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Deep saline aquifers are among the most favorable geological sites for short-and long-term carbon geosequestration. Injection of CO 2 into aquifers causes various hydro-physical, chemical, and geomechanical interactions that affect the injectivity of wellbores. Despite the extensive research conducted on carbon capture and storage (CCS), there exists a lack of focus on the concept of injectivity. The present study aims to identify the gaps by reviewing the major factors contributing to CO 2 injectivity in deep saline aquifers. Moreover, the existing analytical and numerical mathematical models to estimate maximum sustainable injection pressure and pressure build-up are critically reviewed. Concerning the analytical models, the main controversies are related to the general shape of the CO 2 −brine interface that stems from neglecting or assuming CO 2 compressibility, mutual solubility of CO 2 and water, and drying-out effects. Besides, the models predicting waterflooding processes cannot accurately evaluate the injectivity of CO 2 due to the unique features of CO 2 . Furthermore, most models have concentrated on CO 2 storage capacity at the reservoir scale and not on wellbore injectivity. Thermo-poro-elastic effects influencing the in situ stresses and constraining the maximum sustainable injection pressures are also neglected in some cases. Despite the numerous numerical modeling workflows or tools, some mechanisms have not yet gained the desired attention, especially the chemical aspects of CO 2 -rich brine. Finally, while the field data demonstrate the applicability of CO 2 geosequestration in saline aquifers, regular wellbore injectivity monitoring and surveillance is deemed necessary.

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The paper investigates the system of PDEs for two-phase n-component flow in porous media consisting of hyperbolic terms for advective transport, parabolic terms of dissipative effects and relaxation nonequilibrium equations. We found that for several dissipative and non-equilibrium systems, using the streamfunction as a free variable instead of time separates the general (n+1)×(n+1) system into an n×n auxiliary system and one scalar lifting equation. In numerous cases, where the auxiliary system allows for exact solution, the general flow problem is reduced to numerical or semi-analytical solution of one lifting equation.

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