Theoretical study of the synergistic effect between glyceryl monooleate lubricant and carboxymethylcellulose in reducing the coefficient of friction of water-based drilling fluids

Drilling fluids must reduce the coefficient of friction between the drilling equipment and the drilled rock or well casing. Friction forces become particularly relevant in drilling with a high angle gain, in which cases oil-based fluids are generally used. The latter are highly lubricating, but harmful to the environment. For environmental and economic reasons, there is great interest in the development of new additives that enable the use of water-based drilling fluids in all phases of well drilling. Preliminary experimental results show that there is a synergistic effect between glyceryl monooleate (GMO), normally used as lubricant additive, and carboxymethyl cellulose (CMC), a polysaccharide normally used in water-based drilling fluids as a rheological modifier, resulting in extremely low friction coefficients. This work aimed to clarify, through theoretical calculations, the interaction between CMC and GMO, as well as their role in reducing the coefficient of friction between the drilling equipment and the drilled rock when added to water-based fluids.

Calculations based on density functional theory (DFT) were used to predict which, CMC or GMO, preferentially binds to the metal surface. The interactions between the polysaccharide and the surfactant were studied through a combination of classical molecular dynamics and DFT calculations. Finally, dynamic calculations were carried out involving fragments of the polysaccharide, the surfactant and hematite (Fe₂O₃) representing the metal surface, since in the experimental conditions the metal surface will be covered by a primary oxide layer. The results pointed to the preferential binding of CMC to hematite. Regarding the interaction between polymer and surfactant, it was found that the polar part of the GMO interacts with the CMC through hydrogen bonds while the nonpolar carbon chain remains close to the polymer due to hydrophobic interactions. Molecular dynamics calculations showed that GMO increases the binding energy of CMC to hematite and also that this increase in the binding energy is highly influenced by electrostatic interactions.