Injection, Flow, and Mixing of CO2 in Porous Media with Residual Gas

Transport in Porous Media (Impact Factor: 1.55). 10/2010; 90(1):201-218. DOI: 10.1007/s11242-010-9645-1

ABSTRACT Geologic structures associated with depleted natural gas reservoirs are desirable targets for geologic carbon sequestration
(GCS) as evidenced by numerous pilot and industrial-scale GCS projects in these environments world-wide. One feature of these
GCS targets that may affect injection is the presence of residual CH4. It is well known that CH4 drastically alters supercritical CO2 density and viscosity. Furthermore, residual gas of any kind affects the relative permeability of the liquid and gas phases,
with relative permeability of the gas phase strongly dependent on the time-history of imbibition or drainage, i.e., dependent
on hysteretic relative permeability. In this study, the effects of residual CH4 on supercritical CO2 injection were investigated by numerical simulation in an idealized one-dimensional system under three scenarios: (1) with
no residual gas; (2) with residual supercritical CO2; and (3) with residual CH4. We further compare results of simulations that use non-hysteretic and hysteretic relative permeability functions. The primary
effect of residual gas is to decrease injectivity by decreasing liquid-phase relative permeability. Secondary effects arise
from injected gas effectively incorporating residual gas and thereby extending the mobile-gas plume relative to cases with
no residual gas. Third-order effects arise from gas mixing and associated compositional effects on density that effectively
create a larger plume per unit mass. Non-hysteretic models of relative permeability can be used to approximate some parts
of the behavior of the system, but fully hysteretic formulations are needed to accurately model the entire system.

KeywordsGeologic carbon sequestration–Depleted gas reservoir–Enhanced gas recovery–Residual gas–Hysteretic relative permeability

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