| Summary: | This study used numerical simulations of CO<sub>2</sub> storage to identify the benefits of horizontal wells for geological carbon storage, such as enhancing CO<sub>2</sub> trapped in porous media due to relative permeability and capillary hysteresis. Two injection schemes were tested: one using a vertical injector and the other employing a horizontal well. The results revealed two main findings. Firstly, the horizontal injection well effectively prevented or minimized CO<sub>2</sub> penetration into the caprock across various sensitivity scenarios and over a thousand years of CO<sub>2</sub> redistribution. Secondly, horizontal wells provided a safe approach to trapping CO<sub>2</sub>, increasing its entrapment as a residual phase by up to 19% within the storage site. This, in turn, reduced or prevented any unexpected events associated with CO<sub>2</sub> leakage through the caprock. Additionally, the paper proposes a practical method for designing the optimal length of a horizontal well. This method considers a combination of two parameters: the additional CO<sub>2</sub> that can be trapped using a horizontal well and the gravity number. In the case of the reservoir model of this study, a horizontal branch with a length of 2000 m was found to be the most effective design in enhancing CO<sub>2</sub> entrapment and reducing CO<sub>2</sub> buoyancy.
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