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<p>Final radioactive waste disposal in deep crystalline bedrock demands a thorough understanding of flow and transport mechanics in sparsely fractured rock formations. The structural complexity and heterogeneity of crystalline bedrock, and the scarcity of field data for the hydraulic characterization motivates the development of multiple alternative conceptual and numerical models, both to test our understanding and to evaluate prediction uncertainties. Discrete fracture network (DFN) models are widely used in radioactive safety assessment programs in hard crystalline rocks while channel network models offer another representation of flow networks and preferential pathways, in line with indications that flow and transport in deep fractured media are usually dominated by a relatively small number of long preferential pathways. This study applied the channel network modeling approach to understand the hydraulic behavior in a fractured granite system (approximately 450 m deep), at the Äspö Hard Rock Laboratory in Sweden. The channel network model is built from a hydro-structural model of the site including known fracture geometries, with the help of a python scripting library, pychan3d. The study focused particularly on an evaluation of the usefulness of different characterization data to build and calibrate such a channel network model, and to compare this to a calibrated DFN model of the same site. An evolutionary algorithm (CMAES_P implemented in the PEST code) was used to semi-automatically calibrate the channel conductances in the channel network model against the field characterization data (flow rates, drawdowns, and tracer recoveries) in multiple phases. It was observed during the calibration process that some proposed CNM connectivity maps lent themselves to conductance calibration, while others failed to do so. Channel tortuosity and width were then critical to describe transport appropriately in terms of peak arrival and dispersion. The CNM was shown to be more responsive to calibration and to general alterations than a DFN with uniform fracture planes. After calibration, the CNM could match the flow measurements closer than the reference DFN model for the tested characterization phases. The CNM and DFN with the calibrated conductances and fitted geometric parameters were then used to investigate a long-term tracer transport scenario. This comparative study highlights the potential differences and associated uncertainties in the behavior of the two distinct types of models used in the study of crystalline hard rock fractured system.</p>

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The study of fluid flow mechanics in fractured porous rocks is crucial in the area of oil and gas production industries, enhanced geothermal system (EGS), CO2 sequestration, disposal of nuclear waste in deep geological repositories (DGR), etc. There are usually two types of flows in fractured rockmass setting. The dominant flow occurs through the fractures whereas there is also a slow movement of fluid through the matrix block. The fluid movement between fracture and matrix is often continuous across the fracture. The present study focuses on the development of a numerical model which can simulate the flow behavior through fracture and matrix simultaneously, which is also known as dual permeability model. To simulate this problem, a 3D model is built in COMSOL Multiphysics 4.3a where a cylindrical geometry is made, and a fracture is defined parallel to the axis of the geometry. The asperity of the fracture is defined by a variable ‘a’ which varies along the x-axis, in such a way that increases the value of ‘a’ alters the geometry of fracture and increases the roughness of fracture. Darcy flow physics is used to simulate the situation with known parameters like porosity, permeability, storage coefficient, etc. Pressure is applied as a boundary condition at two ends of the geometry which acts as driving force for fluid to flow through the block. The influence of fracture asperity on the flow behavior is examined by doing the parametric study and the study shows the decrement in the velocity magnitude with an increase in asperity. The formation of dual flow velocity regime, one along the defined fracture and the other along with the matrix, indicates the efficiency of the developed dual-porosity and permeability model.

<p>In well-test analysis, the generalized radial flow (GRF) model uses the non-integer flow dimension to describe the change in flow area with respect to radial distance from borehole due to non-uniform flow (Barker, 1988). But the flow dimension not only depends on the change in flow area but also on the permeability variance in the flow medium. Therefore, in our present study, the flow dimension, due to the combined effect of change in flow volume and permeability variance, is termed AFD, the apparent flow dimension.  AFD can be determined as the second derivative of the drawdown-time plot from pressure transient testing and can have variable non-integer values as a function of time. This study presents a comprehensive set of analyses using rectangular channel networks representing multidimensional porous medium, starting from 1D (where the flow volume remains one-dimensional) and proceeding to 3D systems. We investigate the effect of conductance variance between the connected flow channels in a constant flow transient well test with the objective of formulating a relationship between conductance variance and AFD. Results in the one-dimensional case demonstrate that the AFD changes substantially as a function of channel conductance variation. Thus, the AFD increases abruptly when the propagating pressure reaches a high conductance channel, and it decreases when the pressure finds a channel with a lower conductance. The impact of conductance contrast on apparent flow dimension variation is summarized as a generalized plot of AFD upsurge/drop and conductance contrast between successive flow channels. In 2D and 3D systems, the channeling or preferential flow effect of the heterogeneous porous medium is also studied with the help of flow dimension analyses. The heterogeneity is introduced into the 2D network statistically through conductance distributions with varying variance values. The calculated flow dimensions, smaller than the corresponding dimension values, indicate the presence of flow channeling in the network (Verbovšek, 2009).  Channelization in a 2D porous heterogeneous system is examined as a function of the conductance variance, and it is found that channeling tends to result from the larger variance of the conductance distribution. Following the investigation of the 1D and 2D porous media, similar ideas are applied to the 3D channel networks representing 3D systems in order to investigate both steady-state and transient flow problems. Results from this study provide new insight and the possibility of using transient pressure tests to supplement multiple single well tests, interference tests, and tracer transport tests for the characterization of the heterogeneous porous medium.</p><p> </p><p>Barker, J. A. (1988). A generalized radial flow model for hydraulic tests in fractured rock. <em>Water Resources Research</em>, <em>24</em>, 1796–1804.</p><p>Verbovšek, T. (2009). Influences of Aquifer Properties on Flow Dimensions in Dolomites. <em>Groundwater</em>, <em>47</em>(5), 660–668. https://doi.org/https://doi.org/10.1111/j.1745-6584.2009.00577.x</p>