Mechanisms of wetting modification by fluoride to mitigate phase trapping

• Wettability was modified from water wetting to gas wetting by use of a fluoride salt.
• Gas permeability recovery was improved as a result of wettability alteration.
• HPLC confirmed fluoride adsorption onto the rock surface during circulation.
• SEM analysis exhibited a new irregular surface reformed by adsorptions.
• The new surface has a much lower free energy than the original surface.

Wettability alteration has a positive impact on mitigating the damage caused by water trapping. However, the related mechanisms are not quite clear. In this paper, the use of a quaternary ammonium fluoride salt to alter wettability and its potential to mitigate damage caused by aqueous phase trapping in the original water-wet, tight sandstone was investigated. Wettability alteration from water wetting to gas wetting was achieved after the core samples were treated with fluoride. After treatment, the water contact angles were larger than 90°. These contact angles slightly decreased when the temperature rose to 80 °C and 100 °C. The water surface tension decreased from 71.8 mN/m to 20.7 mN/m with the addition of fluoride and later varied over a small range. The contact angles and surface tension tests indicated that the optimal fluoride concentration was 0.1 wt%. The addition of fluoride slightly increased the viscous shear of the drill-in fluid. The flow back rate of the invading liquid with fluoride (85.1%) was almost double that of the liquid without fluoride (45.2%). With the removal of more water, the gas permeability recovery of cores after the circulation of drill-in fluids with fluoride improved by 20%–30% compared with that of drill-in fluids without fluoride.High performance liquid chromatography (HPLC) tests showed that there was hardly fluoride in the filtrate. The results confirmed that fluoride adsorbed onto the rock when the fluid circulated through the sample. This phenomenon was also proved by scanning electron microscopy (SEM) analysis, which showed that uneven molecular aggregation and amounts of adsorptions were more likely to occur at defect points. The adsorption of fluoride onto the rock surface resulted in a new irregular microstructure with a lower surface free energy, which decreased from 72 mJ/m2 to 12 mJ/m2 after fluoride adsorption. The results showed that the modified structure favors water removal.