The primary drainage experiments started from an equilibrated brine saturated Berea sandstone core (60.8 cm long, 7.1 cm diameter). CO2 and brine (both are equilibrated) were co-injected into the core at five water fractional flow steps, which were fw=0.5, fw=0.1, fw=0.01, fw=0.001 and fw=0. For each step, 8 1-cm thick slice of saturation images are shown at 4 cm, 12 cm, 20 cm, 28 cm, 37 cm, 44 cm, 52 cm and 60 cm from the inlet of the core. All saturation images are after 7 by 7 neighborhood mean filtering. In the file name, after the first underscore, it is the water fractional flow value, for instance, fw=0.5 is d5; after the second underscore, it is the distance of this slice to the core inlet, unit is cm.