The Effect of Weighting Materials on mud


Barite  (or  barytes)  is  barium  sulphate,  BaSO4  and  it  is  the  most  commonly  used  weighting
material in the drilling industry. Barium sulphate has a specific gravity in the range of 4.20 -
4.60. The specific gravity of Most commercial barite contain impurities including quartz, chert,
calcite, anhydrite, and various silicates which slower its specific gravity. It is normally
supplied to a specification where the specific gravity is about 4.2.  Barite is preferred to other
weighting materials because of its low cost and high purity. Barite is normally used when mud
weights in excess of 10 ppg are required. Barite can be  used to achieve densities up to 22.0 ppg
in both water- based and oil -based muds. However, at very high muds weights (22.0 ppg), the
rheological  properties  of  the  fluid  become  extremely  difficult  to  control  due  to  the  increased
solids content.

2. Iron Minerals

Iron ores have specific gravities in excess of 5. They are more erosive than other weighting
materials and may contain toxic materials. The mineral iron comes from several iron ores
sources including: haematite/magnetite, illmenite and siderite.The most commonly used iron
minerals are:

Iron Oxides: principally haematite, Fe2O3. Iron oxides have several disadvantages including:
magnetic behaviour which influences directional tool and magnetic logs, toxciticity an
difficulty in controlling mud properties.
Iron Carbonate: Siderite is a naturally occurring ferrous carbonate mineral (FeCO3).
Illmenite: The mineral illmenite, ferrous titanium oxide (FeTiO3).

3- Calcium Carbonates

Calcium carbonate (CaCO3) is one of the most widely  weighting agents especially in non-
damaging drilling fluids. Its main advantage comes from its ability to react and dissolve in
hydrochloric acid. Hence any filter cake formed on productive zones can be easily removed
thereby  enhancing  production.  It  has  a  specific  gravity  of  2.60  -  2.80  which  limits  the
maximum  density  of  the  mud  to  about  12.0  ppg  Calcium  carbonate  is  readily  available  as
ground limestone, marble or oyster shells.

Dolomite is a calcium - magnesium carbonate with a specific gravity of 2.80 - 2.90. The
maximum mud density achieved is 13.3 ppg.

4- Lead Sulphides

Galena (PbS) has a specific gravity of 7.40 - 7.70 and can produce mud weights of up to 32
ppg. Galena is expensive and toxic and is used mainly on very high pressure wells.

5- Soluble Salts

Soluble  salts  are  used  to  formulate  solids  free  fluids  and  are  used  mainly  as  workover  and
completion fluid. Depending on the type of salt used, fluid densities ranging from 9.0 - 21.5
ppg (sg =1.08 - 2.58) can be prepared.


The ability of drilling mud to suspend drill cuttings and weighting materials depends entirely
on its viscosity. Without viscosity, all the weighting material and drill cuttings would settle to
the  bottom  of  the  hole  as  soon  as  circulation  is  stopped.  One  can  think  of  viscosity  as  a
structure built within the water or oil phase which suspends solid material. In practice, there are
many  solids  which  can  be  used  to  increase  the  viscosity  of  water  or  oil.  The  effects  of
increased viscosity can be felt by the increased resistance to fluid flow; in drilling this would
manifest itself by increased pressure losses in the circulating system.

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