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A proppant is a solid material, typically sand, treated sand or man-made ceramic materials, designed to keep an induced hydraulic fracture open, during or following a fracturing treatment, most commonly for unconventional reservoirs. It is added to a fracking fluid which may vary in composition depending on the type of fracturing used, and can be gel, foam or slickwater&#;based. In addition, there may be unconventional fracking fluids. Fluids make tradeoffs in such material properties as viscosity, where more viscous fluids can carry more concentrated proppant; the energy or pressure demands to maintain a certain flux pump rate (flow velocity) that will conduct the proppant appropriately; pH, various rheological factors, among others. In addition, fluids may be used in low-volume well stimulation of high-permeability sandstone wells (20 to 80 thousand US gallons (76 to 303 kl) per well) to the high-volume operations such as shale gas and tight gas that use millions of gallons of water per well.
Conventional wisdom has often vacillated about the relative superiority of gel, foam and slickwater fluids with respect to each other, which is in turn related to proppant choice. For example, Zuber, Kuskraa and Sawyer () found that gel-based fluids seemed to achieve the best results for coalbed methane operations,[1] but as of , slickwater treatments are more popular.
Other than proppant, slickwater fracturing fluids are mostly water, generally 99% or more by volume, but gel-based fluids can see polymers and surfactants comprising as much as 7 vol%, ignoring other additives. Other common additives include hydrochloric acid (low pH can etch certain rocks, dissolving limestone for instance), friction reducers, guar gum, biocides, emulsion breakers, emulsifiers, 2-butoxyethanol, and radioactive tracer isotopes.
Proppants have greater permeability than small mesh proppants at low closure stresses, but will mechanically fail (i.e. get crushed) and produce very fine particulates ("fines") at high closure stresses such that smaller-mesh proppants overtake large-mesh proppants in permeability after a certain threshold stress.[2]
Though sand is a common proppant, untreated sand is prone to significant fines generation; fines generation is often measured in wt% of initial feed. One manufacturer has claimed untreated sand fines production to be 23.9% compared with 8.2% for lightweight ceramic and 0.5% for their product.[3] One way to maintain an ideal mesh size (i.e. permeability) while having sufficient strength is to choose proppants of sufficient strength; sand might be coated with resin, to form curable resin coated sand or pre-cured resin coated sands. In certain situations a different proppant material might be chosen altogether&#;popular alternatives include ceramics and sintered bauxite.
Proppant weight and strength
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Increased strength often comes at a cost of increased density, which in turn demands higher flow rates, viscosities or pressures during fracturing, which translates to increased fracturing costs, both environmentally and economically.[4] Lightweight proppants conversely are designed toals can break the strength-density trend, or even afford greater gas permeability. Proppant geometry is also important; certain shapes or forms amplify stress on proppant particles making them especially vulnerable to crushing (a sharp discontinuity can classically allow infinite stresses in linear elastic materials).[5]
Proppant deposition and post-treatment behaviours
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Proppant mesh size also affects fracture length: proppants can be "bridged out" if the fracture width decreases to less than twice the size of the diameter of the proppant.[2] As proppants are deposited in a fracture, proppants can resist further fluid flow or the flow of other proppants, inhibiting further growth of the fracture. In addition, closure stresses (once external fluid pressure is released) may cause proppants to reorganise or "squeeze out" proppants, even if no fines are generated, resulting in smaller effective width of the fracture and decreased permeability. Some companies try to cause weak bonding at rest between proppant particles in order to prevent such reorganisation. The modelling of fluid dynamics and rheology of fracturing fluid and its carried proppants is a subject of active research by the industry.
Proppant costs
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Though good proppant choice positively impacts output rate and overall ultimate recovery of a well, commercial proppants are also constrained by cost. Transport costs from supplier to site form a significant component of the cost of proppants.
Other components of fracturing fluids
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Other than proppant, slickwater fracturing fluids are mostly water, generally 99% or more by volume, but gel-based fluids can see polymers and surfactants comprising as much as 7 vol%, ignoring other additives.[6] Other common additives include hydrochloric acid (low pH can etch certain rocks, dissolving limestone for instance), friction reducers, guar gum,[7] biocides, emulsion breakers, emulsifiers, and 2-Butoxyethanol.
Radioactive tracer isotopes are sometimes included in the hydrofracturing fluid to determine the injection profile and location of fractures created by hydraulic fracturing.[8] Patents describe in detail how several tracers are typically used in the same well. Wells are hydraulically fractured in different stages.[9] Tracers with different half-lives are used for each stage.[9][10] Their half-lives range from 40.2 hours (lanthanum-140) to 5.27 years (cobalt-60).[11] Amounts per injection of radionuclide are listed in The US Nuclear Regulatory Commission (NRC) guidelines.[12] The NRC guidelines also list a wide range of radioactive materials in solid, liquid and gaseous forms that are used as field flood or enhanced oil and gas recovery study applications tracers used in single and multiple wells.[12]
In the US, except for diesel-based additive fracturing fluids, noted by the American Environmental Protection Agency to have a higher proportion of volatile organic compounds and carcinogenic BTEX, use of fracturing fluids in hydraulic fracturing operations was explicitly excluded from regulation under the American Clean Water Act in , a legislative move that has since attracted controversy for being the product of special interests lobbying.[citation needed]
See also
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References
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Figure 1. A sand-type hydraulic fracturing proppant called frac sand.
Proppant is a gritty material with uniformly sized particles that is mixed in with fracturing fluid during the hydraulic fracturing (fracking) process to hold open fractures made in the ground. There are a variety of different types of proppant, including naturally occurring sand and man-made proppants. Man made proppants include materials such as resin-coated sand or strong ceramic materials. Proppants come in a variety of different sizes and spherical shapes for a variety of different situations.[2]
Recently, demand for proppants has increased as oil and natural gas wells are being made to yield more oil and gas using hydraulic fracturing. One job can require a few thousand tons of this proppant material.[3]
Types
There are several different types of materials used as proppant. The first of these types is known as frac sand, and is simply a high-purity quartz sand with durable, round grains. As a result of its strength it is crush-resistant, and thus is effective as propping open cracks made in the ground during the hydraulic fracturing process. Most of this sand is made from high purity sandstone.[3] Although natural, this frac sand is not used directly out of the ground, rather it requires processing. The processing process involves washing to remove particles that are too small, and then a screening process to ensure the remaining grains are the correct size.
Other types of proppant materials include resin-coated sand and ceramic proppants. Resin-coated sand is simply sand that is coated in a resin material to smooth the surface of the sand and make the shape more uniform. Along with this, coating the sand in resin increases its strength, making it more desirable as a proppant. Finally, ceramic proppants are the most uniform in shape and the strongest of the proppants as their manufacturing is entirely controlled. The uniform shape of this type of proppant ensures that there is more space for the oil and gas to flow through the proppant material and out of the well.[4]
Size and Shape
Proppants come in a variety of different grain sizes, as well as different shapes. The size and shape of a proppant is important as it influences the final permeability in the induced fracture. Frac sand, or naturally occurring sand-type proppant is generally irregular in shape, although this depends on the source. Compared to other types of proppants it has a low strength and packs together closely in fractures, resulting in a lower permeability when compared to other proppant types. Resin-coated sand is more smooth and round in shape, and is stronger than traditional frac sand. As a result of this shape and texture, resin-coated sand does not pack as closely together and thus is more permeable than frac sand. Finally, ceramic proppant is the most uniform shaped and most round proppant. It has a high strength, and as a result of its properties it is also very permeable, allowing trapped oil or natural gas to flow easily out of the fractures.[5]
References
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