Combined inter- and intra-fracture free convection in fracture networks embedded in a low-permeability matrix

• Inter- and intra-fracture convection are simulated separately and in combination.
• A critical Rayleigh number for interfracture convection is proposed.
• Interfracture convection dominates in continuous fracture circuits.
• Combined inter- and intra-fracture convection develops in complex fracture networks.
• Complex fracture networks require 3D simulation to capture free convection.

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.

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