Evolution mechanism of surface energy of Niutitang Formation shale in Northern Guizhou under supercritical CO₂-water-rock interactions

Surface energy is a key factor governing the storage states of gaseous and liquid fluids in shale reservoirs. To investigate the effects of supercritical CO₂-water-rock interactions on shale surface energy under reservoir temperature and pressure conditions, this study takes shale collected from the Niutitang Formation in northern Guizhou as the research subject. Utilizing the contact angle measurement method and the van Oss-Chaudhury-Good (vOCG) theory, combined with analytical techniques such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy and energy-dispersive spectroscopy (SEM/EDS), the evolutionary patterns and influencing mechanisms of shale surface energy under supercritical CO₂-water-rock interaction were systematically studied. The results indicate that the contact angles between shale and the non-polar test liquid (diiodomethane) are significantly smaller than those with polar test liquids (glycerol and distilled water), demonstrating that the tested shale samples possess strong non-polar characteristics. After supercritical CO₂-water-rock interaction, the shale surface energy showed a significant increasing trend, with the polar component increasing more markedly than the non-polar component. The change in the non-polar component of the shale surface energy mainly stems from the increased percentage of non-polar minerals such as albite and quartz in the shale, while the change in the polar component is controlled by alterations in the surface electron-donating capacity resulting from changes in functional groups. The increase in shale surface energy after supercritical CO₂-water-rock interaction enhances its adsorption capacity for CO₂, CH₄, and H₂O molecules, which would lead to CO₂ and CH₄ being more likely to exist in adsorbed states within the shale reservoir, and the water-blocking effect caused by residual water molecules in the micro-nano pores of shale would also be intensified. This is beneficial for the long-term stable sequestration of CO₂, but may adversely affect shale gas recovery. The research findings provide support for the efficient extraction of shale gas and CO₂ geological storage in Guizhou.