BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Figure 2-11
Photomicrograph of Sandstone Impregnated with Clay
Some sandstone such as this one has extensive coatings of clays on the framework grain surface. In
this image, the clay, presumably illite, is the feather-like material filling the pore spaces between the
quartz crystals.
The prevention of clay swelling and resulting problems can be accomplished by minimizing fluid loss
and using inhibitive drilling fluids. Such fluids include certain polymers, those with potassium salts
and oil-base fluids. Clay swelling is prevented by the potassium ion because it fits well into a clay's
interlayer sites and stabilizes its structure. Polymers tend to be adsorbed on the clay's outer surfaces
and prevent chemical alteration in the interior sites.
Because hydroxyl ions tend to hydrate expandable clays, drilling fluids such as lignosulfonates that are
run at high pH can be damaging. This is part of the reason why lower pH fluids such as polymers are
less damaging, and laboratory tests have shown that keeping the pH as low as possible in dispersed
fluids will reduce formation damage in fluid-sensitive rocks. The base oils in oil-base fluids do not
react with clays and, if included water is kept as the internal phase of the emulsion, an oil-base fluid
will be highly inhibitive.
Formation damage can also occur in clastic rocks with non-expandable clays such as kaolinite or illite.
These clay minerals tend to form plates and fibers that are loosely attached to the pore walls (see
Figure 2-12).
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 2-17