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A drilling fluid is any fluid which is circulated through a well in order to
remove cuttings from a wellbore. This section will discuss fluids which
have water or oil as their continuous phase. Air, mist and foam, which can be used as drilling fluids, will not be discussed at this time.
A drilling fluid must fulfill many functions in order for a well to be drilled successfully, safely, and economically.
The most important functions are:
1. Remove drilled cuttings from under the bit
2. Carry those cuttings out of the hole
3. Suspend cuttings in the fluid when circulation is stopped
4. Release cuttings when processed by surface equipment
5. Allow cuttings to settle out at the surface
6. Provide enough hydrostatic pressure to balance formation pore
pressures
7. Prevent the bore hole from collapsing or caving in
8. Protect producing formations from damage which could impair
production
9. Clean, cool, and lubricate the drill bit
Occasionally, these functions require the drilling fluid to act in conflicting
ways. It can be seen that items #1-3 are best served if the drilling fluid has
a high viscosity, whereas items #4-5 are best accomplished with a low
viscosity. Items #6 & 8 are often mutually exclusive because drilled solids
will tend to pack into the pore spaces of a producing formation.
Make-up of a Drilling Fluid
In its most basic form a drilling fluid is composed of a liquid (either water
or oil) and some sort of viscosifying agent. If nothing else is added,
whenever the hydrostatic pressure is greater than the formation pore
pressure (and the formation is porous and permeable) a portion of the fluid
will be flushed into the formation. Since excessive filtrate can cause
borehole problems, some sort of filtration control additive is generally
added. In order to provide enough hydrostatic pressure to balance abnormal
pore pressures, the density of the drilling fluid is increased by adding a
weight material (generally barite).
In summary, a drilling fluid consists of:
The Base Liquid
• Water - fresh or saline
• Oil - diesel or crude
• Mineral Oil or other synthetic fluids
Dispersed Solids
• Colloidal particles, which are suspended particles of various
sizes
Dissolved Solids
• Usually salts, and their effects on colloids most is important
All drilling fluids have essentially the same properties, only the magnitude
varies. These properties include density, viscosity, gel strength, filter cake,
water loss, and electrical resistance.
Normal Drilling Fluids
Though this type of drilling fluid is easy to describe, it is hard to define and
even more difficult to find. In the field, a normal fluid generally means
there is little effort expended to control the range of properties. As such, it
is simple to make and control. General rules include:
1. It is used where no unexpected conditions occur
2. The mud will stabilize, so its properties are in the range required
to control hole conditions
3. The chief problem is viscosity control
Formations usually drilled with this type of mud are shales and sands.
Since viscosity is the major problem, the amount and condition of the
colloidal clay is important. To do this, two general types of treatment are
used:
1. Water soluble polyphosphates
(a) they reduce viscosity
(b) can be used alone or with tannins
(c) if filter cake and filtration control is required
- add colloidal clay to system
2. Caustic Soda and Tannins
(a) they also reduce viscosity
(b) used under more severe conditions than phosphate treatment
The upper portions of most wells can use “normal” muds
1. Care must be taken not to add chemicals which may hinder the
making of special muds later on
2. Native clays used to make the mud are usually adequate
Special Drilling Fluids
These drilling fluids are made to combat particular abnormal hole
conditions or to accomplish specific objectives. These are:
1. Special Objectives
(a) faster penetration rates
(b) greater protection to producing zones
2. Abnormal Hole Conditions
(a) long salt sections
(b) high formation pressures
Lime Base Muds
1. Water base mud
2. Treated with large amounts of caustic soda, quebracho, and lime.
Added in that order
3. Ratio of 2 lb caustic soda, 1.5 lb quebracho and 5 lb lime per 1
barrel of mud
4. Will go through a highly viscous stage, but will become stable at
a low viscosity
5. Good points
(a) can tolerate large amounts of contaminating salts
(b) remains fluid when solids content gets high
6. Weakness - it has a tendency to solidify when subjected to high
bottom-hole temperatures
Lime-Treated Muds
1. Similar to lime based mud - differ only in degree
2. A compromise attempt at overcoming the high temperature
gelation problem
(a) use less lime than lime-base mud
(b) not nearly so resistant to salt contamination
Emulsion Muds - Oil in Water
1. Oil can be added to any of the normal or special muds with good
results
2. No special properties necessary
3. Natural or special emulsifying agents hold oil in tight suspension
after mixing
4. Oils used are:
(a) Crude oils
(b) Diesel
(c) any oil with an API gravity between 25 and 50
5. Oil content in mud may be 1% to 40%
6. Advantages are:
(a) very stable properties
(b) easily maintained
(c) low filtration and thin filter cake
(d) faster penetration rates
(e) reduces down-hole friction
7. Major objection is that the oil in the mud may mask any oil from
the formations
Inhibited Muds
1. Muds with inhibited filtrates
2. Large amounts of dissolved salts added to the mud
3. High pH usually necessary for best results
4. Designed to reduce the amount of formation swelling caused by
filtrate - inhibit clay hydration
5. Disadvantages
(a) need specialized electric logs
(b) requires much special attention
(c) low mud weights cannot be maintained without oil
(d) hard to increase viscosity
(e) salt destroys natural filter cake building properties of clays
Gypsum Base Muds
1. A specialized inhibited mud
(a) contained large amounts of calcium sulfate
(b) add 2 lb/bbl gypsum to mud system
(c) filtration controlled by organic colloids
2. Advantages
(a) mud is stable
(b) economical to maintain
(c) filtrate does not hydrate clays
(d) high gel strength
3. Disadvantages
(a) fine abrasives remain in mud
(b) retains gas in mud
Oil Based Muds
1. Oil instead of water used as the dispersant
2. Additives must be oil soluble
3. Generally pre-mixed and taken to the wellsite
4. To increase aniline value, blown asphalt and unslaked lime may
be added
5. Advantages
(a) will not hydrate clays
(b) good lubricating properties
(c) normally higher drill rates
6. Disadvantages
(a) expensive
(b) dirty to work with
(c) requires special electric logs
(d) viscosity varies with temperature
Inverted Emulsions
1. Water in oil emulsion. Oil largest component, then water added.
Order of addition is important
2. Have some of the advantages of oil muds, but cheaper.
Somewhat less stable
Salt Water Muds
1. Can be used either completely or partly saturated
2. Weight can vary up to 10 lb/gal when saturated
3. No filter cake building properties, easily lost to porous
formations
Silicate Muds
1. Composed of sodium silicate and saturated salt water
2. Has a pickling effect on shales which prevents heaving or
sloughing
3. Will be 12 lb/gal or higher
4. Corrosive, expensive and gives poor electric log results
Low Solids Muds
1. Keeps amounts of clays in the mud at a minimum, which
promotes faster and safer drilling
2. Three ways to remove solids from mud
(a) water dilution
(b) centrifuging
(c) circulate through large surface area pits
3. When clays are removed, a minimum of viscosity control
chemicals are needed
Inverted Emulsions
1. Water in oil emulsion. Oil largest component, then water added.
Order of addition is important
2. Have some of the advantages of oil muds, but cheaper.
Somewhat less stable
Salt Water Muds
1. Can be used either completely or partly saturated
2. Weight can vary up to 10 lb/gal when saturated
3. No filter cake building properties, easily lost to porous
formations
Silicate Muds
1. Composed of sodium silicate and saturated salt water
2. Has a pickling effect on shales which prevents heaving or
sloughing
3. Will be 12 lb/gal or higher
4. Corrosive, expensive and gives poor electric log results
Low Solids Muds
1. Keeps amounts of clays in the mud at a minimum, which
promotes faster and safer drilling
2. Three ways to remove solids from mud
(a) water dilution
(b) centrifuging
(c) circulate through large surface area pits
3. When clays are removed, a minimum of viscosity control
chemicals are needed
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