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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 A gel is a semi-solid that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state, although the liquid phase may still dif ...
,
foam Foams are materials formed by trapping pockets of gas in a liquid or solid. A bath sponge and the head on a glass of beer are examples of foams. In most foams, the volume of gas is large, with thin films of liquid or solid separating the reg ...
or slickwater–based. In addition, there may be unconventional fracking fluids. Fluids make tradeoffs in such material properties as
viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...
, where more viscous fluids can carry more concentrated proppant; the energy or pressure demands to maintain a certain flux pump rate (
flow velocity In continuum mechanics the flow velocity in fluid dynamics, also macroscopic velocity in statistical mechanics, or drift velocity in electromagnetism, is a vector field used to mathematically describe the motion of a continuum. The length of the f ...
) 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 Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...
wells (20k to 80k gallons per well) to the high-volume operations such as
shale gas Shale gas is an unconventional natural gas that is found trapped within shale formations. Since the 1990s a combination of horizontal drilling and hydraulic fracturing has made large volumes of shale gas more economical to produce, and some a ...
and
tight gas Tight gas is natural gas produced from reservoir rocks with such low permeability that massive hydraulic fracturing is necessary to produce the well at economic rates. This natural gas is trapped within rocks with very low permeability, in other ...
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 (1988) found that gel-based fluids seemed to achieve the best results for
coalbed methane Coalbed methane (CBM or coal-bed methane), coalbed gas, coal seam gas (CSG), or coal-mine methane (CMM) is a form of natural gas extracted from coal beds. In recent decades it has become an important source of energy in United States, Canada, Au ...
operations, but as of 2012, 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 Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...
(low pH can etch certain rocks, dissolving
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
for instance), friction reducers,
guar gum Guar gum, also called guaran, is a galactomannan polysaccharide extracted from guar beans that has thickening and stabilizing properties useful in food, feed, and industrial applications. The guar seeds are mechanically dehusked, hydrated, mil ...
,
biocide A biocide is defined in the European legislation as a chemical substance or microorganism intended to destroy, deter, render harmless, or exert a controlling effect on any harmful organism. The US Environmental Protection Agency (EPA) uses a slig ...
s, emulsion breakers,
emulsifier An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Altho ...
s, 2-butoxyethanol, and
radioactive tracer A radioactive tracer, radiotracer, or radioactive label is a chemical compound in which one or more atoms have been replaced by a radionuclide so by virtue of its radioactive decay it can be used to explore the mechanism of chemical reactions by ...
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. Though
sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of s ...
is a common proppant, untreated sand is prone to significant fines generation; fines generation is often measured in wt% of initial feed. A commercial newsletter from
Momentive Momentive Inc. is an American chemical company. Its products include silicones and quartz. Momentive is headquartered in Waterford, New York. The company employs 9,200 employees across more than 50 locations throughout North America, Europe, As ...
cites untreated sand fines production to be 23.9% compared with 8.2% for lightweight ceramic and 0.5% for their product. 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
ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
s and sintered
bauxite Bauxite is a sedimentary rock with a relatively high aluminium content. It is the world's main source of aluminium and gallium. Bauxite consists mostly of the aluminium minerals gibbsite (Al(OH)3), boehmite (γ-AlO(OH)) and diaspore (α-AlO(O ...
.


Proppant weight and strength

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. 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).


Proppant deposition and post-treatment behaviours

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. 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

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

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 Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...
(low pH can etch certain rocks, dissolving
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
for instance), friction reducers,
guar gum Guar gum, also called guaran, is a galactomannan polysaccharide extracted from guar beans that has thickening and stabilizing properties useful in food, feed, and industrial applications. The guar seeds are mechanically dehusked, hydrated, mil ...
,
biocide A biocide is defined in the European legislation as a chemical substance or microorganism intended to destroy, deter, render harmless, or exert a controlling effect on any harmful organism. The US Environmental Protection Agency (EPA) uses a slig ...
s, emulsion breakers,
emulsifier An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Altho ...
s, and 2-Butoxyethanol.
Radioactive tracer A radioactive tracer, radiotracer, or radioactive label is a chemical compound in which one or more atoms have been replaced by a radionuclide so by virtue of its radioactive decay it can be used to explore the mechanism of chemical reactions by ...
isotopes are sometimes included in the hydrofracturing fluid to determine the injection profile and location of fractures created by hydraulic fracturing.Reis, John C. (1976). ''Environmental Control in Petroleum Engineering.'' Gulf Professional Publishers. Patents describe in detail how several tracers are typically used in the same well. Wells are hydraulically fractured in different stages.
Scott III, George L. (3 June 1997) US Patent No. 5635712: Method for monitoring the hydraulic fracturing of a subterranean formation. US Patent Publications.
Tracers with different half-lives are used for each stage.
Scott III, George L. (15-Aug-1995) US Patent No. US5441110: System and method for monitoring fracture growth during hydraulic fracture treatment. US Patent Publications.
Their half-lives range from 40.2 hours (
lanthanum-140 Naturally occurring lanthanum (57La) is composed of one stable (139La) and one radioactive (138La) isotope, with the stable isotope, 139La, being the most abundant (99.91% natural abundance). There are 38 radioisotopes that have been characterize ...
) to 5.27 years (
cobalt-60 Cobalt-60 (60Co) is a synthetic radioactive isotope of cobalt with a half-life of 5.2713 years. It is produced artificially in nuclear reactors. Deliberate industrial production depends on neutron activation of bulk samples of the monoisoto ...
).
Gadeken, Larry L., Halliburton Company (08-Nov-1989). Radioactive well logging method.
Amounts per injection of radionuclide are listed in The US
Nuclear Regulatory Commission The Nuclear Regulatory Commission (NRC) is an independent agency of the United States government tasked with protecting public health and safety related to nuclear energy. Established by the Energy Reorganization Act of 1974, the NRC began operat ...
(NRC) guidelines. 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. In the US, except for diesel-based additive fracturing fluids, noted by the American
Environmental Protection Agency A biophysical environment is a biotic and abiotic surrounding of an organism or population, and consequently includes the factors that have an influence in their survival, development, and evolution. A biophysical environment can vary in scale f ...
to have a higher proportion of
volatile organic compound Volatile organic compounds (VOCs) are organic compounds that have a high vapour pressure at room temperature. High vapor pressure correlates with a low boiling point, which relates to the number of the sample's molecules in the surrounding air, a ...
s and carcinogenic
BTEX In the petroleum refining and petrochemical industries, the initialism BTX refers to mixtures of benzene, toluene, and the three xylene isomers, all of which are aromatic hydrocarbons. The xylene isomers are distinguished by the designations ''or ...
, use of fracturing fluids in hydraulic fracturing operations was explicitly excluded from regulation under the American
Clean Water Act The Clean Water Act (CWA) is the primary federal law in the United States governing water pollution. Its objective is to restore and maintain the chemical, physical, and biological integrity of the nation's waters; recognizing the responsibiliti ...
in 2005, a legislative move that has since attracted controversy for being the product of special interests lobbying.


See also

*
List of additives for hydraulic fracturing The differences between additives for fracking in different countries are the type of chemicals used (hazardous, non-hazardous), the disclosure of chemicals and the composition of fracturing fluid. In 2010, Halliburton announced the creation of f ...
*
Hydraulic fracturing and radionuclides Hydraulic fracturing is the propagation of fractures in a rock layer by pressurized fluid. Induced hydraulic fracturing or hydrofracking, commonly known as fracking, is a technique used to release petroleum, natural gas (including shale gas, tight ...


References

{{Reflist Petroleum engineering Hydraulic fracturing