Fig. 1 Sand proppant mixing into the fracturing fluid passing through a window screen
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Fig. 2 Representation of proppant agents settling into the fractures
Fig. 3 Cost-efficiency &#;pyramid&#; of proppant types
Fig. 4 Sample of artificial glass proppant produced from waste bottles
Fig. 5 Samples of proppants with different density
Fig. 6 Multifunction ceramic proppants coated with bacterial biofilm
Fig. 7 Krumbein-Sloss chart
Fig. 8 Refiguration of proppant shapes
Fig. 9 Chart resuming the basic proppant selection factors
The original version of this article was created by Francesco Gerali, Elizabeth & Emerson Pugh Scholar in Residence at the IEEE History Center
It is recommended this article be cited as:
F. Gerali (). Proppants, Engineering and Technology History Wiki. [Online] Available: https://ethw.org/Proppants
Introduction
Hydraulic fracturing is the process of injecting fluids into oil or gas bearing formations at high rates and pressures to generate fractures in the rocks (Fig. 1). The task of proppants, or propping agents, is to provide and maintain long term efficient conduit for production from the reservoir to the wellbore. Proppants are mixed with the fracturing fluids and injected downhole into the formation (Fig. 2).
Today, proppant materials can be grouped into three main categories: rounded silica sand, resin coated sands, sintered or fused synthetic ceramic materials (Fig. 3). The most common used base materials are sand, ceramic, and sintered bauxite. Calculations about the proppants performance into the formation injected also improve to the well stimulation operation; to quantify proppant performance, specific quality-control procedures have been elaborated in past 30 years by the American Petroleum Institute[1] (API) and the International Standards Organization (ISO). Proppant selection, which includes the proppant type, size and shape, is a very critical element in stimulation design. Proppants have to withstand high temperatures, pressures, and the corrosive environment present in the formation. If the proppant fails to withstand the closure stresses of the formation, it disintegrates, producing fines or fragments which reduce the permeability of the propped fracture.
Availability and cost are the most desirable attributes of proppants, but also physical qualities of both natural (e.g. sand grains) and man-made, or artificially engineered, (such as resin-coated sand and ceramic) proppants are crucial. The structural properties of proppants, both natural and manmade are gauged following specific criteria:
History and recent advancements in proppant technologies (-)
The first experimental use of hydraulic fracturing took place in with the use of about 20,000 pounds (9.1 metric tons) of river sand. The fracturing treatments followed used construction sand sieved through a window screen. There have been a number of trends in sand size - from very large to small - but since the beginning a &#;20/+40 US-standard-mesh[2] sand has been the most popular. Currently, approximately of the 85% of the sand used match this size.
Since then, the process of stimulating and fracturing wells has continually evolved as have the materials and technologies employed to do likewise. In addition to naturally occurring sand grains and walnut hulls, in past 70 years, especially after the s Texas boom, several proppant materials were carefully sorted for size, shape, weight (e.g. ultra-lightweight) and mechanical characteristics (e.g. multifunctional). Numerous propping agents have been evaluated throughout the s and the s, including resin-coated sands, plastic pellets, rounded, Indian glass beads, ashes and steel shot (Fig. 4). Since the s were developed and commercialized aluminum pellets, high strength glass beads, sintered bauxite, and fused zirconium.
Experimentation and new fracking techniques led to great steps forward in proppant studies, peaking in innovative breakthroughs achieved in the s. The manmade proppants provided unique capabilities that natural proppants could not: scientists and manufacturers have developed proppants that can be engineered to mitigate or withstand many of the physical and environmental conditions existing in deep in fractured zones. The new age of proppants - still in combination with traditional proppants - allowed resource developers to match up with conditions and characteristics of their target rock formations with more effectiveness. By adjusting proppant properties such as size, geometry, and weight, the optimal channel for unconventional hydrocarbons production flows can be obtained in different kinds of rocks.
Sand Proppants
Sands have remained, for decades, the most used proppant agent for hydraulic fracturing, mainly due to their low cost and easiness to handle. Fracking sands are extracted from silica sand deposits or riverbeds, then crushed, washed, dried and (but not always) sized. These are mechanical processes and no special (expensive) chemical action is needed. Two types of sand proppants are chiefly in use since the beginning of the practice: white quartz sand (light colored because the few impurities), and brandy brown quartz sand (first experimented in ) with low percentage of silica and high impurity content, which makes it more prone to crushing at lower stress.
The rate of concentration of sands in the drilling fluids (lbm/fluid gal)[3] remained low until the mid-s, when viscous fluids such as crosslinked water-based gel and viscous refined petroleum were introduced. Large-size propping agents were advocated then. The trend then changed from the monolayer or partial monolayer concept to pumping higher sand concentrations. Since that time, the concentration has increased almost continuously, with a sharp increase in recent years. These high sand concentrations are due largely to advances (pression and velocity) in pumping equipment and improved fracturing fluids. For example, today it is not uncommon to use proppant concentrations averaging 5 to 8 lbm/gal throughout the treatment, with a low concentration at the start of the job, increased to 20 lbm/gal toward the end of the job.
Lightweight proppants
The specific gravity[4] (s.g.) of sands has an average value of 2.65; the manufactured ceramic proppants have s.g. around 3.9. Both are heavier than the water (s.g. of 1.0) or brine solutions (s.g. of about 1.2) which are typical base fluids used to carry the proppant to the formation. There are three major trade-offs in using lightweight proppant with high density proppants:
To prevent the settling factor are used high viscosity fracturing fluids to keep the proppant material suspended in order to allow it to penetrate more deeply into the fractures. Ultra-lightweight proppant is preferred in some applications since it reduces proppant settling, requires low fluid viscosity to transport and allows for increased propped length. They can also be more useful in situations where high pump rates or carrier fluids with low viscosities are needed. Several techniques have been used to reduce specific gravity of the proppant. The S.G for lightweight proppant ranges from 0.8 to 2.59.
To make lightweight proppant were selected proppant material which has a lower specific gravity. Walnut shells, pits and husks were the earlier types of lightweight proppant used in the field. Even though such materials would penetrate deeper into the formation, their low structural strength limits their applicability to formations with relatively low closure pressures. Additionally, small particle fragments resulting from crushing of such materials reduce the conductive space available for fluid flow by reducing the fracture network.
Resin coated proppants
Since fracking sand is easily friable and creates fines when it is over-stressed, resin coated sand was developed to enhance the conductivity of sand. Resin-coated sands (RCS&#;s) have been a mainstay of stimulation treatments for more than 40 years. The longevity of RCS&#;s is primarily due to its ability to form a pack within fractures thus preventing proppants from flowing back into the wellbore during fracking and production. It&#;s also stronger than sand and often the choice when compressive strength is needed to prevent crushing in areas of extreme pressure at depth. The main disadvantage of the resin coating is that since the coating material is made of polymers, they tend to have low softening temperatures or low degradation temperatures compared to inorganic materials.
Coating technologies have been applied later to glass beads and ceramic proppants as well. All these proppants belong to resin coated proppant (RCP) category and can also be used as a way to prevent sand production in areas of soft formations where sand control is needed.
Proppants are either pre-coated with resin in a production facility and taken to location (precured) or coated (curable) at the well site by liquid resin coating systems (LRC). The performance of the proppant depends on the properties of the cured resin material. The most commonly used resins used to coat proppants are epoxy resins,[5] furan,[6] polyesters, vinyl esters, and polyurethane. Epoxy resin is the main one used for proppant coating, mainly because it has very good mechanical strength, heat resistance and chemical resistance. Furan has great resistance to heat and water, but it does not provide high mechanical strength. Polyurethane provides also great mechanical strength, good heat resistance and chemical resistance, but just when the application temperature is below 250 F.
The chemical crosslinks that form during the cure of the resin materials do not allow the cured material to melt or flow when re-heated. However, cured/crosslinked resins do undergo a very slight softening at elevated temperatures at a point known as the Glass Transition Temperature[7] (Tg). When the temperature is above the Tg, the mobility of the polymer chains increases significantly and the cured resin changes from a rigid/glassy state to more of a rubbery/compliant state. In this case, the resin system becomes soft and the strength decreases. So Tg has been used as a valuable parameter to determine performance limit of resins.
Ceramic Proppants
Sands showed to be not capable to withstand high closure stresses (up to psi). That gap got solved with the development of higher strength ceramic proppants manufactured since the early s from sintered bauxite, kaolin, magnesium silicate, or blends of bauxite and kaolin. Compared to sands, the synthetic ceramic proppants have higher strength and is more crush resistant especially where closure stresses exceed to 10,000 psi. They are more uniform in size and shape and has higher sphericity and roundness to yield higher porosity and permeability of the proppant bed. Furthermore, ceramic proppants have a remarkable chemical and thermal stability, which is functional to reduce the diagenesis effect. Ceramic proppants were since the beginning an engineered product featured by a more complex manufacturing. Since , lightweight ceramics (LWC) were developed. Few year later followed the intermediate density ceramics (IDC) and high density ceramics (HDC) (Fig. 5).
The alumina content of ceramic proppants correlates well with the pellet strength and the proppant density. The approximate correlation between alumina content and strength is true provided the proppant grains are of high quality and manufactured in a manner which minimizes internal porosity. LWC typically contains 45e50% alumina; IDC contains 70e75% alumina; HDC contains 80e85% alumina. However, in it was first coupled a raw material that is very high in alumina content with a new manufacturing process that creates extremely spherical, mono-sized, fully densified particles. Such proppants are referred to as ultra-high-strength proppant (UHSP) can be rated up to 20,000 psi in crushing strength.
Multifunctional proppants
Multifunctional proppants may be used to detect hydraulic fracture geometry or as matrices to slowly release downhole chemical additives, besides their basic function of maintaining conductive fracture during well production. Multifunctional proppants such as traceable proppants and contaminant removal proppants (filled or costed with chemical additives) have been used to prolong the well performance.
Traceable proppants answered to the need of obtaining detailed information about the stimulation treatment, such as location and geometry of created hydraulic fractures. It is also important to determine if the hydraulic fracture has extended to unwanted zones such as water zones. Nuclear logs are one of the common ways used to trace the extent of the induced fracture detection and it involves the use of radioactive materials (tracers) that can be detected by gamma ray logging tools. The radioactive tracers can be made by coating sand with radioactive materials, mixing with pulverized natural radioactive materials, impregnating radioactive ion exchange resins or ground plastic into proppants. When mixed with regular proppant during the fracturing process, these radioactive materials will emit gamma rays. Then gamma ray detectors, which have been in use in the oil & gas industry since early 's, are used to log after the hydraulic fracturing process. The gamma rays from the radioactive tracers are detected, recorded, and analyzed either in real time during wireline logging run, or recorded into memory and processed later after the tracers are retrieved. This technique is typically very effective and can be used to trace signals from multiple tracers.
Hexion Inc.[8] in has patented (applied in ) an advanced technique which involves non-activated radiation-susceptible materials capable of being activated by a neutron source. These materials can be incorporated into the resin coating of the proppant or into the composite composition of the proppants to be pumped downhole the same way as the radioactive tracers. Then a logging tool which contains a pulsed or continuous neutron source and gamma ray detector is moved past the intervals containing the traceable proppant. The gamma rays emitted from the neutron activated tracers are then detected by the sensor in the tool when it passes through a zone containing the activated material. The advantage of this technique is that it resolves the concerns of the handling, transportation, storage, and environmental concerns of handling a radioactive material.
In it was reported a technique which can detect the fracture geometry without using radioactive elements. This is accomplished by incorporating a high thermal neutron capture compound (HTNCC) at low concentrations into each ceramic proppant grain during manufacturing process. The HTNCC-containing proppant can be pumped downhole and into the induced fractures. Because these high thermal neutron capture compounds absorb neutrons, changes to neutron levels can be detected using conventional compensated neutron logs (CNL) or pulsed neutron capture (PNC) tools. The proppant containing zone is scanned using after-frac compensated neutron logs and the results are compared with the corresponding before-frac logs. The location of the detectable proppants was determined from analysis of before-frac and after-frac compensated neutron logs. This technological advancement will most probably expand the portfolio of tracers, at the same time diminishing the downsides of using radioactive agents.
The concept of using proppant grains to filter, clean, or remove possible contaminants from a production well have also been developed in early s: those were called contaminant removal proppants. Depending on the nature of the contaminants, they can be removed by any chemical, physical, or biological ways. This can be achieved by incorporating chemical contaminant removal component either coated onto the proppant grains or filled in the pores of porous proppants (Fig. 6).
Non-spherical proppants
Traditionally, the ideal proppant shape has been conceived spherical or nearly spherical and non-angular: spherical proppants with narrow size distribution provide fractures with the highest conductivity. A lower Krumbein[9] number indicates a more angular proppant (Fig. 7). Angular and pointed proppant particles tend to break off points, which lead to lower conductivity at higher closure stress. Ceramic proppant and resin-coated ceramic proppants require an average sphericity of 0.7 or greater and an average roundness of 0.7 or greater. All other proppants shall have an average sphericity of 0.6 or greater and an average roundness of 0.6 or greater.
Between and , different shapes of proppants - elongated and rod-shaped - other than conventional spherical shape have been produced (Fig. 8). They present higher conductivity factor due to the higher porosity in the packing. A series of laboratory observation held in late s noted the untapped pack porosity both for spherical and rod-shaped proppants. The conductivity results, 37% spherical vs. 48% rod-shaped demonstrated the benefit of the latter. The variation in rod length and diameter can increase the risks of placement, impact conductivity and affect proppant flowback performance.
In was investigated a different shaped high-drag ceramic proppant based on the relationship that increasing the drag force of the proppant particles will reduce the proppant settling velocity. It was designed and optimized in a way that center of gravity and centroid of volume do not align in a stable manner, so the proppant particles tumble and flutter when settling in a fluid: this design results into a slower settling time compared to conventional spherical sand proppant.
Conclusions
Developing unconventional petroleum and gas plays is very cost-sensitive, especially when using proppants with advanced characteristics. With the development of deeper reservoirs and more complex fracture geometry and formation properties, the performance requirements for proppants have become very demanding (Fig. 9).
See also
References
Acharya, A R. Tue . "Viscoelasticity of crosslinked fracturing fluids and proppant transport". SPE (Society of Petroleum Engineers) Product. Eng.; (United States). Journal Volume: 3:4
Alkhasov, Solomon. . Commercializing A Resin-Coated Proppant. Case Western Reserve University. Master of Science Thesis.
Campos, V. P. P. de, Sansone, E. C., & Silva, G. F. B. L. e. . &#;Hydraulic fracturing proppants&#;. Cerâmica, 64 (370), 219-229.
Gallagher, David. . &#;Hierarchy of Oily Conductivity&#;. Journal of Petroleum Technology 63 (4): 18-20.
Liang, Feng, Sayed, Mohammed, Al-Muntasheri, Ghaithan A., Frank F., Chang, Li, Leiming . &#;A comprehensive review on proppant technologies&#;. Petroleum 2: 26-39
Gallegos, Tanya J., Varela Brian A. . Trends in Hydraulic Fracturing Distributions and Treatment Fluids, Additives, Proppants, and Water Volumes Applied to Wells Drilled in the United States from through &#;Data Analysis and Comparison to the Literature. Scientific Investigations Report &#;. Reston, Virginia: U.S. Geological Survey.
Gidley, John L., Society of Petroleum Engineers. . Recent Advances in Hydraulic Fracturing. 12th ed. Richardson, TX: Society of Petroleum Engineers.
Gurley, D G, Copeland, C T. . Method for forming a consolidated gravel pack in a subterranean formation. United States Patent Number: US . Assignee: Dow Chemical Co.
Halliburton Services. . The FracbookTM Design/Data Manual for Hydraulic Fracturing. Duncan, OK: Halliburton.
Jacobs, James A., and Stephen M. Testa. . Development of Unconventional Oil and Gas Resources: Horizontal Drilling and Hydraulic Fracture Stimulation Techniques. Newark: John Wiley & Sons, Incorporated.
R.R. McDaniel, S.M. McCarthy, M. Smith. . Methods and Compositions for Determination of Fracture Geometry in Subterranean Formations. U.S. Patent No. 7,726,397 B2.
Olsen, T.N., Bratton, T.R., Thiercelin, M.J.: &#;Quantifying Proppant Transport for Complex Fractures in Unconventional Formations,&#; Paper SPE , presented at the SPE Hydraulic Fracturing Technology Conference, The Woodlands, TX, 19-21 January.
Page, J.C., Miskimins, J.L.: &#;A Comparison of Hydraulic and Propellant Fracture Propagation in Shale Gas Reservoir,&#; 09-05-26, J. Can. Pet. Tech., Vol 48, No. 5, May .
Zendehboudi, Sohrab and Bahadori, Alireza (eds). . &#;Exploration and Drilling in Shale Gas and Oil Reserves&#;. Shale Oil and Gas Handbook. Gulf Professional Publishing: 81-121.
Further Reading
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To determine the effect of sintering temperature and ODCPRs contents on the physical&#;mechanical performance of proppants and optimize the sintering conditions, proppants were prepared with various ratios of ODCPRs and sintered at different temperatures. The physical and mechanical performances of the proppants, including the bulk density, apparent density, roundness, sphericity, acid solubility and breakage ratio, and acid solubility were analyzed.
As displayed in Fig. 1a,b, when the ODCPRs contents were improved from 20 to 40%, the bulk density and apparent density of proppants decreased in all sintering temperature points, and the breakage ratio showed the opposite tendency. Whereas the acid solubility of the proppants has fluctuated, which was not only impacted by the ODCPRs contents but also the sintering temperature. The acid solubility was closely correlated to the soluble phases in the proppant, and a large amount of alkaline earth metal oxides was significantly improved with the increment of ODCPRs (such as potassium, sodium, and calcium oxide), which would facilitate the formation of liquid phases under a higher sintering temperature. Thus, under the condition of a relatively high temperature or high ODCPRs dosage, more glass phases would be generated, attributing to a higher acid solubility.
Figure 1The properties of samples with different contents of ODCPRs sintered at various temperatures: (a) bulk density and apparent density; (b) breakage ratio and acid solubility.
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Furthermore, with the increment of sintering temperatures, the optimal sintering temperatures of the A0, B0, and C0 formulas with their lowest breakage ratio and acid solubility all first declined and raised subsequently. The best sintering temperatures of the three formulas could be confirmed as °C, °C, and °C, respectively. According to Table 1, with the increase of the ODCPRs content, both the K value and the molar ratio (Al2O3/SiO2) in initial mixtures were significantly reduced, indicating that the content of flux oxides and silica in the formula was also gradually improved, thus the sintering temperature declined at the higher ODCPRs content. Therefore, formulas of A0 and B0 at °C had exhibited better performance. Noteworthy, the ODCPRs consumption and the cost of raw materials should be comprehensively considered as well, hence formula B0 sintering at °C was identified as the optimal choice with the lowest breakage ratio (6.97%) and acid solubility (4.56%).
Additionally, the photographs of proppants produced in the experiment were displayed in Fig. 2a, it could be observed that the color was darkened as the ODCPRs content or sintering temperature increased. Besides, the Krumbein/Sloss template was used to evaluate the roundness and sphericity of the proppants as shown in Fig. 2b. The average roundness and sphericity degree of all proppants (A0&#;C0) was above 0.8, which could well meet the requirements of the oil and gas industry standard SY/T -.
Figure 2(a) Photograph of proppants under different sintering temperatures; (b) the Krumbern/Sloss template.
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Figure 3a showed the XRD patterns of ceramic proppants of the B0 formula in different sintering temperatures. It can be observed that the crystalline phases of these proppants mainly consisted of corundum and a small quantity of mullite, celsian, and anorthite. Corundum was the predominant crystalline phase that was ascribed to the initial mixtures with a high molar ratio (Al2O3/SiO2), resulting in a highly conducive to the generation of the corundum phase. The diffraction peak of the mullite crystal was weak following that mullite has a relatively low content in this sintering temperature range. Corundum with high crystallinity and granulate mullites can improve the toughness of the ceramic proppants attributing to the reduction of breakage ratio28,32. Thus, as the sintering temperature increased from to °C, the diffraction peak intensities of corundum and mullite crystals both enhanced, which would facilitate the development of high-performance ceramic proppants. Nonetheless, owing to masses of metallic oxides in the ODCPRs, some celsian and anorthite were also generated by the reaction between the BaO, CaO, SiO2 in the ODCPRs, and Al2O3 in bauxite. After the sintering temperature raised over °C, the intensity of the celsian and anorthite phases' diffraction peaks was weakened, which could be because the celsian and anorthite phases gradually changed from the solid to the glass phase with the temperature ascending. But too many liquid phases would eventually exist in the amorphousness of the proppants, contributing to a negative influence on the performance of proppants23.
Figure 3XRD patterns of ceramic proppants: (a) formula B0 sintered at different temperatures, (b) formulas with different ODCPRs contents at their optimum sintering temperatures.
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Additionally, as shown in Fig. 3b, when the ODCPRs content was improved, both the intensities of corundum and mullite diffraction peaks were reduced but the celsian and anorthite phases` diffraction peaks were enhanced at their respective optimal sintering temperature. It was implied that the crystallinity of the corundum and mullite phases decreased with the reduction of the molar ratio (Al2O3/SiO2), while the excessive Ba2+ and Ca2+ in the matrix reacted with Al and Si that formed the celsian and anorthite phases. Conversely, the formation of excessive celsian and anorthite phases might inhibit the development of corundum and mullite phases, and remarkably impair the performance of proppants33. Thereby, the liquid phase was also increased owing to the augment of ODCPRs content, causing the descent of the sintering temperature, such as the formula C0.
Figure S4 displayed the cross-sectional microscopic morphology of the proppants before the acid treatment of formula B0 under different sintering temperatures. With the increase of the sintering temperatures, the prevalence of the inner pores initially decreased and then turned to increase. A loose internal structure with less molten phase and high porosity was observed in the proppant when the optimal sintering conditions were not satisfied, as shown in Fig. S4a. When the sintering temperature gradually improved, a denser structure was achieved, on account of more liquid phases filling in the pores, as exhibited in Fig. S4b,c, and the molten phases also formed a cohesion bonding between the crystal particles and strengthened physical and mechanical properties of ceramic matrix34. Howeveduer, as demonstrated in Fig. S4d, when the sintering temperature was increased over °C, more elongated and connected pores appeared in the interior, which was attributed to generating the excessive liquid phase and promoting the expansion of the ceramic body, resulting in the decrease in density and strength of proppants35,36.
As shown in Fig. 4, the inner appearances of the proppants of formula B0 at different sintering temperatures were also investigated after the acid treatment. Compared to the results from Fig. S4, it can be indicated that the whole equilibrium network structure of the proppant matrix was comprised of the residue glass phase and crystal phases. As seen from Fig. 4a, when the proppant was calcined at °C, the after-eroded net-like phases scattered around the incomplete sintered granular crystal particles. With increasing the sintering temperature to °C, the sample was mainly formed by abundant dispersive small corundum grains with loose internal structures, as shown in Fig. 4b. Subsequently, when the sintering temperature reached °C, as presented in Fig. 4c,d, interlocking columnar and granular shapes of crystals occurred, which were corresponding to the growth of the mullite and corundum crystals. Furthermore, the crystals became bigger under a relatively high temperature ( °C), which illustrated that a proper sintering temperature would enhance the development of corundum and mullite phases. Good growth of corundum has the advantages of high hardness, strength, and resistance to chemical erosion37, thus the proppants of formula B0 at a relatively higher sintering temperature showed better performance.
Figure 4Microscopic cross-sections of the ceramic proppant after the acid treatment of the formula B0 at different sintering temperature: (a) °C; (b) °C; (c) °C; (d) °C.
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Figure S5 displayed the microscopic morphology of the proppants with different ODCPRs contents at their optimum sintering temperature, respectively. Three formulas of the proppants were all well-sintered, meanwhile obvious porous inner structures occurred in all samples as well. Afterward, with the increase of ODCPRs content, more liquid and gas phases were generated during the sintering process. These gas phases were produced from mineral decompositions, leading to an increase in both the number and size of the inner pores of the samples, such as CaCO3 and BaSO4. In parallel with this, the inner crystal appearance of samples with different formulas was also studied after the acid treatment, as displayed in Fig. 5. The results further elaborated that a denser structure composed of granular corundum and rod-like mullite phases was achieved when enough content of Al2O3 and molar ratio (Al2O3/SiO2) was provided, as shown in Fig. 5a,b. However, when ODCPRs contents of the proppant improved beyond 40%, too many ODCPRs in formula C0 formed too much glass phase, as shown in Fig. 5c, causing a relatively higher breakage ratio and acid solubility.
Figure 5Microscopic cross-sections of the ceramic proppants after the acid treatment with different ODCPRs contents at their optimum sintering temperature: (a) A0 at °C; (b) B0 at °C, (c) C0 at °C.
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To guarantee the transition of ceramic proppants into a compacter structure through the sintering reaction, it is necessary to explore the effect of holding time on the performance of proppants. As presented in Fig. 6a, the bulk density and apparent density of proppants under different holding times at °C were carried out. The bulk density and apparent density of proppants exhibited the same tendency which was first increased and then decreased observably with the increase of the holding time, the biggest bulk density and apparent density were obtained at °C and heat preservation for 60 min, as 1.48 g/cm3 and 3.03 g/cm3, respectively. Furthermore, the breakage ratio and acid solubility of proppants initially decreased and gradually raised with the extension of the holding time, as shown in Fig. 6b. The lowest breakage ratio (6.97%) and acid solubility (4.56%) of the proppants were acquired by heating preservation for 60 min, which could well meet the standard of SY/T -. Thus, guaranteeing a proper holding time during the calcination process for preparing ODCPRs-based ceramic proppants at the best sintering temperature was beneficial.
Figure 6The properties of B0 samples sintered at different holding times: (a) bulk density and apparent density; (b) breakage ratio and acid solubility.
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The XRD patterns of formula B0 under different holding times were also shown in Fig. 7, and the modest heat preservation (holding for 60 min) was advantageous to the formation and growth of corundum and mullite crystals. Meantime, it also resulted in the reduction of bauxite and celsian phases. However, the increase in holding time had no obvious influence on the final phase category of the proppant, which deduced that the variation in the performances of the proppant was mainly caused by the microstructure and glass phase content changes of the ceramic proppant.
Figure 7The XRD patterns of B0 samples sintered at different holding times.
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Figures S6 and 8 displayed the microscopic diagrams of the ceramic proppant before and after the acid treatment of the formula B0, which were sintered with different holding times at °C, respectively. As observed in Fig. S6, with the extension of holding time, the pore characteristics of the proppants before acid treatment were similar. Nevertheless, after the elimination of the glass phase, the microstructure of the crystals resulted in obvious differences. When the holding time was only 30 min, plentiful coarse granular corundum grains and a small amount of needle-shaped mullite were observed. After the holding time was improved to 60&#;90 min, both the corundum and mullite crystals grew bigger (Fig. 8a,b). When the holding time reached 120 min, some coarse granular corundum crystals grow up into big particles, which were with excessive growth and an average grain size of about 20 μm. Over the holding time, the abnormal excessive growth of grains would be adverse to gas exhaustion and easily form internal stress failure, which manifested as a change in density and decrease in porosity and strength38,39. Thus, uniformly distributed and smaller crystals improved the mechanical performance of ceramic proppants better.
Figure 8Microscopic diagram of the ceramic proppant after the acid treatment of the formula B0 sintered at different holding times: (a) 30 min; (b) 60 min, (c) 90 min; (d) 120 min.
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Metallic oxide sintering additives were used to promote the liquid phase formation and the liquid viscosity alteration of the ceramic system, which can significantly improve the performance of proppants40,41. In this section, the performances of ODCPRs ceramic proppants with different mass fractions of V2O5 and MnO2 additives at various sintering temperatures were studied.
As plotted in Fig. 9a, when the sintering temperature was below °C, with an increase in V2O5 dosage, the bulk density and apparent density of formula B0 were first increased slightly and then decreased significantly. Moreover, the density of proppants turned out to be much lower when the sintering temperature rose beyond °C. Furthermore, the whole series of BV samples showed a distinct decrease in breakage ratio and acid solubility when the sintering temperature reached °C, as displayed in Fig. 9b. Thereinto, sample BV2 presented the minimum value of breakage ratio (8.40%), and sample BV1 presented the minimum value of acid solubility (4.33%). Although the sintering temperature of the proppant was reduced after the incorporation of V2O5 additives that could reduce the energy consumption, the results also elucidated that the mechanical and acid resistance performance was still undesirable in comparison with sample B0 under its optimum sintering condition. This was ascribed to the promotion effect of the sintering reaction, because of the addition of V2O5, which led to more densification of the proppants with an appropriate amount of liquid phase. However, when under high-temperature conditions, too many liquid phases formed42, while O2 from the decomposition of V2O5 would also happen at a high temperature, finally resulting in the decline of the mechanical performance of the proppant42.
Figure 9The properties of proppant samples with different dosages of V2O5 and MnO2 additives sintered at various temperatures: (a,c) bulk density, and (b,d) breakage ratio.
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Figure 9c showed the bulk density and apparent density of formula B0 with different dosages of MnO2. With the increase in the MnO2 dosage, the sintering temperature and the density showed a descending tendency, indicating that a low-temperature liquid phase was formed during the sintering process, on account of the excellent sintering ability of MnO227,41. Simultaneously, variations of breakage ratio and acid solubility with the sintering temperature of proppants were shown in Fig. 9d. It can be found that the breakage ratio and the acid solubility of the proppants decreased with a sintering temperature up to °C and followed by an increase with a sintering temperature up to °C. The minimum breakage ratio and acid solubility was achieved by formula BM1 as 5.25% and 4.80% at °C, respectively, which was much better than formula B0.
Figure 10 gave the XRD patterns of proppant samples added with different contents of V2O5 and MnO2 at their optimum temperatures, indicating there were no new diffraction peaks detected in the crystalline phases in the product after the addition of V2O5 or MnO2. It also can be observed from Fig. 10a, the diffraction peak intensity of crystal phases has no obvious change with the augment of the V2O5 additive dosage. This implied that the addition of V2O5 additives has no observable influence on the phase composition of the proppant, which would be attributed to V2O5 easily entering the lattice of mullite crystals as a solid solution and promoting the formation of the liquid phase during the high-temperature sintering process43,44,45,46.
Figure 10XRD patterns of ceramic proppants under different sintering temperatures added with (a) V2O5 additives and (b) MnO2 additives.
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According to Fig. 10b, the peak intensity of corundum and mullite phases of samples containing MnO2 additives were very similar to formula B0. The reason was Mn4+ was able to dissolve into the corundum phase and replace Al3+ to form a finite solid solution during the sintering process, which caused the lattice distortion and promotion of grain growth, reduction of sintering temperature, and the densification of structure27,36,41,47. Whereas, the peak intensity of anorthite and celsian was enhanced with the increase of the MnO2 additive, indicating the addition of MnO2 could facilitate the generation of the anorthite and celsian. The formation of celsian and anorthite was related to the abundant BaO and CaO in the ODCPRs, and the excessive glassy phase was generated owing to the fluxing effect of the MnO2 additive could further stimulate the growth of celsian and anorthite48,49.
Figures S7 and S8 showed the cross-sectional microscopic images of the proppants with different dosages of V2O5 and MnO2 after the acid treatment, severally. The results from Fig. S7 revealed that the addition of V2O5 has a remarkable influence on the formation of mullite. The sample without V2O5 only contained a few needle-like mullite crystals (Fig. S7a). As the dosage of V2O5 increased within the range of 0.5&#;1 wt%, anisotropic mullite crystals in significant amounts appeared among the granular corundum particles (refer to Fig. S7b,c) which contributed to enhancing the mechanical strength and acid resistance of the ceramic proppant. With further improvement of the V2O5 dosage, the needle-like mullite crystals changed into bigger rod-like mullite (Fig. S7d). This denoted that V2O5 additives can dramatically drive the in-situ development of spearhead columnar mullite, on account of the increase of liquid phase in the reaction system, which was corresponding to the analysis of XRD results.42, which was corresponding to the analysis of XRD results.
The microscopic diagrams of the ODCPRs proppants with various MnO2 dosages were exhibited in Fig. S8. The observation of corundum phases in samples containing MnO2 was consistent with the B0 sample. Multiple corundum grains were gathered together to form a granular shape, while only a small number of acicular mullite was distributed among the corundum grains, indicating that the addition of MnO2 does not influence the growth of mullite crystals but promoted the development of corundum. Noteworthy, as shown in Fig. S8b, at the MnO2 dosage of 2 wt%, the cluster shape of celsian was observed, which was in accordance with the results of XRD. With further increase in the MnO2 dosage at 3 wt%, the liquid phase dramatically increased and gradually englued the crystal grains, and some incompleted porous glassy phase dispersed in the matrix after the acid treatment, as the remnants left after being corroded that would seriously affect the mechanical strength and density of the ceramic proppants.
The mass change and sintering behaviors of the raw materials were comprehensively evaluated through TG-DSC experiments. As depicted in Fig. 11a,b, the pure bauxite has presented negligible weight variation during the calcination process, indicating a relatively stable chemical structure. However, an exothermic phenomenon occurred above °C, leading to the recrystallization of Al2O3 at high temperatures. Meanwhile, as shown in Fig. 11b, distinct weight reduction stages were observed at 25&#;495 °C, 495&#;698 °C, and 698&#; °C for the ODCPRs, which corresponded to three different sintering processes: firstly, when the heating temperature was raised from room temperature to 495 °C, a primary weight loss of 5.94% occurred according to the evaporation of free, absorbing and crystal water as well as combustion of residue organic constituents (such as oil)50. Subsequently, a steep weight loss of 4.78% occurred when the temperature was above 698 °C that corresponded to an obvious endothermic peak (664 °C), which would be attributed to the violent decomposition of carbonates51,52. Thirdly, an dramatic weight quality loss of 10.53% occurred from 698 °C to °C, and an endothermic peak ( °C) was observed, which might be related to the solid-phase reaction, on account of the generation of a liquid phase or the transformation between crystal forms of salt substances at high temperatures (such as the decomposition of BaSO4)53,54.
Figure 11TG-DSC curves of samples (a) bauxite; (b) ODCPRs; (c) A0; (d) B0; (e) C0; and (f) BM1.
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The thermal behavior of the mixtures with 20 wt%, 30 wt%, and 40 wt% ODCPRs were displayed in Fig. 11c&#;e respectively, which have shown a similar weight descending trend with the pure ODCPRs sample, and all samples included three different weight loss stages. The results suggested that with the increase of the ODCPRs content, greater weight loss of the samples can be achieved. Meantime, the appearance of endothermic peaks was also delayed because more energy was consumed for thermal decomposition reactions. Specifically, the most significant increase in weight loss occurred during the solid-phase reaction process, and the greatest mass loss (6.32%) was generated by C0, which may be attributed to the formation of more molten phases that accelerated the growth of corundum.
Moreover, the thermal behavior of BM1 was demonstrated in Fig. 11f, with the addition of MnO2 powders, a new weight loss stage emerged between 697 and 903 °C. This could be attributed to the decomposition of MnO2 powders, which act as a sintering aid in the raw material and induce molten action between the solid phase within the high-temperature range40. Combined with Fig. 11b, no new phase formation has been observed following the incorporation of MnO2. This phenomenon can be ascribed to the increased formation of the liquid phase at high temperatures, which was caused by the melting of aluminosilicate, on account of the distortion of Mn2+ into the corundum lattice and solid solution formation41. This further promoted the liquid&#;solid reactions, resulting in a greater mass change within the 903&#; °C stage. Consequently, the proppant density and strength were enhanced by optimizing and shrinking the interface among solid phases through the addition of MnO2. Based on the preceding discussions, the proppant underwent various reactions during sintering, including water evaporation, hydrocarbon combustion, carbonate and sulfate decomposition, glass phase melting, and corundum recrystallization, as follows41,52,53,54,55,56:
$${\text{H}}_{{2}} O(l)\mathop{\longrightarrow}\limits^{{100 - 200\;^\circ {\text{C}}}}H_{2} O \uparrow $$
(1)
$$CxHy({\text{hydrocarbon}}) + O_{2} \mathop{\longrightarrow}\limits^{200 - 600^\circ C}CO_{2} \uparrow + H_{2} O \uparrow$$
(2)
$$CaCO_{3} \mathop{\longrightarrow}\limits^{{650 - 750\;^\circ {\text{C}}}}CaO + CO_{2} \uparrow$$
(3)
$$Al_{2} O_{3} ({\text{pseudocorundum}})\mathop{\longrightarrow}\limits^{{900 - \;^\circ {\text{C}}}}Al_{2} O_{3} ({\text{corundum}})$$
(4)
$$ BaSO_{3} + RO_{x} ({\text{metallic oxide}})\mathop{\longrightarrow}\limits^{{ - \;^\circ {\text{C}}}}BaRO_{y} + SO_{2} \uparrow $$
(5)
$$ MnO_{2} \mathop{\longrightarrow}\limits^{{460 - 570\;^\circ {\text{C}}}}Mn_{2} O_{3} \mathop{\longrightarrow}\limits^{{926\;^\circ {\text{C}}}}Mn_{3} O_{4} \mathop{\longrightarrow}\limits^{{\;^\circ {\text{C}}}}MnO$$
(6)
$$2MnO\mathop{\longrightarrow}\limits^{{Al_{2} O_{3} }}2Mn_{{_{Al} }}^{`} + V_{O}^{``} + 2O_{O}^{X}$$
(7)
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