Physics

New research topples a 100-year-old law on how fluids move in rocks


Scans taken of fluids moving through rock have shown engineers have had it wrong when it comes to substances flowing through porous materials.

For the past century, the movement of gases and liquids through rocks has been modelled using a law that assumes they flow in a stable pattern. Under the microscope, it turns out this was the wrong assumption to make.

 

The discovery made by researchers from Imperial College London might not seem all that ground-breaking, but any future technology that aims to capture carbon dioxide and store it in underground reservoirs would need their models on how gases move to be as accurate as possible.

By scanning a block of sandstone with X-rays produced by a synchrotron called the Diamond Light Source, the engineers created high-speed videos of nitrogen gas and liquid salt-water seeping through microscopic channels, giving them the most detailed moving images of the process to date.

Previously X-ray scans took single captures over a number of hours. Using the synchrotron’s bursts allowed them to snap the same images in as little as 45 seconds to create a more accurate animation.

Until now it was thought that the two different phases of fluid would stick to their own channel, flowing in a static, if still complex manner.

This was all thanks to a 19th century French engineer named Henry Darcy, who came up with a hydraulics law to describe how fluids moved through porous materials.

His law was later extended to mathematically describe the relative permeability of a porous material, one that depended on the phase of fluid pushing through it.

 

Darcy’s extended law didn’t take into account interactions between different fluids moving through their own channels, which although an assumption, has been a useful way to describe most hydraulic phenomena.

The images produced by the Imperial College researchers shows that far from being stable, the pathways taken by the two fluids stutter and shift, lasting no more than tens of seconds before taking a new direction.

Take a look at the short clip below to get a better idea of their findings.

The researchers liken it to a tiny road-network with traffic lights.

“The flow of cars through the network can be stopped by a red traffic light, blocking a junction for a short time. When the light turns green – when the local energy balance favours movement again – the flow continues down the same road,” the researchers write.

 

Since the sudden shifts in ‘traffic’ take just a split moment, the researchers hope to eventually capture images on the scale of 100ths of a second, something currently impossible with existing X-ray technology.

They have called this process dynamic connectivity, and it could have significant applications in a variety of contexts.

“Trying to model how fluids flow through rock at large scales has proven to be a major scientific and engineering challenge,” says lead researcher Catriona Reynolds.

“Engineers have long suspected that there were some major gaps in our understanding of the underlying physics of fluid flow. Our new observations in this study will force engineers to re-evaluate their modelling techniques, increasing their accuracy.”

Carbon capture and storage (CCS) is one engineering feat that requires sound understanding of how gases and liquids move through a porous substance.

In simple terms, the practice would see emissions from industrial fossil fuel combustion pumped into stable subterranean reservoirs, such as deleted oil and gas fields or deep saline aquifers.

There are various proposed solutions, including dissolving CO2 in water and pumping it into basalt where it could mineralise, trapping the carbon as a solid.

Such diverse technologies would all have their pros and cons, and pose clear risks and benefits. But most would benefit from knowing what’s happening to the fluids at a microscopic level.

The modifications to the existing models could also have applications in understanding how freshwater moves into aquifers deep underground, or how seawater flows through the bedrock, providing a more accurate insight into the crust’s volatility.

Controversially, it might also help improve processes that extract fossil fuels from underground, providing safer, more efficient fracking processes.

No doubt Mr Darcy would be quite pleased to find it was time his old law was given a good tweak.

This research was published in PNAS.



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