This special sand is one of the keys to producing oil and natural gas from tight shale formations.
A Crush-Resistant Sand for Oil and Gas Wells
“Frac sand” is a high-purity sand with very durable and very round grains of a specific size. It is a highly specialized, crush-resistant material produced for use by the petroleum industry. It is used in the hydraulic fracturing process (known as “fracking”) to produce petroleum fluids, such as oil, natural gas and natural gas liquids from rock units that lack adequate pore space for these fluids to flow to a well. Frac sand can be a natural material made from high purity sandstone. An alternative product is ceramic beads made from sintered bauxite.
|Close-up view of frac sand (on the right) and a typical sand of similar grain size (on the left). Notice how the frac sand has a very uniform grain size, nicely rounded grain shapes and a uniform composition. It is also a very tough material that is highly resistant to fracturing. Photo © BanksPhotos, iStockphoto.|
How is Frac Sand Used?
Some subsurface rock units such as organic shale contain large amounts of oil, natural gas or natural gas liquids that will not flow freely to a well. They will not flow to a well because the rock unit either lacks permeability (interconnected pore spaces) or the pore spaces in the rock are so small that these fluids cannot flow through them.
The hydraulic fracturing process solves this problem by generating fractures in the rock. This is done by drilling a well into the rock, sealing the portion of the well in the petroleum-bearing zone, and pumping water under high pressure into that portion of the well. This water has been treated with a chemical such as guar-gum that creates a viscous gel. This gel facilitates the water’s ability to carry grains of frac sand in suspension.
Large pumps at Earth’s surface increase the water pressure in the sealed portion of the well until it is high enough to exceed the breaking point of the surrounding rocks. When their breaking point is reached they fracture suddenly and water rushes rapidly into the fractures, inflating them and extending them deeper into the rock. Billions of sand grains are carried deep into the fractures by this sudden rush of water. Over one million pounds of frac sand can be used to stimulate a single well.
|Simplified diagram of a natural gas well that has been constructed with horizontal drilling to increase the length of penetration through the Marcellus Shale. Hydraulic fracturing is typically done in the horizontal portion of the well to stimulate a flow of gas from the shale. This well configuration is used in shale plays of the United States.|
Frac Sand as a “Proppant”
When the pumps are turned off, the fractures deflate but do not close completely – because they are propped open by billions of grains of frac sand. This only occurs if enough sand grains to resist the force of the closing fractures have been delivered into the rock.
The new fractures in the rock, propped open by the durable sand grains, form a network of pore space that allows petroleum fluids to flow out of the rock and into the well. Frac sand is known as a “proppant” because it props the fractures open.
Other materials that have been used as a proppant include ceramic beads, aluminum beads and sintered bauxite. Frac sand generally delivers the highest level of performance and it is currently the proppant most frequently used by the petroleum industry.
|Aerial view of a frac sand mining operation in Wisconsin. Frac sand is a highly specialized product that can only be produced from a small number of sand deposits. Photo © BanksPhotos, iStockphoto.|
|Aerial view of a frac sand processing facility in Wisconsin. Photo © BanksPhotos, iStockphoto.|
What Type of Sand?
Petroleum industry proppants must meet very demanding specifications. The characteristics of a high quality frac sand include:
- high-purity silica sand
- grain size perfectly matched to job requirements
- spherical shape that enables it to be carried in hydraulic fracturing fluid with minimal turbulence
- durability to resist crushing forces of closing fractures
Rock units such as the St. Peter Sandstone, Jordan Sandstone, Oil Creek Sandstone and Hickory Sandstone have been potential sources of frac sand material. These rock units are composed of quartz grains that have been through multiple cycles of weathering and erosion.
That long history has removed almost all mineral grains other than quartz and produced grains with very round shapes. This is why sand dredged from rivers, excavated from terraces or removed from beaches is unlikely to produce a good product.
Where these rock units are produced they are usually soft and poorly cemented. This allows them to be excavated and crushed with minimal damage to the quartz grains. High-purity sand from areas such as the Appalachians is often not suitable for frac sand because it has been subjected to tectonic forces which have deformed the rock and weakened the sand grains.
Where is Frac Sand Produced and Used?
A few years ago producers in Wisconsin and Texas were supplying much of the frac sand used by the oil and gas industry. However, a huge spike in demand caused by the natural gas and shale oil boom has motivated many companies to provide this product. Many of these companies are in the central part of the United States where the St. Peter Sandstone and similar rock units are close to the surface and easily excavated. These areas are also where tectonic forces have not caused severe folding of the rock units and weakened the sand grains.
Most of the high-purity silica sands in the United States have been known for decades. They have been used for glass-making and metallurgical uses. The current search for frac sand is not about “discovering new sources of sand,” it is instead about determining which sources produce superior materials.
Frac sand is used to produce natural gas, natural gas liquids and oil from shales and other tight rocks where hydraulic fracturing is required. These include: the Marcellus Shale, Utica Shale, Bakken Formation, Haynesville Shale, Fayetteville Shale, Eagle Ford Shale,Barnett Shale and many other shale plays throughout the United States.
Increasing Demand for Frac Sand
The demand for frac sand in North America has risen sharply in the last few years in response to numerous shale plays developing in many parts of the United States and Canada. The United States Geological Survey reports:
The production increase for silica sand in 2010 was largely attributable to surging demand for hydraulic fracturing sand. The increased demand for hydraulic fracturing sand was the result of ongoing and increased exploration and production of natural gas from various underground shale formations throughout the United States. Additionally, most major silica sand producers had production capacity increases for hydraulic fracturing sand.
Reported average prices for frac sand in the U.S. Geological Survey Minerals Yearbook were between $45 per ton and $50 per ton in 2010. This is significantly higher than the average price of $35 per ton for specialty sand sold outside of the construction industry.
Sintered Bauxite Proppants
Powdered bauxite can be fused into tiny beads at very high temperatures. These beads have a very high crush resistance and that makes them suitable for use as a proppant. The specific gravity of the beads and their size can be matched to the viscosity of the hydraulic fracturing fluid and to the size of fractures that are expected to develop in the rock. Manufactured proppants provide a wide selection of grain size and specific gravity compared to a natural proppant known as frac sand. Frac sand is currently used instead of manufactured proppants because it has a cost and transportation advantage.
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