BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Laboratory Studies
Formation damage can be studied in the laboratory using a return permeameter, a device that can
measure the permeability of rock samples before and after exposure to drilling fluids (see Figure
2-13).
Figure 2-13
Schematic Diagram of a Return Permeameter
The cylinder in the center is a pressure cell that contains the rock sample at a specified temperature
and pressure. A fluid or gas is flowed through from right to left in order to measure permeability, and
then the test drilling fluid is circulated through the left side of the cell where it will contact one end of
the core sample. The permeability is then re-measured to determine the percent return permeability.
This device consists of a pressure chamber that contains the core sample at a certain temperature and
pressure, flow lines for fluids and gasses, and gauges to measure pressures and flow rates in order to
calculate permeability. After the permeability of the sample in its natural state is measured, the core is
then exposed to a drilling or other test fluid for a specified time, and then its permeability is remeasured.
The results of these tests are often given as percent return permeability which is simply the percentage
of the original permeability retained by the core after exposure to the test fluid. In other words, a
sample that experienced no damage would have 100% return permeability. The purpose of these tests
is to evaluate a variety of drilling fluids to see which ones cause the least amount of damage.
Some rocks are more prone than others to formation damage, so samples from the actual formation to
be drilled are necessary to properly evaluate test fluids. Because this is useful information, many
operators will furnish samples for return permeability testing. As more samples from a particular
region are tested, a valuable database can be built up that can be used in selecting drilling fluids for
use in that area.
Besides their use in return permeability testing, formation samples can be examined by X-ray
diffraction, scanning electron microscopy, and thin section analysis to determine the mineralogical
content, pore structure, and how the clay minerals are distributed in the rock. The vulnerability of a
reservoir rock to formation damage can often be predicted by using these techniques. After the cores
are tested for return permeability, they can be examined in the scanning electron microscope for
evidence of permeability loss.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 2-19