Wettability alteration to maintain wellbore stability of shale formation using hydrophobic nanoparticles

ABSTRACT Wellbore stability is a complex issue to achieve safe and efficient drilling operations, especially in shale formations using water-based drilling fluids. The objective of this paper is to maintain wellbore stability by wetting reversal of shale with synthesized hydrophobic nanoparticles (HN). The molecular structure and microstructure of HN were studied by Fourier transform infrared spectroscopy (FT-IR) and transmission electron microscopy (TEM), respectively. Experiments were conducted to analyze the particle size, dispersion stability, and thermal stability of HN. The effects of HN on the wettability of the shale surface were investigated by a series of contact angle measurements. The changes of shale microstructure were observed by scanning electron microscopy (SEM). To evaluate the wellbore strengthening performances of HN, spontaneous imbibition measurements and uniaxial compression strength tests were performed. The effect of HN on the base fluid was also evaluated. The results showed that the HN with an average particle size of 217.4 nm was successfully prepared. High temperature slightly promoted the dispersion of HN in water. HN was still relatively stable at 240 ℃. HN was strongly adsorbed on the shale surface to form a hydrophobic layer, which transformed the shale surface from hydrophilic to hydrophobic. Moreover, the ability of HN to cause a wetting reversal of shale was not susceptible to temperature and could be maintained even at ultra-high temperatures. After the HN treatment, the water absorption of shale decreased obviously, and the uniaxial compressive strength increased significantly. In addition, the addition of HN had no significant effect on the rheology and filtration properties of drilling fluids. The HN presents a high application potential as a nano-structured wetting alteration agent to ensure the stability of the wellbore.