Application of polymers for coating of proppant in hydraulic fracturing of subterraneous formations: A comprehensive review

Journal of Natural Gas Science and Engineering 24 (2015) 197e209

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Review article

Application of polymers for coating of proppant in hydraulic fracturing of subterraneous formations: A comprehensive review Mansoor Zoveidavianpoor*, Abdoullatif Gharibi Universiti Teknologi Malaysia, Faculty of Petroleum & Renewable Energy Engineering, 81310 Johor Bahru, Johor, Malaysia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 November 2014 Received in revised form 17 March 2015 Accepted 18 March 2015 Available online 27 March 2015

Polymers have extensively been employed by petroleum industry to maintain, treat and optimize drilling and production operation in oil and gas wells. Polymers minimize solid deposition in wells, maintain fluid viscosity, thicken water, reduce downtime caused by corrosion and maintenance work, but can also be used to clean well and surface equipment. One of the most widely forms of polymer application in oil and gas industry is related to coating of proppant in hydraulic fracturing (HF) operation. Proppants are small spheres that must have enough strength to withstand to high closure stresses. Application of polymers for coating of proppants is just emerging in recent years and promises higher performance by taking the both of strength and flexibility context into account. Coating of proppant with a thin layer of polymer will results in higher fracture conductivity, considerably reduce fines generation and scaling, thereby improving the quality of HF treatment. This paper reviews the polymer application for coating of proppants in HF treatment. The purpose of this article is three-fold: (1) to update petroleum and chemical scientists and engineers on the latest development in polymer utilization for coating of proppant, (2) to give some basic details on proppants and polymers to help understand why these developments came about and to (3) summarize the latest advances in proppant coating methods and procedures. © 2015 Elsevier B.V. All rights reserved.

Keywords: Polymer Hydraulic fracturing Coatings Resin Proppant

1. Introduction Hydraulic fracturing (HF) is known as the main and effective method for increasing oil and gas recovery (Zoveidavianpoor et al., 2010) and since its inception; it has made significant contributions to the petroleum industry (Veatch and Moschovidis, 1986). The 1970s have seen the resurgence of HF treatment. Much of the resurgence has been due to the decline of the US domestic gas production that turns the interest in improving ways to extract oil and gas from unconventional formations. Various combinations of fracturing fluids and proppants can be designed based on individual well conditions and as can be seen from Fig. 1, proppant is the second abundant constitute with ~9.5 weight percent. As discussed by Holditch (1979), the ideal propping agent has a low density, resistant to crushing and corrosion, is strong and readily available at low cost. Proppant as a derivative of the combination of synthetic based or natural based materials is used to keep open the fracture. Proppant Market report categorizes

* Corresponding author. E-mail address: [email protected] (M. Zoveidavianpoor). http://dx.doi.org/10.1016/j.jngse.2015.03.024 1875-5100/© 2015 Elsevier B.V. All rights reserved.

the global market by three types; sand, ceramic, and Resin Coated Proppant (RCP) that are utilized by 80%, 10% and 10% by volume (see Fig. 1). In general, the availability of the newly created fracture area is limited for production, if no proppants are placed in the fracture to keep it open. The function and the representative compound of the additives used in fracturing fluid are tabulated in Table 1. The importance of HF technology explains why production of unconventional gas reservoirs (i.e., very tight porous formations) which are economically challenging, have proved satisfactory. The substantial portion of the gas production from shale plays has made the United States as the largest natural gas producer in the word, which achieved via the introduction of HF technology (Wang et al., 2014). The boom in the development of proppant coating is being fueled by continued improvements in proppant technologies for fractured wells. Coated proppant has created many benefits for the upstream petroleum industry. Therefore, the present study addresses the most common types of polymers for coating of proppant, advantages and disadvantages of polymer coatings, and their performance in different conditions. Also, this paper provides an overview of common methods for coating of proppant. The present

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Fig. 1. Composition of a typical fracturing fluid (modified from Arthur et al., 2008).

study offers an opportunity for researchers to know more about various types of proppant and their properties, advantages of using polymers for coating of proppant, and processes and procedure of coating proppant.

tons, a 28% increase compare to 2012 and predicted to reach 84.2 million tons by 2019 at a compound annual growth rate of 10.7% from 2014 to 2019. More than 99% of that supply is met with sand, resin-coated sands, and ceramic proppant.

2. Proppant

3. Low-weight proppant evolution

Proppants are small particles used in combination with fracturing fluid to keep the created fracture open during HF treatment. Proppant can be classified in two categories; conventional and advanced. Conventional proppants include sand, ceramic, nutshells, and glass beads. Proppants that are coated with a thin layer of polymer are known as advanced proppants. Comprehensive information about different types of proppants and their properties is presented in Table 2. According to the Proppant Market Report (PMR, 2014), the proppant market in 2013 exceeded 45 million

The relatively low cost, abundance, sphericity, and low specific gravity of high-quality sands have made sand a good proppant for most HF treatments. However, the closure stress on the substrates increases with depth and fracture conductivity has been found to deteriorate rapidly when closure stresses exceed approximately 6000 psi. In response to this problem and to resist the increased closure stress of deeper wells, several higher-strength proppants have been developed including sintered bauxite (Cooke, 1977), ceramic (Sarda, 1981) and resin-coated sand (Sinclair and Graham,

Table 1 Typical composition of HF fluid. Constituent

Representative compound

Purpose

Carrier/base fluid Proppant Acid Clay stabilizer Anti-bacterial agent Friction reducer Surfactant Salt Scale inhibitor pH-adjusting agent Iron control Corrosion inhibitor Biocide Breaker Crosslinker Gelling agent

Fresh water Sand Hydrochloric or muriatic acid Choline chloride Glutaraldehyde Polyacrylamide Isopropanol Potassium chloride Ethylene glycol Sodium or potassium carbonate Citric acid n,n-dimethyl formamide Glutaraldehyde Ammonium persulphate Borate salt Guar gum or hydroxyethyl cellulose

Fracture the rock Holds fractures open after flowback Dissolves minerals and initiates cracks in the rock Prevents formation clays from swelling Eliminates bacteria Minimizes friction between the fluid and the pipe Increases the viscosity of the fracture fluid Creates a brine carrier fluid Prevents scale deposits in pipes Maintains effectiveness of chemical additives Prevents precipitation of metal oxides Prevents pipe corrosion Minimizes growth of bacteria that produce corrosive and toxic by-products Allows a delayed breakdown of gel polymer chains Maintains fluid viscosity as temperature increases Thickens water to suspend the sand

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Table 2 General information about proppants used in HF treatment. Proppant type

Sand

Ceramic

Glass beads

Nut shells

RCP

Surface properties Specific gravity Closure pressure application range (psi) Surface resistance to chemicals Cost Strength

Hydrophilica 2.5e2.69b 6,000c Goodd Cheape Lowf

Hydrophilicg 3.29e3.60b 15,000c Strengthd Expensive than sand and glass beadsh Highf

Hydrophilici 2.2e2.5j 5000e6,000k Goodl Expensive than sandm Lowk

Hydrophobic- Hydrophilicn 0.94e1.697o 6,250p Poorq Cheapp Lowp

Hydrophobicr 1.25e2.61s 10,000e15,000c Strengthq Expensivet Highf

a b c d e f g h i j k l m n o p q r s t

(Tschapek et al., 1983). (Jones et al., 1981). (Betzold, 2001). (Shmotev and Pliner, 2009). (Li et al., 2013). (Han et al., 2014). (Watanabe, 2009). (Urbanek, 2006). (Tsuneda et al., 2003). (Bayerl and Bloom, 1990). (Graham and Sinclair, 1988). (Senturk et al., 2006). (Lesko et al., 2011). (Li et al., 2011). (Akinhanmi et al., 2008). (Gaoxiang, 2012). (Ueno et al., 2014). (Qin and Hu, 2013). (Huang, 2014). (Zhang, 2009).

1981). On the other hand, the higher cost of these substrates has been the most significant factor constraining their widespread use. In addition, the higher specific gravity of ceramic proppants increases the volume cost and enhances proppant settling in the fracture; both consequences are undesirable. Therefore, lowerspecific-gravity proppants are more cost effective and have the potential to improve proppant transport and accordingly may determine the success of a HF treatment (Novotny, 1977). The transport characteristics of proppants during placement, therefore, need to be improved and accordingly works on proppants with lower specific gravities were initiated (Jones et al., 1981; Cutler and Jones, 1982). As shown in Table 2, the specific gravity of ceramic is 3.60 and sand is 2.69; therefore, ceramic is 33.8% more dense than sand. The settling velocity for ceramic proppant, as shown by Cutler et al. (1985), however, is ~65% higher than sand. RCPs are significantly lighter than conventional proppants and exhibit high compressive strength. There are various criteria to select proppants, fracturing fluids, and pump pressure in HF treatment. The main options for proppant selection are sufficient particle strength to withstand the fracture closure stress and median particle diameter. The primarily criterion in selection of fracturing fluid is providing sufficient viscosity to reduce proppant settling in the wellbore and in the created fracture. In addition, pumping rate is limited by the wellbore profile, perforation area and the desire to keep the fracture in-zone. It has been shown that reducing the pressure of fracturing fluid can contribute to limited vertical propagation of the fracture (Simonson et al., 1978; Nolte, 1982), which is of high priority in HF treatment. This pressure can be controlled via two options; treatment pumps and fluid viscosity. Minimizing fluid viscosity is demonstrated to be much more effective method than reducing the pumping rate (Nolte, 1982). Lower-specific gravity proppants can take place in lower-viscosity fluids and also higher volumes of proppant can be pumped. The potential benefits of low-weight proppant application in HF treatment are (1) reduce proppant settling (2) increased effective propped length (3) using fluids of lower viscosity (4) lower pumping rate. As a result, the previous limitations imposed on the fluid and pumping rates by utilizing low density proppants can be

eliminated and/or minimized. Therefore, low-weight proppants can lower proppant costs and result in significant advances in fracture control and thereby increase hydrocarbon production. 4. From conventional to advanced proppants Sand was not capable to withstand high closure stresses (Tschapek et al., 1983); ceramic was introduced to remove this problem but the high specific gravity had restricted its utilization nowadays (Watanabe, 2009). Consequently, proppant settling occurred before reaching the end of the fracture (Shmotev and Pliner, 2009). Utilization of glass beads as another type of conventional proppant was restricted due to high costs of energy and production. Furthermore, its application was restricted to certain depths because of its lower resistance to high closure stress (Tsuneda et al., 2003). Nutshells are deformable under high closure stresses which cause sealing of fracture when closure pressure increases. Problems that are created during utilization of conventional proppants have motivated researchers to focus on using polymers to improve the quality of proppants. Hence, to improve the capability of conventional proppants to withstand high closure stresses, polymers are added in order to (1) prevention of flowback, (2) better cleanup of the formation, and (3) prevention of settling of proppant. Coated proppant is comprised of a substrate (i.e., proppant) and one or two layers of polymer. The portion of the particle that is coated by polymers is known as substrate (Li et al., 2011). Sand, glass beads, ceramics, carbon particles, bauxite, crushed nut hulls, and other natural fibers have demonstrated as good candidates for application as substrate of polymer coating (i.e., RCP) (Qin and Hu, 2013). Coating of lignocellulosic materials such as coconut shell with appropriate polymers leads to a proppant with low specific gravity, high strength, and low velocity settling which are known as light weight and ultra-low weight proppants (Zoveidavianpoor et al., 2014). Modification of the surface properties of substrates is performed by means of polymer coating to obtain favorable effects such as altering surface topography, wettability, and chemical reactivity (Jones and Cutler, 1985). Bounding of the polymer to particle can be

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physical in nature, however may be conducted with chemical bonds that are formed between components of substrate and polymer. Chemical bonds are usually covalent, ionic, or both. It is to be noted that during interacting of polymer and substrate, some components of polymer coating may be diffused, infiltrated, or impregnated on the particle leading to development of physical or chemical bonds, or both (Bayerl and Bloom, 1990). For coating of substrate, investigation of the capability of its surface to repel or adsorb water is required. However, the tendency of various types of proppant to repel or absorb water is not similar. As indicated in Table 2, surfaces of sand, glass beads and ceramic are hydrophilic (Akinhanmi et al., 2008). It means that they have more tendencies to absorb water. Consequently, they require hydrophobic polymers which can modify their surfaces to hydrophobic. Among conventional proppants, nut shells usually have hydrophobic-hydrophilic surfaces. Utilization of hydrophobic polymer is required for coating of nutshells. More information about composition of nut shells can be found in Urbanek (2004). 5. Polymer Polymer is a chemical material comprised of a series of smaller units (monomers) containing a general weight of molecules. Polymers are comprised of a minimum of 3 monomers that are in covalent bond with other monomers (Huang, 2014). The manner of polymer formation is known as polymerization; that can be performed through addition reaction or condensation reaction (Betzold, 2001). Polymers are classified into organic and inorganic polymers (Graham and Sinclair, 1988). Polymers are also classified as thermosetting and thermoplastic in terms of temperature behavior (Gaoxiang, 2012). Organic polymers are the most widely used type of polymer; the main advantages such as easy processibility, high degree of strength, and low density have made this type of polymer the most widely used type of polymer (Graham and Sinclair, 1988). In contrast to their advantages, they suffer from some disadvantages (Shmotev and Pliner, 2009). First, the monomers units from which they are manufactured are often exposed to the vagaries of the petroleum industry (Graham and Sinclair, 1988). These types of polymers have low softening temperatures or low degradation temperatures. Second, most of them are also exposed to degradation from oxygen, ozone, or high-energy radiation (Gaoxiang, 2012). The most common types of organic polymers for proppant coating are different types of resins, furan, polyesters and vinyl esters, and polyurethane. Polymers that used for coating of proppants are presented in Table 3. Interested readers can find relevant information about coating technology in Goldschmidt and Streitberger (2003). Utilization of hydrophobic polymer can provide coating with excellent shear strength because they are less affected by water compared to hydrophilic polymers. As indicated in Table 3, resins

are the main type of polymer used for coating of proppant; it may be because of superior properties of resins compared to other types of presented polymers. Curing agents are used to bind the resin polymeric coating to the particle (Senturk et al., 2006). Properties and different types of curing agents for coating of proppants are presented in Table 4. Interested readers can find relevant information in Hara (1990) and Goldschmidt and Streitberger (2003). Furan resin is another type of organic polymer used for coating of proppant. The derived results from Table 3 indicate that furan has great resistance to heat and water. However, Furan cannot provide enough strength at high closure stresses. Vinyl esters and polyesters are other types of resin that can be employed for coating of proppant (Ueno et al., 2014). In accordance to Table 3, properties that make vinyl esters more useful for coating of proppant are appropriate resistance to water, good capability to make strong bonds that leads to excellent physical strength, and fair resistance to mineral acids. Vinyl esters also have indicated great performance to resistance to a variety range of pH. Polyester coating with high degree of heat resistance, chemical resistance, and high compressive strength can be applied as a good choice for coating of proppant. Another type of polymer commonly used in the proppant industry is polyurethane (Li et al., 2013). It has indicated great potential to coat solid materials such as biomasses, sand, ceramic, glass beads and virtually any type of material that are used as a substrate. As shown in Table 3, polyurethane can provide great chemical and moisture resistance in more rigid systems because of highly cross-linkage of chemical bonds between elements of the polymer. In addition, it has good potential to provide an impermeable barrier between proppant and external medium. Application of polyurethane as coating is recommended when temperature is less than 254  F and pressure is less than 7500 psi (Li et al., 2013; O'Brien and Haller, 2013). Coating of proppant can be performed without using solvent while most of the known methods required solvents (O'Brien and Haller, 2013). From the environmental standpoint, emission of harmful vapors during polymerization will cause damage to human beings (Mcdaniel et al., 2014). Care must be taken because utilization of this type of polymer similar to other chemical coatings is not safe especially when swallowed. According to Table 3, polyurethane coatings cannot provide appropriate heat resistance when temperature is more than 250  F. Comparison of polyurethane and resins indicates that curing of polyurethane is generally faster than resins. In addition, polyurethane coating is capable of curing at lower temperatures and do not have gaseous emissions that require specialized recovery equipment (Urbanek, 2006). Through creation of a balance between these differences, it is possible to choose the best polymer to apply on the surface of proppants. Chemical resistance of resin is desirable and it is capable of taking many acids and bases while chemical resistance of polyurethane is moderate and can only take diluted bases and acids.

Table 3 Coating polymers of proppant and their properties. Organic polymers

Dry temperature (oF)

Resistance to heat

Resistance to acid

Resistance to water

Strength

Aging

Hydrophobic capability

Hydrophilic capability

Resistance to chemical

Epoxy resins Furan Polyester Urea aldehyde Polyurethane Phenol-aldehyde Vinyl esters Furfural alcohol and furfural

250e400 375 212e300 250e400 210e250 250e400 212e300 250e400

Excellent Moderate Fair Excellent Good Excellent Fair Excellent

Good Good Fair Good Fair Good Good Good

Good Good Good Good Fair Good Good Good

Good Poor Fair Good Good Good Fair Good

Very good Good Good Very good Good Very good Good Very good

Good Fair Fair Good Good Good Fair Good

Poor Poor Poor Poor Poor Poor Poor Poor

Good Good Moderate Good Moderate Good Moderate Good

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Table 4 Curing agents and their properties for proppant coating. Curing agent

Curing time

Resistance to heat

Resistance to acid

Cycloaliphatic Amines Polyamine Adducts Polyamides/Amidoamines Aromatic Amines Aliphatic Amines Ketimines

Slow Moderate Slow Slow Fast Slow

High Moderate Low High Moderate Moderate

High Moderate Low High Moderate Moderate

5.1. Resin Coated Proppants Polymers have been widely used in oil and gas industry in composition of fluids and additives to remove certain problems that affect drilling and production of hydrocarbon. As discussed by Mahoney et al. (2013), coating of proppant with a thin layer of polymer can improve the efficiency of HF treatment. This improvement can be achieved through fracture conductivity enhancement (Nguyen et al., 2000), reduction of proppant flow back (Nguyen and Weaver, 1997) and minimizing formation fine production (Gidley et al., 1995), maintenance of long term fracture permeability (Montgomery and Steanson, 1985), and increase in fracture permeability (Blauch et al., 2007). Two different types of RCPs that are known as precured and curable proppant have been offered to the market. Polymeric coating can preserve the particle in operating temperatures and pressures of the subterranean formation. Another important advantage of the polymeric coating is protection of the particle from closure stresses of the subterranean formation (see Fig. 2). Moreover, protection of the particle from ambient conditions and reduced disintegration and/or dusting of the particle can be performed with polymeric coating. The properties and different elements that are involved in the RCP are presented in Table 5. Second column of Table 5 indicates the substrates that are usually used in composition of RCP. Third column shows the polymers that are usually used for coating of substrates. Polymers can be classified to thermosetting and thermoplastic polymers. Thermosetting polymers are comprised of cross-linked compositions and are connected irrevocably throughout molding into an inter-connected molecular (Green et al., 2013). They are not capable of being remolded. Fig. 3 presents an example of thermosetting resin. On the other hand, polymers that are capable to be constantly softened with no alteration in chemical structure are known as thermoplastic polymers (Graham and Sinclair, 1988). They are capable to remold constantly because they can easily keep their structure and

separate after molding. Some of the main advantageous of thermoplastic polymers are: (1) enhanced ductility, superior impact resistance, and great fracture toughness (2) great potential to be recycled and reused (3) excellent corrosion resistance (4) great moisture resistance. More information about chemical structures, physical, and mechanical properties of thermoplastic resin can be found in Tanguay et al. (2011). Fig. 4 indicates an example of thermoplastic polymer. According to the third column of Table 5, thermosetting polymers have been utilized more than thermoplastic types. Their extensive utilization for proppant coating is due to high tendency of epoxy group in composition of thermosetting resins to bind with elements of substrate. Thermosetting can provide more heat resistance compared to thermoplastic polymers. Also, the high flexibility of thermosetting resin for design provides better condition than thermoplastics for coating of proppant. Moreover, thermosetting resins are more cost effective compared to thermoplastic polymers. However, they have suffered from several drawbacks because they cannot reshape, remold and recycle. Coating of nutshells can be conducted with polyesters and thermosetting resins (Zhang, 2009). To improve the capability of polymer for coating, additives that are indicated in the fourth column of Table 5 are used. Last column of Table 5 indicates specific gravity of proppants. Specific gravity of proppant plays an essential role in its transportation with fracturing fluid. Low weight proppant with high strength can provide more effective propped fracture length (McDaniel and McCrary, 2012). Fifth column of Table 5 indicates specific gravity of RCP that is changed in the range of 1.25e3.65. 6. Polymers for proppant coating Polymers (e.g., resin) may be applied to improve proppant strength and act as a glue to hold some of the coated grains together. As shown in Fig. 2, by increasing the closure stress, sand is

Fig. 2. The response of uncoated (left) and coated (right) proppants to closure stress (modified from Stevens, 2014).

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Table 5 General information about different types of substrate and polymers. Authors

Substrate material

Organic polymers applied for coating

Additives

Specific gravity

Graham (1972)

Sand-glass bead

Thermosetting resin, Thermoplastic resin

2.50e2.60

Everett et al. (1975) Graham and Sinclair (1988) Underdown and Glaze (1986) Evans and Sharp (1988) Dewprashad (1995) Johnson and Tse (1997) Sinclair and Richard (1997)

Glass bead Sand Sand Sand Glass beads or sand Sand Sintered bauxite, aluminum and zirconium oxide. In addition, other ceramic material can be used. Sand Ground Walnut shells Sand, lightweight ceramic, bauxite, bio- based material such as different types of nut shells, apricot, and olive Sand, bauxite, ceramic materials, glass materials, Walnut hulls, nutshells, polymer beads. Sand or ceramic particle

Thermosetting Thermosetting Thermosetting Thermosetting Thermosetting Thermosetting Thermosetting

Reinforcing agent such as glass fibers, asbestos, mica, silica, and alumina. Curable agent Curable agent

Dewprashad et al. (1999) Brannon et al. (2002) Sinclair et al. (2006)

Nguyen et al. (2007)

McCrary et al. (2009)

Rediger et al. (2012)

Sand, glass beads, ceramics, carbon particles, bauxite, crushed nut, hulls, manmade polymeric particles

Mahoney et al. (2013) Green et al. (2013)

Sand, ceramic Exfoliated clays, sand, sintered ceramics Sand, ceramic particles

Kuhlmann et al. (2014)

resin resin resin resin resin resin resin

Thermosetting resin Thermosetting resin, polyesters Thermosetting (resin, acrylics, and self-cross linking acrylics), polyurethane

Coupling agent (organo functional silanes), calcium stearate is used for prevention of sintering and mineral oil can be applied for prevention of dust problems, and reinforcing agents. coupling agent (aminosilane)

a coupling agent, fibrous reinforcing agents

Thermosetting resin .

Thermosetting resin, polyester, acrylic and polyurethane. Products of Maillard reaction between a carbohydrate and an ammonium salt of a carboxylic acid, thermoplastic resins, thermosetting resins Hydrogel Thermosetting resin, polyurethane

2.6 2.50 2.56 2.60 2.60 2.50 2.50e3.50

2.65 1.25e1.35 1.25e3.65

1.25e3.65

inorganic filler materials

2.50e3.50

coupling agents include amino, epoxy, and ureidoorgano silanes

1.25e3.65

polyurethane, Thermosetting resin

2.60e3.50 2.50e3.65

2.50e3.65

Fig. 3. Example of thermosetting polymer (epoxy resin).

crushed and when it shatters, fines are released. Fine production and limited fracture conductivity and among others are the result of this failure. The most predominant problems that occur due to improper proppant selection are proppant flowback, fines generation, proppant scaling and low fracture conductivity. Proppant crushing occurs when the fracture closure stress exceeds the strength of the proppant that is placed in the fracture. This leads to the creation of fines which can migrate and plug a proppant pack,

causing diminished well production. Loss of fracture conductivity can occur due to reduced width, which can be further compounded proppant flowback, fine generation and proppant scaling. However, for RCPs this is not the case and the polymer has potential to encapsulate some of the fines (see Fig. 2), thereby RCPs can contribute to the success of HF treatment. How polymer coated proppants (e.g., RCPs) can successfully perform well in the abovementioned problematic situation is described in the following

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proppant in reduction of proppant scales are discussed by Weaver et al. (2008); Weaver (2005); Underdown and Das (1985); Browne and Wilson (2003) and Weaver et al. (2006). The resin coating reduces proppant flowback, fines migration and proppant scaling. These advantages are provided by addition of strength to individual grains that generates uniform stress distribution throughout the pack. Table 6 clearly shows how RCP compares to other conventional proppants. Fig. 4. Example of thermoplastic polymer (polyhydroxy ether).

6.4. Fracture conductivity subsections. 6.1. Proppant flowback As mentioned before, the proppant is used to keep the propped fractures opened and accordingly connect the wellbore with the reservoir. However, high drag forces resulting from high production flow rate can cause proppant to flow out of the fracture and into the wellbore along with the production of oil or gas. RCP is cured under a combination of downhole reservoir temperature and closure stress. Grain-to grain bonding specification keeps the proppant grains from shifting, which correspondingly leads to eliminating proppant flowback. Nguyen et al. (2007) indicate that the bonding technology in cured RCP provides additional proppant pack integrity, enhanced fracture flow capacity, and increased long-term production of the well. The results obtained by Terracina et al. (2010), Browne and Wilson (2003) and Peard et al. (1991) suggest that in contrast to conventional proppants, RCP is the most effective method to prevent proppant flowback. Moreover, Anderson et al. (2002) and Johnson et al. (2005) have demonstrated that RCP is capable of preventing proppant flowback in high temperature, high rate wells, and low temperature wells. 6.2. Fines generation Crushing and fines generation are the effects of low strength proppants that are unable to withstand formation pressure, allow the fracture to close, thereby reduce hydrocarbon flow. According to Cutler and Jones, (1982), 5% fines generation reduces the fracture flow capacity by 60%. As reported by Anderson et al. (2002) and Underdown and Das (1985), RCP does encapsulate proppant grains and is less susceptible to the formation of fines (see Fig. 2). Proppant flow back was experimentally conducted by several researchers (i.e., Parker et al. (1999); Nguyen and Weaver (2003); Weaver et al. (1999); Vreeberg et al. (1994); Rickards et al. (1998); Johnson et al. (2005). Results indicate that adding polymer to proppant can prevent proppant erosion and increase the particles tolerance to higher stress cycles. A proppant pack is subjected to numerous stress cycles throughout the life of a well. These changes in stress can cause the proppant pack to shift, fatigue, and generate fines-leading to decreased conductivity. By using polymers for proppant coating, fines migration is mitigated by encapsulating loose fines within the polymer coating. 6.3. Proppant scaling Due to the hydrophilic nature of the majority of conventional proppants, water can dissolve the substrate surface and consequently scale is generated. On the other hand, the polymer coating provides a hydrophobic layer that prevents water from dissolving the proppant surface and forming scale. The polymer forms a barrier around the proppant that prevents water from reacting with the substrate grain surface and forming scale in the proppant pack pore spaces (see Fig. 2). The promising role of polymer coated

Coated proppant is introduced to the upstream petroleum industry as an effective means to prevent flowback of propping agents. Permeability and width of the propped fracture contribute to the success or failure of HF operation and productivity improvement of the treated wells is directly related to the fracture conductivity. By applying a coating layer to the surface of proppant, a proppant pack with high porosity is created that consequently leads to increasing of conductivity and pack permeability over a wide range of stresses (Dewprashad et al., 1993). As stated by Fink (2013), utilization of coated proppant has improved the fracture conductivity compared to times that uncoated proppants are used. During utilization of coated proppant, it was observed that fracture conductivity has increased that may be due to eliminating of fine production from the proppant and reduction of proppant flowback. The coated proppant also resists packing and settling, resulting in increased pack porosity and permeability (Lehman et al., 2003). When a proppant is coated with a suitable polymer, much more uniform and stable interface will be created between coated proppant and formation materials during utilization of coated proppant for stimulating of subterranean formation (Coulter and Wells, 1972). This uniform and stable interface between coated proppant and formation materials can improve fracture conductivity. Also, as shown by Ali et al. (2000) coating makes the proppant resistant to stress changes resulting from variable production conditions. Accordingly, enhancement of fracture conductivity is continued till ultimate strength is exceeded and propping agent initiates to crush. Many field and laboratory experiences illustrates that conductivity maintenance can extend into an economic benefit by utilization of polymer coated proppants. Glaze and Underdown (1987) introduced a new type of coated proppant that composed of sand as substrate and thermosetting resin as coating layer. During measurement of fracture conductivity, it was observed that coated proppant provided more fracture conductivity compared to uncoated proppant that had substantially the same particle size distribution. Improvement of fracture conductivity may be due to the formation of coated proppant pack with high porosity. Almond et al. (1995) and Weaver et al. (2005) indicated that polymer coated proppant is the best way to increase fracture conductivity. Nguyen et al. (2000) introduced a new approach for improving fracture conductivity. By applying a coating layer on the surface of sand, it is observed that fracture conductivity has improved significantly. It is due to great capability of coated proppant to preserve separated fines, which reserved them from migrating and sealing pore throats of the proppant pack. Nguyen et al. (1998) investigated the effects of applying polymer on the surface of proppant to find its effects on the fracture conductivity. They have observed that by applying polymer on the surface of proppant and addition of additives to control proppant flowback, fracture conductivity is increased and proppant flowback is controlled. The flexible coating layer of polymer encapsulates produced fines from the proppant under compression. As declared by Rickards et al. (2003) and Santos et al. (2009), coated proppant can lead to increase in the fracture conductivity, and that the performance of

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Table 6 Fracture flow capacity of different types of proppants. Factors affecting fracture flow capacity

RCP

Sand

Ceramic

Proppant conductivity Particle size uniformity Strength Proppant flowback control Strength Fines generation mitigation Proppant pack cyclic stress resistance Well depth applicability Proppant scalling

Excellent High High Excellent High Excellent Excellent Deep (>8000 ft) Excellent

Poor Low Low None Low None None Shallow (<4000 ft) Poor

Excellent High High Poor High None None Deep (>8000 ft) Fair

coated proppant for providing desirable fracture conductivity is better than uncoated proppant. Nguyen et al. (2011) observed that utilization of coated proppant has led to providing permeable packs and consequently increase in the fracture conductivity. As results of increase in the fracture conductivity, well productivity has been improved. Coated proppant that is introduced by Mcdaniel et al. (2014) has indicated great crush resistance under compression and accordingly, fracture conductivity has improved during utilization as propping agents.

7. Pre-cured vs curable polymers for proppant coating Since, closure stress of various types of formation is different from 1700 psi in some shale gas wells, up to and above 15,000 psi for deeper wells with high temperature (Soane et al., 2011), various types of polymer coated proppants must be applied according to closure stress of formation. For example, deeper wells require stronger proppants due to the higher closure stresses that exert onto the formation. It seems that utilization of curable polymer coated proppant can be effective because totally curing of this type of coated proppant can occur due to the conditions that dominated on the deeper wells. Sufficient pressure and temperature must be provided for curing of curable coated proppant within the formation. As discussed by Sinclair et al. (2007), many curable polymers cannot develop their full strength till coated proppant becomes totally cured within the formation. It seems that utilization of furan resin for coating of the proppants to apply in deeper wells can be effective. However, managing of furan resin polymer for temperature close to 300  F is difficult (Ellis and Surles, 1997). From the other hand, many shallow wells often suffer from the providing sufficient temperature and pressure to cure coating layer of proppant because they have downhole temperatures lower than 130  F, or even lower than 100  F (Callanan et al., 2001). Therefore, utilization of curable coated proppant cannot be effective. It is better to use pre-cured proppant to keep open the fracture for stimulating of shallow wells. Therefore, utilization of curable coated proppant in the shallow wells is not cost saving for shallow wells. Utilization of both types of pre-cured and curable coated proppant can be recommended within the intermediate wells. In the intermediate depth wells, the closure stress on the propping agent is in the range of 5000 to 10,000 psi (Rumpf and Lemieux, 1990). Therefore, utilization of some type of resins including of phenol-formaldehyde and epoxy for coating of proppant, which are capable of cure in the temperature range of 95e300  F (Pribytkov et al., 2012), is effective. Also, closure pressure of the formation can preserve pre-cured coated proppant within the formation. The applicability of different polymers to different formation depths are tabulated in Table 7. This table is useful for screening the polymer coating techniques for different scenarios. More discussion about the advantages of curable versus pre-cured proppant coating is provided in Section 7.

8. Proppant coating processes 8.1. Impregnation Porous structures are comprised of substrate structure and penetrated pores and impregnation is normally employed for coating of these porous substrates. The aim of impregnation is coating of the internal web structure and filling the porous surface by using diverse types of polymeric coatings (Zhang, 2009). Polymers can fill pores with or without creation of surface coating (Han et al., 2014). Impregnation not only increases proppant's strength (Gaurav et al., 2012) but also add to the substrate the ability to be flexible and capable to absorb shock and stress without deforming (Li et al., 2012). The impregnation process includes two-steps. In the first step, total immersion of substrate in a dip tank that contains polymeric coatings is conducted to impregnate all of the pores. For better control of residence tank, dip tank can employ single or multiple rolls. Key factor which can control and determine the coverage is the flow into the porous surface. The flow into the porous surface can be affected by the amount of applied pressure, permeability of substrate, solution viscosity and the dwell time in the coating dip tank. Second step includes determination of the final coating weight through elimination of extra polymeric coating. For eliminating of excess solution, utilization of several methods such as a pair of metering rolls, web scraper, squeegee, and spray coating is possible. A good reference of the impregnation concept, involved the related processes and mythologies can be found in Fink (2011). 8.2. Coating Two general approaches of proppant coating are employed in proppant industry (Nguyen et al., 2000). In the first approach, proppant is coated in the factory and then transferred to the well site location for application in HF treatment. Second approach includes coating of proppant through activated liquid resin system at the well site that means coating can be applied on the surface of proppant during HF treatment. By increasing the depth of drilling and moving into deeper formations, special conditions such as high temperatures, high flow rates, and high-closure stresses are governed. These special conditions have created more constraints for utilization of curable resin systems (McDaniel and Geraedts, 2003). More discussion about utilization of pre-cured and curable polymers for proppant coating for different well depths is provided in Section 8. 8.3. Methods of proppant coating Several techniques are employed for coating of proppant and the majority of these techniques are categorized into conventional, cold, and hot coating methods. Chemical coating is normally

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205

Table 7 Polymers coating for different well depths. Depth of the wells

Dominated conditions

Performance of polymer systems

Desirable polymers for coating

Required temperature for curing ( F)

Pressure (psi)

Temp. ( F)

Pre-cured

Curable

Pre-cured

Curable

Pre-cured

Curable

Shallow

2500e5000

>100b

Goodb

Poorb

N/A

5000e10,000a

100 > 300a

Goodc

Good

Completely cured before injection into formation 40e95c

N/A

Intermediate Deep

10,000e15,000

300 > 400

Lowe Lowb

Greate

Furan-phenolic resin, Phenolic resin, Epoxy resinf Phenol- formaldehyde resin, Epoxy resinc Epoxy resin Novolak resin plus curing agent (hexamethylenetetramine)d

N/A

370e400d

a b c d e f

&e

Novolak resin and organofunctional silane plus a curing agent includes hexamethylenetetramine, Epoxy resin basedd &e

100e300c

(Rumpf and Lemieux, 1990). (Callanan et al., 2001). (Pribytkov et al., 2012). (Sinclair et al., 2006). (Nguyen and Weaver, 2003). (Armbruster, 1988).

performed in these techniques by means of spraying, dipping or soaking the proppant in a liquid solution of the hydrophobic material. In addition, chemical coating can be conducted by means of applying a thin layer of film such as copolymerized polyvinylidene chloride on the surface of substrate and encapsulating it in an appropriate polymer coating (Jones and Cutler, 1985). Fusing polymer to the proppant through placement of proppant into a fusible powder (glass frit), sputtering, plasma spraying, and fluidizing the proppant in a fluidized bed are other processes of proppant coating (Jones and Cutler, 1985). In recent years, extensive relationship between oil and gas industry with other industries has provided great opportunity for manufactures of proppant to use new methods of proppant coating. 8.3.1. Conventional methods A conventional coating method includes combination of substrates with a liquid hardenable resin. A liquid hardenable resin is composed of a hardenable resin (organic resins), a hardening agent that causes hardening of hardenable resin, a coupling agent (silane) to promote bonding between polymer and particle, a hydrolyzable ester that is used to break viscous fracturing fluid, a surfactant to facilitate coating of polymer on the surface of particle, and a liquid carrier fluid (Rediger, 2011). Liquid hardenable resins cause contamination of equipment used for storage, transport, and injection of proppant. They do not have long shelf time that causes poor support in proppant pack formation. A solution is used to combine particulates of proppant and liquid hardenable resins just prior to injecting the resultant proppant slurry downhole (Vincent, 2005). Several processes are available for coating of proppant; in an embedment, proppant are coated through mixing in a reactor. All of the individual components of polymer and particles are added into mixing vessels (reactor) to produce a reaction mixture. Then, agitating of reaction mixture with agitator speed is performed. Finally, mixing vessel is heated at a temperature that is determined by the polymer coating technology (Urbanek, 2013). Spraying is other method of proppant coating, which begins with pouring of the individual components of polymer into a spray device to form a mixture of coating. Then, prepared mixture is sprayed on the surface of substrates to form RCP. Coating that is performed with spray method has some unique features. Coating layer that is formed during spraying is even and unbroken. In addition, it has sufficient thickness and appropriate integrity.

Batch mixing method is another method that is used for coating of the proppant. Previous batch mixing methods that were used to apply resins on the surface of proppant had indicated great efficiency. However, they were not appropriate to apply as rapid coating that was suspended in continuous streams of a carrier liquid (Hughes et al., 2014). Therefore, continuous transportation of RCP into a fracture with utilization of viscous fracturing fluid for a long period of time is essential. Creation of continuous streams with the batch mixing methods is time-consuming, costly, and accompanied with risks of flow rate interruption or reduction. 8.3.2. Cold coating method 8.3.2.1. Cold Spray. Cold Spray (CS) as a new technique for coating of small particles is classified into the group of thermal spray processes. Cold gas dynamic spraying, kinetic spraying, high velocity particle consolidation (HVPC), high velocity powder deposition and supersonic particle/powder deposition (SPD) are different approaches of this method (Villafuerte, 2010). Interested readers are encouraged to refer to Schmidt et al. (2006) for further investigation on the development of CS technique. 8.3.2.2. Sputtering. Sputtering is a physical method for deposition of thin film on the surface of substrates. It is classified as a physical process because ion bombardment of target material leads to formation of a vapor that is due to a physical process (Ngaruiya, 2004). Creation of film can be conducted through condensation of ejected atom on the surface of substrate. Sputtering has offered some advantages for coating of particles such as better step coverage and less radiation damage to the surface of substrate. In addition, the procedure to deposit films on the surface of particles is simple but expensive. 8.3.2.3. Chemical bath deposition method. Chemical bath deposition (CBD) method is considered an attractive technique owing to its low temperature, low cost and large area deposition capability (Chang et al., 2009). Although CBD has offered a lot of advantages it suffers from some drawbacks. In this method, required heat of chemical reaction is supplied from the solution bath to the substrate surface. It leads to both heterogeneous nucleations at the surface as well as homogeneous particle formation in the bath. The growth solution changes as a function of time that may leads to difficulty in thickness control. The depletion of reactants also limits the attainable thickness. Moreover, the unequal bath-to-surface

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volume is used to form the desired thin film. Therefore, it generates a lot of waste and creates defects in devices (Mark et al., 2005). However, as discussed by Turi (1981), CBD as a chemical deposition method is of greater commercial value than either thermal evaporation or sputtering due to its simplicity, convenience, low cost and low temperature. Recently, as described by Shin et al. (2014), CBD is capable to produce thin films with good quality into surface of substrate materials that is performed through control of precipitation of the compounds from the solution. In addition, CBD has flexibility to control factors which can affect growth of thin films; some of these factors are thickness of film, rate of deposition and quality of crystallites, temperature, and concentration of film. Fig. 5 and Fig. 6 indicate the process of coating with sputtering and CBD methods, respectively.

8.3.3. Hot coating method This technique involves preheating the proppant to several hundred degrees Fahrenheit, slowly adding the polymer to the proppant and mixing the combination until the polymer melts and coats the proppant. Once coated, the proppant is cooled at which time the polymer solidifies (Goldschmidt and Streitberger, 2003). In hot coating method, the surface of preheated proppant (e.g., sand) is subjected to a polymer for coating. Thereafter, the mix is added with a curing agent in the form of an aqueous solution of hexamethylenetetramine. By evaporating water from curing solutions, the temperature of system is decreased up to 230  F, which causes interruption of polymerization process. Then, the RCP is separated from the mixing equipment and cooled down at a maximum high rate up to temperatures less than 120  F (McCrary et al., 2009). Pribytkov (2012) described the drawbacks of this process; first, the second layer of phenol-formaldehyde resin with a curing agent for a certain time is subjected to high temperatures and partially cured. Second, significant curing time for RCPs formed at high temperatures. It causes the actual temperature at the initial stage of polymerization in the well to be more than 158  F, whereas the temperature of the polymerization for the major part of oil beds ranges between 120 and 158  F. Third, polymerization of phenolformaldehyde resin by heating methods causes hazardous production area. The problem cannot be solved even by reheating vapors that are emitted from the polymerization process. It is common method for coating of sand, glass beads, and ceramic proppant but it cannot use for heat sensitive materials (e.g.,

Fig. 6. A schematic of chemical bath deposition method (modified from Choi et al., 2014).

nutshells). 9. Heat sensitive proppants Numerous methods of proppant coating have been used while selection of the best one has always been a challenging issue. There are many differences in the substrate structure of coated proppant. In addition, adaptability of substrates with various polymers is not similar. Thus, care must be taken when choosing the desirable method for coating of substrates. For example, for coating of sand, ceramic and glass bead, it seems that heating of substrate to specific temperature and then addition of polymer is feasible economically. However, this method is not applicable for coating of heat sensitive material such as walnut shell, kernel shell and other types of biomass substrates. Heating causes damage to the structure of substrates and it leads to decrease in their mechanical strength. Instead of heating method, indirect heating and cold curing can be used for coating of heat sensitive materials. In indirect heating, the substrate is not directly contacted with high temperature. It means that substrates can preserve their mechanical strength during coating. Those features which have made this method a good choice for coating of heat sensitive material are simplified production, short heating time, less damage to the structure of substrate, and longer service life. As well as the methods that are mentioned above, cured polymers at room temperature are suitable for coating of heat sensitive materials. Using CBD and sputtering method for coating of heat sensitive

Fig. 5. A schematic of sputtering method (modified from http://www.tcbonding.com/sputtering.html).

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materials is another option. As declared by Hara (1990), CBD has offered several overriding advantages such as uniform film deposition, precise maintenance of deposition temperature, low cost, and non-polluting. Utilization of sputtering as a physical method for coating of proppant produces coating with high quality. It is because of creation of covalent bond between proppant and polymer. However, this method is not cost effective and cannot be used in the range of industrial production.

207

science and technology to increase the strength the proppants, an approach which the polymer industry has used increasingly. Acknowledgment The authors gratefully would like to thank MOHE and UTM (Grant VOTE No. 10J11). References

10. Future considerations Application of polymer coatings that are comprised of industrial polymers and reinforcement agents can enhance the efficiency of HF treatment. In addition, utilization of substrates that have more adaptability with polymers coating can improve the efficiency of HF treatment. Polymers that are used for coating of proppant have negative effects on the health of human beings. For instance, RCP when coated with phenol and formaldehyde polymers emits hazardous materials and causes health damages to those who are exposure to this type of proppant. It is better to decrease these negative effects through using polymers that emit less hazardous materials. As reported by Intergovernmental Panel on Climate Change (IPCC), environmental problems that are related to ceramic factories cause a lot of damages to human beings because these factories cause emissions to be released into air, water and land. In addition, all parts which are involved in the ceramic industry are energy intensive and they consume natural gas, liquefied petroleum gas and fuel oil for firing which leads to production of high amounts of CO2 and other harmful gases (IPPC, 2007). Methods that are used for coating of proppant suffer from some drawbacks that are mentioned in the previous sections; therefore, focusing on those methods that are capable of overcoming these drawbacks can lead to improvement of the efficiency of HF treatment. 11. Conclusions This comprehensive review of the literature has outlined the definitions of proppant and polymer coated proppants and their coating methods, and has discussed motivating factors, profiles, predictors, negative impacts, prevention, and treatment options in relation to each. Organic polymers are used more than other types of polymers for coating of proppant because of easy processibility, high degree of strength and low density. Thermosetting polymers have been applied more than thermoplastic polymers because they can provide more heat resistance and higher flexibility. Of the thermosetting polymers, resins are most widely used because of their great chemical, heat, and corrosion resistance. They are also capable to create outstanding adhesion for protection of proppant. Also, resins have indicated better performance compared to polyurethane because they can provide coating with higher durability. Furthermore, resins can be applied in a wide range of pressure and temperature whereas application of polyurethane is restricted to certain pressure and temperature. In the point of coating method, various types of methods such as conventional, sputtering, CS and CBD method can be used for coating of proppant. Coating of proppant can be performed economically with CBD method. CBD is a simple method and complicated equipment for coating is not required. Also, it causes minimum material wastage. On the other hand, sputtering is capable to provide coatings with high quality but it is not cost effective for coating of proppant. Coating of proppant is usually conducted with conventional method; however, coating of materials that are sensitive to heat must be performed with cold coating method because utilization of the conventional method causes damage to the surface of proppant. This conclusion underlines the need to take advantage of polymer

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