CO 2-wettability of sandstones is a key variable which determines structural and residual trapping capacities and strongly influences multi-phase fluid dynamics in the rock. An increasing number of researchers has now estimated this wettability by conducting contact angle measurements on quartz, however, there is a large uncertainty associated with the reported data. We demonstrate clearly that the main factor which leads to this broad data spread is due to surface contamination. It is clear that typically inappropriate cleaning methods were used which resulted in artificially high contact angle measurements. We used surface cleaning methods typically prescribed in the surface chemistry community and found that the water contact angle θ on a clean quartz substrate is low, 0-30°, and that θ increases with pressure. We conclude that quartz is strongly water-wet at high pressure conditions.

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