classification of drilling fluid

Drilling Engineering – Fall 2012

Prepared by: Tan Nguyen

Drilling Engineering – PE 311

Chapter 2: Drilling Fluids

Classification of Drilling Fluids



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.

Classification of Drilling Fluids

WBMs



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.

WBMs – Nondispersed-noninhibited fluids

Nondispersed-noninhibited fluids



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 water-

Attapulgite or prehydrated bentonite


Spud 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): 0.5 to 1.5 lb/bbl

PAC (low viscosity): 0.5 to 1.5 lb/bbl

Corn or potato starch: 2.0 to 4.0 lb/bbl


Polymer/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.


Extended Bentonite Muds



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.

WBMs – Nondispersed-inhibited fluids

Nondispersed - inhibited fluids



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.


Attapulgite-Starch-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.


Saturated Salt 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)


Potassium Chloride-Polymer 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.

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 lignite-surfactant system.


Lignite-Lignosulfonate 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.


Phosphate-Bentonite Muds



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.

WBMs –Dispersed-inhibited Fluids

Dispersed-inhibited Fluids



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 (Potato or corn starch, polyanionic cellulose, CMC)


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.


Lime 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: As required (Polyanionic cellulose-LV, CMC, potato starch)


Gyp-Lignosulfonate 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 MBTCaustic 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


Seawater-Prehydrated Bentonite Muds



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.

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.

Oil Based System




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.

Oil Based System




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.

Oil Based System

Emulsifiers



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 . A liquid that exhibits a small

contact angle has a strong wetting tendency. If q = 1800, the liquid is said to be completely

nonwetting

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.

Oil Based System

Wettability Control

classification of drilling fluid

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