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

• Super gas-wetting nanoparticle for solving liquid-blocking effect can be prepared.
• The botryoidal structure of gas-wetting particles is vital to gas-wetting alteration.
• The methods for evaluating gas-wetting in CH4-liquid-core system have been studied.
• The surface free energy of cores has been validated by wettability measurements.

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 gas-wetting 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° and 0° to 152° and 127°, 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.

SEM of gas-wetting core surfaces: (a) untreated core, (b) FG40-treated, (c) FP-2-treated, (d) FG40-modified-nano-silica, and (e) FP-2 modified-nano-silica. Fig. (a) shows that almost nothing covers the surface of the untreated core, and the core surface consists of irregular rock particles with sharp edges. Fig. (a) is magnified to 500 nm, where it can be seen clearly that the untreated core with a smooth surface, which can facilitate water or oil spreading on the core surface. In addition, Fig. (a) reveals the reason that liquid easily spreads on the liquid-wetting core, resulting in the contact angle of the liquid on the liquid-wetting surface being less than that of the liquid droplet on the gas-wetting surface. Fig. (b) and (c) shows the surface structure of the core after the gas-wetting alteration by FG40 and FP-2, respectively. As shown in Fig. (b), the core surface is covered by an tight adsorption layer that are mainly aggregated by the fluorosurfactant molecule; Fig. (c) shows that a fluoropolymer adsorption layer-like cave is coated on the core’s surface, which is smoother than the one treated with the fluorosurfactant. Fig. (d) and (e) illustrates that the cores are treated with FG40-modified-nano-silica and FP-2-modified-nano-silica, respectively. The botryoidal gas-wetting nanoparticles can fully absorb on the core surface by van der Waals force, significantly increasing the surface roughness of the core, as can be seen in Fig. (d). Meanwhile, the surface free energy decreases dramatically when the core is treated by FG40-modified-nano-silica. Fig. (e) shows that the botryoidal gas-wetting nanoparticles were distributed in the scattered regions of the core surface, and the structure density of the gas-wetting nanoparticles by FP-2 modified is obviously less than that by FG40 modified. Therefore, a super gas-wetting surface can be achieved by FG40-modified-nano-silica treatment.Figure optionsDownload as PowerPoint slide