Liquid drilling fluids consist of:
•The liquid (continuous phase) – drilling muds
are classified according to their base: water or
Water base muds (WBM)
Solid particles are suspended in water.
Any oil added to WBM is emulsified into the
water phase and is maintained as small,
It is called oil-in-water emulsion or emulsion
The continuous phase can be:
• fresh water
• brackish water
• sea water
• saturated salt water
• another type of brine fluid
Fresh water WBM – Usually available only on
• Commercial clays hydrate more
• Most chemicals are more soluble
• Formation clays hydrate more, which can result in
Brackish water WBM
•Usually in a marine environment
•Higher calcium and magnesium
concentration than fresh water
•Chlorides and hardness varies
•Chlorides in GoM: 15,000 – 30,000 mg/l
•Calcium in GoM: ±400 mg/l
•Magnesium in GoM: ±1200 mg/l
•Hardness in North Sea is much higher
Saturated salt water WBM
•Used primarily to drill through large salt
•Salt must be added to achieve saturation.
•Prevents hole enlargement due to leaching
or dissolving salt from the formation.
•Leaching could result in hole problems and
expensive mud and cement costs.
•Usually used for clay (shale) inhibition.
•Potassium chloride (KCl), calcium chloride
(CaCl2), formates (Na+, K+), bromides.
Oil base muds (OBM)
Diesel or synthetic-base oil is the continuous
Organophilic clay, and trace amounts of water
as the dispersed phase which acts as a polar
activator for the organophilic clay.
If the amounts of water are more than 5%,
then it becomes water-in-oil emulsion (invert
All solids are in OBM are considered inactive
because they do not react with oil.
Advantages of OBM
Good rheological properties at high temperature.
More inhibitive than inhibitive WBM.
Effective against all types of corrosion.
Superior lubricating characteristics.
Permits mud densities as low as 7.5 ppg.
Disadvantages of OBM
• Higher initial cost.
• Requires more stringent pollution-control procedures.
• Reduced effectiveness of some logging tools
• Remedial treatment for lost circulation is more
• Detection of gas kick is more difficult because of gas
solubility in diesel oil.
Reactive – solids that can react with the water
phase and dissolved chemicals.
Reactive commercial clay solids
• Sodium montmorillonite or bentonite
Reactive formation solids
• Montmorillonite (swelling clay)
• Kaolinite (non-swelling clay)
• Chlorite (non-swelling clay)
• Gumbo shale (combination of above clays)
Inert/Inactive – solids that do not react with the
water phase and dissolved chemicals to a
Inert commercial solids
• Barite (barium sulphate) – used to increase mud
density up to maximum of ±22 ppg.
• Hermatite (iron oxide) – used to increase mud density
up to maximum of ±25 ppg.
• Calcium carbonate – used to increase mud density up
to maximum of ±14 ppg. Also as bridging agent in drill-
in, oil and synthetic fluids.
• Lost circulation material (LCM) – Used to bridge off
(seal) formations where whole mud is being lost to the
formation e.g. nut shells (mostly pecan and walnut),
mica, fiber (wood, paper, plastic, etc.).
Inert formation solids
Chemicals are added to “fine tune” drilling
fluids for specific purposes. Examples are:
• Caustic Soda (NaOH)
• Caustic Potash (KOH)
• Lime (Ca(OH)2)
• Chemical de-flocculant (mud thinner)
• Lignosulfonates (organic acid)
• Soda Ash (Na2CO3)
Density – weighing materials are used to
increase mud weights.
Examples are: barite, hematite.
Flocculation – thickening of the mud due to
edge-to-edge association of clay platelets.
Examples of flocculants are: hydrated lime, gypsum,
and sodium tetraphosphates.
Deflocculation – reducing the tendency of a
mud to flocculate.
• Examples of deflocculants (thinners and dispersants)
are: tannins (quebracho) lignitic materials, and various
Viscosity – resistance to flow.
• Examples of viscosifiers are: Attapulgite clays, asbestos
fibers, sodium carboxymethylcellulose (CMC).
pH – the pH of most muds is maintained
between 9.5 and 10.5.
High mud pH is desirable to suppress
corrosion rate, hydrogen embattlement, and
the solubility of Ca2+ and Mg2+. High pH is also
favourable for many organic viscosity control
• Examples are: lime, caustic soda, and bicarbonate
Filtration – tendency of the liquid phase of a
drilling fluid to pass into the formation.
• Filter loss additives include: pregelatinized starch,
CMC, and sodium polyacrylate.
Emulsion – creating a heterogeneous mixture
of two liquids.
• Emulsifiers include: modified lignosulfonates, certain
surface-active agents, anionic (negatively charged) and
non-ionic (noncharged) products.
Some of the functions are:
Cooling & lubrication
Power downhole tools
Support part of drillstring
Where possible hole cleaning should be
achieved by Annular Velocity (AV).
It should be 100 ft/min, higher in deviated
In large hole sections the AV can be as low as
If high AV is not
possible to achieve due
to pump limitations or
due to the risk of
wellbore erosion, then
viscosity must be
Pump rate (bbls/min)
Annular volume (bbls/ft)
Slip velocity (ft/min) Cutting velocity AV
Static condition – the pressure balancing the
Circulation condition – effective pressure is
increased by the pumping pressure. It is given
as the Effective Circulating Density (ECD).
P (psi) 0.052 Depth (ft) Density (ppg)
Annular Pressure Loss (psi)
ECD (ppg) Density (ppg)
0.052 Depth (ft)
Ability of muds to suspend drill cuttings when
the pumps are switched off
Else solids will start to settle.
This can result in:
• Bridging off of the wellbore
• Stuck pipe
• Hole fill
• Loss of hydrostatic.
A gel structure is required to suspend the
cuttings under zero shear conditions.
The gel structure needs to be easily broken or
pressure surges will result when the pumps
are switched on.
This can fracture the formation.
The drilling fluid removes heat from the bit
which is then dispersed at the surface.
Extra lubrication may be required between
the drill string and the casing or wellbore,
especially in directional wells.
• Liquid additives are used, or oil based mud.
• Solid additives are sometimes used such as glass
beads, plastic beads, graphite or nut plug.
• Drill pipe rubbers are sometimes added to reduce
wear between the casing and drill pipe.
Run turbines to turn the bit or power
Transfer information from measurement
equipment to the surface. This is done with a
Turbine motor Positive displacement
Aids in supporting part of the weight of the
drillstring and casing.
The degree of buoyancy is directly
proportional to the density of the fluid.
Effective weight lbm Weight in air lbm BF
Mud density ppg
Buoyancy factor, BF 1
Steel density ppg
Should be environmentally acceptable to the
area in which it is used.
Should not cause corrosion of the drilling
equipment and subsurface tubulars.
Should not damage the productive formations
that are penetrated (filtration property).
Other fine solids
Water 1.00 8.33 350
Diesel 0.86 7.20 300
Bentonite clay 2.60 21.7 910
Sand 2.63 21.9 920
Barite (API) 4.20 35.0 1,470
Densities of some additives.
Mud rheology –
measures viscosity and
gel strength of mud.
The 2 types are: Marsh
funnel viscometer and
Marsh funnel kit
It measures number
of seconds for a
quart of the sample
to run out.
It determines the
(viscosity and gel
strength) of muds in
terms of shear rate
and shear stress.
Mud filtration –
filtration of drilling
Standard API Filter Press
Used at ambient
API FL = 560 cc’s 44 cc’s 15.6 cc’s
125.0 cc’s 22.0 cc’s 9.0 cc’s
1.5 min 7.5 min 10.0 min
16.8% 10.4% 3.0%
Solids Solids Solids
HPHT Filter Press
Used at elevated
Sand contents – sand
volume percent of
sand-sized particles in
the drilling fluid.
Sand Content Kit
API defines sand-sized
particles as any
material larger than 74
microns (200-mesh) in
Resistivity – provides a
rapid means of
detecting soluble salts in
barite and in waters,
such as makeup or
Resistivity of water muds,
filtrates and filter cakes are
routinely applied in
pH (Hydrogen ion
concentration) – The term
pH is used to express the
concentration of hydrogen
ions in aqueous solution. It
can be determined either
using the colorimetric
method (pH paper) or the
electrometric method (pH
The pH paper is impregnated with dyes that exhibit different
colours when exposed to solutions of varying pH.
It determines the pH of an
aqueous solution by
between a special glass
electrode and a reference
Oil, water and
The retort provides a
means of separating and
measuring the volumes of
water, oil, and solids
contained in a sample of
Methylene blue test
(MBT) – The
capacity of a drilling
fluid is an indication
of the amount of
(bentonite or drilled
The methylene blue capacity gives an estimate of the total
cation exchange capacity (CEC) of the solids in the drilling fluid.
lubricity tester is
used to determine
property of OBM.
Aging – aging cells and roller oven are used to
determine the “aging” effects on muds.
Roller Oven Aging Cell