Drilling Engineering – Fall 2012
Drilling Engineering – PE 311
Chapter 2: Drilling Fluids
Classification of Drilling Fluids
Prepared by: Tan Nguyen
Drilling Engineering – Fall 2012
Classification of Drilling Fluids
WBMs
The following designations are normally used to classify water-base drilling fluid systems:
1. Nondispersed-noninhibited systems: spud muds, polymer/bentonite muds, extended bentonite
muds )
2. Nondispersed-inhibited systems: salt muds, KCL-polymer muds
3. Dispersed-noninhibited systems: lignite-lignosulfonate muds, phosphate-bentonite muds
4. Dispersed-inhibited systems: lime muds, gyp-lignosulfonate muds, seawater-prehydrated
bentonite muds.
Prepared by: Tan Nguyen
Drilling Engineering – Fall 2012
WBMs – Nondispersed-noninhibited fluids
Nondispersed-noninhibited fluids
The noninhibited-nondispersed fluids do not contain inhibiting ions such as chloride (Cl), calcium
(Ca2+), or potassium (K+) in the continuous phase and do not utilize chemical thinners or dispersants
to effect flow control. Nondispersed-inhibited fluids do contain inhibiting ions, but do not utilize
chemical thinners or dispersants. Dispersed-noninhibited fluids utilize chemical thinners or
dispersants, but do not contain inhibiting ions. Dispersed-inhibited fluids contain both chemical
dispersants and inhibiting ions.
When referring to a water-base mud system, the term nondispersed means that clay is free to find its
own hydrous dispersed equilibrium in the aqueous solution. It also means that chemical
acceleratives or dispersants have not been added to the system. The term noninhibited refers to the
lack of specific ions such as potassium, calcium, or chloride that would inhibit the ability of the
formation to absorb water. These systems use native waters; they do not use chemical thinners to
affect the solids remaining in the system, or inhibitive ions to prevent the solids from swelling.
Prepared by: Tan Nguyen
Drilling Engineering – Fall 2012
WBMs – Nondispersed-noninhibited fluids
Spud Muds
Spud Muds are used during drilling to:
1. clean the hole;
2. prevent sloughing of the surface hole;
3. provide a viscous sweep to clean gravel/sand from the borehole; and
4. form a filter cake to prevent seepage to the formation.
Formulation:
Water: (Fresh, brackish, salt)
Caustic: 8.5 to 10.5 pH in fresh-water muds 10.5 to 11.5 pH in salt-water muds
Clay: 10 to 35 lb/bbl, depending on mud weight Fresh water-Sodium bentonite. Salt waterAttapulgite or prehydrated bentonite
Prepared by: Tan Nguyen
Drilling Engineering – Fall 2012
WBMs – Nondispersed-noninhibited fluids
Polymer/Bentonite Muds
Polymer/bentonite systems are used primarily in areas where the formations to be drilled contain low
reactive solids. The systems can tolerate low concentrations of calcium. Water containing calcium in
excess of 100 mg/L (ppm) should be pretreated with bicarbonate of soda to precipitate the calcium.
Formulation:
Water (Fresh, salty, light calcium)
Sodium bentonite:
10 lb/bbl
Polymer:
CMC (low viscosity):
PAC (low viscosity):
0.5 to 1.5 lb/bbl
0.5 to 1.5 lb/bbl
Corn or potato starch: 2.0 to 4.0 lb/bbl
Prepared by: Tan Nguyen
Drilling Engineering – Fall 2012
WBMs – Nondispersed-noninhibited fluids
Extended Bentonite Muds
Extended bentonite systems contain chemicals that extend the yield of bentonite and impart the
desired properties to the mud while maintaining minimum solids content, which in turn improves
penetration rates.
Formulation:
Water: (Treat out calcium with soda ash)
Bentonite: 10 to 15 lb/bbl
Polymers:
Polyacrylate: .04 percent by volume ( if the system is weighted, more is required)
Polyacrylamide: 0.5 to 3.0 lb/bbl
Depending on the application, there are many other chemicals that can be used to impart viscosity
and filtration control, such as polyanionic cellulose, xanthum gum, and potato or corn starch.
Prepared by: Tan Nguyen
Drilling Engineering – Fall 2012
WBMs – Nondispersed-inhibited fluids
Nondispersed - inhibited fluids
The systems described below are classified as Nondispersed-Inhibited because prehydrated sodium
bentonite finds its own equilibrium. Chemical dispersants (thinners) are not added to the systems.
Included in these systems are certain muds containing salt ions (NaCl and KCl) that inhibit drilled
formation solids from swelling and breaking into smaller particles as they are transported to the
surface. This makes it easier for the solids-control equipment to remove these particles.
Prepared by: Tan Nguyen
Drilling Engineering – Fall 2012
WBMs – Nondispersed-inhibited fluids
Attapulgite-Starch-Salt Muds
Salt muds are used to improve borehole stability through the inhibiting effects of the salt(s) present
in the makeup water, to minimize hole washout, and to prevent drilled solids from disintegrating as
they are transported to the surface.
Formulation:
1.Seawater or natural brine
2.Caustic soda: pH 9.0, by meter
3.Attapulgite: 10 to 20 lb/bbl
4.Potato or corn starch: 0.5 to 5.0 lb/bbl
5.Polymer: 0.25 to 1.25 lb/bbl (Polyanionic cellulose, CMC, xanthum gum, guar gum)
Attapulgite does not contribute to filtration control; instead, polymers and/or starches must be used
for this purpose.
Prepared by: Tan Nguyen
Drilling Engineering – Fall 2012
WBMs – Nondispersed-inhibited fluids
Saturated Salt Muds
Saturated salt muds are used to prevent solution cavities from occurring in salt domes and stringers
when they are penetrated by the bit, and to minimize hole washout in salt or carbonate beds.
Formulation:
Saturated salt water: 189,500 mg/L NaCl
Attapulgite: 10 to 25 lb/bbl
Potato or corn starch: 0.5 to 2.5 lb/bbl
Polymer: 0.25 to 1.5 lb/bbl (Polyanionic cellulose, xanthum gum)
Attapulgite should be used when filtration control is not required. When drilling other types of
formations, sodium bentonite pre-mix (sodium bentonite prehydrated in fresh water) can be added to
achieve a quality filter cake. To prevent the salt from dehydrating the sodium bentonite clay, a small
amount of lignosulfonate can be added to the pre-mix solution prior to adding it to the mud system.
However, this converts the system to a dispersed system.
Prepared by: Tan Nguyen
Drilling Engineering – Fall 2012
WBMs – Nondispersed-inhibited fluids
Potassium Chloride-Polymer Muds
Potassium chloride (KCl)-polymer muds inhibit clay swelling in thin, moderately active clay
formations. A low percentage of K+ inhibits the swelling and disintegration of drilled solids,
minimizes hole enlargement, and promotes borehole stability.
Formulation:
KCl water (5 to 15% K+ ion): l7.5 to 52.5 lb/bbl
Caustic soda: Low pH (8.5)
Prehydrated bentonite or attapulgite: 10 to 15 lb/bbl starch
Polymer: 0.5 to 5.0 lb/bbl (Potato or corn starch, polyanionic cellulose, xanthum gum, guar gum)
Prepared by: Tan Nguyen
Drilling Engineering – Fall 2012
WBMs –dispersed-Noninhibited fluids
Lignite-Lignosulfonate Muds
In dispersed-noninhibited systems, chemical thinners are added to encapsulate the sodium
bentonite and reactive drilled solids. The systems do not contain inhibitive electrolytes; therefore, the
cuttings are free to disperse as they are transported to the surface.
Lignite-lignosulfonate muds are probably the most versatile exploratory drilling fluids in use today.
Their rheological properties are easily controlled with chemical thinners, and this reduces the risk of
detrimental effects of contaminants, such as salt, anhydrite, and cement, that may be encountered
during drilling. Chemical thinners and filtration-control agents are used to control the high
temperature/high-pressure fluid loss.
Prepared by: Tan Nguyen
Drilling Engineering – Fall 2012
WBMs –dispersed-Noninhibited fluids
Lignite-Lignosulfonate Muds
Formulation:
1.Water: Normally fresh
2.Caustic soda: pH 9.5 to 10,5
3.Sodium bentonite: 10 to 25 lb/bbl
4.Lignite: 3 to 10 lb/bbl
5.Lignosulfonate:
5 to 10 lb/bbl (A yield point of 10 to 16 lb/l00 sq. ft is recommended)
6.Starches or polymers: 0.25 to 2.0 lb/bbl (Potato or corn starch, CMC, PAC)
In general, the lignosulfonate system is very stable, but it shows severe thermal degradation at
temperatures of 350� F and above. One common approach to this problem is the gradual decrease
in the use of lignosulfonate as formation temperatures approach 350� F and conversion to a lignitesurfactant system.
Prepared by: Tan Nguyen
Drilling Engineering – Fall 2012
WBMs –dispersed-Noninhibited fluids
Phosphate-Bentonite Muds
Phosphate-treated muds with mud weights less than 12 lb/gal are used to drill shallow wells in which
bottomhole temperatures will not exceed 150�F. The phosphates normally used in these systems
have specific limitations; therefore, the system should not be exposed to chlorides in excess of 5000
mg/L or calcium in excess of 100 mg/L. The calcium can be controlled with soda ash or bicarbonate
of soda.
Formulation:
1.Water
2. SAPP: 0.5 to 3.0 lb/bbl (per addition)
3.Sodium bentonite: 20 lb/bbl
4.CMC: 0.5 to 3.0 lb/bbl for fluid-loss control
Flocculation may occur at temperatures above 150� F due to phosphate reversion; therefore, the
system should be used for shallow-hole drilling only.
Prepared by: Tan Nguyen
Drilling Engineering – Fall 2012
WBMs –Dispersed-inhibited Fluids
Dispersed-inhibited Fluids
Dispersed-inhibited systems contain chemical dispersants to disperse clays and drilled solids, along
with inhibiting ions to prevent the hydration and the dispersion of formation materials. The fluids do
not contribute to the hydration and dispersion of formation clays, and the cuttings are held together
for removal at the surface. Drilled solids have a minimal effect on rheological properties in inhibited
systems due to the presence of inhibiting electrolytes (Ca2+, K+, CaSO4, surfactants). These
electrolytes suppress the ability of clays to subdivide into numerous interacting particles, making it
easy to maintain the fluid's rheological properties with low treatment concentrations.
Prepared by: Tan Nguyen
Drilling Engineering – Fall 2012
WBMs –Dispersed-inhibited Fluids
Lime Muds
In general, inhibited systems have lower viscosities and low gel strengths. These fluids are used
principally in the drilling of shales or clay formations.
Formulation:
1.Water (Fresh or salty)
2.Sodium bentonite: Not to exceed 15 lb/bbl
3.Caustic soda: 11.5 pH
4.Lignite: 2 to 6 lb/bbl (used at breakover and not in highly weighted systems)
5.Lime: 2 to 3 lb/bbl excess in high-density fluids; 6 to 8 lb/bbl excess in low-density fluids
6. Lignosulfonate: 1 to 10 lb/bbl
7.Starch or polymers: 0.75 to 3.0 lb/bbl
Prepared by: Tan Nguyen
(Potato or corn starch, polyanionic cellulose, CMC)
Drilling Engineering – Fall 2012
WBMs –Dispersed-inhibited Fluids
Lime Muds
Lime muds perform well up to bottomhole temperatures of 250� F, at which point the fluid loss
becomes difficult to control. This leads to dehydration of the system, and solidification can occur. In
most cases the calcium-inhibited system is made from the native mud used to drill the surface hole.
Downhole temperatures aid in converting the system to an inhibited (calcium-bentonite) system. This
procedure is called a breakover. Normally, there is a short period of time during the breakover when
the viscosity may become very high. This is the hump, which is caused by the clay flocculating and
converting to a calcium clay.
Prepared by: Tan Nguyen
Drilling Engineering – Fall 2012
WBMs –Dispersed-inhibited Fluids
Gyp-Lignosulfonate Muds
Gyp muds function as inhibited systems at lower alkalinities than lime muds, and they contain more
soluble calcium (700 mg/L) than lime muds. Therefore, gyp muds are more inhibitive. Gyp muds are
also generally more tolerant of contamination than lime muds, and they have a slightly higher
temperature stability (2750 F)
Formulation:
1.Water: (Fresh or salty)
2.Sodium bentonite: 15 lb/bbl maximum
3.Caustic soda: 9.5 pH
4.Lignosulfonate: 6 lb/bbl
5.Gypsum:
2 to 6 lb/bbl excess
6.Starch or polymer:
Prepared by: Tan Nguyen
As required
(Polyanionic cellulose-LV, CMC, potato starch)
Drilling Engineering – Fall 2012
WBMs –Dispersed-inhibited Fluids
Seawater-Prehydrated Bentonite Muds
The primary advantage of a seawater-prehydrated bentonite system is the easy availability of it’s
source water. Another advantage is that the moderate amount of soluble salts in seawater inhibits
the hydration and dispersion of clays.
Formulation:
1.Seawater
2.MBT: 12 to 18 lb/bbl
3.Prehydrated sodium bentonite to raise MBT
Caustic soda: 0.5 lb/bbl
4.Sodium bentonite: 30 to 50 lb/bbl
5.Lignosulfonate: 4 lb/bbl
6.Caustic soda: 10.5 to 11.5 pH
7.Lignite, starch or polymer: 0.25 to 6 lb/bbl
8.Defoamer
9.Aluminum stearate and nontoxic oil
Prepared by: Tan Nguyen
Drilling Engineering – Fall 2012
Oil Based System
Oil Based Drilling Fluids
The inclusion of water in oil muds is beneficial for the following reasons:
1.Economy: Water is cheaper than oil; the substitution of water for oil usually reduces mud costs.
2.Viscosity and Gelation: Because water acts as a solid in invert emulsions, it helps to increase mud
viscosity. Moreover, the presence of water helps disperse the organophilic clays that are routinely
used to provide gelation properties.
3.Filtration Control:
Again, because the water droplets act as small suspended solids in these
systems, their presence helps to reduce mud filtration.
4.Stabilization: The inclusion of water in the systems allows us to dissolve salts in the water phase
to aid in stabilizing reactive clays and shales.
5.Safety: The presence of water in an oil mud increases the flash and fire points of the fluid. When
high temperatures on surface are encountered, the water begins to evaporate from the system,
thereby helping to insulate the system from oxygen.
Prepared by: Tan Nguyen
Drilling Engineering – Fall 2012
Oil Based System
Oil Based Drilling Fluids
The most common base oils used have been diesel and kerosene. They have an acceptable
viscosity, low flammability, and a low solvency for any rubber in the drilling system. Diesel, however,
is relatively toxic, making the environmental impact of diesel-base muds generally higher than those
of water-base muds.
Mineral oils have replaced diesel oil and kerosene in environmentally sensitive areas of the world.
Mineral oils contain a much smaller percentage of aromatics than diesel or kerosene, and thus are
less toxic to marine life. There is a wide range in aromatic content in mineral oils marketed today.
Crude oil can be used in oil muds; however, it has some drawbacks. For example, crude oil usually
has a significant fraction of light ends, and thus exhibits low flash and fire points. Crude oil may need
to be weathered prior to use. Also, crudes often contain significant amounts of asphaltenes that may
present problems during drilling or completion operations, and may affect the performance of invert
emulsion additives.
Prepared by: Tan Nguyen
Drilling Engineering – Fall 2012
Oil Based System
Oil Based Drilling Fluids
Water present in an oil mud is in the form of an emulsion. A chemical emulsifier must be added to
prevent the water droplets from coalescing and settling out of the emulsion. The emulsifier also
permits water originally present in the rock destroyed by the bit to emulsify easily. A chemical
wettability reversal agent is added to make the solids in the mud preferentially wet by oil rather than
water. Otherwise, the solids will by absorbed by the water droplets and cause high viscosities and
eventually settling of barite.
The emulsified water of an oil mud tends to increase the viscosity of the mud in the same manner as
inert solids. It also causes a slight increase in fluid density. Since the water is much less expensive
than oil, it also decreases the total cost of an oil mud.
Prepared by: Tan Nguyen
Drilling Engineering – Fall 2012
Oil Based System
Emulsifiers
The calcium or magnesium fatty acid soap frequently is used as an emulsifier for oil muds. Fatty
acids are organic asids present in naturally occurring fats and oils that have a structure:
CH3 – CH2 – (CH2)n –COOH
While the fatty acid soaps are the most common type of emulsifier used in oil muds, almost any type
of oil soluble soap can be used. Calcium naphthenic acid soaps and soaps made from rosin (pipe
tree sap) also are common organic acid type soaps.
Prepared by: Tan Nguyen
Drilling Engineering – Fall 2012
Oil Based System
Wettability Control
When a drop of liquid is placed on the surface of a solid, it may spread to cover the solid surface or it
may remain as a stable drop. The shape that the drop assumes depends upon the strength of the
adhesive forces between molecules of the liquid and solid phases. The wettability of a given solid
surface to a given liquid is defined in terms of the contact angle q. A liquid that exhibits a small
contact angle has a strong wetting tendency. If q
nonwetting
Prepared by: Tan Nguyen
= 1800, the liquid is said to be completely
Drilling Engineering – Fall 2012
Oil Based System
Wettability Control
Most natural minerals are preferentially wet by water. When water-wet solids are introduced to a
water-in-oil emulsion, the solids tend to agglomerate with the water, causing high viscosities and
settling. To overcome this problem, wettability control agents are added to the oil phase of the mud.
The wetting agents are surfactants similar to the emulsifiers. This effectively changes the solids from
being preferentially wet by water to preferentially wet by oil.
The soaps added to serve as emulsifiers also function to some extent as wetting agents. However,
they usually do not act fast enough to handle a large influx of water-wet solids during fast drilling or
mud weighting operations.
Prepared by: Tan Nguyen