A Study on a New Type of High-Performance Resin-Coated Sand for Petroleum Fracturing Proppants

Abstract

This study investigates a new type of high-performance coated sand as a petroleum fracturing proppant material. Modified quartz sand was coated with a layer of low-density resin to reduce the overall density of the proppant, thereby improving the suspension of the proppant in the fracturing fluid. Resins play an important role in the preparation of coated sand fracturing proppants. The mechanism of sand formation was studied by examining the phase composition and microstructure of the coated sand proppant. The results demonstrate that when the polyimide resin content is 6% and the curing temperature is 180 °C, the proppant exhibited the best performance with an apparent density of 1.592 g/cm³ and a breakage ratio of only 3.22% under 55.2 MPa. Compared with the widely used epoxy resin-coated support agent and phenolic resin-coated support agent in the early stage, their crushing rate decreased by 5% and their acid solubility decreased by 2%. Hence, this study is worthy of attention.

1. Introduction

In oilfields, especially in the exploration and development of low-permeability oilfields and the transformation of old oil wells, fracturing technology has become the main means of increasing production and enhancing oil recovery, and petroleum fracturing proppants are an extremely important factor in petroleum fracturing technology [1,2,3]. In the exploitation of oil and gas wells, fracturing supports are required to prevent rock fissures deep in the formation [4,5,6]. The fracturing fluid containing the proppant is injected into cracks of the rock layer under high pressure to provide a high-permeability passage for oil and gas circulation, improve the oil guiding rate, and increase oil and gas production [7,8,9,10].

At present, the proppants used in the fracturing process consist of quartz sand, ceramsite, and coated sand [11,12,13,14]. The quartz sand proppant is widely distributed in China, easy to obtain, and relatively inexpensive; however, its surface finish, sphericity, and resistance to breakage are poor and it is easily broken [15,16]. Therefore, the quartz sand proppant is only suitable for shallow closed wells in hydraulic fracturing operations. Ceramsite has a good sphericity, high strength, high compressive strength, and is less susceptible to fracturing [17,18,19,20]. However, its density is relatively high compared with that of quartz sand, and the high energy consumption and price of ceramsite prevents its implementation [21,22,23,24,25,26,27]; ceramic proppants are suitable for deep oil and gas wells with high closure pressures.

In order to solve the insufficient strength of quartz sand proppants and the high density of ceramsite proppants, film-coated proppants were introduced, which have characteristics of a high strength and low density. Herein, modified quartz sand was coated with a layer of low-density resin to reduce the overall density of the proppant, thereby improving the suspension of the proppant in the fracturing fluid. The coating accounted for 5–20% of the total mass of the proppant [28,29,30]. The resulting low-cost proppant balances the relationship between a high strength and low density, and no longer satisfies a single function but realizes a composite of multiple functions [31].

2. Experimental

2.1. Raw Materials

The sand (20–40 mesh) was purchased from Ningxia. The most distinctive property of this type of sand is its high silica content (>98 wt% SiO₂). Polymine resin (C₃₅H₂₈N₂O₇) was purchased from National Pharmaceutical Group Chemical Reagent Co., Ltd. (Shanghai, China).

2.2. Experimental Procedure

Quartz sand (20–40 mesh) and deionized water were added to a three-necked flask, and heated and stirred to boil for 2 h for decontamination. The boiled quartz sand was filtered and dried. A mixture of hydrogen peroxide and concentrated sulfuric acid (98%) was added over 40 min while continuously stirring. Subsequently, the mixture was washed with deionized water until neutral and then dried. An ethanol solution of a silane coupling agent KH560 (γ-(2,3-epoxypropoxy)propytrimethoxysilane) was added and the mixture was stirred at room temperature for 12 h to obtain the modified quartz sand. The modified quartz sand was stirred and heated to 180 °C, and the polyimide resin and lubricant calcium stearate were sequentially added. The uniformity of resin coating does indeed affect the overall performance of the proppant. Therefore, in the production process of the proppant, there should be a clear and developed process, such as a consistent stirring speed during the coating process and the resin should be added multiple times in small amounts during the resin addition process. Grinding and crushing were performed using a 20–40 mesh sieve to obtain the coated sand fracturing proppant.

2.3. Tests

The apparent density, acid solubility, and breaking rate of the samples were tested according to the Chinese Industry Standards SY/T5108-2014 [32] “Fracturing proppant performance index and test recommendation method”.

2.3.1. Apparent Density Tests

A balance (accuracy 0.01 g) was used to accurately weigh the mass m₁ of the 50 mL density bottle after drying and the mass m₂ after filling the bottle with water. Following the addition of a weight of m₃ of proppant to the bottle, the bottle was refilled with water. Once the bubbles dissipated, the bottle was refilled with water again, and the mass m₄ was measured. The apparent density of the proppant ρ was calculated using Equation (1) [26,27].

$$\rho =\frac{m_3-m_1}{m_2+m_3-(m_1+m_4)}{\rho }_w,$$

(1)

where m₁ was the weight of the density bottle (g), m₂ was the weight of the density bottle filled with water, m₃ was the weight of the supporting agent and density bottle added to the density bottle, m₄ was the weight of the water filled with the proppants after adding the proppants, ρ_w was the density of water, and ρ was the apparent density of proppants.

2.3.2. Breakage Ratio Tests

According to the SY/T5108-2014 standard, a crushing chamber with an inner diameter of 50.8 mm was selected [26]. Following crushing, the breaking ratio of the proppant was calculated as follows:

$$\mathsf{\eta }=\frac{m_c}{m_p}\times 100\%,$$

(2)

where m_c is the weight of the crushed sample and m_p is the weight of the sample before the tests [26,27].

2.3.3. Acid Solubility Tests

The acid used for the detection of acid solubility was mud acid (hydrofluoric and hydrochloric acid solution) prepared in a 12:3 ratio (12 wt% HCl and 3 wt% HF) at a specific temperature [11]. First, the appropriate amount of proppant was placed in an oven at 110 °C for 4 h and then placed in a drying vessel for 30 min. Five grams of the proppant (Ws) was added to a 100 mL hydrochloride solution (106.6 g, 20 °C). The hydrofluoric acid mixed solution was placed in a water bath maintained at 65 °C for 0.5 h. The acid-treated sample was then rinsed with deionized water and dried in an oven at 105 °C for 1 h to obtain the acid-treated sample with a weight of Wa. Finally, the acid solubility (S) was calculated using Equation (3) [26].

$$\mathrm{S}=\frac{Ws-Wa}{Ws}\times 100\%$$

(3)

2.3.4. Scanning Electron Microscopy (SEM) and Infrared Spectroscopy (IR)

The morphology of different formulations was analyzed using scanning electron microscopy (SEM, S-4800, 3 kV, WD = 8.4–9.3 mm). The composite coating samples of different formulations were characterized by infrared absorption spectrometry (IR). The solid samples were ground into a powder and tested by KBr blending. The measurement range was 400–4000 cm⁻¹.

3. Results and Discussion

3.1. Reaction Mechanism of the Modification of Quartz Sand

Figure 1 shows the reaction mechanism of the quartz sand surface where two chemical reaction steps are involved. H₂SO₄ and H₂O₂ undergo hydrolysis, while the –Si–O–Si– group on the quartz sand surface is transformed into –Si–OH groups. The silane coupling agent KH560 is hydrolyzed, and strong chemical bonding is achieved between KH560 and quartz sand, owing to the strong chemical interaction (resembling a condensation process) between the reactive silanol groups on the quartz sand surface and the hydroxyl groups of KH560.

3.2. Influence of Polyimide Resin Content on the Performances of the Proppants

To study the optimal addition amount of polyimide, different polyimide resin contents were added to modified quartz sand and stirred at 180 °C, as shown in

Table 1. Experiment formula of coated sand proppants/wt%.

Sample	Curing Temperatures (°C)	Quartz Sand	Polyimide Resin
1#	180	100	0
2#	180	98	2
3#	180	96	4
4#	180	94	6
5#	180	92	8

.

As shown in Figure 2, the performance of the quartz sand is significantly enhanced after coating. However, when the resin content exceeded a certain amount, the proppant performance decreased, which is caused by the agglomerate of the resin on the surface of the proppant, affecting the uniformity of the film. In addition, excessive resin can cause unnecessary energy consumption. Thus, performance degradation occurred beyond the optimal resin content. It can be observed from the figure that when the resin content was 6%, the performance was optimal.

3.3. Influence of the Polyimide Resin Curing Temperature on the Performances of the Proppants

As shown in Figure 2, the acid solubilities and breakage ratios of the five samples initially decreased and then increased. With an increase in the curing temperature, the curing process tended to become more uniform and the properties gradually improved.

As shown in Figure 3, the heating of the polyimide resin is a melting process. When the temperature was higher than the softening point of the resin, the viscosity of the resin decreased as the temperature increased, and the transition from a glassy state to a viscous flow state began. When a certain temperature is reached, the resin is completely transformed into flow dynamics and, thus, the viscosity does not change. The lower the viscosity, the better the fluidity and the more uniform the mixing of the resin and raw sand; therefore, when the resin viscosity reaches a minimum, the original sand can be coated to achieve a uniform coating.

It can be seen from Figure 2 and Figure 3 that the solubility and the breaking rate of the proppant’s acid prepared by the polyimide resin at 180 °C are the lowest. However, at temperatures higher than 180 °C, the solubility and the breaking rate began to increase. This is because the film temperature was high, causing partial coking or oxidation of the resin, affecting the bonding force between the resins and resulting in poor adhesion between the sand particles, thereby reducing the strength of the coated sand. This results in unnecessary energy consumption. Therefore, the optimum curing temperature of the polyimide resin is 180 °C.

3.4. Influence of the Polyimide Resin Curing Temperature on the Microstructure of the Proppants

Figure 4 shows an SEM image of the coated sand proppant (A) at different curing temperatures (B, C, and D). As shown in Figure 4(A1–A3), the surface of the pretreated quartz sand had many irregularities and a large number of open pores and cracks.

From Figure 4(B1–B3), it is evident that the proppant is cured at 160 °C, and pits and cracks on the surface of the quartz sand are filled or covered. However, the curing temperature is low and a small amount of resin on the surface is not crosslinked, thereby affecting the proppant’s performance. In Figure 4(C1–C3), the proppant is cured at 180 °C, the resin and quartz sand were uniformly mixed, and the surface of the proppant was smooth, forming a uniform coating. In Figure 4(D1–D3), the proppant is cured at 200 °C, it can be seen that pores appeared on the surface of the proppant, and the resin agglomerates because of the high film temperature, causing partial coking or oxidation of the resin, affecting the bonding force between the resins, thereby reducing the performance of the proppant. Therefore, the proppant prepared by coating at a curing temperature of 180 °C exhibited the best performance.

3.5. Chemical Composition Analysis of Proppant Surface

As shown in Figure 5, the polyimide resin exhibits an asymmetric carbonyl stretching vibration at 1720 cm⁻¹, and the C–N stretching vibration of the imine ring is observed at 1377 cm⁻¹. The peaks at 1092 and 1116 cm⁻¹ correspond to the deformations of the imide ring (–CO–N–CO–). As shown in Figure 5, the absorption peaks at 1092, 1116, 1377, and 1720 cm⁻¹ weakened, indicating that the amide ring opened, and new absorption peaks appear at 1077, 2935, 3369, and 3692 cm⁻¹. The absorption peaks at 1077 and 2935 cm⁻¹ correspond to the Si–O-based stretching vibration and antisymmetric stretching vibration of methylene, respectively. The hydroxyl group in the broad absorption peak of Si–OH at 3369 cm⁻¹ may have been formed by the hydrolysis of the methoxy group of the silane coupling agent KH560 [33,34].

3.6. Influence of the Polyimide Resin Curing Temperature and Content on the Apparent Density of the Proppants

As shown in Figure 6A, for a curing temperature of 180 °C, as the content of the polyimide resin increases, the apparent density decreases. When the content of the polyimide resin is 6%, the apparent density is 1.592 g/cm³. The reason for the decrease in the apparent density of the proppant after lamination is that the surface of the quartz sand was uneven and there were a large number of holes and cracks. After the film was coated onto the surface of the quartz sand, the pits and cracks were filled or covered to form closed pores and the density of the polyimide resin was relatively small. Therefore, following coating, the volume of coated sand increased, thereby reducing the apparent density of the proppant and reducing the quality of the proppant.

As shown in Figure 6B, when the content of the polyimide resin was 6%, as the curing temperature of the polyimide resin increases, the apparent density of the proppant initially decreases and then increases. Because the apparent density of the proppant decreases with increasing temperature, when the curing temperature is 180 °C, the viscosity of the resin is the lowest (1.592 g/cm³) and the surface of the quartz sand is the most uniform. As the temperature continued to increase, the resin was partially coked or oxidized and, as a result, the surface film was not uniform and some voids could not be covered; therefore, the apparent density increased.

3.7. Comparison of Different Types of Support Agents

In the same test environment, various physical properties of the support agent are supported by the market. The experimental data are shown in

Table 2. Performance of different support agents.

Performance	Quartz Sand	Grain Support Agent	Coating Agent
Crushing rate/%	22.5	5.5	2.7
Volume density/g.cm−3	1.45	1.55	1.31
Visual density/g.cm−3	2.73	2.81	2.31
Acid solubility/%	6.45	4.5	2.2
Golfness	0.7	0.9	0.8

. Compared with the performance of the two supporting agents, it can be seen that the various properties of quartz sand are poor. In the oil and gas well, when the pottery support is high, the pressure resistance increases; but, when the density is high, it causes the settlement speed to be too fast and the corrosion resistance is poor. Table 2 shows the performance of different support agents.

The cost of quartz sand is the lowest, the material is convenient, and the craftsmanship is simple; however, the performance is very poor, the scope of application is narrow, the price of pottery support agents is very expensive, the raw materials are complex and difficult, and the craftsmanship is complicated. For membrane sand, which uses quartz sand as a matrix material and resin as a cover material, the process is simple.

4. Conclusions

Based on the existing support agent research, this paper proposes a high strength, low density proppant with other high performance features. It was mainly studied by analyzing the preparation process, performance analysis, micro structure, and the mechanism of the coating sandwiches. The conclusions are as follows.

• There was a superior performance from the proppant prepared from polyimide-coated quartz sand.
• When the polyimide resin content is 6% and the curing temperature is 180 °C, the apparent density of the proppant is 1.592 g/cm³, the breaking rate is only 3.22% under 52 MPa, and the acid solubility is 3.08%.
• Polyimide-coated sand has many advantages for producing high-quality proppants, such as a simple preparation and superior performance. This study used quartz sand and polyimide resin as raw materials, which are easy to obtain, and the resulting polyimide-coated sand proppant exhibited an improved performance.