Page 35 - Environmental Control in Petroleum Engineering

Drilling and Production Operations 23

Most drilling fluids are composed of 3% to 7% bentonite by volume.
Other clays can be used, but typically do not provide as high a mud
viscosity for the same amount of clay added. During normal drilling
operations, natural clays in the formations can also be incorporated
into the mud, increasing the clay content and mud viscosity over time.
Adding hydratable clays to the water used as a drilling fluid pro-
vides a second important benefit for drilling of wells. Because the
pressure of the mud in the wellbore is normally kept above the
pressure in the formation to prevent blowouts, the water (mud filtrate)
will flow into a permeable formation and be lost. When this occurs,
the suspended clays are filtered out at the face of the formation,
building a mudcake along the walls of the well. The clay particles of
this mudcake are virtually always smaller than the grains of a perme-
able formation, so the resulting permeability of the mudcake is much
lower than that of the formation. This low permeability mudcake acts
as a barrier to minimize subsequent fluid losses to the formation.
Because fluid losses are lower, the total volume of mud needed to drill
the well is reduced.
One difficulty with using clay particles for viscosity control is that
they tend to flocculate (agglomerate) if the mud is allowed to remain
static in the wellbore. When flocculation occurs, the mud viscosity can
significantly increase. If the viscosity becomes too high, the mud can
become too difficult to pump at reasonable pressures and flow rates,
rendering it ineffective as a drilling fluid. Flocculation occurs when
the electrostatic charges along the periphery of the clay particles are
allowed to attract other clay particles. The flocculation rate increases
with an increasing clay content and electrolyte (salt) concentration in
the mud.
A variety of materials are available that can suppress flocculation
of clay particles in drilling muds, although none are totally effective
under all conditions. The most common deflocculants are phosphates,
tannins, lignites, and lignosulfonates. Phosphate deflocculants can be
used when the salt concentrations and temperatures are low. Tannins
are effective in moderate concentrations of electrolyte concentration
and moderate temperatures. Lignites and lignosulfonates can be effec-
tive at high temperatures, particularly if they are complexed with heavy
metals like chromium.
Polymers, like xanthan gum, have also been developed to increase
the viscosity of drilling mud. These polymers have the advantage of