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Density-driven, haline free convection in fractured porous rock is studied in a 3-dimensional numerical model to evaluate the onset and interdependence of intra- and interfracture convection. When the rock matrix allows for solute diffusion, the most likely mode of convection in a regular fracture circuit is interfracture convection along the circuit such that regular fracture circuits can be modeled in a reduced dimensionality (2D) model. The critical Rayleigh number for the onset of interfracture convection in a regular fracture circuit is determined to be half of the critical Rayleigh number for convection in a single vertical fracture. In more complex fracture networks, vertical “dead-end fractures” and additional fracture intersections complicate convective patterns. Combined inter- and intrafracture convection is possible, such that convection is hard to predict. [ABSTRACT FROM AUTHOR]

The effect of fracture network geometry on free convection in fractured rock is studied using numerical simulations. We examine the structural properties of fracture networks that control the onset and strength of free convection and the patterns of density-dependent flow. Applicability of the equivalent porous medium approach (EPM) is also tested, and recommendations are given, for which situations the EPM approach is valid. To date, the structural properties of fracture networks that determine free convective flow are examined only in few, predominantly simplified regular fracture networks. We consider fracture networks containing continuous, discontinuous, orthogonal and/or inclined discrete fractures embedded in a low-permeability rock matrix. The results indicate that bulk permeability is not adequate to infer the occurrence and magnitude of free convection in fractured rock. Fracture networks can inhibit or promote convection depending on the fracture network geometry. Continuous fracture circuits are the crucial geometrical feature of fracture networks, because large continuous fracture circuits with a large vertical extent promote convection. The likelihood of continuous fracture circuits and thus of free convection increases with increasing fracture density and fracture length, but individual fracture locations may result in great deviances in strength of convection between statistically equivalent fracture networks such that prediction remains subject to large uncertainty. [ABSTRACT FROM AUTHOR]