Ceramic Proppant
Ceramic proppants play a critical role in the development of unconventional oil and gas resources. According to Google Search excerpts, ceramic proppants are man-made pellets with superior strength and efficiency compared to natural sand proppants. They are primarily used in the hydraulic fracturing process to keep fractures open and maintain the flow of oil or gas.
With the advent of the development of lower Tertiary formations in the deepwater Gulf of Mexico, a gap in proppant technology was identified at the beginning of the decade. No existing conventional proppant could provide the required conductivity at the anticipated 15,000 psi (and higher) prevailing closure stress. To meet these challenges, a new proppant was developed, with the goal of achieving twice the baseline conductivity of any conventional proppant at 20,000 psi.
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Preparation and Characterization of High-Performance Ceramic Proppants
The Influence of Sintering Temperature and ODCPRs Contents
Performance Test
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, 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. The acid solubility of the proppants fluctuated, impacted by both the ODCPRs contents and the sintering temperature. The acid solubility was closely correlated to the soluble phases in the proppant. More alkaline earth metal oxides resulted in significantly improved solubility with increased ODCPRs, facilitated the formation of liquid phases under higher sintering temperatures. Thus, under high temperature or high ODCPRs dosage, more glass phases would be generated, attributing to higher acid solubility.
Figure 1
The 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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With increasing sintering temperatures, the optimal temperatures for A0, B0, and C0 formulas, having the lowest breakage ratio and acid solubility, first declined and then increased. The best sintering temperatures could be 1320°C, 1320°C, and 1280°C respectively. According to Table 1, with the increase of ODCPRs content, both the K value and the molar ratio (Al2O3/SiO2) in initial mixtures were reduced. Therefore, formulas A0 and B0 at 1320°C exhibited better performance. Considering ODCPRs consumption and raw materials cost, formula B0 at 1320°C was the optimal choice with the lowest breakage ratio (6.97%) and acid solubility (4.56%).
The proppants produced in the experiment exhibited darkened colors as either ODCPRs content or sintering temperature increased (Fig. 2a). The Krumbein/Sloss template evaluated the roundness and sphericity, with all proppants (A0–C0) surpassing 0.8, meeting the oil and gas industry standard SY/T 5108-2014 (Fig. 2b).
Figure 2
(a) Photograph of proppants under different sintering temperatures; (b) the Krumbern/Sloss template.
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Phase Analysis
Figure 3a showed the XRD patterns of ceramic proppants of the B0 formula in different sintering temperatures. It was observed that the crystalline phases mainly consisted of corundum and a small quantity of mullite, celsian, and anorthite. Corundum was predominant, ascribed to the initial mixtures with a high molar ratio (Al2O3/SiO2), resulting in suitable conditions for its formation. As the sintering temperature increased from 1240 to 1360°C, the diffraction peak intensities of corundum and mullite crystals enhanced, promoting high-performance ceramic proppants development. Nonetheless, excessive metallic oxides in the ODCPRs led to the formation of celsian and anorthite by reaction with bauxite. At temperatures over 1320°C, the intensity of these phases' diffraction peaks weakened as they transitioned from solid to glass phases.
Figure 3
XRD 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, increasing ODCPRs content reduced corundum and mullite diffraction intensities, but enhanced celsian and anorthite peaks at their optimal sintering temperature, signifying decreased crystallinity of corundum and mullite phases. Excessive celsian and anorthite could hinder corundum and mullite phases, impairing proppant performance. Thus, sintering temperature decreased with increased ODCPRs content due to more liquid phase generation, such as formula C0.
Morphology Analysis
Figure S4 displayed the cross-sectional microscopic morphology of the proppants before acid treatment, showcasing varied structural characteristics at different sintering temperatures. Initially, with the temperature increase, inner pores decreased, followed by an increase beyond 1360°C, forming a loose internal structure with less molten phase at non-optimal sintering conditions. Gradual improvement in sintering temperature yielded a denser structure with more liquid phases filling pores and strengthening the ceramic matrix. However, too many liquid phases at high temperatures resulted in reduced density and strength due to expanded ceramic body and increased pores.
Post-acid treatment inner structures (Fig. 4) depicted a network of residue glass and crystalline phases, showing growth of corundum and mullite with temperature. At 1240°C (Fig. 4a), incomplete sintered crystals scattered among net-like phases. Improved sintering at 1280°C (Fig. 4b) formed abundant small corundum grains. Optimal growth at 1320°C (Fig. 4c, 4d) resulted in interlocking columnar and granular crystals, primarily consisting of corundum and mullite, enhancing proppant performance due to increased hardness, strength, and chemical resistance.
Figure 4
Microscopic cross-sections of the ceramic proppant after acid treatment of the formula B0 at different sintering temperatures: (a) 1240°C; (b) 1280°C; (c) 1320°C; (d) 1360°C.
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Microstructures of proppants with varied ODCPRs at optimal temperatures (Fig. S5) showed porous structures with increased liquid and gas phases aligned with ODCPRs. Further acid-treated investigations revealed denser structures with enough Al2O3 provided, as evidenced by granular corundum and rod-like mullite phases. Proppants with over 40% ODCPRs exhibited higher breakage ratios due to excessive glass phase formation (Fig. 5).
Figure 5
Microscopic cross-sections of the ceramic proppants after acid treatment with different ODCPRs contents at optimum sintering temperatures: (a) A0 at 1320°C; (b) B0 at 1320°C, (c) C0 at 1280°C.
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The Influence of Holding Time on Proppant Performance
Performance Test
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 1320°C were carried out. The bulk density and apparent density of proppants exhibited the same tendency which first increased and then decreased with the increase of holding time. The highest values were obtained at 1320°C with 60 min of heat preservation, measuring 1.48 g/cm3 and 3.03 g/cm3 respectively. Furthermore, the breakage ratio and acid solubility initially decreased, then gradually increased with extended holding time (Fig. 6b). The lowest breakage ratio (6.97%) and acid solubility (4.56%) were achieved with 60 min of heat preservation, meeting the SY/T 5108-2014 standard. Maintaining a proper holding time during calcination optimized the ODCPRs-based ceramic proppants.
Figure 6
The 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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Phase Analysis
The XRD patterns of formula B0 under different holding times were shown in Fig. 7. A proper heat preservation (holding for 60 min) was advantageous for corundum and mullite crystals formation and growth while reducing bauxite and celsian phases. However, the holding time did not significantly influence the final phase category, suggesting the performance variation was mainly due to microstructure and glass phase content changes.
Figure 7
The XRD patterns of B0 samples sintered at different holding times.
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Morphology Analysis
Figures S6 and 8 showcased microstructural differences of formula B0 ceramic proppants sintered at 1320°C before and after acid treatment under varying holding times. Pore characteristics remained similar before acid treatment. After acid removal, crystals showed significant differences: a 30 min hold produced coarse granular corundum and minor needle-shaped mullite (Fig. S6), while 60–90 min holds created larger, uniform corundum and mullite crystals (Fig. 8a, 8b). At 120 min, corundum grains grew excessively, causing internal stress failure and adverse impacts on gas exhaustion, ultimately reducing density and strength. Consistent, smaller crystals improved proppant mechanical performance.
Figure 8
Microscopic diagram of the ceramic proppant after 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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The Influence of Additives on Proppant Performance
Performance Test
Metallic oxide sintering additives promote liquid phase formation and low-viscosity liquid alterations in the ceramic system, significantly enhancing proppant performance. This section explores the performance of ODCPRs ceramic proppants with different mass fractions of V2O5 and MnO2 additives at various sintering temperatures.
Figures 9a and 9b demonstrated that, below 1280°C, increasing V2O5 improved bulk density and apparent density slightly then decreased significantly. At temperatures above 1320°C, these properties dropped sharply. The BV series' breakage ratio and acid solubility significantly decreased at 1280°C; BV2 had the minimum breakage ratio (8.40%) and BV1 had the minimum acid solubility (4.33%). Although V2O5 reduced sintering temperatures, the mechanical and acid resistance performance was undesirable compared to B0 under optimal sintering. The enhancements from V2O5 were reversed by high-temperature excessive liquid phase formation and O2 decomposition (Fig. 9).
Figure 9
The 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 highlighted bulk and apparent density trends with varied MnO2 dosages up to 1320°C. An increase in MnO2 led densities downward, indicating effective sintering aiding via low-temperature liquid phase generation. Breakage ratio and acid solubility variations in Fig. 9d showed lowest values: 5.25% and 4.80% respectively for BM1 at 1280°C, outperforming B0.
Phase Analysis
Figure 10 illustrated XRD patterns for B0 samples with V2O5 and MnO2 additives at optimal temperatures, revealing no new diffraction peaks in crystal phases. For V2O5 (Fig. 10a), peak intensities remained unchanged, suggesting V2O5's lack of impact on proppant phase composition, promoting liquid phase formation during high-temperature sintering. This facilitated in-situ mullite crystal growth without new phase interference.
Figure 10
XRD patterns of ceramic proppants under different sintering temperatures added with (a) V2O5 additives and (b) MnO2 additives.
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For MnO2, peak intensities resembled B0's except intensified anorthite and celsian diffraction peaks due to MnO2 facilitating their generation through fluxing effects. Mn4+ replacement of Al3+ in corundum phase, lattice distortion, and facilitated liquid–solid proppant reactions at high temperatures enhanced densification and mechanical performance (Fig. 10b).
Morphology Analysis
Figures S7 and S8 revealed differing microstructural influences of V2O5 and MnO2 additives post acid treatment. V2O5 induced significant needle-like to rod-like mullite crystals growth among corundum grains, enhancing mechanical strength and acid resistance. MnO2 maintained corundum grain aggregate shape, with some large celsian clusters observed at 2 wt% dosage (Fig. S8b), aligning with XRD results. Excessive MnO2 at 3 wt% increased liquid phases, reducing performance due to incomplete porous glassy phases post-treatment.
Thermal Behaviors Analysis
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 showed negligible weight variation during calcination, indicating a relatively stable chemical structure. An exothermic phenomenon above 1200°C led to Al2O3 recrystallization. For ODCPRs, distinct weight reduction at 25–495°C, 495
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