Synthetic base fluids are low molecular weight poly-α-olefins (PAOs) with a high viscosity index (Willey et al., 2007). Preferred blends typically combine a high viscosity and a low viscosity component. Preferably, the high viscosity index PAO is the high viscosity component and the low viscosity component is an alkylated naphthalene.

So-called unconventional base stocks can be used as additional base fluids. These fluids are hydroprocessed, highly refined paraffinic base stocks, containing extremely low amounts of aromatic compounds, and sulfur and nitrogen levels relative to conventional hydroprocessed and solvent refined base stocks. They exhibit a high resistance to oxidation and thermal degradation, very high viscosity indices, superior viscosity and film strength at high temperatures, substantially reduced volatility, and improved lubricity (Willey et al., 2007).

Dibasic acid esters also exhibit good thermal stability, but are usually used in combination with additives to enhance the resistance to hydrolysis and oxidation. Polyol esters contain two or more alcohol moieties, such as trimethylolpropane, neopentylglycol, and pentaerythritol esters. They are the reaction product of a fatty acid derived from either animal or plant sources and a synthetic polyol and have excellent thermal stability, and generally resist hydrolysis and oxidation better than other base stocks.

Naturally occurring triglycerides or vegetable oils are in the same chemical family as polyol esters. However, polyol esters tend to be more resistant to oxidation than such oils, and thus tend to function better under severe conditions and high temperatures. The instability normally associated with vegetable oils is generally due to their high content of linoleic and linolenic fatty acids, both of which are unsaturated compounds. As the degree of unsaturation in the fatty acids in vegetable oils increases, the resulting esters tend to be less thermally stable (Willey et al., 2007). Ester-based lubricants have some advantages over oil-based lubricants (Rudnick, 2006, p. 71) in that they:

Extreme pressure agents (EPs) are additives for lubricants that decrease the wear on parts gears that are exposed to very high pressures. They modify the surface of the exposed parts under high pressure conditions by a chemical reaction. EPs include:

There are two types of EP (Willey et al., 2007); firstly compounds that become active at high temperatures, such as lead dithiocarbamate, organosulfur compounds, and organophosphorus sulfur compounds, and secondly inorganic EPs such as molybdenum disulfide, graphite, metal oxides, and powered metals such as copper and lead. The particles of solid EPs act by forming layers between the two bearing surfaces, and so protect them under load by sliding against each other in a way that is similar to cards in a stack sliding against each other. Solid EPs improve the load carrying capacity, but they contribute to excessive seal and hub wear and drill bit seal failure.

Drill bit lubricant compounds containing a copper EP caused seal failures due to copper that was deposited close to the seal area. This accumulated near the seal area until it became abraded by the contact with the copper deposit. Eventually, the grease composition is ejected from the journal area, and a metal-to-metal contact occurs between the roller cone and the journal, which causes a drill bit failure.

On the other hand, lubricants that reduce seal and hub wear do not have a sufficient film strength, i.e., load carrying capacity, to be used as drill bit lubricants. In general, any additives made up from heavy metal complexes exhibit an adverse environmental impact.

Alteratively, zirconium 2-ethylhexanoate or bismuth 2-ethylhexanoate can be added in amounts up to 20% as an additive. These compounds are non-toxic, which gives easier handling in the storage, use, and final deposit of the lubricant.

In addition, zirconium compounds impart some corrosion resistance to metal surfaces, which contributes to reducing the hub wear, and improves the sealing properties. Seals may also wear out more slowly when a zirconium-based lubricant is used.

Zirconium compounds also modify the viscosities of the base stock, which will extend the range of operating temperatures.

Anti-seize compounds are used to mitigate damage from high bearing stresses by providing a dissimilar metal or other material between like substrates. Such a compound inhibits the welding that may otherwise occur from the temperatures, pressures, and stresses normally incurred during proper make-up (Oldiges and Joseph, 2003).

Conventional anti-seize thread compounds include greases, which contain substantial amounts of heavy metals or their oxides, carbonates, or phosphates. The metals include (Oldiges and Joseph, 2003):

However, environmental regulations have begun to discourage or prohibit the use of anti-seize compounds that contain such materials. Organic fluid additives containing antimony, zinc, molybdenum, barium, and phosphorus have also become the subject of environmental scrutiny, partly because of the way that they are used.

Oil field threaded connections are usually coated with an excess amount of the thread compound to ensure complete connection coverage. The excess compound is sloughed off and ends up downhole, where it is then included with the other materials pumped out of the wellhole and into a containment area. From there, materials contaminated with heavy metals must be removed and deposited to a hazardous waste disposal site.

Alternatively, threaded connections can be protected by coating the threads, prior to their make-up, with a solvent-thinned, resin-based coating and bonding composition, composed of a suspending agent, a bonding agent, a thinning agent, and a metallic flake. The coated threads are dried to bond the coating and the bonding composition to the threads. Prior to their make-up, the threads are coated with an excess of an environmentally friendly lubricating composition. A typical formulation is shown in Table 4.1.

Using such a method, the anti-seize metallic film adheres to the thread surface to provide an anti-seize protection while minimizing the amount of metal emitted, since only thread wear discharges metal into the environment. Thus the metal contamination is substantially reduced in comparison to conventional methods.

In summary, the use of anti-seize metallic films, in conjunction with environmentally friendly lubricating compositions reduces the potential for environmental damage, while still providing an optimum protection in critical operations.

Fluorides of alkaline or earth alkaline metals such as CaF₂ have been proposed as anti-seize agents for nickel and chrome ferrous alloys that are prone to galling under high contact stress (Oldiges et al., 2006). Typical grease compositions are formulated as shown in Table 4.2.

Anti-wear additives are divided into two categories (Willey et al., 2007): those activated at a lower temperature than EPs, such as zinc dialkyl dithiophosphate, sorbitan monoleate, chlorinated hydrocarbons, and phosphate esters, and those that become active at lower loads than EPs, PTFE, and antimony trioxide.

Metal deactivators protect against nonferrous corrosion and in some cases also against ferrous corrosion. Common metal deactivators are based on benzotriazole. Ferrous corrosion inhibitors include organic acids and esters, phenolates, and sulfonates (Willey et al., 2007).

Solubility aids make the additives dissolve in the oil or the soap. Common solubility aids include esters, such as trimellitic acid esters (Willey et al., 2007).

Common antioxidants used in grease formulations include zinc dialkyl dithiophosphates, amine phosphates, aromatic amines, phenothiazine, or hindered phenols, such as tert-butylhydroxytoluene.

It is generally preferred not to employ a zinc dialkyl dithiophosphate antioxidant in the lubricating grease if the rock bit comprises an incompatible metal. They are, however, used in other lubricating applications (Willey et al., 2007).

A wide variety of grease compositions are known and often made from refined petroleum or hydrocarbon base stock, which gives them a low viscosity, and provides the base lubricity of the composition. The base stock may constitute about three-quarters of the total grease composition.

This base stock is thickened with a conventional metal soap or a metal complex soap. Light base stocks with low viscosity are used for low temperature greases, and heavier, higher viscosity base stocks are used for high temperature greases (Denton et al., 2007). In order to enhance the film lubricating capacity of base stocks, solid additives such as molybdenum disulfide, copper, lead, or graphite can be added (Newcomb, 1982).

Synthetic polymer EPs and high viscosity synthetic polymers may also be used. Such materials enhance the ability of the lubricant base stock to form a friction-reducing film between the moving metal surfaces under conditions of extreme pressure, and to increase the load carrying capacity of the lubricants.

Molybdenum disulfide is used traditionally in greases for bit lubrication. In addition, polymers of 2-methylpropene (i.e., isobutene) and metal soaps are used to formulate synthetic greases (Denton and Fang, 1996).

A viscosity of 600–750 cP at 120°C is desirable. However, in the severe environment of a rock bit bearing, the viscosity of the composition should be at least 200 cP at 100°C (Delton and Hooper, 1994). Other heavy-duty greases based on molybdenum sulfide also contain calcium fluoride (Landry and Koltermann, 1991a and Landry and Koltermann, 1991b) and metal soaps as thickeners.

Specialized lubricating greases have been developed for the bearing assemblies of roller bits. They are prepared from petroleum oils thickened with alkali and alkaline earth metal soaps (Lyubinin et al., 1995). The greases contain additives and fillers, such as synthetic dichalcogenides of refractory metals, which exhibit the necessary service characteristics. Tests have shown that such greases out perform the initial grease by 7 to 12 times (Table 4.3).

Because of environmental concerns, molybdenum disulfide, regarding alternative compositions for solid lubricants have been developed. These compositions consist of graphite, sodium molybdate, and sodium phosphate (Holinski, 1995; Zaleski et al., 1998), or, more recently, polarized graphite.

Polarized graphite can be used as a lubricant additive for rock bits, since it exhibits extremely good load carrying ability and anti-wear performance.

Graphite consists of carbon in a layered structure, and its lack of polarity prevents graphite powder from forming a lubricant film and adhering to metal surfaces. The polarization of graphite results allows it to adhere to metal and thus form a lubricant film that can carry extremely high loads without failure.

Ordinary graphite has a laminar hexagonal crystal structure and the closed rings of carbon atoms do not normally have any electrical polarization. Hence, graphite has good lubricity because the layers may slip or shear readily, but the lack of polarity leads to a poor adhesion to metal surfaces.

Graphite can be treated with alkali molybdates or tungstenates to impart a polarized layer at its surface. Alternating positive and negative charges are formed. The treated graphite shows an extremely good load carrying capacity and anti-wear performance, somewhat similar to molybdenum disulfide, as well as a good adhesion of particles on metal surfaces and good film-forming properties (Denton and Lockstedt, 2006).

The use of ellipsoidal glass granules instead of spherical glass beads increases the contact surface of antifrictional particles, reduces their ability to penetrate deeply into the mud cake, and increases their breaking strength (Kurochkin et al., 1990; Kurochkin et al., 1992a and Kurochkin et al., 1992b; Kurochkin and Tselovalnikov, 1994).

Drilling muds have been changed significantly over the last few years due to environmental pressures and the increase in drilling operations in more extreme environments, where conventional grease formulations cannot withstand the enhanced demands. For these reasons, new materials have appeared that (Oldiges et al., 2007):

Typical grease thickeners are calcium acetate, lithium stearate, lithium 12-hydroxystearate, anhydrous and hydrous calcium soaps, sodium soaps, organophilic clays, and silica. It is important for a grease to be stable over a range of pHs, since the pH of drilling mud increases as oil well depths increase.

During drilling operations, the threaded connections are exposed to drilling fluids, which include drilling muds and shavings from the drilling operations. These fluids and shavings tend to dissolve, erode, or ablate the grease compounds thus removing their protection and increasing the likelihood of damage to the threaded connections.

Calcium-sulfonate-based grease formulations are a suitable alterative to conventional greases. These high performance sulfonate greases are superior carriers for controlled friction properties in oil field drilling and production thread compounds.

By reducing the thickener content, a cost-competitive calcium sulfonate grease can be formulated. Preferred base stocks include PAOs, polybutenes, polyolesters, vegetable oils, and animal oils (Oldiges et al., 2007). Thixotropic greases or grease-like overbased calcium sulfonate compositions have corrosion inhibiting properties (Olson et al., 1994).

When a drill bit is used in hard, tough formations, high pressures and temperatures are encountered. The total useful life of a drill bit in such severe environments is in the order of 20–200 h for bits of 6–28 in diameter, at depths of about 1,500–6000 m. Useful lifetimes of about 65–150 h are typical. When a drill bit wears out or fails as a borehole is being drilled, it is necessary to withdraw the drill string to replace the bit, which is a very expensive process. Prolonging the lives of drill bits minimizes the lost time in round tripping the drill string for replacing them (Denton et al., 2007).

The replacement of a drill bit can be required for a number of reasons, including wearing out or breakage of the structure contacting the rock formation. The journal bearings on which the roller cones are mounted may fail or wear severely. These bearings are lubricated with special formulations so that they may survive the severe conditions. A lubrication failure can sometimes be attributed to misfit of bearings or seal failure, as well as a problem with the grease itself (Denton et al., 2007).

Purified paraffins are non-toxic and biodegradable (Halliday and Clapper, 1998) and may be used as lubricants, rate of penetration enhancers, or spotting fluids for water-based drilling mud (WBM).

Olefin isomers containing 8–30 carbon atoms are suitable, but isomers having fewer than 14 carbon atoms are more toxic, and isomers having more than 18 carbon atoms are more viscous. Therefore olefin isomers having 14–18 carbon atoms are preferred (Halliday and Schwertner, 1997).

In aqueous drilling fluids, phospholipids are effective lubricating agents (Patel et al., 2006). They are naturally occurring compounds, for example, lecithin belongs to the class of phospholipids. An introduction to phospholipid chemistry was given by Hanahan (1997). They also find use as polymers (Nakaya and Li, 1999). The structural units of a phospholipid are shown in Figure 4.1.

Because of their ionic nature, some phospholipids are soluble in water. A preferred compound as lubrication additive for aqueous drilling fluids is cocoamidopropyl propylene glycol diammonium chloride phosphate (Patel et al., 2006). Phosphatides or phospholipids are environmentally safe lubricating additives (Garyan et al., 1998).

2-Ethylhexanol can be epoxidized with 1-hexadecene epoxide. This additive also helps reduce or prevent foaming. By eliminating the need for traditional, oil-based components, the composition is non-toxic to marine life, biodegradable, environmentally acceptable, and capable of being disposed of at the drill site without the need for costly disposal procedures (Alonso-Debolt et al., 1995).

Esters are compounds of interest, as alternatives with better biodegradability. Some are listed in Table 4.5.

The use of esters in water-based systems, particularly under highly alkaline conditions, can lead to considerable difficulties. Ester cleavage can result in the formation of components with a marked tendency to foam, which then introduces unwanted problems into the fluid systems.

Sulfonates of vegetable oils, in particular soya oil sulfonate, are also used as lubricants. Soya oil sulfonate can be used in water- and oil-based systems, but shows significant foaming, especially in water-based fluids, which restricts its usefulness (Müller et al., 2004a).

A lubricating composition that comprises components that can be obtained from by-products of manufacturing processes, so providing a use for them would also be advantageous (Breeden and Meyer, 2005).

A lubricating composition has been proposed that is obtained by reacting glycerol component comprising glycerol, glycerol oligomers, and a fatty acid component. The reaction product is neutralized with potassium hydroxide or ammonium hydroxide. The composition of the glycerol component is shown in Table 4.6.

The fatty acid component can be obtained from vegetable oils, wood pulp processing, animal fats processing, etc.

Catalysts for esterification include sulfuric acid, hydrochloric acid, nitric acid, and p-toluene sulfonic acid, although concentrated sulfuric acid is preferred (Breeden and Meyer, 2005). The composition can also be subjected to chain extension using diacids, such as maleic acid, succinic acid, or glutaric acid (Breeden and Meyer, 2005).

Synthetic and natural polymers suitable for drilling muds are listed in Table 4.9 and Table 4.10, respectively. The structures of morpholine and methylenebisacrylamide are drawn in Figure 4.5.

Polyacrylamides (PAMs) are eventually hydrolyzed over time and temperature, leading to a lack of tolerance toward electrolyte contamination and to rapid degradation. Modifications of PAM structures have been proposed to retain thermal stability at higher temperatures. Monomers such as 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS) or sulfonated styrene/maleic anhydride can be used to prevent acrylamide (AAm) comonomer from hydrolyzing (Audibert and Argillier, 1995).

Starch is a high molecular weight, natural polymer composed of repeating 1,4-α-D-glucopyranosyl units. It is typically a mixture of linear and branched polymers. Amylose is the linear component with a molecular weight of around 200 k Dalton, and amylopectin is the branched component with a molecular weight of around 1 M Dalton (Fanta et al., 2002).

Normal dent cornstarch contains about 25% amylose, and commercial cornstarch varieties are available that range in amylose content from 0% (waxy cornstarch) to about 70% (high amylose cornstarch).

Starch, as isolated in its native state, is insoluble in water at room temperature because of hydrogen bonding between polysaccharide macromolecules and areas of crystallinity within the starch granule. When a solution of starch is heated, granules initially take up water with limited swelling, then, at a definite temperature, typically about 70°C, the granules swell rapidly and irreversibly, and areas of crystallinity within the granule are lost. The temperature at which this occurs is referred to as the gelatinization temperature.

Near this temperature, the amylose component becomes soluble and diffuses out of the granule matrix. As the temperature is increased beyond about 70°C, a greater percentage of the starch becomes soluble. The granules become highly swollen, until, at a temperature of about 90–100°C, a viscous dispersion of starch in water is obtained. However, despite the overall apparent solubility, the starch is only partially soluble in water and usually occurs as highly swollen granules, thus granule fragments that may be easily separated from such a starch solution by centrifuging.

True solutions of starch in water are difficult to prepare using conventional cooking techniques, and require the application of specialized methods, such as autoclaving at elevated temperatures and pressures.

Steam jet cooking is another technique for preparing starch solutions. This is simpler and more economical than autoclaving, and is suitable for continuous processing. Because of these processing advantages, jet cooking has been used for decades to prepare starch solutions for commercial applications. The method involves pumping a water slurry of starch through an orifice located in a heating chamber, i.e., a hydroheater, where the starch slurry contacts a jet of high temperature, high pressure steam (Fanta et al., 2002). The amount of steam is carefully controlled in the process to achieve complete steam condensation. This means that only little or no excess steam passes through the cooker.

In the excess steam jet cooking technique, the steam entering the hydroheater exceeds the amount required to achieve the required cooking temperature and pressure, thus allowing considerable amounts of excess steam to pass through the cooker along with the cooked starch solution. The intense turbulence caused by the passage of this excess steam promotes mechanical shearing and degradation of starch molecules, especially those having the highest molecular weight, and produces starch solutions with a reduced viscosity (Fanta et al., 2002 and Fanta et al., 1999b).

The high degree of turbulence and mechanical shear of the excess steam jet cooking process also converts the water-immiscible lubricant phase to a homogeneous aqueous dispersion of micrometer-sized oleaginous droplets. These unique, aqueous, starch-oil dispersions form the basis for lubricant compositions that are suitable for oil field applications.

An inherent property of starch pastes and solutions is their tendency to form gels on cooling, and this property is commonly referred to as retrogradation. The phenomenon is caused by aggregation of starch molecules through hydrogen bonding and crystallization. The tendency of starch solutions to retrograde and form gels increases with its amylose content, because amylose is a straight chain polymer with little or no branching.

Although retrogradation has also been observed in amylopectin solutions, it is much slower here, and is generally observed only after such solutions have been allowed to stand for prolonged periods of time (Fanta et al., 2002). The starch can be crosslinked with epichlorohydrin or phosphorus oxychloride (Sifferman et al., 2001 and Sifferman et al., 2002).

Starch-oil compositions are prepared by mixing starch, water, and lubricating oil at room temperature, and then passing this mixture through an excess steam jet cooker. Alternatively, mixtures of starch and water are precooked in the steamcooker, and, after admixing the lubricating oil, the composition is again guided in a steamcooker.

For lubricant oils, a base olefin, a high molecular weight base olefin, a high molecular weight base olefin with ester, olefin blends with ester, or a viscous, liquid polybutene are used (Fanta et al., 2002). The resulting jet cooked compositions are stable with respect to separation and coagulation of oil droplets and are comprised of microscopic droplets of oil, about 1–10 μ in diameter, which are uniformly distributed in the starch-water phase (Fanta et al., 2002).

No emulsifying agents, dispersing agents, or surface active agents are used in the process. If the oil content is held within the preferred range of 20–40 phr, jet cooked compositions can be easily dried by drum drying. Outwardly dry, flake like products are obtained that can be easily reduced in size by milling. No separation of the oil from the dried starch matrix is observed.

These compositions can be easily dispersed in water to form smooth, stable, lump-free dispersions. Water dispersions do not phase separate into their oil and aqueous components on prolonged standing because of a thin layer or shell of starch that spontaneously forms around each oil droplet during the jet cooking process (Fanta et al., 1999a).

Laboratory tests indicated that starch lubricant compositions lower both API and high temperature/high pressure fluid loss values. Results are represented in Table 4.11. The coefficients of friction are up to 45% lower than those of the untreated base muds; similar to those of OBMs. Only 0.5% of starch lubricant need be added to get satisfactory results (Sifferman et al., 2003).

This group of nitrogen-containing additives comprises phenolic Mannich bases, phosphoric acid (Umutbaev et al., 1993), and oxalkylated alkyl phenols with nitrogen containing additives (Koshelev et al., 1993). Vinyl amide monomers for synthetic muds are shown in Figure 4.6.

Extensive laboratory work has been carried out to determine the performance of a number of lubricants including tests to determine the potential for formation damage of several types of drilling fluids, as well as the reduction in the friction coefficient.

Certain polymer additives are also effective as lubrications as a side effect, but in many cases, additional lubricants must be added for the fluid to be successful in drilling to the total intended depth (Knox and Jiang, 2005).

Lubricants for water-based drilling are primarily chosen for their technical performance and environmental acceptability. Hydrocarbons and fatty acids were used mostly in the past, but nowadays a trend to more environmentally acceptable alternatives can be seen, in particular to esters and naturally occurring vegetable oils.

These chemicals are highly lubricating materials as they significantly reduce the coefficients of friction of both metal/metal and metal/rock contacts in water-based fluid environments, by up to 70%. Clearly, effective additives exhibit a high degree of surface activity. This property improves their adhesion to the metal casing or the drilling mud solids. On the other hand, this surface activity makes the lubricants more prone to reacting with other components of the mud.

Lubricants may act as an emulsifier in the presence of even small quantities of oil. Such a composition may turn into an invert emulsion, with the consistency of cottage cheese (Knox and Jiang, 2005). Of course, such events are highly undesirable as the formation of a highly viscous material is definitely a drilling hazard, and the production zone may be damaged.

Apart from this, the lubricant may react with divalent or multivalent ions, forming the ionic bonds as found in ionomers. This reaction results in the formation of a grease-like precipitate, which may formed at concentrations of calcium or magnesium ions as low as 1000 ppm, depending on the chemical nature of the lubricant. Such ionic concentrations are frequently observed even in fresh water. All these issues must be taken into account in the selection of suitable lubricants for water-based drilling fluids (Knox and Jiang, 2005).

Silicate-based muds are notorious for their high coefficient of friction against rock or metal in comparison to oil or synthetic-based muds. However, the latter cause environmental concerns, and in certain locations it is not allowed to drill using lower friction muds. Thus, silicate-based muds are preferred for environmental reasons (Albrecht et al., 2008).

Synthetic muds are also more expensive than silicate-based muds. So it is desirable to lower the coefficients of friction of these muds in order to increase the drilling rates.

Suitable lubricants can be selected from glycosides, which may be functionalized. The amount of lubricant is typically in the range of 1–15%. Specific examples of glycosides are listed in Table 4.12 and the structure of glucopyranosides is shown in Figure 4.7.

Alkylated polyglucosides act as surfactants in microemulsions (Ryan and Kaler, 2001). Microemulsions are thermodynamically stable, isotropic mixtures containing water, oil, and surfactant. They are utilized in a variety of industrial applications besides oil field applications, e.g., in solvent delivery, and polymerization techniques (Hill et al., 1997; Kjellin and Johansson, 2010; Ryan and Kaler, 2001).

A study of the effect of various additives on pipe sticking is available (Pandey and Joshi, 1995), which investigates the effect of various available oil field additives in reducing downhole friction and their optimal concentration. As previously mentioned, the frictional forces present at the string-borehole interface are of prime importance. The friction at the string-borehole interface can be reduced through various chemicals incorporated in the drilling fluid system.

To obtain a mud cake in which sensitivity of various chemicals could be studied, a highly sticky cake was prepared from mud containing gypsum, kaolinite, sand, and shale powder. The ability of various mud additives to minimize friction at the string borehole interface, and thereby reduce the sticking tendency, was evaluated systematically with time. The study was extended to several mud systems.

Various additives have been proposed to assist in freeing a stuck drill pipe, the most common of which is diesel oil, added directly to the drilling mud as a spotting fluid. However, this is not always successful.

An additive comprising an oil-in-water microemulsion has been proposed. Sodium dodecyl benzene sulfonate may be used as a surfactant. Ethylene glycol or diethylene glycol act as cosurfactants (Davies et al., 1997).