## 7 Conclusions  
The main conclusions are summarized below:  
* The CO2 intrusion into the caprock was mainly dominated by diffusion of the dissolved CO2 in the brine. This can become more critical, if this solution accesses active faults and fractures of the shale caprock;
* The impact of mineral precipitation in the caprock and its integrity was negligible, with only an insignificant precipitation of about 0.0005% of the total injected CO2 after 3000 years. Therefore, considering reservoir parameters such as mineralogy, pH, injection rate, pressure, and temperature, there was a small risk for caprock integrity due to mineralization/dissolution;
* Horizontal wells were more effective in controlling the CO2 buoyancy due to two mechanisms: \(\bigcirc\) enhancement of the CO2 entrapment as a residual phase by up to 19% due to relative permeability/capillary pressure hysteresis where the more spread plume contacted new portions of the aquifer pore volume; \(\bigcirc\) the better balance between gravity and viscous forces provided by horizontal wells compared to by vertical wells, with up to an 18% reduction in this balance compared to the vertical well case. The conclusion was based on simulations of homogeneous and heterogeneous sandstone saline aquifers over thousands of years. * A sensitivity test showed the following results: \(\bigcirc\) hypothesis was mainly valid for low- and mid-permeability aquifers (<200 mD); for long-term evaluation, there was no significant impact on the injection rate for the same CO2 mass injected; \(\bigcirc\) the natural flux in aquifers can affect the plume propagation, triggering its contact with the caprock even with horizontal wells; and \(\bigcirc\) these conclusions regarding horizontal wells persisted over tens of thousands of years. * A practical method was proposed to design the optimum length for horizontal wells, combining the maximum CO2 saturation trapped as the residual phase and the gravity number. These results present an exciting opportunity to further enhance CO2 entrapment in CCS projects. The study's findings provide a basis for exploring alternative project options. These alternatives may involve optimizing well geometry, considering uncertainties in trapping properties and exploring different geological realizations. By incorporating well geometry optimization techniques and accounting for uncertainties, it is possible to improve CO2 entrapment and storage efficiency. This approach allows for a more comprehensive assessment of potential benefits and risks associated and opens up promising perspectives for advancing CCS projects. Conceptualization, M.V.B.M., M.D. and K.S. ; methodology, M.V.B.M. ; software, M.V.B.M. ; validation, M.V.B.M., M.D. and K.S. ; formal analysis, M.V.B.M., M.D. and K.S. ; investigation, M.V.B.M. ; resources, M.V.B.M. and M.D. ; data curation, M.V.B.M. and M.D. ; writing--original draft preparation, M.V.B.M. ; writing--review and editing, M.D. and K.S. ; visualization, M.V.B.M. ; supervision, M.D. and K.S. ; project administration, M.D. and K.S. All authors have read and agreed to the published version of the manuscript. This research received no external funding. The data used in this study are provided within the article. However, the models used are not accessible to the public due to PETROBRAS' policy. The authors would like to thank PETROBRAS for authorizing the use of models and [PERSON] for thorough English revision. **Conflicts of Interest:** Author [PERSON] is employed by the company PETROBRAS. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.