Oil production from shale reservoirs has gained attention particularly since the early 2000’s. Oil recovery factors from multi-stage hydraulically fractured horizontal wells have generally been reported to range between 5 and 10% during primary production. This thesis shows that oil recoveries can be significantly improved by conducting huff ‘n’ puff gas injection in shale reservoirs performing under geologic containment, i.e., reservoirs where hydrocarbon fluids are in an inverted position vis-à-vis gravity within the same layer(s) in a given structure. In this research, huff ‘n’ puff CO2, H2, and flue gas injection are investigated using reservoir simulation leading to the following original contributions applicable to shale petroleum reservoirs: (1) a significant increase in oil recovery, (2) safe storage of CO2 on a permanent basis, (3) safe storage of H2 on a temporary basis, and (4) safe storage of flue gas on a permanent basis. In addition, an original multiple sweet spot detection method is developed using a combination of machine learning (ML) and modified Picket plots. All these findings contribute to a possible future with net-zero emissions. The proposed methodology calls for the following procedure (using CO2 as an example): (1) start huff ’n’ puff CO2 injection after the well goes on oil production for 2 or 3 years; thus, the shale reservoir is essentially undepleted, (2) store CO2 gradually in the shale reservoir during the huff periods, and (3) store CO2 continuously once the huff ’n’ puff project is finalized until reaching the initial reservoir pressure. The simulation model includes a history match period with actual production data from a pilot horizontal well of the Eagle Ford Shale in Texas, and forecast periods with huff ’n’ puff CO2, H2, and flue gas injection. The initial reservoir pressure is never exceeded during the life of the project as a safeguard against possible leaks stemming from the creation of new fractures or reactivation of faults. The inclusion of Life Cycle Assessment in the proposed methodology helps to achieve an outcome that is beneficial for improving economics and externalities and consequently helps to balance important pillars of sustainability: society, the environment and economics.