CO2 – brine – sandstone wettability evaluation at reservoir conditions via Nuclear Magnetic Resonance measurements

Abstract

CO2-rock wettability is a key parameter which governs CO2 trapping capacities and containment security in the context of CO2 geo-sequestration schemes. However, significant uncertainties still exist in terms of predicting CO2 rock wettability at true reservoir conditions. This study thus reports on wettability measurements via independent Nuclear Magnetic Resonance (NMR) experiments on sandstone (CO2–brine systems) to quantify Wettability Indices (WI) using the United States Bureau of Mines (USBM) scale. The results show that CO2 (either molecularly dissolved or as a separate supercritical phase) significantly reduced the hydrophilicity of the sandstone from strongly water-wet (WI ≈ 1) to weakly water-wet (WI = 0.26), and associated with that the water-wetness of the rock for the two-phase systems. This was caused by additional protonation of surface silanol groups on the quartz, induced by carbonic acid. Capillary pressure and relative permeability curves and residual CO2 saturation were also measured; these results were compared with literature data, and general consistency was found. NMR T2 distribution measurements also demonstrated preferential water displacement in large pores (r > 1 µm) following scCO2 flooding, while no change was observed for smaller pores (r < 1 µm). These insights add confidence to the assessments of CO2-rock wettability and therefore reduce project risk. This work thus aids in the implementation of large-scale CO2 sequestration.