Numerical Investigation Of Impurity Effect On Carbon Dioxide Geological Storage

Carbon capture and storage (CCS) is one of the major methods to mitigate the effects of greenhouse gas emissions, and CO2 geological storage is recognized as the most efficient way to significantly reduce net carbon emissions in the atmosphere in the short-to-medium term. Among the three kinds of potential geological formations, including deep saline aquifers, oil and gas reservoirs and unminable coal beds, deep saline aquifers are the most attractive ones for their largest storage capacity, wide distribution around the world as well as their relatively short distance from the carbon capture sites.The first step of the sequestration of CO2 in geological formations is to capture CO2 from large industrial emission sources. However, no matter which carbon capture technology is employed, the captured CO2 streams always contain a variety of impurities, such as N2, O2, Ar, SOx, NOx, H2S and H2. It is not technically difficult to provide CO2 streams with high purity. However, high purity requirements, reducing the number of non-CO2 species and/or the level of impurities, would greatly increase the capture cost, while the high capture cost is currently one of the major challenges to large-scale CCS deployment. Geological storage of CO2 with a certain amount of impurities might be a cost-effective way to reduce the total cost of CCS by dramatically lowering the requirements on the capture side. However, there are concerns over negative effects of impurities in CO2 streams on the transport, injection and geological storage of CO2.The research contents of this thesis on the impure CO2 geological storage include the following three aspects:(1) The effects of impurity on the development of the density-driven convective mixing process in the solubility trapping mechanism. (2) The partitioning phenomenon of CO2 and multiple impurities in the saline formations. (3) The effects of impurity on the migration and distribution of the impure CO2 plumes in the layered formations.Both two-and three-dimensional numerical simulations are carried out to evaluate the effects of N2, O2 and SO2 impurities on the density-driven convective mixing process in the solubility trapping mechanism. When dissolved into the formation water, the co-injected N2 or O2 is found to reduce the density increase of the formation water resulting from the dissolution of CO2, while the dissolution of the SO2 impurity would increase the density increase. Furthermore, the effect of SO2 on the density increase is more significant than that of N2 at the same impurity concentration, while the effects of N2 and O2 are similar. The evolution of the dissolution rate of CO2 can be divided into three periods:the diffusion-dominated period, the convection-dominated period and the decay of the convection period. The effect of the N2 or SO2 impurity on CO2 dissolution rate is insignificant during the diffusion-dominated period and the decay of the convection period. During the convection-dominated period, however, it is obvious that the dissolution rate of CO2 decreases with increasing N2 or O2 concentrations or decreasing SO2 concentrations in the injected CO2 streams. With a higher N2 or O2 concentration, the onset of convection is later and the decay time of convection is also delayed. Furthermore, total amount of CO2 dissolved in the formation fluids decreases with increasing concentrations of the N2 or O2 impurity. In contrary to N2, the SO2 impurity is implied to enhance the solubility trapping mechanism and it has more significant effects compared to N2 at the same impurity concentration. Through the comparison of the two-and three-dimensional results, it can be observed that although the two-dimensional model can significantly reduce computational costs, the discrepancies of its results from the three-dimensional results cannot be neglected.The partitioning behaviours of CO2 with two or three kinds of common impurities, i.e., N2, CH4 and H2S, in the formation brine are investigated by numerical simulations. When N2, CH4 or H2S impurities are co-injected with CO2 into the saline formations, the very leading gas front is usually made up of less soluble impurities, such as N2, CH4 or the mixture of N2 and CH4, while more soluble species such as H2S has dissolved preferentially in the formation brine. The results also indicate that the effects of N2, CH4 or the mixture of N2 and CH4 at the same concentrations are generally similar. The effects on the partitioning phenomenon are weaker with decreasing concentrations of N2 and/or CH4 in the injected CO2 streams. The migration distances and the separations between different gas species change linearly with time on the whole, as confirmed by simulation results in a longer model. The broad behaviours identified from the small-size simulations are valid and may be used to predict the long-distance migration and partitioning phenomenon of the gas plume in long-term storage.In terms of the effects of impurity on the migration and distribution of the CO2 plume in the layered formations, different concentrations of the N2 or H2S impurity are considered in the present study. The effects of the magnitude of the capillary force in the sand and shale layers are investigated in the first place. The results show that capillarity does not have much impact on the upward or horizontal migration during the injection period. However, the size of the capillary force plays an important role after the injection has stopped. The existence of the capillary force would hinder the backflow of the formation brine to a certain degree. As a result, the contact area between the CO2 plume and the formation brine is smaller, leading to a decreasing amount of dissolved CO2. Furthermore, total dissolved CO2 decreases with increasing magnitude of the capillary force. With given capillary force, the impacts of N2 or H2S on the migration and distribution of the CO2 plumes in the sand and shale layers are studied. The results indicate that, the horizontal migration distances and the distribution ranges of the supercritical plume increase with increasing N2 concentrations. On the other hand, different concentrations of the H2S impurity do not seem to have much influence. The inclusion of the N2 or H2S impurity would reduce the total amount of injected CO2, while N2 has more significant effects compared to H2S at the same impurity concentration. On the other hand, total amount of dissolved CO2 in the layered sand-shale formations increases with increasing N2 concentrations and decreases with increasing H2S concentrations.