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
4. When viscosity and gel strength become too low, clay solids are replaced by organic or suspended material - polymers
5. Other advantages
- (a) good for drilling with large pumps and high mud volumes
- (b) always give faster drilling
6. Problems
- (a) excessive dilution a problem
- (b) can become expensive
