Analytical Model for CO_2 Injection into Brine Aquifers-Containing Residual CH_4

摘要

During CO_2 injection into brine aquifers-containing residual and/or dissolved CH_4, three distinct regions develop: (1) a single-phase, dry-out region around the well-bore filled with pure supercritical CO_2; (2) a two-phase, two-component system containing CO_2 and brine; and (3) a two-phase, two-component system containing CH_4, and brine. This article extends an existing analytical solution, for pressure buildup during CO_2 injection into brine aquifers, by incorporating dissolved and/or residual CH_4. In this way, the solution additionally accounts for partial miscibility of the CO_2-CH_4-brine system and the relative permeability hysteresis associated with historic imbibition of brine and current drainage due to CO_2 injection and CH_4 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 CH_4 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 CH_4 is dissolved (no free CH_4), these effects are not as important as they are in the residual gas scenario. Relative permeability hysteresis increased the CH_4 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 CH_4 is beneficial or disadvantageous to CO_2 storage capacity and injectivity in a candid brine aquifer.