Lubricant additives are added to drilling fluids to lower the drag and torque between the drill strings and rock formation. They impart lubricating properties into two moving surfaces contacts under extreme temperature and pressure conditions. Currently, there is a significant interest in developing biolubricants derived from organic sources. Biolubricants offer several valuable friction-reducing physicochemical properties, including high lubricity, wide viscosity range, low pour point, high flash point, high thermal stability, and biodegradability when applied in drilling operation. This study reviews the application of general lubricants in drilling fluids and the potential of biolubricants derived from vegetable oils in exceeding the performance of hydrocarbon and mineral-based lubricants. Overall, biolubricants possess most of the physicochemical properties required as a lubricant for drilling fluids. The utilization of organic ester-based biolubricant might result in faster and deeper drilling, lower bioaccumulation and high biodegradability characteristic, less waste volume, and reduced in overall operation cost. Among the available biolubricants, polyol esters of vegetable oils demonstrate the most suitable lubricants for many drilling conditions, even though their application is still very limited.

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This study evaluates the performance of polyol esters as thinners and lubricity enhancers in invert emulsion synthetic-based drilling mud (SBM). Three types of polyol esters, namely pentaerythritol ester (PEE), trimethylolpropane ester (TMPE), and neopentyl glycol ester (NPGE), were prepared at various concentrations of 1, 2, and 3% (v/v) in SBM. The results showed that polyol esters reduced the rheological properties of the drilling mud, such as yield point and gel strength, after the hot rolling test at 135 °C. The rheology data was fitted to the Herschel-Bulkley model to describe the shear thinning behaviour of drilling mud. The model parameters showed that the polyol ester resulted in lower yield stress of SBM, which indicated lower forces are needed to initiate a fluid to move. Their relative effectiveness as a lubricity enhancer was evaluated by measuring the coefficient of friction (COF), wear scar diameter, and scar topography. It was found that polyol esters reduced the COF of SBM by 22% at a concentration of 1% (v/v). PEE provided the best friction reduction and anti-wear performance due to a higher number of the ester group and molecular weight, in comparison with NPGE and TMPE. The lubricity improvement for SBM is recently required by off-shore regulations.

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