While the oil and natural gas industry has had its ups and downs, many speculate it is again poised for growth due to the current political climate. Subsequently, there is a renewed interest around proppants &#; the hard microspheres injected into well bores to &#;prop open&#; rock fissures allowing natural gas and oil to flow out. According to the Fredonia Group, North American proppant demand looks set to grow at an annual rate of 7.6% through .
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Frac sand, or silica sand, a naturally made, abundant quartz sand, is the most commonly used proppant due to its low cost and high availability. However, while frac sand offers an economic solution in most cases, some settings require a bit more ingenuity on behalf of the proppant.
Ceramic proppants &#; a man-made proppant, offer a tailored solution in more demanding drilling settings.
Benefits of Ceramic Proppants
While ceramic proppants cost more than their naturally-made counterparts, they do offer advantages that make them the preferred choice in some settings, because they are capable of maximizing the yield from a given well. These advantages include:
As well bores get deeper, the pressure on proppants gets higher, and frac sand is not always up to the task.
Ceramic proppants can withstand a much greater crush strength than traditional frac sand products &#; up to 10,000 psi.¹ This allows drillers to access much deeper shale formations.
Ceramic proppants also boast a number of characteristics that translate to better conductivity, or the effectiveness at which the petroleum product can flow around the proppants in the rock fissure.
Improved uniformity in size, shape, and overall sphericity allows natural gas or oil to more easily flow around ceramic proppants in the well bore.
Conductivity is further improved due to the stable thermal and chemical nature of ceramic proppants; there is much less risk for the proppant to thermally or chemically react with the shale formation causing an undesirable deposit to form, which would ultimately clog the fissure.¹
Because they are manmade, the characteristics of ceramic proppants can be much more tightly controlled than with sand products. Proppants can be selected based on a variety of engineered characteristics to offer a more tailored solution to the specific needs of a well site.
Within the category of ceramic proppants, proppants are also available in three sub-categories, with higher density proppants able to withstand greater stress:
Ceramic Proppant Production
Ceramic proppants can be made from a variety of materials. While most are made from bauxite, kaolin (aka China Clay), or a blend of the two materials, a variety of other materials may be used, including magnesium silicate and fly ash. Various additives may also be included.
The manufacturing process of ceramic proppants can differ, but in general, proppants follow the process described below.
Raw materials are crushed into powder form. Materials and additives may be mixed in to develop specialized formulations in order to maximize performance of the end product.
The powdered raw material is then pelletized to create a uniform, round pellet/granule product. Proppant sizing is a critical factor in the performance of a proppant at a given well and as such, producers often provide a wide offering of mesh sizes. Proppants are typically between 8 and 140 mesh and it is not uncommon to use multiple sizes at a given well.¹
Once agglomerated, pellets are dried to reduce the moisture content of the material. This is typically carried out in a rotary dryer or other type of industrial drying system. Pellets are classified via screening.
Perhaps the most important step in the process, on-size pellets are sintered in a rotary kiln to &#;cure&#; the pellets. This high temperature sintering, which occurs at around &#; º F, causes chemical and crystalline changes in the material, causing it to harden further, ultimately imparting the crush strength that will be required for high pressure drilling applications.
The hot material exits the kiln and is fed to a rotary cooler. The cooler cools the hot product so it can move on to storage or transport.
Ceramic Proppant Development
When working with ceramic proppants, feasibility testing and process development are often an integral part of developing specialized proppants. Source materials are often tested first at batch scale to confirm feasibility, then at continuous pilot scale in rotary kilns designed to simulate production conditions, depending on the capabilities of the testing facility.
Testing not only confirms feasibility of the intended process with the unique source material, but it also provides the data necessary, such as time and temperature profiles, for process scale-up and the design of a commercial scale unit.
Conclusion
While ceramic proppants make up a small portion of the overall proppant market compared to frac sand, they are still a valuable tool in high stress settings. Their thermal and chemical stability, combined with a more uniform shape and size, and their high conductivity, make them a worthwhile alternative for more specialized applications.
FEECO provides feasibility testing and process development around the manufacture of ceramic proppants. In addition to our testing services, we engineer and manufacture custom rotary kilns to meet your proppant sintering needs, as well as rotary dryers and coolers for the proppant production process. For more information, contact us today!
1.F. Liang et al. A Comprehensive Review on Proppant Technologies. Petroleum 2 () 26 &#; 39. Web. Feb. .
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.
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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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