CO2 Sequestration

[Hydrodynamic Trapping; Capillary Trapping; Imbibition; Convective mixing; Convective Fingering; Critical Capillary Number]

1. In CO2 sequestration, since, both imbibition (residual state) as well as drainage (initial state) could be engineered (unlike an oil reservoir, where, only imbibition can be engineered), whether, the maximization of capillary trapping of scCO2 – would require the analysis on both drainage as well as imbibition processes @ field-scale?

If so, then, do we have to manage the flow rates and volumes of both CO2-injection (drainage) and its associated brine-imbibition in order deduce a favorable capillary trapping?

2. How long does it take for (a) Hydrodynamic trapping (the upward migration of scCO2 plume being prevented from escaping through the top low-permeability seal) to be dominant; and (b) Capillary trapping (where, the buoyant scCO2 plume migrates not only upwards, but also, laterally, while brine displaces scCO2 @ its trailing edge, leading to trapping of disconnected scCO2 blobs in pores) to be dominant – following the injection of super-critical CO2 into saline aquifers?

3. When exactly free-phase CO2 would start diffusing and dissolving into the underlying resident-brine, following the accumulation of the injected CO2 under cap-rock/seal (by buoyant forces)?

4. How exactly to measure the enhancement in the density of brine @ CO2-brine boundary (as a function of reservoir pressure/temperature/salinity) – following the dissolution of CO2 - @ field-scale?

5. When could we expect the commencement of ‘convective mixing’ (associated with gravitational instability) resulting from the density difference between CO2-rich brine @ 2-phase boundary and the underlying CO2-free brine?

Feasible to capture ‘convective fingering’ (formation and development of a pattern of dissolved CO2 concentration resembling fingers) - @ field-scale?

6. While estimating Capillary number, which defines the balance between viscous-forces (product of viscosity and velocity of CO2-phase) and capillary-forces (IFT between CO2 & brine) with respect to CO2 (as opposed to brine), whether, the concept of ‘critical Capillary number’ (as a function of imbibition process; and which reverses the amount of trapped CO2-phase) would remain to be sensitive?

7. How exactly to delineate the over-lapping of hydrodynamic trapping and capillary trapping with that of

(a) Dissolution trapping (dissolution of scCO2 into brine across scCO2-brine interface)?

(b) Mineral trapping (chemical reactions between rock-minerals and dissolved CO2, leading to the conversion of dissolved CO2 into solid-phase)?

(c) Convection trapping (downward convective mixing of dissolved CO2 associated with the enhancement in brine density)?

Or

(a), (b) & (c) have completely different time-scales, which no more overlap with hydrodynamic & capillary trapping?

Or

only (c) would remain to dominate, once the buoyant scCO2 plume reaches the top seal (leaving aside the mineral dissolution, which remains associated, only with, very large time levels)?

Suresh Kumar Govindarajan

https://home.iitm.ac.in/gskumar/

https://iitm.irins.org/profile/61643