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We discuss the pros and cons of fracking and debate if the dangers of fracking offset its economic benefits.

What is "fracking"? History and evolution

Hydraulic fracturing (ʺfrackingʺ) is the fracturing of rock by a pressurized liquid. This technique consists on injecting water, sand and chemicals at high pressure in order to create small fractures and facilitate the extraction of gas and oil. Hydraulic fracturing can report important economic benefits to many countries but entails important environmental and public health risks.

The history of fracking can be traced back to . During the American Civil War, Col. Edward Roberts realised that oil could be extracted through the use of explosives. In , he created the patent of the "Exploding Torpedo." This method used an iron case with powder which exploded under the surface and propelled water, fracturing the subsoil and contributing to increase the production of gas and oil wells. Later, liquid nitroglycerin was used in the process, until, in the s, drilling companies, replaced it by with a non-exploding liquid called acid. 

The expansion hydraulic fracturing began in the s following a successful experiment contucted by Floyd Farris for the Sanolind Oil and Gas Corporation in . Floyd injected gelled gasoline (napalm) and sand in a gas producing formation at the Hugoton gas field in Kansas. A patent for this new method was issued in . Halliburton Oil Well Cementing Company acquired the exclussive license and implemented the first commercial fracturing treatments in Archer County, Texas, and in Stephens County, Oklahoma. The USSR introduced fracking in , and later other countries gradually adopted this technique.

In the late s, a new technique called massive hydraulic fracturing (or high volume fracturing) was introduced in the USA. It consisted of injecting over 136 metric tonnes of proppant and help make economically viable the expoitation of gas saturated sandstones with low permeability. Massive fracturing became popular in Western Canada, Germany, Netherlands and the UK.

Fracking continued to grow and evolve. President Gerald Ford included the development of shale oil resources as a prioritiy in his energy plan. In the US Government lauched the Eastern Gas Shales Project and public funding was devoted to the Gas Research Institute, which undertook many experiments and projects in the following years. In the s, oil companies realised that horizontal wells were more effective in producing oil than vertical ones. In , Nick Steinsberger, an engineer from Mitchell Energy, discovered a more efficient slickwater fracturing technique. By applying higher pump pressure and more water, gas extraction became economically viable in the Barnett Shale in Texas.

Today, there are horizontal wells using massive hydraulic fracturing  in the USA, Canada and China. Thanks to these latest techniques, shale gas extraction has become economically viable. However, some countries, such as France and Tunisia, aware of the environmental and health dangers of fracking have decided to ban it. But is the impact of fracking mostly positive or negative? Should governments continue promoting innovation in this area?

Fracking pros and cons

Defenders of hydraulic fracturing usually explain that this technique has some benefits:

• The main benefit is the impact on the economy. The technique allows companies to access oil and gas difficult to get by traditional drilling methods. Therefore it would contribute develop a new industry and the economy in areas where gas extraction was not possible before. It can also contribute to increase the offer of gas and oil, on which our economy is so dependent, keeping consumer prices down.
• Fracking is positive in terms of energy security as countries may become less dependent on oil and gas imports and avoid the impacts of fluctuation in oil prices.
• Fracking may have an indirect positive impact on environment. If you take into consideration that the technique is mostly used to extract natural gas, which produces far less carbon dioxide emissions than other commonly used sources of energy, such as coal.

On the flip side, criticism against fracking includes some of these arguments:

• Environmental risks are often cited as the main problems. Fracking is associated with air and water contamination in the production area. Some of the chemicals used in fracking, such as hydrochloric acid, can be very dangerous for the environment. More than 40 different chemicals are used in the proces, such as acids, bioacides, breakers, stabilizers, corrosion inhibitors, crosslinkers, gelling agents, and surfactants. Additionally, hydraulic fracturing is a very noisy activity, and it can cause small local earthquakes. 
• In addition, hydraulic fracturing is criticized for wasting a considerable amount of fresh water, when drought risks are increasing. Clean water precious assets for life.
• The economic argument presented by defenders of fracking is also often criticized by opponents. Since oil and gas ressources are limited, using hydraulic fracturing is only delaying the inevitable end of fossil energy, and increasing the problem of climate change. Maybe efforts and investments should be dedicated to sustainable energies, such as the solar or hydroelectric energy, and not to alternative drilling techniques.

Watch these videos on the benefits and dangers of fracking

Emerging questions: Should fracking be used despite its environmental risks? Should fracking be banned? Should governments continue investing in research and innovation to make fracking less harmful for the environment?

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A fracturing proppant whose bulk density is less than 1.5 g/cm3 and apparent density is approximately 2.5 g/cm3 can be regarded as a ULW fracturing proppant (Wu ). On the one hand, it can reduce the amount of guar gum used in the fracturing fluid, which reduces the damage to a reservoir (Cheng and Li ); on the other hand, it can reduce the energy loss during the fracturing process and thus form a high-conductivity fracturing crack (Gao et al. ; Li ). Proppants with ultralow density, high closure pressure, and good heat resistance are urgently needed in the process of unconventional oil and gas resource exploitation. The ULW proppants reported in the literature (Table 1) is mainly divided into three categories in accordance with raw materials, including ULW-1 (organic polymer), ULW-2 (impregnation of nutshells, coated), and ULW-3 (porous ceramsite coated with resin). Each type of proppant has its own advantages and disadvantages. They have been widely used in different conditions depending on geology, availability, prices, and government regulations. The following is a basic introduction to each proppant type.

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2.1

Basic properties of ULW proppants

ULW-1 (Brannon et al. ; Brannon and Starks ) is a heat-treated nanopolymer microsphere with an apparent density of 1.05 g/cm3, a glass transition temperature of approximately 145 °C, a closure pressure of 45 MPa, and a size of 14/40 mesh and 40&#;80 mesh (Fig. 3). The acid solubility rate is less than 2%, and the sphericity is greater than 0.9. The disadvantage of ULW-1 is that it is prone to deformation compared with traditional fracturing proppants. Zhang used graphite, fly ash, and reinforcing carbon black to polymerize with polystyrene to form a nanocomposite ULW polymer microsphere (Zhang et al. ). The glass transition temperature reached above 250 °C, and the crush resistance was less than 2% at 52 MPa. Parker et al. also developed a new ULW proppant from thermoplastic aluminum alloys with stable chemical properties (Parker et al. ). However, it can only be applied to a reservoir with low closure pressure (approximately 7 MPa) because of the strength limit. The density of this proppant is approximately 1.05&#;1.08 g/cm3.

ULW-2 (Bestaoui-Spurr and Hudson ; Han et al. ; Parker et al. ) is a highly angular particle (such as husks and walnut shells), which yields a high permeability at low closure stresses, and no fines are produced as stress increases (Fig. 4). The raw material is necessary to impregnate or wrap with resin to improve the closure stresses. The ULW-2 proppant has an apparent density of 1.25 g/cm3. It can withstand closure stress of 42 MPa at 79 °C and 28 MPa at 146 °C.

Photograph showing the angularity of a 1.25 specific gravity ULW proppant (Rickards et al. )

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ULW-3 (Coker and Mack ; Jardim Neto et al. b; Rickards et al. ) is a porous particle, such as hollow glass microspheres and hollow spheres. It has the same surface roughness as conventional ceramic proppants, as shown in Fig. 5. This type of proppant has an average porosity of approximately 50% and can form a ULW proppant with a stereoscopic density of approximately 1.75 g/cm3. The closing stress of 56 MPa can be tolerated at 121 °C. Nonetheless, this proppant type exhibits a tendency to produce fine particles, leading to the plugging of pores.

Picture showing the sphericity of ULW-3 (Jardim Neto et al. b)

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Table 2 compares the bulk density, bulk porosity, and sphericity of the above three proppants. ULW-3 is the heaviest proppant, whereas ULW-1 is the lightest. As shown in Fig. 6, ULW-1 is basically spherical, ULW-2 is polygonal, and ULW-3 is intermediately rounded. The porosity of packing with ULW-1 is the highest among the three types of proppants. Figure 7 shows particle size distribution of the three proppants. It can be seen that ULW-2 has a wide particle size distribution and a poor uniformity coefficient, and the two other distributions are relatively concentrated.

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Two-dimensional close-up images of ULW with a magnification of 23×&#;(Gaurav et al. )

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Sieve size distribution of ULW proppants (Gaurav et al. )

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2.2

Settling speed of ULW proppants

The results of different types of proppant settlement experiments are shown in Fig. 8. The proppant type varied, and slick water with a relative density of 1.0 and a viscosity of 1&#;3 cps was used as the fracturing fluid. The relative viscosity of the fracturing fluid can be set to fixed values. From Fig. 8, the settling speed of 20/40 traditional quartz sand and ceramsite reaches or exceeds 16.5 ft/min. The settling speed of 40/80 mesh coated lightweight ceramic (LWC) proppant is 8 ft/min, whereas the settling speed of 40/100 ULW proppant is 0.08 ft/min. Under the same conditions, the settling speed of the ULW proppant is much lower than those of quartz sand and ceramsite (Brannon and Starks ).

Settling rate for proppant types and size (Brannon and Starks )

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2.3

Strength and conductivity of ULW proppants

Proppant crushing experiments were conducted at 25 °C and 95 °C under the pressure of 103 MPa, and the stress was continuously loaded for 2 min. Individual particle strengths were also tested at 90 °C (Gaurav et al. ; Gu et al. ). The fine particle content was further analyzed after the test was completed. As shown in Table 3, the experimental results show that ULW-1 and ULW-2 produced only a small number of fine particles, while ULW-3 produced relatively more fine particles. In addition, the single-particle strength test shows that ULW-1 is shaped and easily deformed, and the difference among particles is large; ULW-3 is brittle, and a single particle has the lowest damage point. The strength characteristics of ULW-2 are in between those of ULW-1 and ULW-3.

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Figure 9 shows that the conductivity of 0.02 lb/ft2 ULW-1.05 proppants at psi closure is 3 times greater than that of 1.0 lb/ft2 pack of sand. However, the three types of proppants have opposite changes in displacement efficiency. Figure 10 illustrates the simulation result of displacement efficiency of different proppants. The sand distribution is highly nonuniform, while ULW proppants approach the upper areas as they move further from the wellbore into the reservoir. Among the ULW proppants, ULW-1 generates a proppant bed with the lowest conductivity, but it exhibits the best proppant placement efficiency, i.e., the largest propped area with a uniform conductivity; ULW-3 builds a proppant bed with the highest conductivity, but the bed length is shorter and smaller than that of ULW-1. In short, the use of ULW proppant can obtain a large effective fracture support area, improve the production degree and conductivity of the reservoir, especially the tight reservoir with serious vertical heterogeneity, and enhance the effect of increasing production.

Proppant conductivity vs. closure stress (Brannon and Starks )

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Conductivity distributions for different proppants in 0.1 µD shale (Gu et al. )

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2.4

Propped fracture area and increased production effect of ULW proppants

Compared with the application of conventional proppants, the application of 40/80 mesh ULW proppants combined with slick water provides better proppant transport capacity, conductivity, and borehole performance. Table 4 compares the fracturing effects of conventional and ULW proppants. The simulation results show that the effective fracture area and productivity of fractures in wells with ULW fracturing are significantly higher than those of ordinary proppants. Although the unit price of ULW proppant is high, the ULW technology can achieve full fracture support and high conductivity by using low sand paving concentration. Therefore, the overall cost of fracturing operations has not changed much (Brannon and Starks ).

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