Forecasting evolution of formation water chemistry and long-term mineral alteration for GCS in a typical clastic reservoir of the Southwestern United States

Abstract

Groundwater chemistry and rock properties can change dramatically following CO2 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 CO2 storage, water-alternating-gas, or so-called “WAG” injection schemes are preferred to maximize CO2 mobility and minimize viscous fingering of CO2. Under WAG schemes, reactive transport processes and resulting water quality changes and rock property changes may differ when compared to continuous CO2 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 CO2 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 CO2 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 CO2 injection for enhanced oil recovery (EOR) and sequestration, the Morrow Sandstone Formation in the Anadarko Basin of Texas was selected as representative of a typical clastic CCS candidate. A numerical reactive transport model of a 5-spot well pattern in the Morrow Formation was developed and used to simulate WAG injection and subsequent geochemical processes. Initial conditions of flow and geochemistry were based on actual measurements from the Morrow Formation within the Farnsworth EOR field in northern Texas. The simulation design period included WAG injection for 25 years (injection phase) followed by 975 years of post-injection monitoring (arbitrary post-injection phase). Simulation results suggest that formation water chemistry (pH, aqueous species Ca2+, Mg2+, Fe2+, HCO3−) dramatically changes after CO2 arrival, and mineral dissolution (with an increase in porosity and permeability) is greatest near the injection well during the injection phase. The simulated increase in porosity is approximately 2.7%, with a maximum permeability increase of almost 8.4% at the end of the injection phase. The possibility of halite mineral precipitation, a phenomenon observed in many injection scenarios, was specifically examined. However, no simulations yielded halite precipitation. Mineral precipitation increases in the long term (hundreds of years), resulting in both increased CO2 trapping by mineralization as well as decreased porosity and permeability.Finally, the analysis of impacts of WAG injection during the injection phase suggests that the extent of CO2-rock geochemical interactions following WAG increases compared to CCI scenarios. Specifically, the extra water injected (in WAG) facilitates aqueous reactions compared to CCI, which “dries out” the formation. Changes in porosity and permeability for CCI schemes are much less than those for WAG schemes, a factor to consider with respect to how these different schemes may impact injectivity.