Frac sand is a specialized type of sand that is added to fracking fluids that are injected into unconventional oil and gas wells during hydraulic fracturing (fracking or hydrofracking), a process that enhances petroleum extraction from tight (low permeability) reservoirs. Frac sand consists of natural sand grains with strict mineralogical and textural specifications that act as a proppant (keeping induced fractures open), extending the time of release and the flow rate of hydrocarbons from fractured rock surfaces in contact with the wellbore.
The physical properties of frac sand, as defined by the American Petroleum Institute (API), are quite specific. The optimal frac sand is a naturally occurring, unconsolidated silica sand or friable sandstone that has a nearly pure quartz composition, crush-resistant grains, high sphericity/roundness of grains, and a uniformly medium- to coarse-grain size.
The principal sources of “Northern White” or “Ottawa” sand in the upper Midwest are the Middle and Upper Ordovician St. Peter Sandstone and the Lower Ordovician and Upper Cambrian Jordan Formation, with the Upper Cambrian Wonewoc and Mount Simon Formations gaining in importance. Additional frac sand sources to the south include the Upper Cambrian Hickory Sandstone Member of the Riley Formation in Texas, which is referred to informally as “Brown” or “Brady” sand, and the Middle Ordovician Oil Creek Formation in Oklahoma.
More than 40 United States industry operators are involved in the mining, processing, transportation, and distribution of frac sand to a robust market that is fast-growing in the United States and throughout the world. Factors that influence the economics of mining are the deposit’s areal extent and thickness, textural uniformity, accessibility at or near the surface, nearness to trucking and rail transportation routes, and proximity to the active unconventional petroleum basins.
Frac Sand Destinations
The North American shale plays are commonly referred to by the names of the producing rock units and are located within differently named subsurface structural basins. In the United States, the giant shale gas plays include the Barnett in the Ft. Worth Basin, the Fayetteville in the Arkoma Basin, the Haynesville in the Texas-Louisiana-Mississippi Salt Basin, and the Marcellus in the Appalachian Basin. The principal shale oil plays are the Bakken in the Williston Basin, the Avalon (or Avalon-Bone Spring) in the Permian Basin, the Barnett-Woodford in the Permian and Marfa Basins, and the Eagle Ford in the Western Gulf Basin.
In Canada, the principal tight (sandstone or shale) oil and shale gas reservoirs are in the western shale basins of Alberta and British Columbia and in the eastern shale basins of Quebec, Nova Scotia, and New Brunswick.
Although a shale gas play, shale oil play, or a tight sand oil play may be developed in a particular reservoir in a particular basin, unconventional petroleum basins are not all clearly defined as solely shale gas-producing or tight oil producing basins. This is because the nature of the hydrocarbon production changes within the reservoir over time.
Ceramic Alternatives and Resin Coating
In addition to the abrupt rise in frac sand mining and distribution, a new industry has emerged from the production of alternative proppants, such as coated sand and synthetic beads. The higher performance of synthetic proppants under higher pressure conditions in reservoirs at greater depth has made the higher cost of the ceramic materials cost-effective in some shale basins.
Ceramic proppants are manufactured from non-metallurgical bauxite or kaolin clay that is sintered (powdered and baked in high-temperature kilns) to reduce water content and increase density, roundness, and strength. In this process, the sintered bauxite is mixed with additives such as aluminum oxide, silicate, and iron-titanium oxide. Despite their higher cost compared to natural frac sand, ceramic proppants are more homogenous in composition and more uniform in size and roundness, giving them a higher fracturing strength in wells at greater depths and higher pressures, and they have greater conductivity than either natural frac sand or resin-coated proppants. An additional advantage of synthetic proppants is that their specific gravity and grain size can be matched to the viscosity of the fracking fluid and to the anticipated size of fractures in the rock to develop from the fracking treatment.
The compressive strength (crush resistance) of sand grains is increased by resin coating, which shields the grains from fracture closure stresses, prevents shattering, and contains any fines produced. Another advantage of resin-coated proppants is that they can be made available in a range of mesh sizes. Resin-coated sands consist of less-optimal silica sand (as a seed or substrate) that has been coated with either phenolic or non-phenolic resin to reach the desired shape, grain size, and other properties. Resin coatings can be either pre-cured (bonded to grains before going downhole) or curable (bonds grains together in response to high downhole pressures and temperatures) to minimize or prevent proppant flow-back. Curable resin treatments can be applied to both sand and ceramic proppant.
Mining, Production and Transport
Ideally, frac sand mines are open pit quarries with minimal overburden in which loosely cemented to friable sand is often removed by large excavators or power shovels. Commonly, it may be necessary to include blasting or “bumping”, along with additional overburden removal techniques. Following excavation, mined portions of the pit are backfilled. As well as open pit mining, other types of mining may include contour mining, underground mining, and hydraulic dredging. In the case of more tightly cemented sandstone, the raw material may be disaggregated by crushing, high-pressure water-jetting, or grinding before washing and sorting. Once removed from the mine, friable sand is transported by truck or conveyor belt to either on-site or distant processing plants where it is washed, dried, screened, sorted into different grades, and shipped or stored. The raw product may be directly transported to the market, or it may undergo coating treatments before being ready for market.
Trucks are used to transport frac sand to the plant, to rail trans-loading facilities, and sometimes directly to the customer. In some cases, where feasible, sand is delivered to the market downriver by barge. Rail is the preferred method of transporting sand from the mine or from the processing plant to the location of final use. Sand deposits that have ready access to trucking, river, and rail transportation routes that serve petroleum basins provide cost advantages for the operator.
Common environmental concerns of mining, processing, transport, storage, and application of frac sand or other proppants at the well site include water-use volume, water quality, air quality, noise, scarring of terrain, devaluation of real estate, impact of transportation on road infrastructure, and increased traffic congestion. Permits are required prior to developing a mine and processing plant, and State, county, and local jurisdictions issue and enforce regulations.
Air quality controls related to silica sand mining, processing, and transportation include fugitive dust and emission sources and controls, characteristics of particulates, and air quality monitoring. There is the potential health hazard of inhaling silica dust that is classified by the Occupational Safety and Health Administration (OSHA), in regulation 29 CFR 1910.1200, as a human carcinogen that may be generated during the manufacture, handling, and use of frac sand and the coated substrate sand proppants.
The phenolic resin commonly used in the manufacture of coated proppants contains traces of formaldehyde that is also listed by OSHA as a potential human carcinogen (29 CFR 1910.1048), which, in the form of concentrated dust, can also be combustible and can become a potential fire hazard.
Neither silica dust nor the phenolic resin dust is considered to be an ecotoxin (toxic to the environment).
Are you currently in the process of building a Frac Sand plant, or do you have an existing plant in need of maintenance? Reach out to us today and see all we can do to help maximize the performance of your equipment.