Study On Solute Transport Characteristics Of Three-Dimensional Crossed Fractures Considering The Influence Of Roughness

Quantitative description of fluid flow and solute transport properties of crossed fractures is the fundamental issue for understanding the characteristics of fracture networks.At present,there are few studies about the characteristics of seepage and mass transfer in three-dimensional crossed fractures.In order to simulate the fluid flow and solute transport through three-dimensional crossed fractures,this research firstly obtained the surface morphological data of a natural rock fracture by three-dimensional profilometer,and then three-dimensional crossed fractures model with certain roughness is generated by three-dimensional reconstruction technology.The NavierStokes equations can be solved by assuming the solute transport follows the Fick's law.Then we can simulate the transportation of fluids and solutes in three-dimensional crossed fractures.By comparing the simulation results of the rough-walled model with the parallel-plate model,it was found that the surface roughness has a significant effect on fluid distribution and flow state.Fluid flow and solute transport states in crossed fractures under different geometric shapes also indicate that the geometrical morphology of crossed fractures can significantly affect the solute mixing behavior.These results revealed that the widely used parallel-plate model will lead to remarkable errors in the assessment of solute transport behavior of fractured rock masses especially at intersections.In the future,it is necessary to establish a modified model according to the geometric characteristics of crossed fractures in order to improve the accuracy of the evaluation.The main conclusions are as follows:(1)In the rough fracture model,the heterogeneity of geometric morphology at the intersection and the difference of flow capacity of each branch lead to fluid channeling,which promotes the mixing of solutes,and the fluid tends to flow to the dominant channel.The specific channeling location is closely related to the geometric morphology at the intersection.(2)The mixing behavior of solutes is closely related to Pe value.In theory,the mixing ratio measured at the exit decreases from 0.5 to 0 with the increase of Pe value.Because the effect of channeling,the mixing ratio in this study varies greatly as the increase of roughness,which shows that the dominant seepage channel has a controlling effect on the material transport characteristics.(3)When Pe value is small,the solute transport is mainly diffusion,and the solute mixes completely at the intersection.With the increase of Pe value,the mixed mode of intersection gradually changes from the completing mixing mode to the streamline path mode.When Pe value is more than 50,the mixing ratio does not change any more.It can be considered that the solute transport is mainly about convection,which is at least one order of magnitude smaller than the theoretical critical value.It shows that the complex geometry of intersection has a great impact on the solute mixing behavior.(4)The intersection angle of crossed fractures also affects the solute transport process.In this study,the models of 30,45,60,90 degrees are selected.It is found that the mixing ratio at 30 degrees is the largest under the same roughness and Pe value.When the intersection angle increases,the mixing ratio decreases.