Investigation on Capillary Force Recovery of Nanoparticle-Coated Shale after Supercritical CO2 Exposure: Implications for CO2 Sequestration

The surface wettability and morphology of shale may both be modified by SiO2 nanofluid (SNF), directly influencing the capillary force and CO2 geo-storage. However, the literature requires more information and studies with respect to nanoparticles' effect on shale surface at reservoir conditions of high pressure and temperature. Here, we investigate the stability of nanoparticle layers formed by SNF on Longmaxi (marine) and Yanchang (continental) shale in ScCO2 and nanoparticles' efficiency of wettability reversal at different nanofluid aging times and concentrations. Besides, low-pressure nitrogen gas adsorption (LP-NA) was performed to evaluate the effect of nanoparticles on shale pore structures after ScCO2 exposure. Results indicate that the nanoparticle structure could be stable in ScCO2 and render the wettability of shale samples to be more hydrophilic, while the quartz-rich Longmaxi shale surface shows a denser nanostructure than the clay-rich Yanchang shale surface does and presents a stronger water wettability. More micropores are transformed into mesopores or macropores for ScCO2- and SNF-treated Longmaxi samples than solely ScCO2 samples, but the pore structure parameters show trivial alteration in Yanchang samples after SNF treatment. The difference in the capillary force recovery between Longmaxi (recovered by 660.5%) and Yanchang (recovered by 500.6%) shale implies that silica nanofluid may be more suitable for Longmaxi formation in the CGS application.