The success of geological carbon storage (GCS) projects is dependent upon the sealing integrity offered by the caprock. The adsorption of carbon dioxide (CO2) onto shale, the most common type of shale, is a key consideration for evaluating its integrity. A review of literature indicates a lack of studies regarding CO2 adsorption onto shale, particularly in the context of deep saline aquifers. Furthermore, investigations on the CO2 solubility in brine, leading to acidic conditions, on the CO2 adsorption onto shale are limited. This study employed the volumetric approach to investigate the adsorption of CO2 onto Eagle Ford and Terengganu shales under different storage conditions, particularly acidic conditions. The results indicated that CO2 adsorption capabilities of the Eagle Ford and Terengganu shales varied between 0.183 to 2.63 mmol/g, and 0.103 to 2.08 mmol/g, respectively, when the pressure changed from 250 to 1100 psia at 333 K in non-acidic environment. Moreover, CO2 exhibits a stronger preference for both Eagle Ford and Terengganu shales under acidic conditions, suggesting that enhanced CO2 adsorption in acidic environments is independent of shale mineralogy and aquifer conditions. This study establishes that the performance of shales in retaining CO2 is reduced in the presence of acidic conditions, higher pressure, and elevated brine salinity.