Dynamics of Crustal-Scale Fluid Flow: Interaction between Darcy and Hydrofracture Fluid Transport

Fluid flow through the crust can be described as “bimodal”. At low hydraulic head gradients, fluid flows slowly through the rock porosity, which can be described as diffusional. Hydraulic breccias such as the massive Hidden Valley Breccia in South Australia or those in the Black Forest are evidences for very high fluid velocities, which can only be achieved by localized fluid transport, via hydrofractures. Hydrofractures propagate together with the fluid they contain, and high fluid fluxes during ascent indicate that fluid flow must have been highly intermittent. The propagation of hydrofractures and simultaneous fluid transport can be seen as a “ballistic” transport mechanism, which is activated when transport by diffusion alone is insufficient to release the local fluid overpressure. The activation of a ballistic system locally reduces the driving force, by allowing the escape of fluid.

 

We use a numerical model to investigate the properties of the two transport modes in general, the transition between them in particular, as well as the resulting patterns of this “bimodal transport” (de Riese et al., 2020). When hydrofracture transport is activated due to a low permeability relative to the fluid flux, many hydrofractures develop which do not extend through the whole system. When hydrofracture transport dominates, the system self-organizes and the size-frequency distribution of these hydrofractures follows a power-law size distribution. These hydrofractures organize the formation of large-scale hydrofractures. The large-scale hydrofractures ascend through the whole system and drain fluids in large bursts. Their size distribution shows “dragon-king”-like large hydrofractures that deviate from the power-law distribution. With an increasing contribution of porous flow, escaping fluid bursts become less frequent, but more regular in time and larger in volume.

 

The observed fluid transport behaviour may explain the abundance of crack-seal veins in metamorphic rocks, as well as the development of hydrothermal hydraulic breccia deposits at shallower crustal levels. Fluid transport through the crust is a highly dynamical process. A better understanding of the dynamics and pathways of fluid migration in the crust is of major interest, e.g. to avoid human induced seismicity. The bimodal-transport concept may apply to many systems with a slow and steady transport mechanism and a fast one that is triggered at a certain threshold (e.g. fault zones: slow creep and earthquakes).

 

de Riese, T., Bons, P. D., Gomez-Rivas, E., & Sachau, T. (2020). Interaction between Crustal-Scale Darcy and Hydrofracture Fluid Transport: A Numerical Study. Geofluids, 2020.