Friction Challenge in Hydraulic Fracturing

: Hydraulic fracturing has become one of the most popular techniques for exploring sustainable energy sources. However, friction is associated with the entire fracturing process, presenting signiﬁcant challenges for development. Facing the huge friction challenges, this review is elaborated in the following three aspects: (1) the fundamentals of hydraulic fracturing, including three aspects of rock fracture mechanism, fracturing ﬂuid, and proppant; (2) the friction challenges in hydraulic fracturing, which mainly exist in friction along the path and friction near wellbore; (3) hydraulic fracturing drag reduction technologies, which are considered proppant segment plug, fracturing ﬂuid viscosity enhancement, and proppant surface modiﬁcation technologies. Therefore, we should not only understand the challenges in hydraulic fracturing but also know how to mitigate them. Ad-ditionally, we call for a strong focus on environmentally friendly, green friction-reducing technologies for oil and gas ﬁelds in the future development of the fracturing industry.


Introduction
Hydraulic fracturing (fracking) is a key process to the exploitation of oil and gas and plays an important role in enhancing oil recovery. It consists of four major processes: downhole pressure-out, fracture formation and extension [1], proppant filling [2], and oil/gas channel formation. For each fracturing process, there exists serious friction challenges to the fracturing machinery.
Fracking not only changes the situation of oil/gas fields but also enhances the production of oil/gas [3,4]. In practical applications, the effects of fracturing are often important indicators to evaluate the maturity of the fracturing. In the last few years, hundreds of fracturing wells [5] have been installed in the oil/gas fields of the China National Petroleum Corporation (CNPC). However, the results were not as expected. It is worth noting that all most of those problems are caused by frictional resistance [6], which leads to insufficient pressure at the bottom of the well, reducing the capacity to form fractures. Therefore, the challenge posed by friction are a foundational and ubiquitous problem in the fracking process.
The use of fracturing fluid imposes a couple side effects. During the formation of rock fractures, it requires breaking down the forces in the rocks formation, by shearing and relative motion, which induce frictional behaviors and energy lose [7]. The fracturing fluid could lubricate and reduce drag, but the proppant filling and propping are still affected by friction, which cause wear and tear. When flowback, the fracturing fluid and proppant of couple are subjected to friction. On one hand, the fluid reacts with the formation to form residuals and filter caking, increasing formation blockage and friction loss. It will cause water sensitivity damage to the formation. On the other hand, the accumulation of proppant particles in the embedded fractures can cause clogging, thereby reducing formation permeability. At the same time, the proppant can erode the formation surface and clamp surface of formation permeability. At the same time, the proppant can erode the formation surface and clamp surface of downhole tool at a certain velocity, causing surface erosion wear. Therefore, the challenge is how to reduce friction during fracturing. Understanding the friction challenges and friction reduction mechanism [8] is the key to improving hydraulic fracturing technology and stimulating innovations, as well as boosting the profit of oil/gas production.
In this review, the following three aspects are mainly described as follows: (1) hydraulic fracturing fundamentals, including rock fracture mechanism, fracturing fluid, and proppant; (2) friction challenges in hydraulic fracturing mainly exist in friction along and near wellbore; (3) hydraulic fracturing drag reduction technology-this review considers proppant slug technology, fracturing fluid viscosity enhancement technology, proppant surface modification technology, etc. Based on the summary and expression of the above three aspects, we know that friction challenges in hydraulic pressure are a difficult problem that cannot be ignored. Facing this challenge, we still need to redouble our efforts and try to find new methods, techniques, and theories to alleviate the friction problem in fracturing.

Hydraulic Fracturing Basics
Hydraulic fracking is one of the most important technologies for extracting shale oil/gas [9]. Traditionally, fracking injects fracturing fluid with a high viscosity to create a large displacement at the bottom of the well, in order to generate the fracture. Subsequently, high conductive proppant is filled to form non-closed or incomplete closed fractures. Eventually, the mixture of oil/gas flows to the wellhead ( Figure 1).

Mechanism of Hydraulic Fracturing Rock Fracture
Fracking was unsuccessfully tested for the first time in the United Kingdom in 1880s [10]. Decades of research and improvement of the technology made a successful breakthrough in the United States, which has been applied to various fields ever since [11]. In comparison, fracking was studied and applied relatively late in China. Through continuous efforts, the technological gap becomes smaller and smaller; fracking is widely utilized in the oil/gas fields of China.

Mechanism of Hydraulic Fracturing Rock Fracture
Fracking was unsuccessfully tested for the first time in the United Kingdom in 1880s [10]. Decades of research and improvement of the technology made a successful breakthrough in the United States, which has been applied to various fields ever since [11]. In comparison, fracking was studied and applied relatively late in China. Through continuous efforts, the technological gap becomes smaller and smaller; fracking is widely utilized in the oil/gas fields of China.
Fracture formation and extension is the core technology of fracking, which is related to the distribution of in situ stresses, the mechanical properties of rocks, the properties of the fracturing fluid and the proppant properties [12]. According to the Mohr-Coulomb theory, the formation friction is directly related to the formation stress [13], which affects the formation and extension of fractures. When the shear stress of the formation is greater than the tensile stress of the rock, cracks are formed [14]. In general, the stress distribution in the stratum is heterogeneous. At the same time, the inhomogeneity of formation stress will make pressure of the string increase, leading to equipment for high-load work. Hence, during the fracturing process, the homogeneity of the formation stress should be as close to perfection as possible, which not only reduces chances of friction resistance but also improves the fracturing effectiveness.
It is well known that in situ stress is mainly composed of stress from the formation fluid and fracture from the rock skeleton. The direction of the stresses is primarily horizontal and vertical, as shown in Figure 2. There is a horizontal stress shown in Figure 2a. On the other hand, there is a vertical stress shown in Figure 2b,c, in the X and Y directions, respectively. In most cases, the horizontal stress in the fracture is 1-3 times larger than the vertical stress, while the vertical stress is 1.5-3 times larger than the gravitational stress. The crack tips of the formation are destroyed easily, which bring about energy loss and causes fatigue failure in the fracturing equipment. It is important that the proper fracking force be used, and it is a critical factor in the success of technology. Fracture formation and extension is the core technology of fracking, which is related to the distribution of in situ stresses, the mechanical properties of rocks, the properties of the fracturing fluid and the proppant properties [12]. According to the Mohr-Coulomb theory, the formation friction is directly related to the formation stress [13], which affects the formation and extension of fractures. When the shear stress of the formation is greater than the tensile stress of the rock, cracks are formed [14]. In general, the stress distribution in the stratum is heterogeneous. At the same time, the inhomogeneity of formation stress will make pressure of the string increase, leading to equipment for high-load work. Hence, during the fracturing process, the homogeneity of the formation stress should be as close to perfection as possible, which not only reduces chances of friction resistance but also improves the fracturing effectiveness.
It is well known that in situ stress is mainly composed of stress from the formation fluid and fracture from the rock skeleton. The direction of the stresses is primarily horizontal and vertical, as shown in Figure 2. There is a horizontal stress shown in Figure 2a.
On the other hand, there is a vertical stress shown in Figure 2b,c, in the X and Y directions, respectively. In most cases, the horizontal stress in the fracture is 1-3 times larger than the vertical stress, while the vertical stress is 1.5-3 times larger than the gravitational stress. The crack tips of the formation are destroyed easily, which bring about energy loss and causes fatigue failure in the fracturing equipment. It is important that the proper fracking force be used, and it is a critical factor in the success of technology. (c) vertical fracture in direction σz.

The Mechanism of Fracture Formation
After many years of research and analysis by scholars, the mechanism of the fracture formation by hydraulic fracturing has been well understood. It provides a theoretical basis for hydraulic fracturing tests. Hubberts and Willis [15] established the H-W formation stress prediction model and studied the minimum principal stress perpendicular to the fracture formation. Scott and Williams researched the maximum principal stress of fractures parallel to the formation and further studied the influence of fractures formed in the formation, as well as the fractures of the rock itself. Considering the theory of fracture initiation and elongation under multi-factor conditions, Haimson and Fairhurst [16] built the H-F model and calculated the pressure required for formation of fracture. Eaton et al. [17] set up a model for the relationship between a rock pressure's gradient and formation depth, for which they used the formation stress coefficient and formation Poisson's ratio to perfect the model. Anderson considered the influence of the formation stress concentration and referred to optimize the H-W model. Bradley [18] analyzed the relationship between shear stress and wall cracks, which demonstrated that shear force is the decisive factor in the formation of cracks. Yew et al. [19] utilized a three-dimensional elastic model to describe the impact of stress distribution around the wellbore on the fractures. Fallah- After many years of research and analysis by scholars, the mechanism of the fracture formation by hydraulic fracturing has been well understood. It provides a theoretical basis for hydraulic fracturing tests. Hubberts and Willis [15] established the H-W formation stress prediction model and studied the minimum principal stress perpendicular to the fracture formation. Scott and Williams researched the maximum principal stress of fractures parallel to the formation and further studied the influence of fractures formed in the formation, as well as the fractures of the rock itself. Considering the theory of fracture initiation and elongation under multi-factor conditions, Haimson and Fairhurst [16] built the H-F model and calculated the pressure required for formation of fracture. Eaton et al. [17] set up a model for the relationship between a rock pressure's gradient and formation depth, for which they used the formation stress coefficient and formation Poisson's ratio to perfect the model. Anderson considered the influence of the formation stress concentration and referred to optimize the H-W model. Bradley [18] analyzed the relationship between shear stress and wall cracks, which demonstrated that shear force is the decisive factor in the formation of cracks. Yew et al. [19] utilized a three-dimensional elastic model to describe the impact of stress distribution around the wellbore on the fractures. Fallahzadeh [20] studied the influence of the stress distribution around the casing and cementing sheath around the fractures. Lv et al. [21] established a mathematical model for determining crack pressures based on the elastic mechanics theory. Therefore, in order to obtain effective fracture germination for hydraulic fracturing, it is necessary to overcome the friction between rock layers. Moreover, to avoid friction between rock layers, we need further carry out fracturing fluid lubrication.

The Mechanism of Fracture Extension
Fracture extension refers to the extension and expansion of the fracture, which signifies the expansion of the high-pressure region to the low-pressure region and increases the effective area of the fracture. The extending direction is always perpendicular to the direction of the minimum principal stress, which is influenced by the lithology of the rock formation [22,23], the rock's fracture surface, and natural fractures. According to the linear elastic fracture mechanics model, the fracture stress and strain field at the top of the seam are proportional to the stress intensity factor. Anderson [24] thought that rock strength was the main factor affecting fracture extension from high strength to low strength. Warpinski [25] studied the laws of hydraulic and natural fractures to analyze the mechanism of crack fracture. In doing so, he calculated the extension direction of hydraulic fractures. Martin [26] researched the process of the fracture formation in soft rocks, deriving the mathematical formula for the opening threshold of cracks on the plastic fracture surface. In brief, the friction of the formation causes different degrees of fracking slips, which are horizontal slips and shear slips. It is in facing the challenge of friction on the rock surface that the formation of cracks requires fluid lubrication, which makes it easy to form cracks.
Fracture formation and extension are significant steps in the fracturing process, which are deeply influenced by downhole friction, fracture fluid filling [27], and proppant propping [28]. The greater the friction, the more difficult it is to crack and extend. However, the high viscosity of fracturing fluids can transfer energy to the formation and extension of cracks. Fracturing fluids carry proppant and maintain conductivity. The high viscosity fracturing fluids can compensate for the energy dissipation during fracturing and form rock crevices and extensions. Meanwhile, high-strength proppants sustain the crack, so that it does not close. However, low intensity proppant is more likely to flowback and accumulate, and crack extension is more difficult.

Hydraulic Fracturing Fluid
Fracturing fluid [27,29] is a high viscosity fluid, which plays a role in lubrication and transportation during fracturing. Common examples include water-based [30], oilbased [31], emulsified, foam [32][33][34][35], alcohol-based [36][37][38][39] acid-based fracturing fluids, and so on. The most widely used is a water-based fracturing fluid [36], which is shown in Figure 3.  Water is mainly used in fracturing fluids [30] as the dispersion medium. It needs to add water-soluble polymers and various reagents (thickening agents [40], cross-linking agents [41], and gel breaking agents [42] to form the working fluid required for the process. Water accounts for 99.2% of the weight percentage. The processing is safe, reliable, and has low production costs. When compared to oil-based and foam fracturing fluids, the friction resistance of water-based fracturing fluids is lower. Under atmospheric pressure, the fluid is controlled easily, due to the high hydrostatic pressure of water-based fracturing fluid.
Water-based fracturing fluid consists of many additives, including thickening agents [40], cross-linking agents [41,43], and gel breaking agents [42]. So, the first step involves Water is mainly used in fracturing fluids [30] as the dispersion medium. It needs to add water-soluble polymers and various reagents (thickening agents [40], cross-linking agents [41], and gel breaking agents [42] to form the working fluid required for the process. Water accounts for 99.2% of the weight percentage. The processing is safe, reliable, and has low production costs. When compared to oil-based and foam fracturing fluids, the friction resistance of water-based fracturing fluids is lower. Under atmospheric pressure, the fluid is controlled easily, due to the high hydrostatic pressure of water-based fracturing fluid.
Water-based fracturing fluid consists of many additives, including thickening agents [40], cross-linking agents [41,43], and gel breaking agents [42]. So, the first step involves adding thickening agents, mainly to increase the viscosity, reduce its filtration performance, reduce its frictional loss, and improve the carrying capacity. The cross-linking agents can fuse with thickening agents to improve the proppant carrying capacity and reservoir permeability. After the gel breaking are added, these agents mainly reduce the viscosity of the crosslinking agent to ensure the dredging ability of the crack and reduce the damage of reservoir. Finally, these fracturing reagents need to be recycled back to reduce the permeability loss to the reservoir. The classification of thickening agents, cross-linking agents, and gel breaking agents are shown in Table 1.
According to the apparent viscosity theory [44], the viscosity of a fracturing fluid is an important factor effecting the friction. However, viscosity has two effects on friction. On one hand, it can increase the shear stress between the fluid and the pipe wall, leading to an increase of friction losses. On the other hand, with the increase of the viscosity, the polymer solution produces a transition delay effect, which inhibits turbulence and reduces friction loss. Controlling the fluid's viscosity becomes the key to reducing friction losses (which have an important role in reducing the suspended sand), fluid friction, and filtration loss. During the pumping stage, the fluid viscosity needs to be moderated to achieve a state of suspended sand and low friction. During the fracture formation stage, the fracturing fluid viscosity needs to increase. The increased fracturing fluid has enough fluid pressure to complete the expansion [65] and extension of fractures. During the drainage stage, the fluid viscosity needs to be reduced. A low viscosity fluid can flow back to avoid blockage collapse of the stratigraphic structures.

Hydraulic Fracturing Proppant
Fracturing proppant is an indispensable filler in the fracking process. The proppant [9] supports fractures and prevents their closure by forming a highly dredging oil/gas flow channel. The proppant type and particle characteristics (i.e., size, strength, sphericity, and roundness) determine the flow conductivity of the fractures. The stronger the flow conductivity, the higher the oil/gas production capacity. However, based on the fitting formula theory, the proppant particle size and ratio to sand have an impact on the friction. This causes abrasion near the well and sand blockages in the crack.
In the fracturing process, ideal proppants ( Figure 4) have high compressive strengths and a small crushing rate. Most proppants are spherical particles with uniform size [65] and smooth surface, which are easy to transport in fracturing fluid. When the proppant supports the fracture, it does not react with the formation fluid. The origin of the proppant is river sand, which comes from the Arkansas River in the United States. Until the mid-1950s, natural quartz sand [66], which is shown in Figure 4a, was widely used in fracking. After a few decades, ceramic proppants [67,68] were utilized in the fracturing process. Ceramic proppants have the advantages of having high strength, as well as good corrosion resistance and roundness. From the 1980s till now, coated [69], biological, and self-suspending proppants continue to improve proppant performance [70]. Propant transportation schedule is shown in Figure 4. Proppants are carried by the fracturing fluids, also known as sand-carrying fluids. The fluids are transported to the formation, via coiled tubing, in order to realize the fracturing operation. Unfortunately, walls of coiled tubing suffer from proppant erosion and wear, causing the structure to fail. In line with Dean's theory [71], the flow rate and viscosity of the fluids correlate with the degree of erosion and wear. As the flow rate and viscosity of the fluid increases, the erosive and wear of the tubing increases. During the conveying process, the roller will produce centrifugal forces on the fluid. Under the centrifugal force, the fluid will rotate and float and can increase the friction of the tubing. Therefore, researchers pay close attention to the friction and erosion of proppants. Carpenter proposed that new proppants [72] need to reduce the concentration of the fluid, in order to reduce friction and erosion of proppants.

Friction Mechanism in Hydraulic Fracturing (Problem)
The impacts of friction in hydraulic fracturing cannot be ignored. Researchers began to pay the attention to the friction problem in the 1980s. Excessive friction will cause serious pressure loss in the fracturing process, leading to increase the risk in the fracturing construction. Friction loss in the process is mainly composed of two parts: the friction along the path and near the wellbore. Proppants are carried by the fracturing fluids, also known as sand-carrying fluids. The fluids are transported to the formation, via coiled tubing, in order to realize the fracturing operation. Unfortunately, walls of coiled tubing suffer from proppant erosion and wear, causing the structure to fail. In line with Dean's theory [71], the flow rate and viscosity of the fluids correlate with the degree of erosion and wear. As the flow rate and viscosity of the fluid increases, the erosive and wear of the tubing increases. During the conveying process, the roller will produce centrifugal forces on the fluid. Under the centrifugal force, the fluid will rotate and float and can increase the friction of the tubing. Therefore, researchers pay close attention to the friction and erosion of proppants. Carpenter proposed that new proppants [72] need to reduce the concentration of the fluid, in order to reduce friction and erosion of proppants.

Friction Mechanism in Hydraulic Fracturing (Problem)
The impacts of friction in hydraulic fracturing cannot be ignored. Researchers began to pay the attention to the friction problem in the 1980s. Excessive friction will cause serious pressure loss in the fracturing process, leading to increase the risk in the fracturing construction. Friction loss in the process is mainly composed of two parts: the friction along the path and near the wellbore.

Friction along the Path
Installing coiled tubing is one of the vital steps in the fracturing process. During the process, the coiled tubing comes into contact with the borehole wall, resulting in frictional resistance along the path. Friction loses becomes more serious as the depth of tubing increases. The main factors affecting the friction along the path is the coiled tubing diameter, fracturing fluid viscosity, and proppant concentration.

Coiled Tubing Friction
Coiled tubing is required to be installed to a depth of more than seven kilometers. As the depth increases, the tubing starts to act like a soft rope in the borewell. As a result, experts build soft rope models to analyze the tubing stress. Wojtanowicz et al. [73] studied the effects of a borehole's friction on the tubing's stress, in which the high friction in the wellbore leads to a stress concentration in the tubing, resulting in structural failure of the tubing and thread seal on the casing. In 1983, Johancsik et al. [74] first performed a stress analysis on the tubing and established the micro-element soft rod for structural mechanics model. Sheppard et al. [75] considered friction and torque factors when designing and optimizing a borehole's trajectory. Based on the Sheppard model, Maida's team [76] considered the effect of viscous resistance of a fluid on the friction of the tubing. Daring et al. [77] established a two-dimensional model to analyze the deformation of the tubing in the wellbore. His team [78] built a three-dimensional model to analyze the force on the tubing in the wellbore. In conclusion, the increase of tubing depth and friction along the pipeline is a problem that cannot be ignored. In order to further alleviate the oil and gas pipeline's structural wear, the main way to prevent damage during drilling is to reduce friction with lubrication.
During the process of tubing friction and wear, the coefficient of friction (COF) also becomes a crucial factor to determine the stability and reliability of the tubing. Ho et al. [79] considered the effect of large deformations on the COF of the borehole. Sheppard used theoretical models to research the influence of borehole geometry on frictional resistance. Dikken et al. [80] introduced the effect of pressure gradient along the borehole on the COF. Landman and Goldthorpe [81] studied the relationship between uniform flow and the COF, which established the relevant prediction model. Johancsik et al. [74,82] investigated the effects of the type of mud and wellbore conditions on friction coefficients. When researching the influence of the COF on pipeline wear, not only should the sliding friction be considered but also the comprehensive friction, including drilling fluid lubrication and rock properties. Therefore, the COF usual empirical value is summarized in Table 2. The data in Table 2 are derived from the measured calculation in the study of the friction coefficient of the string in a large displacement well.

Fracturing Fluid Viscosity
After the fracturing fluid enters the formation, the balance of forces in the formation breaks, causing different degrees of shearing damage and energy loss in the formation. The losses are mainly caused by water damage and water sensitivity damage and are related to the fluid's viscosity. At certain locations of the pipe the friction becomes severe.
Water damage of the fracturing fluids mainly occurs in low permeability reservoirs. Some water is locked in the formation, due to the low permeability and poor porosity of the formation; as a result, this causes water damage in the formation. Water damage can lead to an increase in the viscosity of the fluid and, as a result, the friction resistance increases within the well. Bahrami et al. [83] studied the mechanism of water lock damage, in order to analyze the factors of increasing viscosity. Ni's team [84] used nuclear magnetic resonance to research the water damage in coal seams. They proposed protective measures for surfactants, in order to reduce the viscosity. Fan et al. [85] invented a waterproof sealing agent, used to reduce water damage and decrease the viscosity of the fluid. Bijeljic's team [86] utilized the multi-component porosity model to analyze the effects of water damage on multiphase oil/gas. Holditch et al. [87] used the capillary pressure changes of tight sandstone formations to study the combined effects of hydraulic fracturing intrusive formation permeability damage and relative permeability damage The greater the pressure, the more severe the water lock damage. Lin et al. [88] used a combination of experimental and numerical simulations to research the influence of water lock damage on formations. Therefore, the phenomenon of water lock damage is a non-negligible phenomenon that causes shale formation damage. To a certain extent, water lock damage leads to increased formation pressure. The inner surface of downhole devices and pipelines are eroded and worn down by proppant particles, resulting in surface stress concentrations. Under the comprehensive stress caused by the pressure of the medium in the pipe, as well as other external forces, it is feasible to form cracks and expand, which can eventually lead to the collapse of an oil and gas pipeline. In order to mitigate this damage, the main measures focus on effectively reducing the viscosity of the fluid and friction between downhole devices, pipes, and borehole walls.
Besides water lock damage, water sensitivity damage can also cause damage in the formation. Water sensitivity damage is caused by the chemical reactions between the fracturing fluid and minerals in the formation. This causes erosion of the formation, which causes the particles in the formation to fall off, accumulate, and increase the tubing friction along the path. Researchers began to focus on this type of damage in the 1930s. Monaghan et al. [89] studied the interactions between the aqueous phase and clay minerals in the reservoir. They found that water-sensitive damage was severe, but the reservoir permeability decreased. Mungan et al. [90] analyzed the fluid's pH and salinity when the reservoirs were damaged. Tchistiakov et al. [91] investigated the water sensitivity damage of the charged particles around clay. Aradeiba et al. [73] researched the water sensitivity damage of strata without expansive clay minerals. However, the impact of water sensitivity damage on the formation was not great. Effective prevention and protection can decrease the friction along the path and, as a result, reduce the impact of the water sensitivity damage on the formation.

Proppant Concentration
Proppant, like the Great Wall, supports cracks formed by the fracturing fluid. The ability to embed and accumulate proppant particles can damage the original formation structure, resulting in erosive wear and different degrees of friction in the cracks [92]. Holditch et al. [93] studied the effects of different concentrations of proppant in the deep well formation. They determined that the higher the concentration, the more serious the erosion wear. Lacy's team [94] established a proppant embedment model to research the relationship between the depth of proppant embedment and degree of formation damage under different formation rocks. Cook et al. [95] investigated the relationship between fluid viscosity and the depth in which the proppant embedded itself within complex formations. Using the shunt loading method, Penny et al. [96] studied the formation damage caused by proppants. Therefore, it is inevitable that proppant embedment will cause damage within the formation, resulting in erosion and wear of the formation and equipment. This is a problem that should be avoided and prevented.
Failure caused by the wear of the downhole device, pipeline surface, and accumulation of metal particles in the pipeline caused the blockage in the formation. Blockage can cause local formation pressure to be too high and is the main way to erode and destroy the formations. In the process of proppant filling cracks, the movement of fluid assists proppants with large diameters to fill the cracks, and some proppants with small diameters will also accumulate and block. Subsequently, the pressure and friction in the bottom of the well would increase. In the oil-loosened sandstone formation, Wong et al. [97] found that proppants are easy to accumulate after the sandstone particles fall off the multi-layer formation skeleton and damage the formation. Carroll et al.'s [58] research found that broken proppants would block the crack even more, reducing its permeability. However, in the clay mineral formation, the clay would set off the hydration and expansion reaction. Hayatdavoudi et al.'s [98] research showed that after the reaction, proppants block the cracks, thereby increasing the friction along the cracks. Khilar and Fogler [99] studied the friction loss of salt chemical proppants to the formations. Civan et al. [100] established the mathematical model of clay hydration expansion, finding that more serious clay hydration would have more proppant accumulation and, therefore, more frictional losses in the formation. The damage mechanism of the proppant is shown in Figure 5

Friction near Wellbore
During hydraulic fracturing, the fluid enters the cracks of the formation from the pump through surface lines, coiled tubing, and perforation holes. The whole process is influenced by frictional resistance, leading to pressure loss. Excessive friction causes crack length to grow, resulting in serious sand plugging. In the 1980s, near-wellbore friction refers to the concentration of stress around the perforations and wellbore, which caused near-well friction. Near-well friction is mainly divided into perforation friction and fracture bending friction.

Perforation Friction
During the fracturing process, an insufficient number of perforations and perforation pollution will give rise to perforate friction and occur in the perforation hole. The schematic diagram of perforation abrasion is shown in Figure 6. It can be seen from Figure 6 that the number of perforations is insufficient; as the perforation resistance increases, proppant will settle faster, leading to blockages. Economides and Nolte [101] showed

Friction near Wellbore
During hydraulic fracturing, the fluid enters the cracks of the formation from the pump through surface lines, coiled tubing, and perforation holes. The whole process is influenced by frictional resistance, leading to pressure loss. Excessive friction causes crack length to grow, resulting in serious sand plugging. In the 1980s, near-wellbore friction refers to the concentration of stress around the perforations and wellbore, which caused near-well friction. Near-well friction is mainly divided into perforation friction and fracture bending friction.

Perforation Friction
During the fracturing process, an insufficient number of perforations and perforation pollution will give rise to perforate friction and occur in the perforation hole. The schematic diagram of perforation abrasion is shown in Figure 6. It can be seen from Figure 6 that the number of perforations is insufficient; as the perforation resistance increases, proppant will settle faster, leading to blockages. Economides and Nolte [101] showed from reservoir stimulations that friction causes pressure loss. Daneshy et al. [102,103] performed simulations on perforation fractures and found physical relationships between perforation friction and the number of perforations. After that, Crump and Conway [104] established the friction model for hole perforations and discovered that different perforation parameters generated different perforation friction types. Cramer et al. [105] found a linear relationship between the hole's diameter and amount of proppant passing through the hole. Wu's team [106,107] analyzed the effects of the size of the diameter, as well as the effect that the number of perforations had, with regard to friction. Lecampion et al. [108,109] showed that pressure drop would cause friction from the perforations to decrease. El-Rabaa and Shah [110] established a mathematical model of the perforation friction on the hole. This model showed that an insufficient number of perforations will cause an increase in friction. Therefore, in hydraulic fracturing, the ideal case is to maintain the number of perforations within a reasonable range. When the number of perforations is insufficient, the friction (as a result of perforations) is larger, resulting in a lower fracturing rate, in which the solid particles of proppant flushed the surface of the perforation, causing serious erosive wear of the surface of the perforation. Meanwhile, erosive wear can also generate perforation pollution, which is the main cause of perforation wear. Long's team established a perforation wear model to explore the effect of perforation wear on crack propagation.

Fracture Bending Friction
The friction caused by bend after a fracture is another friction mechanism near the well. In hydraulic fracturing engineering, bending friction is mainly caused by inclination, the number of fractures, an improper perforation stage, and other factors. Zhu's team [111] established a dynamic frictional torque model for large, deviated wells, in order to study the influence of inclination of friction resistance on oil/gas well. They determined that the higher the inclination, the greater the friction wear. According to the Besilianke formula, a mathematical model of friction is built and can be used to more easily analyze the influence of the inclination on the friction coefficient [112]. All in all, we need to further consider the influence of wellbore inclination and friction. In order to reduce the corresponding frictional resistance, it is necessary for us to reasonably reduce the wellbore inclina- When the number of perforations is insufficient, the friction (as a result of perforations) is larger, resulting in a lower fracturing rate, in which the solid particles of proppant flushed the surface of the perforation, causing serious erosive wear of the surface of the perforation. Meanwhile, erosive wear can also generate perforation pollution, which is the main cause of perforation wear. Long's team established a perforation wear model to explore the effect of perforation wear on crack propagation.

Fracture Bending Friction
The friction caused by bend after a fracture is another friction mechanism near the well. In hydraulic fracturing engineering, bending friction is mainly caused by inclination, the number of fractures, an improper perforation stage, and other factors. Zhu's team [111] established a dynamic frictional torque model for large, deviated wells, in order to study the influence of inclination of friction resistance on oil/gas well. They determined that the higher the inclination, the greater the friction wear. According to the Besilianke formula, a mathematical model of friction is built and can be used to more easily analyze the influence of the inclination on the friction coefficient [112]. All in all, we need to further consider the influence of wellbore inclination and friction. In order to reduce the corresponding frictional resistance, it is necessary for us to reasonably reduce the wellbore inclination.
On the other hand, multiple cracks are a factor that effect the bending friction from fracture. Multiple cracks, caused by the improper phase of natural fractures and perforations, can result in the increase of bending friction in near-well friction. Overbuy et al. [113] confirmed the existence of multiple cracks. Lyle et al. [114] discussed the causes of multiple cracks. According to Michael's theory, the fracture bending friction is related to the total displacement of the fracturing fluid [115]. McDaniel at al. [116], Brumley et al. [117], and Mahrer et al. [118] observed multi-crack morphology and found that the greater frictional resistance, the more serious the damage of the construction. Lu studied the relationship between the crack width and severity of wear (due to friction). Using a numerical model, Jeffrey et al. [119] proved that the formation pressure of the second crack was 7% higher than that of the first crack. As a result, how many cracks would be reasonable is known. More cracks mean more pressure drops and a more serious friction wear at the formation.

Friction Reducer
Friction along the path and near the well are two forms of friction that generate varying degrees of fracture losses. Because the hydraulic fracturing process is relatively complicated, a problem to solve is how to effectively avoid friction challenges in the fracturing process. There are several techniques to reduce fracturing friction, which include proppant slug technology, fracturing fluid viscosity increasing technology, proppant surface modification technology, and so on.

Proppant Slug Technology
Proppant slug technology is the most common way to reduce fracturing friction. This technology is mainly used to chock off the imperfect perforations and curved fractures, so that the flow path of the cracks become perfectly smooth, reducing the formation filtration. Therefore, the technology can reduce the frictional resistance and pressure losses. Cleary et al. [120] first proposed the proppant bridge slug technique. This technology was then widely adopted in engineering practices. Malhotra et al. [121] used an alternative slug technology to reduce the friction and provide the deepest and most uniform proppant distribution in the cracks. Medvedev et al. [122] utilized multi-proppant slug technology to reduce the friction during fracture processes. The proppant bridge slug technology will also clog the natural fractures, in order to reduce the formation filtration and ensure the extension of the primary fracture. This technology improves the efficiency of fracturing operations and reduces the friction losses. At the same time, the technology to increase the fluid viscosity, as well as proppant surface modification technology, will become more widespread.

Fracturing Fluid Viscosity Increasing Technology
Fracturing fluids can cause serious friction wear to the formation. Optimizing the fracturing fluid's viscosity is an important technology to improve the lubrication of the fluid. Increasing the viscosity of the fluid [123] not only decreases the frictional resistance but also increases oil/gas production. Huang et al. [124], Liang et al. [28,125], and Guzmán et al. [126] used nanomaterials to hybridize with the fracturing fluids to increase the viscosity, control friction losses, and change rock filtration properties and the lubricating property of fluid. Using Gemini surfactant technology, Blanzat et al. [127], Chu et al. [128], and Lu et al. [129] improved the thermal resistance, shearing resistance, recovery, and sand-carrying properties of the fracturing fluids. This increased the lubrication of the fracturing fluid and avoided friction and formation damage. Fracturing fluid viscosity enhancement technology is now used in oil and gas fields in many countries, such as the Vaca Muerta formation in Argentina [130], the Fat Achimov formation in Russia [131], and many others. To a large extent, it overcomes pipeline resistance.
New fracturing fluids in the future will greatly improve the lubrication. When the fluid enters the high temperature complex formation, the fluids need more lubricity and high temperature resistance. At the same time, efforts are underway to invent green and environmentally friendly fracturing fluids, which not only lubricate and reduce the frictional resistance but also protect the environment. For example, the addition of D-Limonene [132], which is a green surfactant, to the pressure fluid improves the viscosity of the fluid, while reducing wellbore friction by 50%.

Proppant Surface Modification Technology
The friction losses in the fracturing process can also be improved by proppant surface modification technology. Proppant surface modification technology improves the lubrication of proppant surfaces, in order to change their performance. While researching proppant surface modification technology, Boyer et al. [133] proposed the proppant transport modifier technology (PTM, coating of hydrophobic surfactant on proppant surface), which modified the proppant surface to increase the concentration of the fracturing fluid and reduce friction losses. Songire's team [134] used a surface modifying agent (SMA) emulsion coating to invent a new proppant and found that the coating surface was beneficial to reduce particle migration and accumulation, in order to improve the surface lubrication of proppants. Palisch et al. [70] researched the neutral lubricity of proppant surfaces. Lu's team [135] studied the application of the proppant consolidation assistant (PCA) to propose a proppant-controlled reflux technology. Wang et al. [136] used the surface coating the sand upgrade agent (SUA) method to improve surface pressure resistance and lubricity. Therefore, proppant surface modification techniques not only improve proppant concentrations and lubricity but also reduce friction during fracturing.
The proppant surface technology still needs to be further refined. First, an evaluation system of proppants needs to be constructed. Afterwards, proppants need to improve their temperature resistance and pressure resistance under complicated working conditions, improve surface lubricity, and reduce proppants concentration, in order to reduce the friction.
Friction reduction techniques, a major issue in the hydraulic fracturing challenge, include not only the methods described above but also the addition of anionic and cationic [137] drag reducing agents [138] to the fracturing fluid to reduce resistance. Alternatively, optimization algorithms are used to improve fracturing systems [139], and additives [140] are used to increase production and preserve value. We need to do our best to reduce friction challenges to research the fracturing mechanism, propose new fracturing technology and materials, study the friction mechanism, and reduce friction in fracturing, as shown in Figure 7. We should pay attention to the challenges of friction in hydraulic fracturing and constantly explore the friction performance and material structure optimization techniques in fracturing. In the future, the green development of hydraulic fracturing will fundamentally reduce the harm of friction. are used to increase production and preserve value. We need to do our best to reduce friction challenges to research the fracturing mechanism, propose new fracturing technology and materials, study the friction mechanism, and reduce friction in fracturing, as shown in Figure 7. We should pay attention to the challenges of friction in hydraulic fracturing and constantly explore the friction performance and material structure optimization techniques in fracturing. In the future, the green development of hydraulic fracturing will fundamentally reduce the harm of friction.

Conclusions and Perspective
In the production of oil/gas, hydraulic fracturing has become an indispensable technology, suitable in a variety of different environments. Although hydraulic technology has become very mature in the oil/gas fields, the friction challenges in fracking have not been fully resolved. This paper analyzed the mechanism of fracture formation, in order to better understand the fracture and extension processes. Formation fractures are formed under the influence of formation friction, and the fractures are further extended under the

Conclusions and Perspective
In the production of oil/gas, hydraulic fracturing has become an indispensable technology, suitable in a variety of different environments. Although hydraulic technology has become very mature in the oil/gas fields, the friction challenges in fracking have not been fully resolved. This paper analyzed the mechanism of fracture formation, in order to better understand the fracture and extension processes. Formation fractures are formed under the influence of formation friction, and the fractures are further extended under the comprehensive stresses generated by the pressure of the fracturing fluid, as well as other external forces delivered in the tube. It is undeniable that friction has a huge impact on hydraulic fracturing. Its main exposition has the following points: (1) Basic research of hydraulic fracturing, fracture extension, proppant filling, and fracturing fluid filling are all related to friction problems in the fracturing process. (2) The friction problem of hydraulic fracturing mainly lies in the resistance challenge of wellbore and near wellbore, which destroys the water sensitivity of formation and causes formation instability. (3) In the face of friction challenges in fracturing, three technologies, such as proppant slug technology, fracturing fluid viscosity enhancement technology, and proppant surface modification technology are proposed to alleviate friction problems in fracturing and improving oil and gas production and preservation.
Additionally, friction reduction is the main challenge in the hydraulic fracturing process. In the future developments, hydraulic fracturing fundamentals, friction challenges, friction mitigation, and new fracturing techniques and materials will continue to be a more ancient theme. So, further research is needed on the following points.
(1) Fracturing fluid performance improvement. Green and recyclable fracturing fluids are proposed to not only reduce friction losses in the fracturing process, but also to break through the development of new high-performance materials. All in all, the future of hydraulic fracturing is expected to be highly efficient, environmentally friendly, and economic. Reducing the friction in the fracturing process will require a continuous effort and breakthroughs in developing newly high-performance materials. Successfully addressing the frictional challenge of hydraulic fracturing will drive the development of 'green' technologies in the production of oil/gas, as well as in other related engineering applications.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.