Rock Products

MAY 2015

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www.rockproducts.com Frac Sand Insider May 2015 | 37 Geology 30/50 are most popular for oil fracking; and 40/70 and 70/140 are most commonly used for gas fracking (Zdunczyk, 2014). The use of 40/70 and the fner 100 mesh sands has increased in response to growth in gas development in the Barnett, Fayetteville, Haynesville, and Marcellus shale plays (Beckwith, 2011). HIGH SPHERICITY/ROUNDNESS Shape factors are based upon the Krumbein/Sloss criteria for sphericity and roundness (Krumbein and Sloss, 1963), with the ISO 13503-2/API RP19C standards for frac sand at ≥0.6 for each. The greater roundness/sphericity provides better porosity/permeability between grains, allowing better fow of oil and gas from the fractures to the wellhead (Zdunczyk, 2014). HIGH CRUSH RESISTANCE Crush resistance testing for proppants is measured in weight percent (wt. %) of fnes generated under specifc loading pressures up to 9,000 pounds per square inch (psi), and it varies for each grain size (mesh) range. For example, at 4,000 psi, the 6/12 mesh should yield ≤20 wt. % fnes; 16/30 mesh and 20/40 mesh should yield ≤14 wt. % fnes; 30/50 mesh should yield ≤10 wt. % fnes; and 40/70 mesh should yield ≤6 wt. % fnes (Zdunczyk, 2007). The pressure test results in a k-value that indicates the highest value of psi pressure X 1,000 that generates ≤10 wt. % fnes (American Petroleum Institute and others, 2008). For example, a k-value of 8 means that, at 8,000 psi pressure, no more than 10 wt. % fnes were generated, but more than 10 wt. % fnes were generated at the next higher tested pressure. Different types of frac sand can bear different ranges of crush resistance (stress ranges) and are assigned classes recognized by API that refect this. Class C sands (such as modern aeolian sands) have a stress range of 0–4,000 psi, Class D sands (such as the Hickory Sandstone Member of the Riley Formation) have a stress range of 0–5,000 psi, and Class E sands (such as the St. Peter Sandstone) have a stress range of 2,000–6,000 psi (Herron, 2006). Modifed and synthetic proppants have higher stress ranges than frac sand; resin-coated proppants will bear 4,000–12,000 psi, and ceramic proppants will bear 10,000–16,000 psi of stress loading (Herron, 2006). Crush resistance of frac sand is dependent upon hardness of grain; grain crystallinity; absence of weak planes that could have developed during tectonism; absence of deep pitting of grains, and absence of authigenic overgrowths on quartz grains (Zdunczyk, 2007). To reduce the presence of weaker grains, the ISO 13503-2 proppants standard for percent clusters is ≤1.0. LOW SOLUBILITY Acid solubility of a sandstone is determined by the amount of soluble cement or soluble mineral grains that it contains. Low solu- bility requires a high percent silica, as quartz tends to be insoluble under ordinary conditions, becoming increasingly soluble when the pH exceeds nine, or when exposed to hydrofuoric acid (Missou- ri Department of Natural Resources, 2014). Frac sand solubility is measured by percent weight loss from a test in which a 5-gram sample of proppant is soaked in a 12/3 mixture of HCl/HF for 0.5 hr at 150°F, after which the sample is rinsed, dried, and reweighed (American Petroleum Institute and others, 2008). The ISO 13503-2/ API RP19C solubility standard for proppants varies according to grain size ranges; 6/12 through 30/50 is <2.0 percent weight loss, and 40/70 through 70/140 is <3.0 percent weight loss. LOW TURBIDITY Low turbidity is defned as the absence of clay, silt, or other fne grains and impurities. The amount of suspended particles or other fnely divided matter is measured in scattered light in a formaz- in-based solution at 90° angles, and is recorded in Nephelometric Turbidity Units (NTU) or Formazin Turbidity Units (FTU). (optek-Dan- ulat, Inc., 2014). The ISO 13503-2 turbidity standard for frac sand is ≤250 NTU. Low turbidity is a result of mineralogical maturity and grain-size sorting in the natural depositional environment. Fine suspended matter in the mined sand is usually washed out during processing, so this property can be controlled for the fnal product (Zdunczyk, 2007; O'Driscoll, 2012; Buchsbaum, 2013). DENSITY/SPECIFIC GRAVITY Bulk density is the density of both the proppant and the po- rosity, and it is measured by flling a known volume with dry prop- pant and measuring the weight. Apparent density excludes the extra-granular porosity by placing a known mass in a volume of fuid and determining the amount of fuid that is displaced. Absolute density is the density that the material would have if no intra-granu- lar porosity were present (American Petroleum Institute and others, 2008). Specifc gravity of liquids and solids is defned as a dimen- sionless unit which is the ratio of density of a material to the density of water at a given temperature. Specifc gravity of quartz is 2.65 (Hurlbut, 1971). The standard, ISO 13503-2, describes how density is mea- sured, but gives no requirement for frac sand. "Bulk density/specifc gravity" is listed as an indicator of quartz purity of sand when its value is similar to that of quartz (Wolfe, 2013). GOOD FRIABILITY Unconsolidated deposits of "soft, loose" sand or poorly con- solidated, poorly cemented (friable) sandstone is most desirable (Runkel and Steenberg, 2012). Such material can be mined with- out blasting and is removed by large excavators or power shovels (Maslowski, 2012).

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