Imaging of Steady-State Intermittent Flow Pathways in a Carbonate Rock with 1 Second Time Resolution

Fluid flow in porous media is ubiquitous in nature and is of key importance to a range of societal challenges including the geological storage of carbon dioxide and hydrocarbon recovery. Traditionally, macroscopic fluid flow behaviour in porous media is described using Darcy's law extended to multiphase flow. However, this relies on the assumption that the interface between fluid phases is invariant at a steady-state, fixed saturation, such that once a pathway is establish, it will remain stable [1-2].

However, recent observations show that flow pathways are intermittent, even at steady-state and at flow rates typical in the subsurface where capillary forces dominate [3-4]. We have shown that intermittency is favoured by high capillary numbers and low mobility ratios, and is likely to be prevalent during the injection of carbon dioxide underground, or during natural gas production [4]. We need to understand the pore-scale mechanisms controlling this behaviour to understand the impact that it has on energy dissipation and fluid transmissibility. This will allow us to link the presence of intermittent flow pathways to impacts on larger scale flow processes and measured properties such as relative permeability and capillary trapping.

In this work we present observations of unsteady and steady-state 2-phase immiscible fluid flow observed at a temporal resolution of 1 second and a spatial resolution of 2.75 μm. We used synchrotron X-ray imaging (at the Swiss Light Source) to image intermittent flow pathways in a heterogeneous carbonate. With this temporal resolution we are able to fully resolve periodic pathways at scales ranging from the μm to mm, including interface velocities. We identify different types of intermittent flow pathways depending on the connectivity of the pore space and connectivity of the fluid phases at a particular location. We capture the time-evolution of intermittency during unsteady-state flow and the approach to steady-state.

[1] Armstrong et al., Physical Review E 94, 043113 (2016).

[2] Blunt, Multiphase Flow in Permeable Media: A Pore-Scale Perspective (2017).

[3] Reynolds et al., Proceedings of the National Academy of Sciences 114, 8187 (2017).

[4] Spurin et al., Physical Review E (submitted 2019).