The effect of fluorosurfactant-modified nano-silica on the gas-wetting alteration of sandstone in a CH4-liquid-core system

In gas condensate reservoir, liquid condensation near the wellbore region might kill gas well deliverability, research shows that gas-wetting alteration can be one of effective methods to alleviate liquid condensation in porous media. However, the wettability of cores can only be altered from liquid-wetting to intermediate gas-wetting, making the improvement for gas well deliverability very limited. To achieve super gas-wetting alteration, nano-silica was modified functionally by the fluorosurfactant FG40. To evaluate the influence of gas-wetting alteration on cores, the contact angle measurement, Owens two-liquid method, capillary rise, and imbibition spontaneous tests in the CH4-liquid-core system were conducted, respectively. The results showed that the wettability of the core can be altered to intermediate gaswetting or super gas-wetting after treatment by the FG40, FP-2, and FG40 modified nano-silica. The contact angles of the brine and decane on the core surface increased from 23 degrees and 0 degrees to 152 degrees and 127 degrees, respectively, after the 0.5% FG40 modified nano-silica treatment; these angles are obviously higher than in those cores treated with FG40 and FP-2 solutions at an equivalent concentration. Meanwhile, the surface free energy of the cores sharply decreased from about 70 mN/m to approximately 0.61 mN/m after the FG40 modified nano-silica treatment. The results of the capillary rise and imbibition tests were also consist with that of the contact angle measurement; the liquid levels of brine and decane significantly decreased from 27 and 13.5 mm to -20 and -7 mm, respectively, after treatment by the FG40 modified nano-silica solution. The imbibition of brine and oil in the core also decreased sharply due to the super gas-wetting alteration. Results of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) demonstrated that the botryoidal structure of gas-wetting particles plays a vital role in gas-wetting alteration, which can alter the wettability of the core from strong liquid-wetting to super gas-wetting. (C) 2016 Elsevier Ltd. All rights reserved.