Analytical Model for CO2 Injection into Brine Aquifers-Containing Residual CH4

During CO2 injection into brine aquifers-containing residual and/or dissolved CH4, three distinct regions develop: (1) a single-phase, dry-out region around the well-bore filled with pure supercritical CO2; (2) a two-phase, two-component system containing CO2 and brine; and (3) a two-phase, two-component system containing CH4, and brine. This article extends an existing analytical solution, for pressure buildup during CO2 injection into brine aquifers, by incorporating dissolved and/or residual CH4. In this way, the solution additionally accounts for partial miscibility of the CO2-CH4-brine system and the relative permeability hysteresis associated with historic imbibition of brine and current drainage due to CO2 injection and CH4 bank development. Comparison of the analytical solution results with commercial simulator, CMG-GEM, shows excellent agreement among a range of different scenarios. The presence of residual CH4 in a brine aquifer summons two competing phenomena, (1) reduction in relative permeability (phase interference), which increases pressure buildup by reducing total mobility, and (2) increase in bulk compressibility which decreases pressure buildup of the system. If initial CH4 is dissolved (no free CH4), these effects are not as important as they are in the residual gas scenario. Relative permeability hysteresis increased the CH4 bank length (compared to non-hysteretic relative permeability), which led to further reduction in pressure buildup. The nature of relative permeability functions controls whether residual CH4 is beneficial or disadvantageous to CO2 storage capacity and injectivity in a candid brine aquifer.