Drilling fluid is one of the important components in oil and gas exploration. It serves essential functions including the removal of cuttings; lubricating and cooling the drill bit; sealing permeable formations; suspending solids even after drilling (and consequently fluid flow) has halted, and controlling subsurface pressure to prevent formation damage. Due to rising energy demand, exploration has extended into the delicate marine region. Nevertheless, existing diesel and mineral oil-based hydrocarbon have very poor biodegradability and high toxic leading to the deterioration of benthic marines’ lives through cuttings disposal or spilled.

Ester, hydrocarbon derivatives (poly-alpha-olefin, linear olefin) and ether have been introduced as a substitution to diesel to reduce the environmental impact. Findings show that degradation of base oil from hydrocarbon derivatives are fast in aerobic condition but slow and in some cases are not fully degraded in anaerobic. Nevertheless, esters are fully biodegradable and non-toxic due to the absence of aromatic compounds.

Before selecting an ester as the base oil, some critical criteria such as kinematic viscosity, flash point, pour point, and thermal and hydrolytic stability should be determined. These properties are close-related to the compositions and structures of the ester. The compositions of ester depend on plant origin, climate, geographical factor, and so forth. Interestingly, the esters’ structures can be modified through chemical modification such as transesterification. Transesterification is a process of converting an ester to a new ester by the reaction of an ester with alcohol.

Based on the previous studies, a low kinematic viscosity ester can be synthesized by reacting any feedstock that possesses high content saturated fatty acids such as coconut oil and palm kernel oil with methanol or ethanol to produce coconut or palm kernel oil methyl/ethyl esters. Even though these kinds of esters significantly less viscous than palm oil methyl ester, they still have an issue with pour point. Saturated fatty acids generally have a high pour point. Similar to high viscosity, high pour point is also undesirable for drilling fluid’s base oil because it will significantly change the drilling fluid’s rheology at cold temperature, particularly for the deepwater drilling operation.

To overcome this problem, modification of ester’s structure can be an alternative to decrease its pour point. An ester with unsymmetrical structure has a lower pour point than asymmetrical structure.  Another vital issue is susceptibility to hydrolysis. Thermal and hydrolytic stability of an ester can be improved by using branched alcohol because the presence of branched structure creates a steric effect. Naturally, ester has a superior flash point compared to hydrocarbon-based synthetic base oil due to the presence of the carboxylic group ester linkage that tends to impart polarity, which helps to offset the high volatility attribute. Besides highly biodegradability and eco-toxicity, ester-based drilling fluid provides superior lubricity which supports in deviated and extended reach operations.

Good base oil generally yields a drilling fluid with good rheology and filtration properties. Current commercial ester-based drilling fluid is able to drill up to 300 °F bottom hole temperature. Therefore, there are still having rooms of improvement for the development of ester as a base oil for environmentally friendly and high pressure-high temperature drilling fluid system.

Siti Zulaika Razali
Research Officer
Materials Processing and Technology Laboratory (MPTL)
Institut Teknologi Maju, UPM
zulaika@upm.edu.my

Date of Input: 18/07/2019 | Updated: 01/08/2019 | nursyahirah