BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Conventional (Basic) Fluid Systems
Spud Muds
A spud fluid, often referred to as a spud mud, is normally a very simple inexpensive fluid for
drilling the first and sometimes second intervals from surface. Under certain drilling
environments such as reactive shales, water flows, surface gas, bit balling, and poor penetration
rate, the spud mud requirements can be more complex. However, a typical spud mud is one of
simplicity, low cost, and can be easily and rapidly mixed, due to the large volume requirements
associated with large diameter surface hole in a traditional wellbore design.
Spud fluids can vary considerably depending on lithology, make-up water, and hole size. When
freshwater is available and surface formations contain considerable quantities of reactive clays,
the well may be spudded with water only. As a contrast, when surface formations consist of
loosely consolidated sand and/or gravel, it may be necessary to spud with a drilling fluid having
good wall cake properties and sufficient viscosity to keep the hole clean. Some typical spud fluid
practices are described below.
Freshwater / MILGEL ® (Bentonite)
Make-up water should be checked for calcium and magnesium. If the hardness check indicates a
calcium ion concentration in excess of 150 mg/L, the water can be pre-treated with soda ash. As
a rule of thumb, 10 lb of soda ash will lower the calcium by 100 mg/L in 100 bbl of water. Be
careful to avoid over treatment with soda ash. This product can change from being beneficial in
removing the calcium contaminant, to being detrimental by increasing the carbonate radical anion
which is a clay flocculent After the make-up water quality is acceptable, a good spud fluid may
be mixed by using one of the following procedures.
1. Mix 20 to 25 lbm/bbl of MILGEL ® . Mixing rate will vary between two to 10 minutes per
sack dependent upon the efficiency of the mixing equipment. In some cases, small
quantities of lime may be added to bentonitic spud fluids (after bentonite is hydrated) to
thicken the fluid and improve hole cleaning. If optimum cake quality and filtrate is
desired it is better to use additional viscosifiers and avoid the addition of lime, a
recognized clay flocculent and source of calcium ion.
2. Add 10 to 12 lbm/bbl of SUPER-COL ® . The yield of SUPER-COL is approximately
twice that of MILGEL. SUPER-COL ® results in a lower solids concentration than
MILGEL formulations, and uses half the volume of product which may be critical on
some wells.
Note:
SUPER-COL should not be used where chlorides are above 2000 mg/L or where hardness
cannot be reduced to less than 100 mg/L.
3. Bentonite extenders (e.g. BENEX) added to MILGEL slurries increase the viscosity of the
suspension. Normally, two sacks of BENEX and 10 to 12 sacks of MILGEL per 100 bbl
of fluid result in an excellent freshwater spud fluid. Depending on the quality or amount
of peptizing on the bentonite in use these extenders may not perform as efficiently. The
best results are obtained with BENEX when combined with a non-treated bentonite, such
as MILGEL ® NT. A disadvantage of this system is the increased mixing complexity.
Normally, a mixing tank with a chemical feed pump is required for mixing and adding
extenders. Care must be taken to hydrate the bentonite extenders thoroughly to remove
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 3-31