Page 233 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 209

Drilling Fluids Program [ ]
2.5.4

tain drilled clays in an aggregated state or at least suppress clay disper-
sion. This is similar to attempting to disperse bentonite in hard or
brackish water.
Gyp lignosulfonate muds would not provide the stability to very
reactive shales as would, for example, a potassium chloride nondis-
persed polymer system (which uses a different mechanism of inhibi-
tion). However, gypsum lignosulfonate systems can still be relevant
against some reactive shales. Engineering a gyp lignosulfonate mud is
similar to engineering an invert emulsion oil-based mud. Every com-
ponent of the mud has its particular function. The concentration range
of each component chemical is closely defined and, if correctly main-
tained, can result in a very stable mud. Each chemical interacts with
each other.
The basis of gyp mud is 15 to 18 ppb of bentonite prehydrated and
fully dispersed in fresh water; 6 ppb of gypsum is added to the system
and 5 to 6 ppb of lignosulfonate deflocculant with caustic soda is added
to counter the flocculating effect of the gypsum on the bentonite. In
adding 6 ppb of gypsum, it is intended that 2 ppb of gypsum will sol-
ubilize in the mud and 4 ppb will remain in suspension as a “reservoir”
for the supply of calcium ions. The pH will control how much gypsum
will go into solution and how much will remain in excess (in reserve).
As mentioned above, the means of inhibition is the presence of soluble
calcium ion and the objective is to get as much into solution as possi-
ble. At a pH range of 9.5 to 10.0, it should be possible to achieve a fil-
trate calcium concentration of 1200 to 1500 ppm. The higher the pH,
the less soluble calcium will be present.
The lower pH range for gyp mud is defined as that which is neces-
sary to allow lignosulfonate to solubilize. Normally, this is around a pH
of 9.5. If you used another deflocculant that would be active at a lower
pH and you used a lower pH, more calcium would be in solution. At
the pH range, however, of 9.5 to 10.0, 2 ppb of gypsum will be solubi-
lized and 4 ppb will be excess gypsum.
The amount of caustic soda (sodium hydroxide) that has to be
added to a gyp lignosulfonate mud to obtain a pH of 9.5 is quite sub-
stantial (usually around 3.5 ppb). This is because of the acidic nature
of lignosulfonate and, to a lesser extent, gypsum. Filtration control in
gypsum mud is normally achieved with the addition of 5 ppb of CMC

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