Intermittent fluid connectivity during two-phase flow in a heterogeneous carbonate rock

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 modeling 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. Little is known about the pore structure controls or the impact of intermittency on continuum scale flow properties. Here we investigate the impact of intermittent pathways on the connectivity of the fluids for a carbonate rock. Using laboratory-based micro computed tomography imaging we observe that intermittent pathway flow occurs in intermediate-sized pores due to the competition between both flowing fluids. This competition moves to smaller pores when the flow rate of the nonwetting phase increases. Intermittency occurs in poorly connected pores or in regions where the nonwetting phase itself is poorly connected. Intermittent pathways lead to the interrupted transport of the fluids; this means they are important in determining continuum scale flow properties, such as relative permeability. The impact of intermittency on flow properties is significant because it occurs at key locations, whereby the nonwetting phase is otherwise disconnected.

Figure 1

Schematic of the flow loop used for the experiments. Only the top part of the sample was imaged.

Figure 2

(a) Grayscale data for the air-saturated scan, (b) grayscale data for the $f_w$ = 0.5 high-Ca experiment, (c) grayscale data for $f_w$ = 0 high-Ca experiment, and (d) histogram for the grayscale values in the pore space for (b) and (c) and the brine-saturated scan. Red circles highlight the rearrangement of fluid between scans.

Figure 3

Intermittency classification in three dimensions for a subvolume within the sample: (a) $f_w=0.7$ high-Ca nitrogen experiment and (b) $f_w=0.25$ high-Ca nitrogen experiment. Green is a region of intermittent flow behavior and blue is stable nitrogen. The pore space is overlain. The average flow direction is top to bottom.

Figure 4

The role of intermittency on the connectivity of the nonwetting phase for all experiments: (a) percentage volume of the pore space identified as intermittent, (b) fractional reduction in the number of ganglia through the inclusion of intermittent flow pathways as stable nonwetting phase, (c) brine saturation, and (d) change in the brine saturation through the inclusion of intermittent flow pathways as stable nonwetting phase.

Figure 5

The role of intermittency in the connectivity of the nonwetting phase for a different subvolume and for a repeat experiment for the low-Ca nitrogen experiments: (a) Percentage volume of the pore space identified as intermittent, (b) fractional reduction in the number of ganglia through the inclusion of intermittent flow pathways as stable nonwetting phase, (c) brine saturation, and (d) change in the brine saturation through the inclusion of intermittent flow pathways as stable nonwetting phase.

Figure 6

Pore occupancy for all experiments for the $f_w=0.7$ scan, where the most intermittency was identified. Pore diameter was classified in bins of 5 $\mu \mathrm{m}$: (a) high-Ca nitrogen, (b) low-Ca nitrogen, and (c) decane.

Figure 7

Mean pore size for the nonwetting phase filled pores and intermittent pores for all experiments. The black line defines the mean pore size for all pores within the subvolume.

Figure 8

Coordination number against pore occupancy of the nearest-neighbor pores: (a) All pores for all nitrogen observations, (b) intermittent pores for the low-Ca nitrogen observations and, (c) intermittent pores for the high-Ca nitrogen observations.

Figure 9

Potential displacements for a pore, with a coordination number of 2, identified as intermittent in a scan. WP denotes the wetting phase (brine).