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

Abstract

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.

Representative Elementary Volumes, Hysteresis, and Heterogeneity in Multiphase Flow From the Pore to Continuum Scale

“…Intermittent fluid connectivity generally occurs in non-negligible volume fractions only when µ nw /µ w << 1, i.e. in gas-water systems (Berg et al, 2016;Kogure et al, 2017;Reynolds et al, 2017;Gao et al, 2017;Spurin et al, 2019aSpurin et al, , 2019b. We note there may be some small-scale intermittent fluctuations in the fluid distribution, highlighted by the pressure signals in the SI, that are not captured by the time-averaged nature of the scanning.…”

Mechanisms controlling fluid breakup and reconnection during two-phase flow in porous media

“…Injection of the fluids continued during a scan. A more detailed description of the experimental procedure is given in [21].…”

“…rL K defines the pore geometry, which plays an important role in generating intermittent pathways [21]. However, as it is a constant in our experiments and rather ambiguous to define at the pore scale, we follow previous work which has no measure of pore geometry in the definition of the capillary number [6,22,23].…”

“…Intermittent pathway flows are relevant for geological carbon storage, and the production of natural gas. They should therefore be considered in modeling attempts as intermittent pathways impact larger scale flow properties [21]. Conventional oil production may be controlled by connected or intermittent pathway flow depending on the setting.…”

The development of intermittent multiphase fluid flow pathways through a porous rock

“…The two phases compete for occupancy leading to intermittent occupation. It has been previously shown that intermittency is also more likely to occur in regions of the pore space that are poorly connected (5).…”