Ester-based drilling fluids deliver outstanding performance, even under extreme borehole and formation conditions.

Rising world energy demand has led to exploration for oil and gas in increasingly difficult environments. Exploration is becoming more and more complex, especially when offshore operations are involved. Drilling in deep water is increasing. Extended, deviated wells and borehole instability in challenging geological formations are some of the obstacles encountered. Not only are new technologies constantly being developed, but drilling is now being carried out at extreme conditions - and at considerable time and cost pressure. Accordingly, today's drilling fluids have to satisfy higher and more demanding technical standards. And they must satisfy increasingly stringent requirements in regard to safety and environmental protection. All of these performance requirements are found in drilling fluids that are based on renewable natural resources.
"Ester Quality" (EQ) stands for a new generation of drilling fluids. They are based on vegetable esters derived from natural raw materials like palm kernel oil. The emblem was established as a symbol of the defined and original ester quality standards developed by the specialty chemicals company Cognis. The EQ emblem ensures that drilling fluids comply with the recently revised and far more stringent specifications of the US EPA (Environmental Protection Agency) regulations as laid down in the NPDES-permit.
Deeper, faster, safer: These are the most important requirements for modern drilling. This means that drilling fluids must be cold-resistant even in extreme water depths. Despite the changing exterior conditions, they must also be mechanically stable, withstanding, for example, differences in pressure and temperatures at various depths. Specially designed vegetable ester fluids even remain stable at temperatures above 350°F (177°C). Also, rate of penetration (ROP) is an important economic factor. Modern drilling fluids must help to rapidly discharge the cuttings and dependably ensure borehole stability. Even at high drilling speeds, they must be able to cool and lubricate the drill bit sufficiently.
Benefits of ester-based drilling fluids at a glance
• Faster drilling
• Reduced drilling costs
• Superior lubricity
• Excellent hole cleaning
• Protection of drilled formations
• Compliance with the new US-EPA regulations
• Proven track record
Especially for sensitive marine ecosystems, an additional aspect is constantly gaining in importance. Drilling fluids must not have a negative effect on people or the environment. They should not smell, be non-toxic upon inhalation, or cause skin irritations. Drilling fluids must also be as biodegradable as possible, under both aerobic and anaerobic conditions. Even if substances should pass into the food chain, they must not have a toxic effect on living creatures.
Retrospection
Shale- and clay-rich geological formations remain a constant challenge in drilling due to their instability and water sensitivity. Various drilling fluids have been applied to meet this issue. Oil-based drilling fluids, in use since the late 1960s, were based on diesel and/or mineral oil. These oil-based muds are invert emulsions with the oily phase forming the outer phase, incorporating the inner, brine phase stabilized by emulsifiers and other conditioning additives. Oil-based mud systems perform much better than water-based muds in borehole stabilization and allow faster penetration rates. Additionally, one of their important features is their high lubricity. Lubricity is of great importance when drilling extended and deviated wells, increasingly common in offshore exploration.
A major drawback of oil-based muds is their poor biodegradability, both in aerobic and in anaerobic conditions, as well as their toxic properties. Mineral oils and most petrochemical synthetic fluids can pollute the marine environment if the drilled cuttings are dumped overboard or in cases of accidental spillage because of:
• Low biodegradability;
• High toxicity from the presence of aromatic compounds;
• Tainting of marine life, affecting its edibility by humans or marine species; and
• Severe impact on the seafloor sediments and food chain.
In direct response to the need for a high-performance and environmentally safe alternative to petroleum-based materials, common vegetable and fish oils have been tested. However, none of them offers the necessary chemical stability or rheological properties required. Notwithstanding this, after considerable research and extensive testing, Cognis has formulated a high-performance and biodegradable vegetable ester from palm kernel oil and/or palm oil. This ester, based on vegetable raw materials, has been produced in Malaysia since 1995 and has found widespread acceptance in offshore applications. The biodegradable ester is an alternative to petroleum-based muds. This is because petroleum muds are costly and troublesome to handle and often involve recovering and transporting the drill cuttings to shore for treatment and disposal. Vegetable esters drill cuttings, in contrast, can be safely discharged into the ocean without harming the ecosystem.
What makes vegetable ester fluids so unique?
The unique ecotoxicological properties of esters have dramatically reduced the impact of drilling fluids on the marine environment. Ester-based fluids meet the highest of the US-EPA standards, satisfying the 275 days biodegradation test and Leptocheirus toxicity test. Seafloor surveys in the North Sea, Gulf of Mexico, offshore Australia and Brunei and ongoing toxicity testing with various marine species have confirmed the esters' environmentally benign properties. All seabed surveys have documented short-term seafloor recoveries after ester cutting disposal.
Beside the ecotoxicological advantages, the technical performance also highly favors the ester-based fluids. Thanks to the esters they contain, the drilling fluids demonstrate high reliability - under extreme conditions. Ester-based drilling muds enable high-performance drilling.
Ester fluids can now be formulated in such a way that during drilling operations in very deep and cold water, they remain fully pumpable and lubricate the drill bit efficiently even when well bores are highly deviated. The use of esters gives drilling muds sufficiently low viscosity to ensure the greatest possible equipment safety, when high pressure and extreme temperature fluctuations are involved.
Drilling with ester fluids results in exceptionally well-gauged boreholes. Esters strongly protect the geological formation, preventing the swelling of reactive clay and shale formations. Their excellent lubrication adds to these advantages, providing support in deviated and extended reach operations. The polar ester group and balanced vegetable C-chains are the main factors conferring these properties. Adequately formulated, esters are against hydrolytic decomposition at typical drilling conditions. Ester-based drilling fluids can therefore be tailored to the most demanding needs. They can be used straight and in a broad spectrum of blends at any ratio.
Since their introduction in 1990 in Norwegian waters, vegetable esters have surpassed all expectations. Although the chemical and drilling industry has produced alternatives to esters (mostly based on synthetic petrochemicals), esters have consistently outperformed them by:
• Posing a much smaller risk to human health;
• Having been used for over 10 years without any epidemiological issues; and
• Not tainting marine life, not accumulating and not having any negative effects on algal photosynthesis.
The "green" strategy decisively reduces contamination of the environment by not using non-natural hydrocarbons, thus minimizing or avoiding long lasting contamination and future liabilities from, in particular, wastes on the seafloor.
Commercial issues
The cost of ester fluids is higher than mineral oil or synthetic petroleum based drilling fluids, reflecting the expense of producing them. The higher cost is offset by technologically superior properties, in particular by an increased rate of penetration, fewer drilling problems, reduced liabilities on spillage, lower disposal cost and superior environmental properties. Thus, taking all related costs into account, ester carrier fluids offer a more economical solution. In regards to future liabilities, vegetable ester-based fluids are the least expensive technology in the long run.
What's next?
New, low-viscosity esters and new ester-based drilling fluids have been developed. Continuous improvements in process technology and increasingly strict regulatory rulings are enhancing the use of esters in drilling fluids worldwide. Also, certain blends with vegetable esters offer additional opportunities to satisfy present needs in several offshore regions.

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