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www.rockproducts.com Frac Sand Insider May 2015 | 67 Domestic Frac Sand fracking of the surrounding rock is performed. Multi-stage fracking is performed by temporarily isolating portions of a single wellbore multiple times with plugs in order to produce high water pressure and get maximum fracturing results in the most geologically produc- tive intervals (Judice, 2012; Schlumberger, 2014b). The increase in the number of stages and greater depth resulted in greater con- sumption of proppant per hole and a greater variety of proppants to maximize conductivity. In the deepest parts of the Williston Basin, where depths are on the order of 3,400 m below the surface, there is a greater proportion of RCS and ceramic as a proppant with the deepest holes which approach over 3,000 m in depth using pre- dominantly ceramic or solely ceramic proppant. Some wells also have different proportions of proppants for different stages (DTC Energy Group, Incorporated, 2013; Nordeng and Helms, 2010; Petroleum News Bakken, 2014; Schaefer, 2013; U.S. Geological Survey, 2013). As shown in Table 2 and Figure 2, consumption of frac sand in the Appalachian region in the Northeastern United States and the Permian Basin, mostly in Texas, consumed approximately 6.8 Mt (22 percent) and 5.3 Mt (17 percent), respectively (PacWest, 2014b). Technological Advances Increase Proppant Consumption Per Well The surge in frac sand consumption relative to the number of ac- tive horizontal drilling rigs has increased substantially over the last several years. This results from numerous factors that include: 1) application of advanced technologies such as: (a) multi-stage and higher density hydraulic fracturing per well, which increased from an average of about 3.4 hydraulic fracturing stages in 2008 to over 13 at the beginning of 2012. In 2014, wells with 30 stages were not uncommon and some wells have as many as 50 stages; (b) methods that result in more extensive fracturing in bedrock; and (c) reservoir stimulation of older wells by hydraulic fracturing; 2) im- proved effciencies by drilling multiple holes from one site with closer spacing; and 3) refreshing of previously fracked wells by re-fracking (CBC News, 2014; Helman, 2014; McDivitt, 2014; Nangia, 2013; Schaefer, 2009; Schlumberger, 2014a; Spencer, 2014; Tucker, 2013). These advances have increased the average proppant consumption per well. For example, in 2008, the average amount of proppant, which was nearly all sand, used per horizontal well was approximately 900 t for a 1,500 m well. In 2010, the average amount of sand used was closer to 2,300 t for a well completed on a 3,000-m length measured horizontally. In 2014, an average hor- izontal well consumed from 4,100 to nearly 5,000 t of proppant of which over 90 percent, by weight, was sand, equivalent to 40 to 50, 100 short-ton capacity train car loads. In a few recent cases, wells required about 9,000 t of sand (Cadre Proppants, 2013; Fielden, 2013; Rock Products, 2014). Also, a well may be refracked multiple times over its life to increase production or refresh the well (Street- wise Reports, 2013; Tate, 2014). For the purpose of comparison, from 2011 through mid-2014, the amount of proppant required for fracking a vertical drill hole, nearly all of which was sand, remained essentially level, at about 230 t per well (Down Hole Trader, 2014; Geiver, 2014; PacWest, 2014b). The average amount of proppant used per unit distance for horizontal holes is expected to continue to climb with improved fracturing technologies, closer-spaced and increased number of stages per drill hole, and refreshing of previously developed wells. Frac Sand Reserves and Resources Published data pertaining to the total amount and distribution of frac sand reserves and resources in the United States are not available. Reserve data are published by some publicly traded companies in annual reports and other media mostly as aggregated estimates for multiple operations. For example, U.S. Silica, a publicly trad- ed company, and among the largest producers of frac sand in the United States, reported combined reserves of 124 Mt among 15 operations across the country that meet API specifcations for frac sand and control about 260 Mt of overall reserves (Yahoo Finance, 2014). Meeting API standards suggests that this is high-quality frac sand. In 2012, U.S. Silica stated that they have 40 to 45 years of frac sand reserves (Rigzone, 2012). In some cases, companies provide site specifc data. For example, Hi-Crush Partners LP, also a publicly listed company, announced that they have a reserve of high quality frac sand at a company-owned facility in Wisconsin of about 57 Mt and an interest in another frac sand facility, also in Wisconsin, with about 43 Mt of reserves (ETF Channel, 2014). Some privately owned companies do not make reserve infor- mation public, however Preferred Sands, LLC, recently published a frac sand reserve estimate of nearly 24 Mt in Wisconsin and 73 Mt in Nebraska (Younger, 2013; Preferred Sands, 2014). Northern Frac Proppants reports a reserve in excess of 35 Mt at their Alma Center, Wisconsin, frac sand operation and over 50 Mt at their undevel- oped Jackson, Wisconsin, site (Northern Frac Proppants, 2014). Sierra Frac Sand, LLC, also published a frac sand reserve estimate - eralized trend of proppant types used in fracked wells. Mixtures of proppants in conductivity and con- conditions associated with the depth of wells (see map inset in upper left cor- ner). Depths are shown as feet below sea level. Add approximately 1,800 ft. (550 m) for depth below surface. modifed from EIA, 2014b and provided cour-