Impact of Matrix Diffusion on Heat Transport Through Heterogeneous Fractured Aquifers

Silvia de Simone (1) , Olivier Bour (1) , Philippe Davy (1)

1 GR - Géosciences Rennes

Transport in subsurface fractured rocks is strongly affected by the variability of the velocity field, which in complex fracture networks may vary broadly. Because the rock matrix is almost impervious, the flow occurs in the network of connected fractures, which exhibits multiscale features spanning several orders of magnitude, that is, fracture size ranging from millimeters to hundreds of meters, with a small number of fractures acting as preferential flow paths where most of the fluid flow channelizes (Bonnet et al., 2001; Goc et al., 2010; C. F. Tsang & Neretnieks, 1998). Heterogeneity at different scales in fracture aperture and hydraulic conductivity implies strong heterogeneity in flow rates and diffusive processes, leading to behaviors that cannot be captured by traditional theories, and field-scale observations that are difficult to predict (Becker & Shapiro, 2003). Understanding the impact of multiscale heterogeneity and network connectivity on advective and advective-diffusive transport is critical for many scientific and engineering applications including geothermal energy, nuclear waste disposal, waste-water injection, and water resource protection. It is key to improve our capacity of characterizing the geometry of the fracture network and identifying connected structures by means of field tests, such as conservative solute or heat tracer experiments. They typically comprise the interpretation of breakthrough curves (BTCs) to yield information on the structure of the fractured systems which drive flow and transport processes. In natural fractured media, transport of both solutes and heat behaves different from traditional Fickian models, where a constant dispersion coefficient can describe the impact of flow fluctuations occurring at the small scales on solute and thermal spreading. Solute transport through fractured media often exhibits anomalous (non-Fickian) behavior, with non-symmetrical BTCs characterized by power-law post-peak tails (Berkowitz & Scher, 1995). This behavior is mostly controlled by the structure of the interconnected network rather than by the variability in fracture aperture, the latter leading to local flow fluctuations within an individual fracture, but not significantly influencing flow, and thus advective transport, at the network scale (Frampton et al., 2019; Makedonska et al., 2016). Yet, linking structural and hydrodynamic properties to large-scale transport behavior is challenging because of the complexity of the fracture network (Hyman, Dentz et al., 2019a, 2019b; Kang et al., 2020). Besides the heterogeneity of advective displacement, transport is also affected by diffusive exchange between the fluid circulating in the fractures and the rock matrix, which can be assumed as a non-flowing (or immobile) region. While molecular diffusion of solutes has negligible effects on the observed response (Becker & Shapiro, 2000), in the case of heat transport the fluid-rock diffusive exchange strongly modifies the BTC (Carrera et al., 1998),

Domaines: Mécanique des fluides [physics.class-ph]; Hydrologie

Silvia de Simone, Olivier Bour, Philippe Davy. Impact of Matrix Diffusion on Heat Transport Through Heterogeneous Fractured Aquifers. Water Resources Research, 2023, 59 (2), pp.e2022WR033910. ⟨10.1029/2022wr033910⟩. ⟨insu-04000584⟩