Subsurface fluid flow is ubiquitous in nature, and understanding the interaction of multiple fluids as they flow within a porous medium is central to many geological, environmental, and industrial processes. It is assumed that the flow pathways of each phase are invariant when modelling subsurface flow using Darcy's law extended to multiphase flow; a condition that is assumed to be valid during steady-state flow. However, it has been observed that intermittent flow pathways exist at steady-state, even at the low capillary numbers typically encountered in the subsurface. In this thesis we use both laboratory-based and synchrotron-based micro-CT imaging to capture the pore-scale flow dynamics that arise when multiple fluids flow simultaneously through the pore space of a rock.

Using laboratory-based micro-CT we observed that intermittent flow pathways occur in intermediate sized pores due to the competition between both flowing fluids. This competition moves to smaller pores when the flow rate of the non-wetting phase increases. Intermittency occurs in regions where the non-wetting phase is poorly connected. Intermittency leads to the interrupted transport of the fluids; the impact on flow properties is significant because it occurs at key locations, whereby the non-wetting phase is otherwise disconnected. The amount of intermittency expected during flow is dependent on the capillary number and the viscosity ratio of the fluids.

Using fast synchrotron X-ray tomography, with 1~s time resolution, we imaged the pore-scale fluid dynamics as the macroscopic flow transitioned to steady-state, and then during steady-state. We observed distinct behaviour during transient flow, with the intermittent fluid occupancy largest and most frequent during the initial invasion into the rock. Our observations suggest that transient flows require separate modelling parameters. We observed that, during steady-state flow, intermittent fluid transport allows the non-wetting phase to flow through a more ramified network of pores. While a more ramified flow network favours lowered relative permeability, intermittency is more dissipative than laminar flow through connected pathways, and the relative permeability remains unchanged for low capillary numbers, where the pore geometry controls the location of intermittency. As the capillary number increases further, the role of pore structure in controlling intermittency decreases, resulting in an increase in relative permeability. These observations can serve as the basis of a model for the causal links between intermittent fluid flow, fluid distribution throughout the pore space, and its upscaled manifestation in relative permeability.