In addition to the most common mud systems discussed previously, there are other muds that do not fall neatly into one category or another in the classification scheme.
Low-Density Fluids and Gaseous Drilling Mud (Air-Gas Drilling Fluids)
The basic gaseous drilling fluids and their characteristics are presented in Table 1.
| Type of Mud | Density, ppg | PH | Temp. Limit ◦F | Application Characteristics |
| Air/gas | 0 | – | 500 | High-energy system. Fastest drilling rate in dry, hard formations. Limited by water influx and hole size. |
| Mist | 0.13-0.8 | 7-11 | 300 | High-energy system. Fast penetration rates. Can handle water intrusions. Stabilizes unstable holes (mud misting). |
| Foam | 0.4-0.8 | 4-10 | 400 | Very-low-energy system. Good penetration rates. Excellent cleaning ability regardless of hole size. Tolerates large water influx. |
This system involves injecting air or gas downhole at the rates sufficient to attain an annular velocity of 2,000 to 3,000 ft/min. Hard formations that are relatively free from water are most desirable for drilling with air-gas drilling fluids. Small quantities of water usually can be dried up or sealed off by various techniques.
Air-gas drilling usually increases the drilling rate by three or four times over that when drilling with mud, as well as one-half to one-fourth the number of bits is required. In some areas, drilling with air is the only solution; these are (1) severe lost circulation, (2) sensitive producing formation that can be blocked by drilling fluid (skin effect), and (3) hard formations near the surface that require the use of an air hammer to drill.
There are two major limitations with using air as a drilling fluid: large volumes of free water and size of the hole. Large water flows generally necessitate converting to another type of drilling fluid (mist or foam). The size of the hole determines a volume of air required for a good cleaning. Lift ability of air depends on annular velocity entirely (no viscosity or gel strength). Therefore, large holes require an enormous volume of air, which is not economical.
Mist Drilling Fluids
Misting involves the injection of air and mud or water and foam-making material. In the case of “water mist,” only enough water and foam is injected into the air stream to clear the hole of produced fluids and cuttings. This unthickened water can cause problems due to
the wetting of the exposed formation, which can result in sloughing and caving of water-sensitive shale into the wellbore. Mud misting, on the other hand, coats the walls of the hole with a thin film and has a stabilizing effect on water-sensitive formations. Amud slurry that has proved adequate for most purposes consists of 10 ppb of bentonite, 1 ppb of soda ash, and less than 0.5 ppb of a foam-stabilizing polymer such as high-viscosity CMC. If additional foam stability is needed, additional foamer is used.
Non-dispersed (Low-Solids) Muds
The term low-solids mud covers a wide variety of mud types, including clear water (fresh, salt, or brine), oil-in-water emulsions, and polymer or biopolymer fluids (muds with polymer
and no other additives).
Extended Bentonite Muds
Low-solids non-dispersed mud is generally prepared from freshwater with little or no drilled solids and bentonite, along with a dual-action polymer for extending the bentonite and flocculating drilled solids. This type of mud is designed for low-solids content and to have low viscosity at the bit for high drilling rates. The polymers used greatly increases the viscosity contributed by the bentonite and serve as flocculants for native clay solids, making them easier to remove by solids-control equipment. These polymers or bentonite extenders permit the desired viscosity to be maintained with about half of the amount
of bentonite normally required.
No de-flocculant is used, so a flocculated system is maintained. The flocculation and lower solids content permit the mud to have a relatively low viscosity at the bit and at the bottom of the hole, where shear rates are high, and relatively high viscosity at the lower shear rates in the annulus for good hole cleaning. One problem with this type of fluid is that filtration rates are fairly high because the solids are flocculated and their quantity is low. This means that they do not pack tightly in the filter cake. Sodium polyacrylates or small amounts of CMC may be added for filtration control.
The temperature limitation of extended bentonite fluids is 200–275F◦. Other benefits include improved hydraulics and less wear on bits and pump parts.
Inhibitive Salt/Polymer Muds
An inhibitive mud is one that does not appreciably alter a formation once it has been cut by the bit. The term covers a large number of mud systems, among them saltwater muds with
more than 10,000 mg/L of sodium chloride, calcium-treated muds (lime and gyp), and surfactant-treated muds.
Under the category of inhibiting salt/polymer muds, however, we are speaking specifically about muds containing inhibitive salts such as KCl, NaCl, or diammonium phosphate along with complex, high-molecular-weight polymers. In these muds, prehydrated bentonite and polymer are added for viscosity and gel strength, polyanionic cellulose (PAC) or CMC are added for fluid loss control, and corrosion inhibitors and oxygen scavengers often are used to protect tubular goods.
These muds are used for drilling and protecting water-sensitive formations and are good for minimizing formation damage due to filtrate invasion when the formation contains hydratable clay solids. Good hole cleaning and shear thinning are characteristics of these fluids. High-solids concentrations cannot be tolerated, however, making good solids control very important.
Temperature limitations of 200–250◦F are also characteristic. Among the muds of this type is KCl/lime mud. This mud system uses pre-hydrated bentonite or KCl for inhibition, lignosulfonate and/or lignite as a thinner, KOH (caustic potash) or caustic soda for alkalinity, lime for alkalinity and inhibition, and polymers such as CMC or PAC for filtration control.
Surfactant Muds
Surfactant muds were developed primarily to replace calcium-treated muds when the high temperature becomes a problem. The term surfactant means surface-acting agent, or a material that is capable of acting on the surface of a material. In drilling muds, surfactants are additives that function by altering the surface properties of the liquid and solid phases of the mud or by imparting certain wetting characteristics to the mud.
The composition of the surfactant mud system tends to retard hydration or dispersion of formation clays and shales. The pH of these muds is kept from 8.5 to 10.0 to give a more stable mud at higher temperatures.
The surfactant mud usually encountered is a lignite surfactant mud system. This mud is made up of freshwater using bentonite, lignite, and the surfactant. Small amounts of defoamer may be required with the addition of the lignite. The pH of this mud is maintained within closely fixed limited (8.5 – 10.0) for maximum solubility of the thinner (lignite). Tolerance to salt, gyp, and cement contamination is limited.
To retain satisfactory flow properties at high temperatures, the clay content of the mud must be kept low (1–1.6 CEC capacity) through the use of dilution and solids-control equipment. The combination of lignite with surfactant in this mud enables its use at extremely high bottom-hole temperatures.
This is due to the temperature stability of lignite and the effect of the surfactant in providing viscosity control and minimizing gel strength development at higher temperatures.
High-Temperature Polymer Muds
The development of a high-temperature polymer system evolved from a need for a mud system with low solids and nondispersive performance at higher temperatures. System capabilities:
- Good high-temperature stability
- Good contaminant tolerance
- Can formulate temperature stable non-dispersed polymer mud system
- Can be used in a wide variety of systems for good shale stability
- Minimum dispersion of cuttings and clays
- The flexibility of general application
Application of the high-temperature polymer system primarily consists of five products: (1) polymeric de-flocculant, (2) acrylamide copolymer, (3) bentonite, (4) caustic soda or potassium hydroxide, and (5) oxygen scavenger. Barite, calcium carbonate, or hematite is then used as a weighting agent.
The polymeric de-flocculant is a low-molecular-weight, modified polyacrylate de-flocculant used to reduce rheological properties of the system. If differs from lignosulfonates in that it does not require caustic soda or an alkaline environment to perform. Limited amounts of the polymer may be used in low-mud-weight systems, but larger additions will be needed at higher mud weights and when adding barite to increase the fluid density.
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