Page 232 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 208

[ ] Well Programming
2.5.4

is a highly dispersive mud and has no inhibitive properties. This mud
can cause substantial formation damage if there are clay minerals pre-
sent in the reservoir sand. The problem for the formation is that the lig-
nosulfonate does not know when to stop its dispersive action. If it is
necessary to run a dispersed mud, it is always better to use one (such
as a gyp mud) that has some inhibition.
Normally lignosulfonate is added until the rheological properties
and gels are brought to the required levels. Usually 2 to 3 ppb of lig-
nosulfonate is used. The pH of a caustic lignosulfonate is normally
maintained above 9.3 or whatever level is required for the lignosul-
fonate in use to solubilize. Filtration control is normally obtained with
CMC or starch. The use of any other polymer for filtration control such
as PAC is a waste of money since any special benefits that might be
seen in a nondispersed system will be completely suppressed in a caus-
tic lignosulfonate mud. Caustic lignosulfonate muds are only relevant
for nonreactive shales and sands where formation damage is not impor-
tant. In such formations, it may be used where inferior rig solids con-
trol equipment may cause a solids buildup. Quite often a nondispersed
polymer system that fails will end up, at least temporarily, as a caustic
lignosulfonate mud.
Gypsum lignosulfonate mud. Originally, gypsum or gyp lignosul-
fonate muds were developed from other bentonite-based systems for
drilling through gypsum (CaSO 2H O) and anhydrite (CaSO ) zones.
2
4
4.
Substantial contamination of the bentonite muds was experienced and
they became highly flocculated. It was decided to contaminate these
muds on surface with gypsum to stabilize their properties. In particu-
lar, they had to be treated with a high concentration of lignosulfonate
to ensure that they remained deflocculated and that their rheology
remained the same. A correctly formulated gyp mud would therefore
suffer no further contamination and deterioration of properties when a
gypsum or anhydrite sequence was drilled.
The use of gyp lignosulfonate mud has been taken a lot further
than its original role in drilling gypsum and anhydrite formations (e.g.,
the Guadelupe Mountains of the United States). Because of the inhibi-
tive properties of the presence of substantial filtrate calcium, it can sta-
bilize moderately reactive shales. Of all the available dispersed mud
systems, gyp lignosulfonate is probably the most inhibitive. The mech-
anism of inhibition is that the divalent calcium ion attempts to main-

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