Types of Base Oils Used
In many areas, diesel is used to formulate and maintain oil muds. In remote areas where diesel may not be plentiful, crude oil can be used instead of diesel. Crude oils should be tested prior to their use, since they may pose safety problems. Mineral oils and the new synthetic fluids are replacements for diesel, especially because of their lower toxicity compared to diesel. However these fluids are usually higher in cost compared to diesel.
There are a wide variety of oils available for use in oil muds. The properties of these oils can have a significant effect upon the physical properties of the oil mud. The properties of the oil which are tested are:
- Flash point - a measure of the volatility of the base oil. The higher the flash point of the oil, the less likely the oil mud will catch fire. The flash point of an oil will change with age as the more volatile components of the oil vaporize into the atmosphere. Addition of water will generally cause the flash point of the finished oil mud to be higher than that of the base oil. The flash point of the oil should be greater than 150°F.
- Aniline point - an indication of the relative aromatic content of the base oil. The aromatic components are particularly detrimental to the rubber parts of the circulation system. The aniline point should be at least 140°F. Certain oil mud products such as the organoclay viscosifiers are affected by the amount of aromatic components in the base oil. As the aromatic
content is decreased, more viscosifier will generally be required or a different viscosifier will have to be used.
- Base oil viscosity - a measurement of the viscosity of the base oil and will vary considerably with the type of oil. Crude oils usually have very high viscosities because of higher asphaltic components, whereas the refined oils such as diesel and mineral oils have considerably lower viscosities. Addition of brine and solids to an oil increases its viscosity substantially, but the viscosity of any mud is generally proportional to the viscosity of the base oil. Lower viscosity oil muds will generally cause an increase in penetration rates.
- Aromatic content - a measure of the quantity of aromatics or benzene-like compounds in the oil. These are the compounds that will affect the toxicity of the base oil with the higher content of aromatics, the more toxic the oil mud will be. Most of the mineral oils now used in oil muds have a aromatic content less than 1% by weight.
The types of base oils used in oil-based muds are described below:
Crude Oils - crude oil can be used in place of diesel as the base oil in areas where diesel may not be available in sufficient quantities to formulate and maintain an oil-based mud system. There are some disadvantages with using crude oil as listed below.
- crude oils have lower flash points and fire points compared to diesel or other refined oils.
- crude oil usually has higher base viscosity compared to diesel and the overall viscosity of the oil mud will be higher than that offered by diesel or other refined oils.
- crude oil needs to be weathered before use since the crude has low flash points because of the light ends contained in crude.
- the aniline point of the crude is usually low which can cause the rubber parts of the circulating system the deteriorate.
- crude oil may contain impurities that may require higher concentration of emulsifiers therefore pilot testing with the crude oil is required to design the proper formulation.
Refined Oils- the refined oils are those such as diesel or kerosene which is the most commonly used oil to formulate and maintain oil-based muds.
- check the aniline point of the diesel to determine if it will be damaging to the rubber parts.
- some diesels may contain additives used to "winterize" the diesel and can have an affect on the emulsifier used to formulate oil muds.
- diesels have higher aromatic content than the mineral oils and these components will have a direct effect on the toxicity of the oil mud.
Mineral Oils - the mineral oils have lower aromatic content than diesel and are considered less toxic than diesel.
- mineral oils have higher flash points than diesel and are safer to use especially in high temperature applications.
- mineral oils have a low viscosity compared to diesel and crude oils which will affect the overall viscosity of the oil-based mud.
- mineral oils have a low aromatic content (<1.0%) compared to diesel, which makes
- them more environmentally acceptable than diesel. Also, the lower aromatic content makes mineral oils more attractive to use and less hazardous to personnel in the event of contact. Low-toxicity mineral oils, like ESCAID110 (Exxon) have aromatic contents of <0.1%.
- mineral oils unlike diesel, do not contain surfactants that could change wettability of the formation.
Synthetic Fluids - The base fluids in synthetic muds are non-petroleum organic compounds that act like petroleum-derived oils in drilling operations but appear to biodegrade readily in the ocean. Like most OBMs, synthetic muds are invert emulsions, with the synthetic fluid serving as the external, or continuous, phase and a brine serving as the internal phase. Several base fluids, mainly in the size range C16-C24, have been introduced in the marketplace during the last few years.
- PETROFREE (Baroid) - an ester made by the reaction of palm kernel fatty acids with a proprietary alcohol. This was the first commercial synthetic fluids. [PETROFREE may be considered a synthetic vegetable oil]
- AQUAMUL (Anchor) - a di-acetal (di-ether) made by condensation of alcohols. [Lower molecular weight diacetals are used as solvents, cosmetics, perfumes and flavoring agents]
- NOVASOL (M-I) - a straight-chain non-aromatic hydrocarbon made by polymerization of ethylene. [Also known as a PAO, for Poly-a-Olefin, this product resembles a highly refined low-toxicity mineral oil not unlike Amoco SUPERLA White Mineral Oil No. 7]. Other synthetic hydrocarbons similar to NOVASOL, like ISOTECH (Chevron) and ULTIDRILL (Dow-ell) have also been introduced recently.
- BIOMUL (Baker-Hughes Inteq) - a detergent alkylate composed of benzene attached to a saturated hydrocarbon chain. [Detergent alkylates, which are used in the manufacture of detergents, are also called linear alkylbenzenes and might be considered hybrids of diesel and low-toxicity mineral oil]
Properties - In the field, synthetic drilling fluids are thought to perform as well as conventional OBM's. Some differences in their properties remain, however, which may be desirable attributes (+) or which may pose some limitations on handling and use (-):
- Synthetic fluids are severalfold more expensive than oils used in conventional OBM's; (-)
- Synthetic muds appear to be more biodegradable and disperse in seawater more readily; (+)
- Synthetic muds are considerably more viscous at ambient temperatures, but they also thin more readily with increasing temperature; (-)
- Synthetic muds are not as thermally stable; (-)
- Synthetic muds do not hydrate or dehydrate shales as rapidly; (-)
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