STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drilling Muds and Completion Fluids 667
Mud temperature should be lower than 130°F.
Salt contamination should be lower than 5000 pprn chloride.
Calcium concentration should be kept as low as possible.
pH should be 8 to 9.5; in continuous use, the pH of some phosphates may
decrease below the recommended limits so that pH maintenance with
caustic soda is required.
Lignite Muds
Lignite muds are usually considered to be high-temperature-resistant since
lignite is not affected by temperatures below 450°F. Lignite constitutes an
inexpensive chemical for controlling apparent viscosity, yield point, gel strength,
and fluid loss of a mud. Since lignite is refined humic acid (organic acid), caustic
soda (sodium hydroxide) is usually necessary to adjust the pH of the mud to
above 8; the treatment normally consists of adding 1 part of NaOH to 4 to 8
parts of lignite. If precausticized lignite (alkali + lignite) is being used, there is no
need for the addition of caustic soda. The main limitations on lignite muds are
* hardness lower than 20 ppm
pH of 8.5 to 10
mud temperature below 450’F
Quebracho-Treated Muds
Quebracho-treated freshwater muds were used in drilling at shallow depths.
The name of “red” mud comes from the deep red color imparted to the mud
by quebracho. Muds treated with a mixture of lignite and quebracho, or a
mixture of alkaline organic polyphosphate chemicals (alkaline-tannate treated
muds), are also included in the quebracho treated muds. The quebracho thinners
are very effective at low concentrations, and offer good viscosity and filtration
control. The pH of “red” muds should be 8.5 to 10; mud temperature should
be lower than 230°F.
Quebracho muds are used to increase the resistance to flocculation caused
by contaminating salts, high pH (11 to 11.5). These muds can tolerate chloride
contaminations up to 10,000 ppm.
Lignosulfonate Muds
Lignosulfonate freshwater muds contain ferrochrome lignosulfonate for
viscosity and gel strength control. These muds are resistant to most types of
drilling contamination due to the thinning efficiency of the lignosulfonate in
the presence of large amounts of hardness and salt.
Lignosulfonate muds can be used efficiently at a pH of 9 to 10, and have a
temperature limitation of about 35OoF, above which lignosulfonates show severe
thermal degradation. The recommended range of rheological properties of
freshwater-base muds is shown in Figure 4-111 [29].
Dispersed Inhibited Systems. Dispersed inhibitive fluids attempt to combine
the use of dispersed clays and deflocculants to derive the fundamental properties
of viscosity and fluid loss with other features that will limit or inhibit the hydration
of the formation and cuttings. It will be realized these functions are in opposition;
therefore the ability of these systems to provide a high level of shale
inhibition is limited. However, they have achieved a high level of success and in