Experimental investigation on molecular-scale mechanism of wettability alteration induced by supercritical carbon dioxide-water-rock reaction

Wettability is a deciding attribute in enhance oil recovery research, thus plays a vital role in unconventional reservoir development. Previous studies were focused on wettability modulation of the reservoir rocks by changing the properties of fluids. However, the wettability alteration mechanism of tight sandstones remains unclear. In this study, the influence of molecular groups on wettability is discussed by measuring the contact angle of samples from the Upper Triassic Yanchang Formation after being subjected to the supercritical carbon dioxide-water-rock dynamic reaction. The mineral composition of the samples under different reaction times was characterized by XRD. Zeta potential measurements and the Fourier-transform infrared spectroscopy (FTIR) technique were combined to determine the type and amount of molecular groups on the rock surface. Results show that the sample becomes more hydrophilic with the reaction time because of the varied composition of rock minerals induced by the chemical reaction. However, it is not easy to discern the influence of different mineral content on wettability only through the XRD experiment. The Supercritical carbon dioxide - water - rock dynamic reaction reduced the absolute value of zeta potential and weakened the effect of the electrostatic force on wettability, yet increased the content of hydrophilic groups (C-O and C--O), and thus reduced the contact angle (altered the wettability towards more water-wet). This study can advance the understanding of the wettability alteration mechanism in tight sandstones and promote the development of carbon dioxide geologic sequestration and enhanced oil recovery technologies by the regulation of wettability.