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Design, synthesis and application of low-density proppant (LDP) are of great significance for efficient and clean exploitation of low permeability oil and gas. On the basis of a brief introduction of hydraulic fracturing and the application of traditional proppants, this review systematically summarized the polymer application progress in LDP, including coated sand, coated ceramics, coated nutshells, especially for polymer composites based ultra-low density proppant (ULDP). Finally, the existing problems and future development direction are also prospected.
Keywords:
LDP, polymer, sand, ceramics, nutshell, ULDP, fabrication, property
As one of the most important technology for the exploitation of oil and gas, hydraulic fracturing has been widely used in the exploitation of unconventional low permeability reservoir, and secondary exploitation of old oil and gas wells ( ) [1]. Hydraulic fracturing can stimulate hydrocarbon production by creating a network of highly conductivity fractures surrounding a wellbore. In the hydraulic fracturing, proppant is brought into the fractures generated by hydraulic fracturing with fracturing fluid, therefore effective conductivity and high output of oil or gas can be obtained [2]. Among the materials used in the hydraulic fracturing, proppant was considered to be the key materials to enhance the exploitation efficiency of oil and gas, especially for the old wells and low permeability reservoir [3].
Open in a separate windowProppant is a solid particle with certain sphericity and roundness, strength, corrosion resistance and stability, which can prevent the reclosing of fractures generated by hydraulic fracturing, maintain the smooth flow of oil and gas, and improve the productivity [4,5]. In the hydraulic fracturing process, the shape, strength, acid solubility and turbidity of the proppant can influence the integrity of the newly created fractures, thus determined the efficiency of oil and gas flow out of the well [6]. The sphericity and roundness are very important because it can influence the strength of the proppant. The sphericity is the degree of the proppant close to the sphere, and the roundness refers to the relative sharpness of proppant edges and corners. The angular and pointed proppant tends to break easily, whereas the well-rounded proppant will be mechanically stable. The proppant with high sphericity and roundness can provide high conductivity, because larger passages can be formed for this kind of proppant. The ability to withstand compressive loading is also necessary for proppant. The proppant mustn’t break into fines and block the fractures, which will down the production rate, thus the proppant should with low crushing rate. In addition, the proppant must endure harsh environment such as acid mixture pumped into wells to generated crack, thus the low acid solubility and low turbidity is also very important for the proppant.
Since the natural sand, glass bead and nutshell has been used as proppants in the hydraulic fracturing, new and efficient proppants have been developed rapidly [7]. After 1970s, due to the excellent performance in oil exploitation, synthetic ceramics proppants sintered with bauxite as the main materials was rapid developed and promoted. In order to further improve the comprehensive properties, polymer coated sand and coated ceramic proppant were developed after 1980s [8]. However, traditional natural sand and synthetic ceramics is not suitable for the eco-friendly hydraulic fracturing, because they are settled quickly in low-viscosity fracturing liquid ( ) [9]. In addition, the inertia of these high density proppants makes them hard to turn from the wellbore to perforations [10].
Open in a separate windowThe oil and gas production is highly correlated with the propped length and area, and the fracture conductivity of hydraulic fracturing. As reported in the literature [11], 3D fracturing model shows that ULDP cover higher and longer fracture areas with a smaller propped width. The ULDP can improve the propped length and area in low permeability shale reservoirs, while mainly improve the propped area in high permeability shale reservoirs. On the other hand, the fracture conductivity was highly dependent on the proppant size, flow back, and fines generation [6]. Generally, large proppant size, low flow back and low fines generation can provide higher fracture conductivity.
As the traditional hydraulic fracturing fluid possesses a low viscosity, which has a poor carrying ability for high density proppant, such as natural sand, ceramic and glass bead, which has high density and high fines generation [12]. In order to improve the carrying capacity of traditional proppant, it is necessary to increase the flow rate or viscosity of fracturing fluid. Though these methods can solve the problem of carrying ability to some extent, yet they will cause new problems such as difficult treatment of fluid flow back and great damage to reservoirs [13]. Therefore, in order to thoroughly solve the problem of proppant transport, it is of great significance to develop novel proppant with low density, or new fracturing technology such as liquid self-propping [14]. According to the difference of density, proppants can be divided into following categories ( ) [15].
Till now, there are three main strategies to obtain low density proppant: (1) make the traditional HDP into hollow or porous structure, so as to reduce the apparent density of the proppant [5,15]. (2) organic polymer coating for traditional HDP, which can not only reduce the density to some extent, but also can improve the mechanical strength, sphericity, hydrophobicity and corrosion resistance [6,7,8]. (3) develop ULDP with organic polymer, modification with inorganic filler is always needed to improve the mechanical and thermal properties [15]. Though the porosity of traditional HDP can significantly reduce the density, yet the mechanical strength and corrosion resistance will also be affected. Therefore, increasing the content of low density organic polymer in the proppant, including coated proppant and polymer composite proppant, became the most important development direction of high performance LDP. As mentioned above, this review firstly introduced the HDP based on natural sand and synthetic ceramics, then systematically summarized the application of polymer in coated sand, coated ceramic, coated nutshell, especially for the polymer composites based proppant.
At present, the most used proppant in hydraulic fracturing is high-density natural sand and synthetic ceramics, the typical properties of these proppants are listed in [6]. Because of its low cost and relative low density, the natural sand was also widely used in the hydraulic fracturing [7]. However, the natural sand possesses high crushing rate, high turbidity and low sphericity, which is only suitable for the exploitation of shallow oil and gas wells with low closing pressure. With the depth and pressure increase of the oil well, the natural sand can not meet the requirements, high performance synthetic ceramic proppant came into being. Though high-strength ceramic proppant can be made from aluminous minerals and other industrial wastes, yet the fabrication of ceramic proppant needs to be sintered under high temperature, leading to the increase of cost [16,17]. Most of the ceramic proppant was made with high density bauxite as raw materials, leading to the high density of traditional ceramic proppant. To enhance the carrying ability for high density ceramic proppant, the increase of displacement or viscosity for the fracturing fluid is always needed, while this will bring about the problems of difficult treatment of fluid flow back and great damage to oil and gas reservoirs [18].
In order to reduce the density of synthetic ceramic proppants, novel ceramic proppant with hollow or porous structure were developed ( ). Template based method is the most commonly approach to prepare hollow ceramic proppant, and volatile materials are always used as the templates, which can be volatilized through heating in the process of sintering [19]. With the addition of porogen in the fabrication, porous ceramic proppant can be made with the method similar to hollow ceramic proppant [20]. Though the porosity of high density ceramic proppant can significantly reduce the density, the mechanical strength and corrosion resistance will also be affected [15].
Open in a separate windowThe coating of traditional proppant with organic polymer can not only decrease the density of the proppants, but also can improve the sphericity and prevent the flow back of proppant ( ) [7]. In addition, the polymer coating can serve as a protective layer to improve the chemical resistance and hydrocarbon affinity, providing interaction between proppant to prevent leakage, as well as wrapping the breaks proppant after the proppant broken [8].
Open in a separate windowPolymers are obtained through the polycondensation or addition polymerization of small monomers. According to the chemical composition, polymers can be divided into organic polymers and inorganic polymers. According to the thermal behavior, the organic polymers can be divided into thermosetting polymers and thermoplastic polymers. As the thermosetting polymers have many advantages such as easy processing, high strength and low density, they have been widely used in the proppants coating [8]. On the other hand, the thermosetting polymers also possess some disadvantages, including low resistance to oxygen, ozone and radiation, as well as the low thermal stability. The properties of the polymers mainly used in the proppant coating are listed in [12], including epoxy resin, phenolic resin, urea-aldehyde, and so on. Among this polymers, epoxy and phenolic resin was the most promising candidate, due to its high performance and low cost.
According to the difference of curing style, the coated proppant can be divided into precuring proppant and curable proppant [7]. The precuring proppant was made by traditional heat curing, individual proppant particle with smoother surface can be obtained. In addition, the compressive strength, acid solubility, sphericity and roundness of the proppant can be improved. For the curable proppant, the polymer was coated on the substrate firstly, then injected into the formed fracture, finally cured and formed network structure under high temperature and pressure in the well. The formed proppant network can not only prevent flow back, but also can decrease the proppant embedding into the clay or shale [8].
Proppant is the key materials for hydraulic fracturing, with the increase of low permeability reservoir and the high requirements of environment protection, the demand for LDP is growing rapidly, especially for the ULDP. However, the density of traditional natural sand and synthetic ceramic proppant is too high, and the properties of sphericity, crushing rate, acid solubility and turbidity is not ideal. Though the coating of high density proppant can reduce the density to some extent, yet the obtained coated sand and coated ceramic only can be regarded as LDP.
In order to obtain ULDP, organic matrix based materials was rapidly developed for its construction, including nutshell and polymer composites. Though the nutshell based proppant is renewable and cheap, yet the sphericity and compressive strength is very low, the properties of acid solubility, turbidity and heat resistance is also not ideal. In addition, the coating of nutshell also has limited effect on improving the performance. Based on suspension polymerization, ultra-low density (less than 1.25 g·cm−3) polymer composite microsphere with very high sphericity (more than 0.9) can be obtained. In addition, the compressive strength and heat resistance of polymer composites based proppant can be improved by chemical cross-linking and inorganic filler incorporation. Therefore, polymer composite based ULDP will be the most promising candidate.
Now, PS and PMMA composite microsphere based ULDP has been widely investigated. With the development of polymer composites, besides the low density and high sphericity, polymer composite microsphere with improved crushing rate, acid solubility, turbidity and heat resistance has also been reported, which has good application prospect in the field of hydraulic fracturing. However, there are also some problems need to be addressed in the investigation and application. Firstly, there are few researches about the practical application performance of this kind of ULDP, especially for the conductivity. Secondly, the cost of polymer materials is higher than that of sand, ceramic and nutshell. It is hoped that these problems can be solved in the future research, so the industrial application of polymer composites base ULDP can be realized.
This work was supported by grants from the National Natural Science Foundation of China (51903080), Foundation of Hubei Provincial Department of Education (Q20222802, D20212801, B2021223), Scientific Research Foundation of Hubei University of Science and Technology (BK202003, 2022T03, 2021ZX15, 2022ZX13).
Conceptualization, J.G. and T.C.; methodology, X.H.; software, T.L.; investigation, G.H.; formal analysis, Y.Z.; resources, G.H.; original draft preparation, J.G., T.C. and T.L.; review and editing, Y.Z. and X.H.; project administration, T.C. All authors have read and agreed to the published version of the manuscript.
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