BAKER HUGHES - Drilling Fluids Reference Manual

BOREHOLE PROBLEMS
Should the pipe become stuck in coal, and circulation is possible, experience shows that
spotting a high pH pill around the coal can help to free the pipe.
Where the coal seam is not tectonically stressed and geological information regarding the seam
is required care must be taken with fluid properties and drilling practices. Seat earths and
marine bands both provide valuable information about the coal but both are easily washed out.
When coring with water based drilling fluids a low filtrate should be utilized and jet velocity
should be minimized. There is some evidence that these fractured rocks can be stabilized with
products such as Gilsonite and SULFATROL ® .
INHIBITIVE WATER BASED DRILLING FLUIDS
There has been extensive research into inhibitive drilling fluids in an attempt to control
wellbore instability by means of shale control. Shale control continues to dominate research due
to the fact that shale instability remains one of the largest contributors to troublesome drilling
and increased costs.
“Today's definition of an inhibitive fluid is much broader than simply the inhibition of the
swelling of highly expanding clays such as Montmorillonite. It means a fluid that exhibits
minimal reactivity with the borehole and more specifically with the broad range of shales and
other argillaceous formations.” This statement by B.G. Chesser of Baker Hughes Drilling
Fluids typifies the type of thinking that has guided the evolution of inhibitive fluid systems over
the years.
The goal of inhibitive water based drilling fluid design is generally to prevent
hydration/swelling of the clay minerals and to minimize pore pressure transmission.
Shale Hydration
All classes of clay minerals absorb water, but smectites take up much larger volumes than do
other classes, because of their expanding lattice. For this reason, most of the studies on clay
swelling have been made with smectites, particularly with montmorillonite.
Two swelling mechanisms are recognized: crystalline and osmotic. Crystalline swelling (also
called surface hydration), results from the adsorption of mono-molecular layers of water on the
basal crystal surfaces – on both external, and, in the case of expanding lattice clays, the interlayer
surfaces. The first layer of water is held on the surface by hydrogen bonding to the
hexagonal network of oxygen atoms. Consequently, the water molecules are also in hexagonal
coordination. The next layer is similarly coordinated and bonded to the first, and so on with
succeeding layers. The strength of the bonds decreases with distance from the surface, but
structured water is believed to persist to distances of 75 – 100 Å from an external surface.
The structured nature of the water gives it quasi-crystalline properties. Thus, water within 10 Å
of the surface has a specific volume about 3% less than that of free water. (Compared with the
specific volume of ice, which is 8% greater than that of free water.) The structured water also
has a viscosity greater than that of free water.
The exchangeable cations influence the crystalline water in two ways. First, many of the
cations are themselves hydrated, i.e., they have shells of water molecules (exceptions are NH 4 + ,
K + , and Na + ). Second, they bond to the crystal surface in competition with the water molecules,
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 7-11