资源环境科技发展态势分析平台

Flow and transport in three-dimensional discrete fracture networks with internal variability in aperture and permeability are investigated using a numerical model. The analysis is conducted for three different texture types representing internal variability considering various correlation lengths and for an increase in domain size corresponding to an increase in network complexity. Internal variability in discrete fracture networks generally increases median travel times and delays arrival of bulk mass transport when compared against reference cases without texture, corresponding to smooth fractures. In particular, internal variability textures with weak connectivity increase travel times nonlinearly with domain size increase, further delaying bulk mass arrival. Textures with strong connectivity can however decrease median travel times, accelerating bulk mass arrival, but only for limited ranges of correlation length and domain size. As domain size increases, travel times of textures with strong connectivity converge toward travel times obtained for classical multivariant Gaussian textures. Thus, accounting for internal fracture variability is potentially significant for improving conservative estimates of bulk mass arrival, flow channeling, and advective and reactive transport in large-scale discrete fracture networks. Further, early mass arrival can arrive significantly earlier for textures with strong connectivity and classical Gaussian textures corresponding to intermediate connectivity but are only slightly affected by textures with weak connectivity. Thus, accounting for internal variability in fractures is also important for accurate estimates of early solute mass arrival. The overall impact on predictive transport modeling will depend on the extent of, or lack of, internal fracture connectivity structure in real-world fractured rocks.

Plain Language Summary This study investigates transport of waterborne substances in subsurface fractured bedrock, a topic which is of relevance to applications such as subsurface disposal of spent nuclear fuel, storage of carbon dioxide, and disposal of other hazardous material. A physically based numerical model for simulating water flow in the fractured bedrock system is used. Many model-based studies assume fractures to be smooth planes, which are an acknowledged simplification; however, real-world fractures are known to have rough surface asperities. In our study, we account for fracture roughness by assuming textures with different connectivity structure and investigate how this impacts waterborne transport in bedrock. We demonstrate that this type of fracture roughness can control important features of flow and waterborne mass transport. Specifically, most of the mass will generally arrive later than expected when compared to a smooth fracture plane assumption. However, we also observe that a small percentage of mass can, under certain circumstances, arrive earlier than what would be expected if smooth fracture planes are assumed. This means that the assumption of smooth fracture planes should generally be considered a conservative simplifying assumption in the context of subsurface storage, but it is less likely to be accurate when considering early mass arrival.