OBM oil-based mud requires special chemicals additives to ensure that the emulsion is extremely stable and can withstand conditions of high temperature and contaminants. Oil mud products must be dispersible in the external oil phase. We believe that you might also like this article (Baroid Drilling fluids).
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The major OBM oil-based mud chemicals additives that will be discussed in this article are:
- Primary Emulsifier
- Secondary emulsifiers
- Organophilic lignites
- Organophilic gellants
- Wetting agents
- Polymeric viscosifiers
- Rheological modifiers
- Weighting Agents
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calcium soaps are the primary emulsifier in oil muds. These are made in the mud by the reaction of lime and long-chain (C-16 to C-22) fatty acids. Soap emulsions are very strong emulsifying agents but take some reaction time before the emulsion is actually formed.
Wetting agents prevent solids from becoming water-wet while the emulsion is forming. Emulsifiers surround the water droplets and prevent their coalescence. Invert emulsion drilling fluids are three-phase systems: a continuous phase of oil (diesel, mineral oil, or alternative oil) and two discontinuous phases of solids and emulsified water. Since the oil is the continuous phase, the oil surrounds the water droplets and the solid particles. The water will not dissolve in the oil, but rather remains suspended as small droplets. Putting mechanical energy into the system creates such emulsions.
The energy breaks down the emulsified phase into small droplets. These drops, however, only remain stable for a short time before they begin to coalesce and form larger droplets. Over time the water and oil will separate, with water migrating to the bottom and oil to the top. Stable emulsions, like those found in properly formulated oil-based invert emulsion muds, should not exhibit phase separation. Surfactants (emulsifiers) are used to stabilize these emulsions. Solids, particularly clay solids, also function as emulsifiers for water-in-oil. Muds that are stabilized with emulsifiers and/or clays will be oil continuous but may exhibit settling of solids if the viscosity is too low.
A surfactant is a molecule with two chemical groups having different natures, joined to form a tadpole-like structure. The “tail” is a fatty oil-compatible group containing carbon and hydrogen atoms while the “head” is a polar, water-compatible group. The polar group may be an alcohol, amine, amide, acid, or salt of an acid. The ability to change the combination of oil compatible groups and water compatible groups makes it possible to create a great variety of surfactants. As two water droplets come together they repel each other, rather than colliding and combining to form one larger droplet. The type of surfactant and the amount of mechanical energy used to mix the mud determine the size of the water droplets. With increased agitation, smaller droplet form and a tighter emulsion are produced. Smaller water droplets resulting in a larger surface area and a greater area of oil/water contact.
Secondary emulsifiers Additives For Oil-Based Drilling Mud
The secondary emulsifiers are very powerful oil wetting chemicals. Generally, these products do not form emulsions as well as the primary emulsifiers, but the oil-wet solids before the emulsion are formed. Used to readily emulsify any water intrusions quickly. Typically, these additives are polyamides or imidazolines.
The organophilic lignites are used as high-temperature fluid loss additives. They also will aid in the emulsification of water especially at high temperatures. Lignite is treated with an amine to make it oil dispersible. It controls fluid loss by plugging and can be used at high concentrations without causing excessive viscosities (20-lb/bbl +/-). Oil-based mud Asphaltic fluid loss additives – generally consist of Gilsonite or asphalt derivatives. Gilsonite has high-temperature stability (400°F) whereas asphalt is not as temperature stable (350°F). High concentrations of Gilsonite can cause excessive viscosity and gelation of the mud. The treatment level will not usually exceed 8 lb/bbl.
Organophilic gellants Chemicals For Oil-Based Drilling Mud
Invert emulsions derive limited viscosity from the base oil and submicron-sized emulsified water, but this viscosity will be Newtonian in character. The major non-Newtonian viscosity is derived from organophilic clays (also known as organoclays). These are bentonites in which the inorganic exchangeable cations, such as sodium, calcium, and magnesium, have been displaced by fatty quaternary amines. These viscosity builders are made from bentonite, hectorite, or attapulgite treated with an amine to make them oil dispersible. Bentonite is most commonly used and is compatible with diesel and mineral oils up to 350°F. For temperatures above 350°F, especially in mineral oil formulations, hectorite-based clay should be used. Organophilic attapulgite is used to improve the suspension properties of packer fluids without appreciably increasing the viscosity.
The separation of the clay platelets can be helped by adding chemical species that will adsorb between the inter-laminar sites and begin to weaken the bonding forces between the sheets can help the separation of the clay platelets. These chemicals are called polar activators. They also act on un-coated sites. The polar activator most often used is water. Freshwater is more effective than calcium chloride brines so the order of addition of the salt can be important. This is why adding water with the organophilic clay will enhance the viscosity and dispersion. These materials appear to function by adsorbing onto organophilic clay particles and enhancing interaction among them. Polar activators enable the formation of a network or structure that increases viscosity. The nature of the network is thought to be different than the viscosity-building structure that forms among clay particles in water-based muds. Although the water in itself is an excellent polar activator, its effect on viscosity is diluted somewhat by the emulsifiers that are always present in invert muds.
Aromatics are present naturally in diesel and are responsible for the ease with which organophilic clays (OPC) yield in diesel oil. With the advent of low-toxicity mineral oils as replacements for diesel, yielding of OPC’s was made more difficult. That problem was originally solved by reformulating the OPC’s. More recently, similar results have been obtained by adjusting the emulsifier packages. The new generation of oils, namely the esters and ethers, exhibit viscosities that are several-fold higher than the mineral oils, but they approach the mineral oils insofar as their inability to yield OPC’s.
Wetting agents Additives For Oil-Based Drilling Mud
The viscous properties of the fluid may be derived from other solids such as drilled solids and weighting material. These solids will adsorb the surfactants and will likely be mainly oil-wet. However, if insufficient surfactants are present, the solids can contact each other through a layer of water resulting in high viscosities being generated. Water-wet solids, particularly barite, can give viscosity problems at the solids removal equipment ultimately resulting in the removal of the barite. The problem can be avoided by maintaining the correct level of emulsifier and can possibly be cured by the addition of surfactants called oil-wetting agents. These are supplemental additives to quickly and effectively oil-wet solids that become water-wet. Drill solids and weighting agents will naturally water-wet and the wetting agents will strip off the water and replace it with an oil layer.
The Polymeric viscosifiers additives that increase the viscosity of oil muds in the presence of organophilic bentonite, especially when the organophilic bentonite performance is reduced by high temperatures; they work up to 400°F. High molecular weight sulfonated polystyrene becomes effective only when the temperature exceeds 250°F.
Rheological modifiers Chemicals For Oil-Based Drilling Mud
Generally, oil-based muds behave as pseudoplastic fluids. They thin with increasing shear in a manner similar to clay-containing water-based muds. However, absolute viscosities tend to be considerably lower in oil-based muds. A consequence of this is that oil-based muds generally do not suspend solids, such as weighting materials and drill cuttings, as well as similar viscosity water-based muds. This property is particularly important at low shear rates and is related to the low-shear viscosity and elastic properties of the mud. To enhance viscosity and cut- tings-carrying (hole cleaning) capacity, oil-mud polymers may be added. Care must be taken when adding these products as over-treatment results in extremely high viscosities. Often these products will not fully yield until reaching a certain temperature. It is very difficult to decrease excessive viscosities caused by the addition of these polymers.
Alternative viscosity builders are a class of fatty acid dimers and trimers, which appear to build low-shear viscosity with less effect at high shear rates. These are particularly effective in deviated wells to ensure suspension of both weighting materials and cuttings during periods of little or no circulation. Rheology modifiers of this type typically increase low-shear-rate viscosity values as determined by the 3 and 6 rpm readings on a viscometer. Barite can “sag” (Barite Sag)or slide down the hole, especially on deviated wells; these additives will minimize or eliminate this “sag”. Increases in total mud viscosity are avoided when using these additives.
used to increase the density of the oil mud. The most commonly used weighting agent is barite. A mud weight of around of 21.0 lb/gal is the highest achievable with barite. Hematite, with an s.g. of 4.85 can also be used to increase the density of the oil mud. A mud weight of around 24.0 lb/gal can be achieved with hematite. For the same mud weight, the solids content of the oil mud weighted with hematite will have a lower solids content than weighted with barite because of the higher S.G. of the hematite.