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Multiphase fluid flow in permeable media is a complex pore-scale phenomenon, which has many applications in natural and industrial processes, such as water infiltration in soils, oil recovery from reservoir rocks, geo-sequestration of supercritical CO2 to address global warming, and subsurface non-aqueous phase liquid contaminant transport. At the pore scale, the two most important processes that compete during the displacement of a non-wetting (hydrophobic) fluid by a wetting (hydrophilic) fluid are pore-filling or piston-like displacement and snap-off; this latter process can lead to trapping of the non-wetting phase1. The trapping of the non-wetting phase is important: for CO2 storage, a maximum trapping efficiency is desired whereas for oil recovery, less trapping is preferable for efficient production. Although, there are several studies reported on two-dimensional visualisation of fluid displacement and trapping, the dynamics of imbibition processes (displacement of a non-wetting fluid by a wetting fluid) leading to snap-off have not been investigated in such details in realistic three-dimensional geometry. The time-dependent information is important to validate models of pore-scale displacement and to quantify how the balance of viscous and capillary forces controls the exact nature of trapping.
We conducted dynamic flow experiments in a 3.8 mm diameter and 10 mm long Ketton limestone sample. The sample was first saturated with the brine. The system was pressurised to 10 MPa, followed by injection of oil (a drainage process for water-wet porous media) from the top of the sample by establishing a pressure gradient of 50 kPa. The sample was imaged in three dimensions continuously during drainage using a pink beam (with photon energies up to 30 keV) at beamline I13-2. We used a voxel size of 3.28 μm and a time-step of 38 s between each three-dimensional data collection. After the completion of the drainage process (identified when there was no longer any visible change in fluid saturations), the flow was reversed by injecting brine (imbibition process) from the base of the sample at a pressure gradient of 22 kPa. The sample was imaged continuously during imbibition.
Figure 1 shows the oil phase at various time steps during drainage (Fig. 1a-d) and imbibition (Fig. 1e-h). The residual oil at the end of imbibition contains a number of disconnected oil ganglia (trapped phase), represented by different colours (Fig. 1h). A complete three-dimensional image sequence of drainage and imbibition processes is shown in videos (Video 1 and 2)6.
In this study, our aim was to obtain a complete understanding of various pore-scale displacement and snap-off events that result in trapping of the non-wetting phase in the pore space. Figure 2 shows snapshots of one of the pore-filling and trapping events during imbibition. The complete three-dimensional sequence of the oil displacement process is shown in the video (Video 3)6. During brine injection, the brine-oil interface moves from pore to pore in a piston-like displacement without oil trapping, until it reaches the pore space marked by the red circle (Fig. 2e). Here, the brine (not shown for effective visualisation) in the adjacent oil-filled throats starts to swell. The brine layers in the throat corners continue to grow until the brine-oil interface is no longer stable, resulting in snap-off of the interface and trapping of the oil.
Beyond point B (Fig. 3), there is an apparent disparity in capillary pressure across the system, which drives brine in layers to flow from the ganglion to the connected side. This represents a swelling of wetting layers in the throat, as can be seen by an incremental reduction in oil volume in the throat subset in Figure 3b (open symbols). Eventually at point C (Fig. 3a, 3c), we see snap-off, resulting in the trapping of oil ganglion in the pore body. When this happens, there is a rapid rise in capillary pressure as the ganglion rearranges to find a lower energy configuration. The remarkable feature of this analysis is that the swelling of wetting layers proceeds over approximately 14 minutes. This is many orders of magnitude slower than the sub-millisecond filling observed during a Haines jump in drainage3,4. We have investigated three different snap-off events for different pore topologies and fluid configurations, which are reported in Singh et al.5. The time-scale for brine layer swelling in all the studied events are of the order of tens of minutes, followed by instantaneous snap-off processes. After snap-off the local pore-scale capillary pressure rises as the ganglion rearranges to find a lower energy configuration.
In conclusion, the time-resolved X-ray micro-tomography provides new insights into pore-scale fluid displacement in three-dimensional porous media. This has led us to investigate the pore-filling and snap-off events that lead to the trapping of the non-wetting phase. These findings have important implications in many fields such as oil recovery, CO2 sequestration and remediation of oil contaminated sites.
Funding acknowledgement: We gratefully acknowledge funding from the Qatar Carbonates and Carbon Storage Research Centre (QCCSRC), provided jointly by Qatar Petroleum, Shell, and Qatar Science & Technology Park. We thank Diamond Light Source for providing experimental time on Beamline I13-2 (MT11587).
Corresponding author: Dr Kamaljit Singh, Imperial College London, kamaljit.singh@imperial.ac.uk
Singh K, Menke H, Andrew M, Lin Q, Rau C, Blunt MJ, Bijeljic B. Dynamics of snap-off and pore-filling events during two-phase fluid flow in permeable media. Scientific Reports 7, 5192, doi: 10.1038/s41598-017-05204-4 (2017).
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