Invert emulsion oil-based drilling fluids have demonstrated significant superiority over water-based drilling fluids in improving the deep and deviated hole drilling performance. However, the disposal of drilled cuttings coated with mineral oil from offshore rigs has dramatic impact on marine environment. Mineral oil has also been known to cause skin cancer if exposure is prolonged. With this regard, biodegradable invert emulsion ester-based drilling fluids are preferred. However, most of these ester oil-based systems tend to encounter alkaline hydrolysis resulting in solidification of the drilling fluid systems.
This research studied the properties of the ester oilbased drilling fluid systems by using the blended mixture of ester and synthetic mineral oil. The continuous oil bases of the drilling fluid consists of Malaysian palm oil ester derivatives (methyl laureate ester or isopropyl laureate ester) blended with commercially available synthetic mineral oil. Mineral oil is added to the esterbase drilling fluid systems in order to reduce the alkaline hydrolysis problem. However, the mixture of methyl laureate ester with mineral oil was found unstable against alkaline hydrolysis at high temperature. Whereas the isopropyl laureate and mineral oil blended system showed a promising stability towards the hydrolysis process up to temperature of 250 °F. Introduction of mineral oil into the isopropyl laureate ester-based system had seem to destabilise the drilling fluid system by weakening the intermolecular bonding of the ester molecules which resulting in an significant increase of the rheological properties.
Drilling fluids or commonly known as drilling muds are used in the oil wells drilling operations to control the downhole pressure, transport drilled cuttings to the surface, lubricate drill string and bit and etc.
Nowadays, there are three types of drilling fluids, which are based on water, oil and gas. Water-based and oil-based drilling fluids are more commonly used in the drilling operations. In comparison, oil-based drilling fluids have several advantages over water-based drilling fluids, such as increased down hole stability, longer bit life, low reservoir damage factors, reduced torque on deviated wells and excellent stability to high temperatures and in the presence of contaminants. Even though they may be two to three times more expensive than water based drilling fluids, their use is justified by better performances and savings on mud maintenance (Degouy et. al, 1993).
The increased use of oil-based fluids especially from diesel or mineral oil has given rise to concern about their effect on the marine ecosystem. The major problem lies in the disposal of oil-contaminated cuttings into the sea. This has a dramatic effect on seabed life over a distance up to 500 metres from the rig or platform, although little impact is observed beyond 2000 metres. Furthermore, mineral oils have been known to cause certain type of skin cancer if exposure is prolonged (Hodder et. al, 1991).
Application of environmental friendly synthetic oil, which can be biodegraded under aerobic and anaerobic conditions, is considered to be the ideal solution for the problems. Previous research found that some complex esters form stable invert emulsion fluid systems are readily biodegradable under marine environments (Peresich et. al, 1991).