Effect of Flushing Fluid Composition on Rate of Penetration in Limestone: Surface and Interface Aspects

Rate of penetration (ROP) is considered as a crucial economic and technical indicator in the drilling process. An enhancement in ROP, even if it is small, will result in substantial cost savings. Although the existence of influence of water-based flushing media composition on ROP has been well-known, further understanding about this effect needs to be established, especially in terms of surface and interface aspects. This is mainly due to the complexity of drilling process in chemical fluid environment. In this paper, the effects of several commonly used materials in flushing fluid on ROP were investigated to provide valuable information about chemically enhanced drilling. Using a modified coring machine with a thin walled diamond impregnated coring bit, drilling experiments in limestone were performed in the laboratory. Fluids with different composition including a variety of surfactants and polymers were used as the flushing media, and the corresponding ROPs were observed. Important properties of fluid, rock surface and rock-fluid interface, such as surface tension, indentation hardness, Zeta potential, contact angle, Gibbs free energy changes of wetting and spreading, were measured or calculated to gain an insight into the variation of ROP. The results suggested that ROP was obviously influenced by additive type and concentration. Some surfactants (such as CTAB and BS-12) and polymers (such as polyoxyethylene) have the ability to improve drilling efficiency, and there are optimal concentrations for the enhancement of ROP. On the basis of discussion on these observations, the change of ROP is ascribed to the integrated influence of surface Gibbs free energy change, Zeta potential, indentation hardness and fluid lubricity. These factors, closely related to the fluid composition, act jointly during drilling. This paper is helpful to design fast drilling fluids. Through adjusting chemical composition of the fluid, the interfacial properties of fluid and rock can be controlled to facilitate rock fragmentation, which may be of great significance for engineering practice.