Mitigation and control of borehole pressure at the bottom of an injection well is directly related to the effective management of well injectivity during geologic carbon sequestration activity. Researchers have generally accepted the idea that high rates of CO 2 injection into low permeability strata results in increased bottom-hole pressure in a well. However, the results of this study suggested that this is not always the case, due to the occurrence of localized salt precipitation adjacent to the injection well. A series of numerical simulations indicated that in some cases, a low rate of CO 2 injection into high permeability formation induced greater pressure build-up. This occurred because of the different types of salt precipitation pattern controlled by buoyancy-driven CO 2 plume migration. The first type is non-localized salt precipitation, which is characterized by uniform salt precipitation within the dry-out zone. The second type, localized salt precipitation, is characterized by an abnormally high level of salt precipitation at the dry-out front. This localized salt precipitation acts as a barrier that hampers the propagation of both CO 2 and pressure to the far field as well as counter-flowing brine migration toward the injection well. These dynamic processes caused a drastic pressure build-up in the well, which decreased injectivity. By modeling a series of test cases, it was found that low-rate CO 2 injection into high permeability formation was likely to cause localized salt precipitation. Sensitivity studies revealed that brine salinity linearly affected the level of salt precipitation, and that vertical permeability enhanced the buoyancy effect which increased the growth of the salt barrier. The porosity also affected both the level of localized salt precipitation and dry-out zone extension depending on injection rates. High temperature injected CO 2 promoted the vertical movement of the CO 2 plume, which accelerated localized salt precipitation, but at the same time caused a decrease in the density of the injected CO 2 . The combination of these two effects eventually decreased bottomhole pressure. Considering the injectivity degradation, a method is proposed for decreasing the 123 398 K.-Y. Kim et al.pressure build-up and increasing injectivity by assigning a 'skin zone' that represents a local region with a transmissivity different from that of the surrounding aquifer.

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The primary purpose of this study is to understand quantitative characteristics of mobile, residual, and dissolved CO 2 trapping mechanisms within ranges of systematic variations in different geologic and hydrologic parameters. For this purpose, we conducted an extensive suite of numerical simulations to evaluate the sensitivities included in these parameters. We generated two-dimensional numerical models representing subsurface porous media with various permutations of vertical and horizontal permeability (k v and k h ), porosity (φ), maximum residual CO 2 saturation (S max gr ), and brine density (ρ br ). Simulation results indicate that residual CO 2 trapping increases proportionally to k v , k h , S max gr and ρ br but is inversely proportional to φ. In addition, the amount of dissolution-trapped CO 2 increases with k v and k h , but does not vary with φ, and decreases with S max gr and ρ br . Additionally, the distance of buoyancy-driven CO 2 migration increases proportionally to k v and ρ br only and is inversely proportional to k h , φ, and S max gr . These complex behaviors occur because the chosen sensitivity parameters perturb the distances of vertical and horizontal CO 2 plume migration, pore volume size, and fraction of trapped CO 2 in both pores and formation fluids. Finally, in an effort to characterize complex relationships among residual CO 2 trapping and buoyancy-driven CO 2 migration, we quantified three characteristic zones. Zone I, expressing 123 808 W. S. Han et al.the variations of S max gr and k h , represents the optimized conditions for geologic CO 2 sequestration. Zone II, showing the variation of φ, would be preferred for secure CO 2 sequestration since CO 2 has less potential to escape from the target formation. In zone III, both residual CO 2 trapping and buoyancy-driven migration distance increase with k v and ρ br .