Rheology-controlled hydraulic selection in fracture-matrix heat transport: mechanisms and thermal signatures

Abstract

Geological fractures exhibit heterogeneous aperture fields that localize flow along preferential pathways and produce nonuniform fluid-matrix contact times. Heat transport results from channelized advection coupled to conductive exchange with the rock matrix. For non-Newtonian fluids, this coupling is constitutively dependent: shear thinning biases the aperture-to-flux mapping toward larger apertures, while yield stress suppresses flow below the mobilization threshold. This study examines rheology-controlled hydraulic selection in fracture-matrix heat transport using thermal-front advance, longitudinal spreading, and outlet breakthrough as diagnostics. A stochastic semi-analytical channel model represents aperture classes as parallel pathways with constitutively determined fluxes, and the thermal response is obtained by flux-weighted superposition of channel-scale advection-conduction solutions for a semi-infinite matrix. This separation allows late-time scalings and response amplitudes to be analysed independently. Rheology affects observable spreading not only through mean velocity, but also through high-order flux-weighted aperture moments that set the amplitude of persistent inter-channel variance. Matrix diffusion sets the late-time scalings of breakthrough curves and front moments, while aperture variability and rheology control amplitudes, crossover behavior, and inter-channel spreading. Global sensitivity analysis shows that shear thinning controls flux reweighting, while yield stress controls hydraulic accessibility and retained flow. After normalization to a fixed flux-weighted mean velocity, aperture variability and the flow index jointly redistribute heat-carrying flux and shift the residence-time spectrum. The model defines an interpretable reference limit separating rheology-controlled hydraulic selection from matrix-controlled thermal transport.