BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
commonly used unit of measure in the oil and gas industry is the millidarcy, which is 1 / 1000 of a Darcy.
Devices which measure permeability, called permeameters, are used extensively to measure the
permeabilities of formation samples.
Causes and Remedies
As was stated earlier, it is obvious that a rock formation's permeability should be reduced as little as
possible when it is drilled and completed. This is difficult to avoid, however, and the reduction of
permeability is referred to as formation damage. In the drilling fluids industry, the principal concern is
formation damage caused by drilling fluids. Formation damage caused by drilling fluids involves the
interaction of the drilling fluid or the filtrate with a permeable rock formation that results in a loss of
permeability. After a formation is drilled, pressure and flow tests can sometimes be carried out to
determine if it has experienced damage.
The simplest form of formation damage involves the flow of the entire drilling fluid (liquids and
solids) into the pores of the rock; this is usually called fluid invasion. If enough solid material such as
bentonite or weighting agents accumulate in the pores, blockage and permeability loss can result. This
type of damage can occur due to one or a combination of the following factors:
• Poor fluid loss control
• Excessive overbalance
• High-formation permeability
An example of a sandstone which experienced such invasion is shown in Figure 2-9. Fluid invasion
can be avoided if the drilling fluid forms a stable filter cake on the borehole wall that serves as a
barrier to invasion. One possible remedy in extreme cases is the use of bridging agents, which are
particulate materials such as calcium carbonate that block the formation’s pores at or near the borehole
wall.
Figure 2-9 SEM Photomicrograph of Venezuelan Sandstone
The permeability of this sandstone was greatly reduced by fluid invasion during a return permeability
test. The light gray material labeled “S” in the center of the photograph are drilling fluid solids
(barite and gel) that have plugged a pore space. Some open pore space can be seen in the lower left
corner.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 2-15