Groundwater chemistry and rock properties can change dramatically following CO 2 injection in a geologic sequestration system. A favored target for subsurface sequestration is clastic reservoirs, due to their limited tendency to impact water quality or porosity and permeability due to dissolution or precipitation compared to carbonate reservoirs. However, most clastic reservoirs will exhibit geochemical changes, especially during the injection phase and over the long term. And, in most oil reservoirs targeted for enhanced recovery and concomitant CO 2 storage, water-alternating-gas, or so-called "WAG" injection schemes are preferred to maximize CO 2 mobility and minimize viscous fingering of CO 2 . Under WAG schemes, reactive transport processes and resulting water quality changes and rock property changes may differ when compared to continuous CO 2 injection (CCI) schemes.The purpose of this paper is to analyze and quantify the extent of geochemical changes to both water chemistry and rock properties, specifically for the "low hanging fruit" of CO 2 storage targets: a sandstone formation using a WAG injection scheme.Specifically, the objectives of this study are: (1) to evaluate the evolution of formation water chemistry and mineral alteration induced by WAG injection in a typical southwestern U.S. sandstone reservoir; (2) to quantify CO 2 trapping mechanisms and associated porosity and permeability evolution over the long term following injection; (3) to investigate whether different injection schemes (WAG vs. CCI designs) may affect the evolution of water chemistry and mineral alteration during the injection phase. Because it is not just a candidate formation, but rather is already undergoing CO 2 injection for enhanced oil recovery (EOR) and sequestration, the Morrow Sandstone Formation in the