Oil-base muds are composed of oil as the continuous phase, water as the dispersed phase, emulsifiers, wetting agents, and gellants. Other chemicals are used for mud treatment, such as , filtrate reducers, and weighting agents.
The oil for an oil-based mud can be diesel oil, kerosene, fuel oil, selected crude oil, mineral oil, vegetable esters, linear paraffins, olefins, or blends
of various oils. There are several desired performance requirements for any oil:
- API gravity = 36◦ – 37◦
- Flash point = 180◦F or above
- Fire point = 200◦F or above
- Aniline point = 140◦F or above
Emulsifiers are very important in oil-based mud because water contamination on the drilling rig is very likely and can be detrimental to oil mud. Thinners, on the other hand, are far more important in water-based mud than in oil-based mud; oil is dielectric, so there are no interparticle electric forces to be nullified.
The water phase of oil-base mud can be freshwater or various solutions of calcium chloride (CaCl2), sodium chloride (NaCl), or formates. The concentration and composition of the water phase in oil-base mud determines its ability to solve the hydratable shale problem.
The external phase of oil-base mud is oil and does not allow the water to contact the formation; the shales are thereby prevented from becoming wet with water and dispersing into the mud or caving into the hole.
The stability of an emulsion mud is an important factor that has to be closely monitored while drilling. Poor stability results in coalescence of the dispersed phase and the emulsion will separate into two distinct layers. Presence of any water in the HPHT filtrate is an indication of emulsion instability.
The advantages of drilling with emulsion muds rather than with water-base muds are:
- High penetration rates;
- Reduction in drill pipe torque and drag;
- Less bit balling;
- Reduction in differential sticking.
Oil-base muds are generally expensive and should be used when conditions justify their application. As in any situation, a cost-benefit analysis should be done to ensure that the proper mud system is selected. Oil-based fluids are well suited for the following applications:
- Drilling troublesome shales that swell (hydrate) and disperse (slough);
- Drilling deep, high-temperature holes in which water-base muds solidify;
- Drilling water-soluble formations such as salt, anhydride, camallite, and potash zones;
- Drilling the producing zones.
For additional applications, oil muds can be used:
- As a completion and workover fluid;
- As a spotting fluid to relieve stuck pipe;
- As a packer fluid or a casing pack fluid.
Drilling in younger formations such as “gumbo,” a controlled salinity invert fluid is ideally suited. Gumbo, or plastic, flowing shale encountered in offshore Gulf of Mexico, the Oregon coast, Wyoming, West Africa, Venezuela, theMiddle East, Western Asia, and the Sahara desert, benefits from a properly designed salinity program. Drilling gumbo with water-based mud shale disperses into the mud rapidly, which reduces the drilling rate and causes massive dilution of the mud system to be required. In some cases, the ROP must be controlled to prevent plugging of the flowline with hydrated “gumbo balls.” Solids problems also are encountered with water-based fluid drilling gumbo such as bit balling, collar balling, stuck pipe, and shaker screens plugging.
Properly designed water-phase salinity invert fluidswill pull water from the shale (through osmosis), which hardens the shale and stabilizes it for long-term integrity.
Generally, oil-base mud is to delivered to the rig mixed to the desired specifications. In some cases, the oil-base mud can be mixed on location, but this process can cost expensive rig time. In the latter case, the most important principles are (1) to ensure that ample energy in the form of shear is applied to the fluid and (2) to strictly follow a prescribed order of mixing. The following mixing procedure is usually recommended:
- Pump the required amount of oil into the tank.
- Add the calculated amounts of emulsifiers and wetting agent. Stir, agitate, and shear these components until adequate dispersion is obtained.
- Mix in all of the water or the CaCl2-water solution that has been premixed in the other mud tank. This requires shear energy. Add water slowly through the submerged guns; operation of a gun nozzle at 500 psi is considered satisfactory. After emulsifying all the water into the mud, the system should have a smooth, glossy, and shiny appearance. On close examination, there should be no visible droplets of water.
- Add all the other oil-base mud products specified.
- Add the weighting material last; make sure that there are no water additions while mixing in the weighting material (the barite could become water wet and be removed by the shale shakers).
When using an oil-base mud, certain rig equipment should be provided to control drilled solids in the mud and to reduce the loss of mud at the surfaces:
- Kelly valve—a valve installed between the Kelly and the drill pipe will save about one barrel per connection.
- Mud box—to prevent loss of mud while pulling a wet string on trips and connections; it should have a drain to the bell nipple and flow line.
- Wiper rubber—to keep the surface of the pipe dry and save mud.
Oil-base mud maintenance involves close monitoring of the mud properties, the mud temperature, and the chemical treatment (in which the order of additions must be strictly followed). The following general guidelines should be considered:
- The mud weight of an oil mud can be controlled from 7 lb/gal (aerated) to 22 lb/gal. A mud weight up to 10.5 lb/gal can be achieved with sodium chloride or with calcium chloride. For densities above 10.5 lb/gal, barite, hematite, or ground limestone can be used. Calcium carbonate can be used to weight the mud up to 14 lb/gal; it is used when an acid-soluble solids fraction is desired, such as in drill-in fluids or in completion/workover fluids. Iron carbonatemaybe used to obtain weights up to 19.0 lb/gal when acid solubility is necessary (Table 1).
- Low-gravity solids contents of muds should be kept at less than 6%v/v. Although oil muds are more tolerant solids contamination, care must be taken to ensure that solids loading does not exceed the recommended guidelines. A daily log of solids content enables the engineer to quickly determine a solids level at which the mud system performs properly.
- Water-wet solids is a very serious problem; in severe cases, uncontrollable barite setting may result. If there are any positive signs of water-wet solids, a wetting agent should be added immediately. Tests for water-wet solids should be run daily.
- Temperature stability and emulsion stability depend on the proper alkalinity maintenance and emulsifier concentration. If the concentration of too low, the solubility of the emulsifier changes, and the emulsion its stability. has to be established and controlled by alkalinity testing. The recommended range of lime content for oil-based muds is 0.1 to 4 lb/bbl, depending on base oil being used. Some of the newer ester-base muds have a low tolerance for hydroxyl ions; in this case, lime additions should be closely controlled.
- CaCl2 content should be checked daily to ensure the desired levels of inhibition are maintained.
- The oil-to-water ratio influences funnel viscosity, plastic viscosity, and HTHP filtration of the oil-based mud. Retort analysis is used to detect any change in the oil-water ratio because changes to the oil-water ration can indicate an intrusion of water.
- Electrical Stability is a measure of how well the water is emulsified in the continuous oil phase. Because many factors affect the electrical stability of oil-based muds, the test does not necessarily indicate that particular oil-base mud, the test does not necessarily indicate that a particular oil-base mud is in good or in poor condition. For this reason, values are relative to the system for which they are being recorded. Stability measurement should be made routinely and the values recorded and plotted so that trends may be noted. Any change in electrical stability indicates a change in the system.
- HTHP filtration should exhibit a low filtrate volume (<6 ml). The filtrate should be water free; water in the filtrate indicates a poor emulsion, probably caused by water wetting of solids.
| Mud Weight, ppg | 8–10 | 10–12 | 12–14 | 14–16 | 16–18 |
| Plastic Viscosity, cP | 15–30 | 20–40 | 25–50 | 30–60 | 40–80 |
| Yield Point, lbs/sq ft2 | 5–10 | 6–14 | 7–16 | 10–19 | 12–22 |
| Oil-Water Ratio | 65/35 –75/25 | 75/25 –80/20 | 80/20 –85/15 | 85/15 –88/12 | 88/15 –92/8 |
| Electrical Stability | 200–300 | 300–400 | 400–500 | 500–600 | Above 600 |
