Real-time imaging reveals distinct pore scale dynamics during transient
and equilibrium subsurface multiphase flow
Abstract
Many subsurface fluid flows, including the storage of CO underground or
the production of oil, are transient processes incorporating multiple
fluid phases. The fluids are not in equilibrium meaning macroscopic
properties such as fluid saturation and pressure vary in space and time.
However, these flows are traditionally modelled with equilibrium (or
steady-state) flow properties, under the assumption that the pore scale
fluid dynamics are equivalent. In this work, we used fast synchrotron
X-ray tomography with 1s time resolution to image the pore scale fluid
dynamics as the macroscopic flow transitioned to steady-state. For
nitrogen or decane, and brine injected simultaneously into a porous rock
we observed distinct pore scale fluid dynamics during transient flow.
Transient flow was found to be characterised by intermittent fluid
occupancy, whereby flow pathways through the pore space were constantly
rearranging. The intermittent fluid occupancy was largest and most
frequent when a fluid initially invaded the rock. But as the fluids
established an equilibrium the dynamics decreased to either static
interfaces between the fluids or small-scale intermittent flow pathways,
depending on the capillary number and viscosity ratio. If the fluids
were perturbed after an equilibrium was established, by changing the
flow rate, the transition to a new equilibrium was quicker than the
initial transition. Our observations suggest that transient flows
require separate modelling parameters. The timescales required to
achieve equilibrium suggest that several metres of an invading plume
front will have flow properties controlled by transient pore scale fluid
dynamics.