The porous media with higher cementation degree have wider velocity spreads. The solute transport becomes more anomalous with increasing cementation degree. It is the changing characteristics of the flow field that lead to more anomalous solute transport. The pore structure of porous media varies significantly with the change in the degree of cementation among solid grains, and recent studies have shown that pore structure greatly influences anomalous solute transport. Nevertheless, limited effort has been devoted to explore the change in the degree of anomalous solute transport with increasing cementation. In this study, we performed pore‐scale numerical simulations to investigate the anomalous dispersion in computer generated two‐dimensional porous media with different degrees of cementation. The simulated pore‐scale fluid fields show that porous media have wider spreads of velocities when the cementation degree increases. When the percentage of cemented solid grains increases from 0.0 to 57.8%, the stagnant regions' area extends more than 11 times, and preferential flow region increases by a factor of more than 6. These changing characteristics of the flow field as the cementation degree increases make solute transport more anomalous. Therefore, we should be careful when applying an advection–dispersion equation (ADE) to predict solute transport in highly cemented porous media.

We quantitatively explore and explain the different time behavior of anomalous solute transport in three‐dimensional (3D) porous media with different cementation degrees. We find that the temporal evolution of the breakthrough curves (BTCs), spatial moments and propagators representing the time behavior of the transport varies significantly in each case with rising cementation degree. For example, the early breakthrough and long tail in the BTCs are enhanced simultaneously when the cementation degree increases, implying that the solute transport is anomalous. Correspondingly, instead of both the first and second central moments growing linearly with time as described by the traditional advection‐dispersion equation (ADE), the relationship between the second central moment M2 and time gradually transitions from an approximate linear relationship into a nonlinear relationship. This indicates that the scale effect is enhanced in the porous media with larger cementation degree. In addition, the propagator is characterized by a more remarkable and more persistent stagnant concentration peak when the cementation degree rises. Our simulations also demonstrate that the CTRW model can capture the simulated BTCs and quantitatively predict the temporal evolution of the first two moments of the solute plume. Finally, we explain the mechanism of the different time behavior of solute transport via the characteristics of the flow field. Core Ideas Time behavior of solute transport varies radically with rising cementation degree. CTRW model can characterize the different time behavior of solute transport. The changing characteristics of flow field cause the different time behavior.

Anomalous dispersion of solute in porous media can be explained by the power-law distribution of waiting time of solute particles. In this paper, we simulate the diffusion of nonreactive tracer in dead-end pores to explore the waiting time distributions. The distributions of waiting time in different dead-end pores show similar power-law decline at early time and transit to an exponential decline in the end. The transition time between these two decline modes increases with the lengths of dead-end pores. It is well known that power-law distributions of waiting time may lead to anomalous (non-Fickian) dispersion. Therefore, anomalous dispersion is highly dependent on the sizes of immobile zones. According to the power-law decline, we can directly get the power index from the structure of dead-end pores, which can be used to judge the anomalous degree of solute transport in advance.

In this study, a pore-scale simulation method is applied to quantitatively study the variation of solute dilution through porous media with different cementation degrees and explore the corresponding mechanisms. The study results indicated that the cementation degrees of the solid grains had a significant effect on the solute dilution process and that the influence was very complicated. The complexity was manifested in that the effect of rising cementation degree on the solute dilution process would be different or even completely opposite in the porous media in which the solid grains cement slightly with that in porous media with a higher cementation degree. For example, for the porous media in which the solid grains were slightly cemented (the percentage of the cemented solid grains P c is less than 40%), the dilution effects became enhanced with the increase of cementation degree. Then, after P c increased to about 55%, the dilution effect was obviously weakened, and the solute was in an incomplete dilution state for a long period of time. In addition, this study found that the properties of the flow fields may vary greatly in porous media with different cementation degrees and that those differences in the flow fields resulted in the distinct behavior of the solute dilution. It is interesting to note that a more heterogeneous flow field had not necessarily led to the enhancement of the dilution process.