BAKER HUGHES - Drilling Fluids Reference Manual

BOREHOLE PROBLEMS
and thus tend to disrupt the water structure. Exceptions are Na + and Li + , which are lightly
bonded and tend to diffuse away.
When dry montmorillonite is exposed to water vapor, water condenses between the layers, and
the lattice expands. The energy of adsorption of the first layer is extremely high, but it deceases
rapidly with succeeding layers.
Osmotic swelling occurs because the concentration of cations between the layers is greater than
that in the bulk solution. Consequently, water is drawn between the layers, thereby increasing
the c-spacing and permitting the development of diffuse double layers. Although no semipermeable
membrane is involved, the mechanism is essentially osmotic, because it is governed
by a difference in electrolyte concentration. Osmotic swelling causes much larger increases in
bulk volume than does crystalline swelling.
Shale Hydration Inhibition
To minimize the effects of water on shale, two aspects of the hydration can be modified. One is
to replace sodium and calcium ions on the exchange sites of the swelling clay, montmorillonite,
with a cation (very often potassium) or with an amine such as those in CLAY-TROL and
MAX-GUARD. The second approach is to encapsulate shale cuttings with PHPA (NEW-
DRILL®) and thus hinder their physical degradation. This encapsulation makes it easier for
the surface solids equipment to remove these solids from the drilling fluid.
Various inhibitive systems are formulated with water soluble salts such as: sodium chloride,
potassium chloride, potassium carbonate, calcium sulfate, calcium chloride and calcium
hydroxide. The cations contained in these systems are adsorbed onto the clay surface through a
cation exchange process. The adsorption of these cations onto the clay surface reduces the
ability of the clay to hydrate and swell. The reduction in clay swelling is due to the amount or
thickness of the water film associated with that particular cation.
When clays containing low order replacement cations (monovalent cations) are exposed to
water, the cations hydrate and dissociate from the clay surface forcing the clay platelets apart.
The distance that the platelets separate is governed by the hydration radius of the cation. A
higher replacement order cation having a smaller hydration radius and a greater affinity for the
clay structure can now displace the low order cation reducing the separation between the
platelets and reversing the swelling tendency.
The most commonly occurring cations may be ranked according to their replacement power as
follows,
Li + , Na + , K + , Mg ++ , Ca ++ , Al +++
Increasing Order of Replacement
Potassium chloride systems have been used with a great deal of success around the world while
calcium-base systems (lime and gypsum fluids) have been used with varying degrees of
success.
The mechanisms by which PHPA polymers stabilize shale cuttings are adsorption, bridging,
and flocculation. The long chain polymer can attach itself to shale particles through adsorption,
sometimes referred to as encapsulation. This encapsulation process forms bridges between
many shale particles and thus links these particles together preventing or greatly limiting
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 7-12