The saturation of gas trapped in porous rocks by capillarity depends on many factors. Herein, we focused on the effect of gas saturation at flow reversal on capillary trapping saturation. To investigate gas trapping in various sandstone cores, experiments were carried out under supercritical conditions. Residual gas saturation increased with increasing initial gas saturation. The local residual gas saturation fluctuated with heterogeneity due to the sedimentary structure. To evaluate the effect

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... of the initial gas saturation on the residual gas saturation at the pore scale, experiments were also carried out under room temperature. For a fixed capillary injection flow rate, the initial gas saturation depended on the pore size distribution and heterogeneity due to the sedimentary layers. For vertical Berea sandstone cores, the capillary entrance pressure, associated with the layered structure, caused injected gas to enter in the porous layer of the core. However, for horizontal cores, injected gas flowed through a few layers with high permeability. On the other hand, for Kimachi sandstone cores, injected gas only entered the large pores, whereas for Tako sandstone cores, it entered both large and small pores. Therefore, high initial gas saturation can be achieved. Journal of Fluid Science and Technology may provide vast additional capacity for CO 2 storage. Before the injection of CO 2 , the pore space of the porous rock is filled with water. Because CO 2 displaces water during the CO 2 injection, the saturation of CO 2 after the injection stopped (hereafter, the initial gas saturation) depends on many factors such as the injection flow rate, viscosity ratio of CO 2 and water, buoyancy, and so on. After that CO 2 would move upward due to buoyancy, but some fraction of CO 2 is trapped in porous rocks by capillarity. We define the saturation at this moment as the residual CO 2 saturation. The experimental information on the residual CO 2 saturation as well as the relation between the initial gas saturation and the residual CO 2 saturation is very limited for a supercritical CO 2 and water system. For Berea sandstones, the residual gas saturations are 24.8%-28.2% in a supercritical CO 2 and water system (20) . Krevor et al. (21) observed the relative permeability and residual CO 2 trapping in four sandstone rock samples and showed the relation between the initial CO 2 saturation and the residual gas saturation based on slice-averaged values of X-ray CT images. Akbarabadi and Piri (22)-(23) carried out core flooding experiments at various flow rates in three different sandstone rock samples with an X-ray CT scanner. Pentland et al. (6) measured of the capillary pressure-saturation characteristic curve in addition to observing the residual trapping as a function of the initial CO 2 saturation in a Berea sandstone core. The residual CO 2 saturation of 35 % was observed for the maximum CO 2 saturation of 85 % (6) . Iglauer et al. (24) measured the residual CO 2 saturation of 25 % in a homogeneous Doddington sandstone by using computer microtomography. In addition to that, they analyzed the cluster size distribution of trapped CO 2 bubbles (25) . The residual CO 2 saturation also depends on a number of factors, such as the entry mechanism of the wetting fluid (either by forced or spontaneous imbibitions), the rate of imbibition, the initial CO 2 saturation, and the properties of the porous media (26)-(30) . We focused on the initial gas saturation, which strongly affects residual gas saturation. Recently, the relations between the initial gas saturation and the residual gas saturation were obtained for several sandstones including Berea at supercritical conditions (6), (21)-(23) . In this paper, to clarify the relationship between the initial gas saturation and residual gas saturation in sandstone cores, experiments were separately carried out under supercritical conditions and room temperature. To obtain information of gas distribution in the cores, an X-ray computed tomography (CT) scanner was used. For the room-temperature experiments, the direct visualization of pores and trapped gas bubbles was achieved at the pore scale. In natural sandstone cores, both drainage and imbibition processes are influenced by the capillary pressure difference due to the pore size distribution and the heterogeneity of the cores. The effects of these factors on the initial gas saturation and residual gas saturation will be discussed.