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The Great Sand Dunes National Park in Colorado, USA, flanked by the Sagre De Cristo Mountains.
The Great Sand Dunes National Park in Colorado, USA, flanked by the Sagre De Cristo Mountains.

Blown away: Nature's frac sand processing system

In reviewing his enormous output over nearly two decades, Bill Langer provides an update on a story he previously submitted about proppant – or frac sand …

Back in 2006 I wrote an article called “Sand and the winds of time” (in Quarry 2006: 14[5]:50).

It described the eolian processes that make sand dunes. Eolian – also spelled aeolian or æolian – is an adjective referring to something that was carried, formed, eroded or deposited by the wind, such as sand dunes.

Last summer we took a trip with our grandkids to the Great Sand Dunes National Park in Colorado, USA. That gave me the opportunity to wow them with some geology. I told them some of the special properties of dune sand, especially as it relates to being used as frac sand, also known as proppant.

"Frac sand is a special type of silica sand that is pumped down into deep wells to prop open rock fractures and allow oil or gas to flow to the well."

I started by telling them frac sand is a special type of silica sand that is pumped down into deep wells to prop open rock fractures and allow oil or gas to flow to the well. Frac sand is pumped under tremendous pressure into a well along with water and other fluids. The pressure opens up tiny fractures in the rock, and the tiny sand grains flow into the fractures where they prop them open. Hence the name proppant.

Obviously, the sand grains need to fit into the tiny fractures in the rock being fracked, so frac sand needs to be sieved into specific sizes. It is very important that the top (largest) and bottom (smallest) sand size are closely spaced – within 0.4mm of each other or less – depending on the conditions of the rock and the gas or oil being developed. A mixture of different sizes of grains serves to clog up the fractures and restrict the flow of gas or oil to the well.

To keep the kids interested I exclaimed: “Geology to the rescue!” Eolian processes are nature’s sand sieving machine. Wind near the Earth’s surface commonly is unable to move particles larger than sand. However, it can suspend particles smaller than sand and scatter them aloft as dust or haze. This leaves a very clean sand deposit.

The grandkids figured out that the sand grains have to be very strong because they have to keep the fractures from closing back up. Once again, eolian processes help. If you look closely at a sand dune on a windy day, you will see that the surface of the dune appears to be in motion.

Bill Langer’s grandchildren on the sand plain.
Bill Langer’s grandchildren on the sand plain.

This is because most eolian sand grains move by a process called saltation, where they bounce along as they are lifted into the air, fall back to the ground, and bounce up again. The repeated impact of the grains falling on one another smashes the weaker grains to smithereens. The hard, durable sand grains, generally those made of quartz, survive the collisions with other grains.

Finally, I told them frac sand grains need to be round – the closer they are to the shape of a ball bearing, the better. The little tips on angular grains can break off, making particles that clog up the pathways for the oil and gas. In addition, it is much easier to screen round sand grains than angular grains into the exact size requirements for proppant. The round grains slip right through the screens, while more angular ones can clog up the screen.

Again, eolian processes help. All that abrasion from the sand grains smashing against one another knocks the angular protrusions off the grains, resulting in spherical grains.

You might want to pass this story on to some unsuspecting kids you know. They will be blown away!












ABOUT THE AUTHOR
Bill Langer
Independent Research Geologist

Bill Langer is a freelance writer and retired Senior Research Geologist for the U.S. Geological Survey. Bill is now an independent researcher specialising in aggregate resources. Click here to email Bill or visit his website.








Monday, 20 August, 2018 01:25pm
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