A comprehensive review of ultralow-weight proppant ...

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

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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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