Page 53 - Well Logging and Formation Evaluation

Quicklook Log Interpretation 43

• Pressure/volume/temperature (PVT) properties of the oil and gas in the
reservoir
• Formation-water salinity
• Additional mobility/permeability information

In the conventional mode of operation, a probe is mechanically forced
into the borehole wall and chambers opened in the tool into which the for-
mation flows. Pretest chambers are small chambers of a few cubic cen-
timeters that can be reemptied before the next pretest station. For downhole
3
sampling, larger chambers are used, typically 2 / 4 or 6 gallons. Since the
first fluid entering the tool is typically contaminated by mud filtrate, normal
practice is to make a segregated sample; that is, fill one chamber, seal it,
and then fill a second chamber (hopefully uncontaminated). Once the
chambers are retrieved at surface, they may be either drained on the well-
site or kept sealed for transferring to a PVT laboratory.
Optional extra modes in which the tools can typically be used include
the following:

1. As an arrangement of packers in order to isolate a few meters of the
borehole wall, thereby providing a greater flow area
2. As a pumpout sub while sampling in order to vent the produced fluids
into the wellbore until it is hoped that the flow is uncontaminated by
mud filtrate
3. To monitor the fluid properties (resistive, capacitant, optical) while
pumping out to determine whether oil, water, or gas is entering the
chamber
4. As dual packer assemblies run to create a “mini-interference test” that
can be used to assess the vertical communication between different
intervals.

Pretests and sampling are often not successful. Moreover, the fact that
the tool is stationary in the hole for long periods means that there is a
higher than usual chance of getting the tool stuck in the hole. One of these
problems can occur:

• Seal failure. The rubber pad surrounding the probe, which provides a
seal between the mud pressure and the formation pressure, may fail,
resulting in a rapid pressure buildup to the mud pressure.
• Supercharging. Tight sections of the formation may retain some of the
pressure they encounter during the drilling pressure (which is higher