Oil-based muds were developed to prevent water from entering the pore spaces and causing formation damage. There are several advantages and disadvantages of this type of mud system. The advantages include the following:
- Shale inhibition—In highly smectitic or “gumbo” shales, the borehole maintains stability and cuttings samples are generally intact.
- Reduction of torque and drag problems—Since oil is the continuous phase, the borehole and the tubulars are wetted with a lubricating fluid. This is a distinct advantage in deviated wellbores.
- Thermal stability—Oil-based muds have shown stability in wells, with BHTs of 585°F
- Resistance to chemical contamination—Carbonate, evaporite, and salt formations do not adversely affect the properties of an oil mud. CO2 and H2S can easily be removed with the addition of lime (CaCO3).
Disadvantages of oil-based mud systems include the following:
- High initial cost—The oil fraction alone of a barrel of oil mud may cost 40-70 USD per barrel. This is considerably higher than most water-based muds at any weight.
- Slow rates of penetration—Oil muds historically have had lower rates of penetration as compared to water-based muds.
- Pollution control—Most areas where oil muds are used have environmental restrictions. Rig modifications may be necessary to contain possible spills, to clean up oil mud cuttings, and to handle whole mud without dumping.
- Disposal—Oil mud cuttings may have to be cleaned up before dumping. Some regulatory agencies require cuttings be sent to a designated disposal area.
- Kick detection—H2S, CO2 and CH4 are soluble in oil muds. If gas enters the wellbore, it can go into solution under pressure. As the gas moves up the wellbore, it can break out of solution at the bubble point and rapidly evacuate the hole, blowing the mud with it.
- Formation evaluation—Some wireline logs should not be run in oil-based muds. Also, additional steps are needed to remove oil coatings from cuttings before they are described. (For more information on wireline tool compatibility with drilling fluid composition, and for more on removing oil coatings from cuttings.)
Oil-based muds contain three phases: oil, brine, and solids phase.
The oil phase is the continuous phase in which everything else in the system is mixed. The oil can be diesel, mineral oil, or one of the new types of synthetic oils.
The brine phase is present in the system as a high concentration salt solution that is emulsified into the base oil. Usually a solution of calcium chloride is used because it gives a greater flexibility in adjusting the concentration of the salts. This phase is difficult to control because, if the salt concentration nears saturation, the emulsifiers and oil-wetting compounds precipitate.
The solids phase includes the weight material, viscosifiers, and fluid loss reducers. A primary requirement for this phase is that it remain oil wet. Compounds exclusively developed for this purpose are included in the oil mud make-up. If the solid phase ever becomes water wet, the system is said to have “flipped” and the consequences are severe and operationally expensive. The system will separate into two phases: solid and liquid. The solid phase will pack and plug the wellbore, necessitating remedial drilling.