Evaluation of enhancing CO2 sequestration by post-brine injection under different scenarios using the E300 compositional simulator

To accelerate the CO2 trapping in geological storage sites, several injection strategies have been proposed by researchers yet. However, the question remains unanswered as to which one of these injection strategies is the most efficient in terms of immobilizing CO2 and most importantly, how these strategies might be improved. In this paper, we attempt to simulate a typical geological CO2 storage scheme in an aquifer using an E300 compositional reservoir simulator and subsequently investigate and compare the impact of various injection strategies on CO2 immobilizing efficiency. Secondly, the impact of adding a short period of post-brine injection on various strategies is newly investigated.

Our results reveal that using a relatively short period of post-brine injection can significantly improve total CO2 trapping efficiency in all the strategies. In general, by using post injection of brine, more CO2 is spread out through the aquifer and, as a consequence, by increasing the interfacial area of the CO2 plume, the amount of dissolution as a result of mass transfer increases significantly. Moreover, the effect of convection can become stronger in the case of post-brine injection creating a stronger density instability and thus a more rapid initiation of convection. Furthermore, when brine is injected into the system, CO2 is displaced laterally away from the well which results in a forced imbibition process by injected brine and thereby enhancing the capillary trapping efficiency. In particular, the post-injection of brine has other effects in horizontal injection and simultaneous CO2/brine injection using different intervals. In this regard, post-brine injection creates a stronger downward pressure gradient that counters the tendency of the CO2 plume to rise and therefore retards the CO2 in reaching the top of the aquifer and increases the time that the CO2 can be in contact with the fresh formation brine accordingly.

We envisage that the method could enhance the total trapping efficiency of CO2 from 26%, 30.8%, 39.8% and 59.1% to 47.7%, 44.2%, 62% and 63.9%, when post-brine injection was added into different strategies of CO2 continuous injection, simultaneous injection of CO2/brine in the same and different intervals and a horizontal system, respectively. However, our findings show that the effectiveness of post-brine injection may be reduced in high vertical permeability values and in this respect, capillary trapping can be more affected by vertical permeability variation. Furthermore, the results show that selection of the rate and duration of post-brine injection can have considerable effects on total CO2 trapping efficiency.