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HIGH DEMAND FOR SANDS
As oil prices rise to all-time highs, producers search for means to increase production in existing wells, and exploit new oil and gas fields, which only a few years ago were not economically viable. This trend to maximize production has caused an unprecedented demand for hydraulic fracturing sand (frac sand).
Understanding the American Petroleum Institute (API) frac sand specifications (see sidebar), and what is required geologically for a frac sand deposit to make grade is key for helping producers identify which deposits have potential. The next step is to consider what Mother Nature has provided, and determine if processing techniques can be used to meet those specifications laid out by the API.
What makes a good deposit?
Frac sand flowsheets
A typical frac sand flowsheet consists of both wet and dry portions. The wet flow sheet is designed to both remove clay slimes that can increase the turbidity (muddiness created by stirring up sediment or having foreign particles suspended) of the final product, and break-up any agglomerates. Additionally, the wet process functions as a preliminary sizing step by rejecting excess fines - a processing step much lower in cost if performed wet rather than dry. The sizing step can also be used to pre-classify materials for blending in the dry plant to meet varying market conditions. The dry processing section is designed to size the sand into the various frac sand products, and if needed, remove any magnetic contaminants, which are generally softer, non-quartz minerals that negatively affect crush and acid solubility.
Usually, the first step in wet processing is to liberate any clays and thus allow for their removal during the desliming operation.
Washing is the simplest and lowest cost method for cleaning frac sand. In some of the very pure deposits, washing is the only wet process needed to produce an acceptable final product. In this process, water is added to the sand, which is then pumped to a cyclone for desliming. The movement of the slurry passing through the pump and pipeline is sufficient to loosen the small amount of fines or clay, which can then be removed through a variety of methods.
Attrition scrubbing is used when the clay or silts are more tightly bound to the silica grains, or when ‘clay balls’ exist that are similar in size to the silica sand grains.
In frac sands, slimes are considered the -100μm (-140 mesh) material that is generally in the form of clays or very fine silica. As slimes are detrimental to frac sand processing they are removed, mainly through the use of hydrocyclones and hydraulic classifiers.
The dry plant is crucial for all frac sand plants as it is where final screening is conducted. When excess levels of non-quartz minerals are present, sizing is sometimes followed by Rare Earth Roll (RER) magnetic separation.
Both the glass and frac sand markets have similar requirements of pure quartz with few contaminating minerals. Frac sand requires more closely sized particles, while glass sand is more stringent on levels of purity. Regardless, the required technologies for producing high quality products are the same. The key is how those technologies are staged in a process flowsheet.
The most economical approach to processing a frac sand deposit is ‘crystal clear’ – team up with a technology partner that can evaluate what Mother Nature has provided and test for best processing techniques. When done right, these tests will naturally turn into a process flowsheet based on the unique nature of the deposit. The result is an operation that can produce desired products through the sands of time.
Contributed by Jim Sadowski, Manager - Process Solutions, Global Physical Separation and Misty Dobbins, Marketing Communications Manager, North America both of Outotec.
Outotec is a critical partner in the process development planning exercises for Victory Nickel’s Frac Sand deposit. Outotec develops and provides technology solutions for the sustainable use of Earth’s natural resources. As the global leader in minerals and metals processing technology, Outotec has developed over decades several breakÂthrough technologies. The company also offers innovative solutions for the chemical industry, industrial water treatment and the utilization of alternative energy sources. Outotec shares are listed on the NASDAQ OMX Helsinki. www.outotec.com
Why Frac Sands
Frac sands are used as a proppant, or sized particles mixed with fracturing fluid to hold fractures open after a hydraulic fracturing treatment. This treatment, known as hydrofracing, is the forcing of a concoction of frac sands, viscous gel and other chemicals down a well to prop open fractures in the subsurface rocks thus create a passageway for fluid from the reservoir to the well.
The following are the API criteria for frac sands with relation to available minerals processing options:
1. GRAIN SIZE: API specifics that 90 wt% of the sand must fall within a specified size range for a particular product. The generally defined frac sand products are 12/20, 20/40. 40/70 and 70/140. (For example, to meet the requirements of a 20/40 product, 90wt% must be - 20 + 40 mesh.) This can be achieved through high efficiency screening.
2. SPHERICITY AND ROUNDNESS: Round or spherical quartz grains are a result of the deposition of the quartz deposit. Most deposits containing these preferred grains are geologically very old because, over time, the quartz grains have been rounded and non-quartz type minerals removed. From a practical standpoint, there is no processing route that can change the grain shape.
3. CRUSH RESISTANCE: The resistance to crushing is a key consideration as it relates to the amount of fines generated after a product is subjected to a particular pressure, as defined by API. “Good crush deposits” tend to be older geologically because aging allows for the creation of the more pure quartz that is void of other, softer minerals. There are some processing routes that can improve the crush results by removing the majority of the softer minerals.
4. ACID SOLUBILITY: For a product to have low acid solubility, it must be primarily quartz with little to no other minerals present. There are some processing routes that can improve the acid solubility.
5. TURBIDITY: The amount of silt and clay-sized particulate matter is also important. Some deposits are naturally low in fines, but when they are present, there are processing routes that can improve these criteria.
6. CLUSTERS OR AGGLOMERATES OR DEPOSITS with many agglomerated grains cannot be economically processed to meet the API specification, which is < 1% clusters.
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