Channeling and eddy flow behaviors in 3D rough-walled rock fractures enhanced by shear created asperity contacts; Nonlinear relationship between flowrates and pressure gradients in the 3D fractures; Nondeterministic relationship between effective transmissivity and local apertures; Limitations of simplified 2D models in modeling nonlinear flow behaviors in roughwalled fractures.
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.
Fluid flow and solute transport in a 3D rough-walled fracture-matrix system were simulated by directly solving the Navier-Stokes equations for fracture flow and solving the transport equation for the whole domain of fracture and matrix with considering matrix diffusion. The rough-walled fracture-matrix model was built from laser-scanned surface tomography of a real rock sample, by considering realistic features of surfaces roughness and asperity contacts. The numerical modeling results were compared with both analytical solutions based on simplified fracture surface geometry and numerical results by particle tracking based on the Reynolds equation. The aim is to investigate impacts of surface roughness on solute transport in natural fracture-matrix systems and to quantify the uncertainties in application of simplified models. The results show that fracture surface roughness significantly increases heterogeneity of velocity field in the rough-walled fractures, which consequently cause complex transport behavior, especially the dispersive distributions of solute concentration in the fracture and complex concentration profiles in the matrix. Such complex transport behaviors caused by surface roughness are important sources of uncertainty that needs to be considered for modeling of solute transport processes in fractured rocks. The presented direct numerical simulations of fluid flow and solute transport serve as efficient numerical experiments that provide reliable results for the analysis of effective transmissivity as well as effective dispersion coefficient in rough-walled fracture-matrix systems. Such analysis is helpful in model verifications, uncertainty quantifications and design of laboratorial experiments.
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