small pretest chamber. Chamber volume is 20cc but this can be adjusted if necessary to lower values
3. The probe module contains a strain and quartz pressure gauge which monitors pressure in flowline. Prior to setting the probe the gauges will show mud hydrostatic pressure. Once probe is set gauges will show pressure in flow line that is connected to formation via the probe. The rate of build of pressure is related to the permeability of the formation. If pressure doesn't build up to stabilized formation pressure within a few minutes then this indicates that the formation is low permeability/tight and usually the pretest will be aborted rather than wait hours (days?) for pressure to stabilize.

The MDT is a modular tool and there are a variety of modules that can be added to the basic pretest configuration. Eg. pumpout modules, sample chambers and fluid analyzers would be added for sampling. For tight formation there are specialized probes and a dual packer module.