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Article

Graphene and Resin Coated Proppant with Electrically Conductive Properties for In-Situ Modification of Shale Oil

1
State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, SINOPEC Research Institute of Petroleum Engineering Co., Ltd., Beijing 102206, China
2
State Key Laboratory of Petroleum Resources and Prospecting, Beijing Key Laboratory of Biogas Upgrading Utilization, Harvard SEAS-CUPB Joint Laboratory on Petroleum Science, China University of Petroleum (Beijing), Beijing 102249, China
*
Author to whom correspondence should be addressed.
These authors contributed equally to this work.
Energies 2022, 15(15), 5599; https://doi.org/10.3390/en15155599
Submission received: 6 July 2022 / Revised: 27 July 2022 / Accepted: 27 July 2022 / Published: 2 August 2022
(This article belongs to the Special Issue Energy Storage and Conversion Based on Low-Dimensional Nanostructure)

Abstract

:
Proppant is an essential material in hydraulic fracturing, and it can support artificial fractures for a long time. However, few people have applied proppant and conductive materials in the in-situ modification of shale oil. Here, we developed a graphene and resin coated (GRC) proppant with electrically conductive properties. The electrical conductivity of the GRC proppant improved by four orders of magnitude. The GRC proppant has a 54.58% improvement in suspension and 22.75% increase in settlement time at 0.25 wt% concentration compared with uncoated proppant. The GRC proppant’s adhesion reached 68.34 nN under 1 μN load force, increasing by 63.13% compared to uncoated proppant. This new electrically conductive proppant can be used as a conductive carrier to improve the efficiency of electric heating in in-situ modification technology of shale oil.

1. Introduction

In recent years, with the shortage of conventional energy, the dependence of oil and gas has been increasing yearly [1,2,3]. Exploiting unconventional energy like shale oil is of great significance in solving the energy security problem [4,5,6]. Before the 21st century, shale oil resources were mainly developed and utilized by surface distillation mining technology [7,8,9,10] with a restriction such as heavy environmental pollution and poor economic benefits [11,12,13,14], so more green and efficient underground in-situ mining technology was gradually developed [15,16].
Heating technology is the most critical in underground in-situ mining technology, which can be divided into electric heating method, convection heating method [17,18,19], combustion heating method [20], and radiation heating method [21] according to heating methods [22,23,24]. The electric heating method mainly converts electric energy into heat energy in-situ and then produces shale oil by pyrolysis of organic matter. In the late 1970s, Shell pioneered the innovative In-Situ-Conversion (ICP) Process, which inserts electric heaters underground to heat shale oil reservoirs [25,26,27,28,29]. ExxonMobil proposed ElectrofracTM Process to heat shale oil reservoirs by in-situ hydraulic fracturing and filling the fractures with electrically conductive materials to form heating elements [30,31,32,33]. Ahmmed et al. coupled the subsurface flow and transport simulator and geoelectrical simulator without sacrificing computational performance. This work will provide an important reference for our future studies on transferring information of fracturing fluid and shale oil from the field/experiments to continuum-based process models and their applications to compute the bulk electrical conductivities formed after proppants placement [34,35]. Figure 1 illustrates this electric heating method and preset electrically conductive proppants as conductive materials to facilitate shale oil in-situ modification. In this electric heating method, proppants can be used to support artificial fractures or as electric heating elements to facilitate shale oil in-situ modification. Diversiform functional proppants such as self-suspending proppants, hydrophobic proppants, and tracer proppants have been developed to meet production needs in various geological conditions. Still, the combination of proppants and in-situ electric heating technology has not been studied [36,37,38,39,40]. Because of the particularity of underground in-situ mining technology, how to improve the heat conduction efficiency and speed up the in-situ modification efficiency of shale oil after hydraulic fracturing has become an important scientific problem, which is of great significance in the process of middle and low maturity shale oil exploitation.
In this work, we have successfully designed and prepared a graphene and resin coated (GRC) proppant with electrically conductive properties for in-situ modification of shale oil. Compared to uncoated proppant, the GRC proppant has a low broken rate, 54.58% and 33.39% increase in suspension and liquid conductivity, which can withstand high temperatures. The electrically conductive proppant improved its electrical conductivity by four orders of magnitude. In addition to promoting the migration and support effect of proppant in hydraulic fractures, this new GRC proppant can further improve the in-situ modification effect of middle and low maturity shale oil, which is of great significance for improving the efficiency of shale oil production.

2. Materials and Methods

2.1. Experimental Materials

Ceramic proppant was 40/60 mesh bought from Henan Kaibeidi Environmental Protection Technology Co., Ltd. in the market. Epoxy resin and supporting curing agent were supplied from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Deionized water was produced by the water deionizer supplied from Jining Yuze Industrial Technology Co., Ltd (Shandong, China). Ethanol absolute was purchased from Tianjin Zhiyuan reagent (Tianjin, China). Silane coupling agent was supplied from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Graphene was supplied from Shanghai Macklin Biochemical Co., Ltd. Guar gum was purchased from West Asia reagent.

2.2. Preparation of Electrically Conductive Proppant

The proppants were washed clean with deionized water and dried in an oven at 100 °C for 1 h. Then poured proppants into a beaker containing 300 mL silane coupling agent KH560. The coupling agent was kept stirring at 600 rpm for 20 min to modify proppants’ surface. The surface-modified proppant was poured into a solution of epoxy resin and supporting curing agent with a mass ratio of 3:1 and stirred for 10 min. The GRC proppant was poured into graphene, stirred evenly, and cured in a cuboid mold at 80 °C for 8 min. The block sample was removed from the oven and cooled to room temperature, then ground and screened to obtain the desired electrically conductive proppant.

2.3. Suspension and Sedimentation Experiment

Guar gum was poured into deionized water and stirred at 600 rpm for 10 min to prepare the solution with different mass concentrations. Slowly pour proppants into the solution and let it sit for 2 min after stirring the mixed solution at 500 rpm for 10 min. The proppant suspended on the surface and sedimented to the bottom was collected and weighed after drying, respectively, to calculate the proppant suspension capacity.

2.4. Electrical Conductivity Experiment

After the block sample is removed from the mold, its electrical conductivity is tested by an electrochemical workstation before grinding. After electrifying both sides of the sample, the electrical conductivity is calculated by the current, voltage, and size of the sample.

2.5. Characterization

The scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDX) images of the sample were respectively captured by Zeiss Sigma 500 and Bruker XFlash 6/30. Atomic force microscopy (AFM) images were performed on a Bruker Dimension Icon with Scan Asyst. Contact Angle measuring instrument (SDC-200) was used to measure the contact angle between the sample and liquid. The liquid conductivity of the sample was tested by a conductivity measuring instrument (FCS-842). Other abbreviations appearing in the article and their full spelling can be found in the Abbreviations section.

3. Results and Discussion

3.1. Morphology Characterization

The scanning electron microscope (SEM) results of uncoated and GRC proppants are shown in Figure 2. The surface morphology of the uncoated proppant (selected from ceramic proppant) was not evenly distributed with pores and grooves, which showed a rough, porous structure. The GRC proppant’s surface was smoother than the uncoated proppant. It can be seen that the polymer coating on the surface of the ceramic proppant fills in the holes and remarkably reduces the surface roughness of the proppant.
The energy-dispersive X-ray spectroscopy (EDX) results of uncoated and GRC proppants are shown in Figure 3. The surface of the uncoated proppant contains a large number of silicon and oxygen. After coating, silicon and oxygen are partly reduced and a large number of carbon and nitrogen elements are detected, indicating that the polymer coating covers the uncoated ceramic proppant well.

3.2. Hydrophobic Properties

The contact angle (CA) between proppant and liquid reflects the wettability of proppant. Figure 4 shows the contact angle between various liquids like deionized water or 0.2 wt% guar gum solution and proppant. According to Figure 4a,b, it can be observed intuitively that the hydrophobicity of GRC proppant is enhanced. The contact angles of the uncoated proppant and GRC proppant are 44.58° and 81.93° in the deionized water, respectively. The contact angle between GRC proppant and guar gum solution is 105.28°, while the contact angle between uncoated proppant and guar gum solution is only 69.74°. The contact angle of GRC proppant increased by 83.78% and 50.96% in deionized water and guar gum solution, respectively. The reason for the increase in the contact angle between the proppant and the sand-carrying liquid is that the coating material has a certain hydrophobic branch, which makes the surface change from hydrophilic to hydrophobic. The contact angle tests hydrophilicity and hydrophobicity between the proppants and liquid. The property shows the ability of proppants to pass through the shale oil and underground water.

3.3. Suspension and Settlement Properties

The surface coating of proppant changes the mineral surface from a normally hydrophilic and water-wet condition to a hydrophobic condition. This mineral surface property change results in a stabilization of proppant and fine movement, as well as a surface that is not conducive to scale deposition. In order to promote proppant migration to the fracture depth, we studied the suspension and settlement properties of proppant. The suspension of the proppant before and after the coating is shown in Figure 5a. The proppant suspension effect improved with the increase of guar gum solution concentration before coating. However, the suspension effect of GRC proppant did not change much, which even in deionized water has a good suspension effect. When the mass concentration of guar gum solution is 0.25 wt%, the suspension capacity of GRC proppant increases by 54.58%, as indicated by the mass percentage of proppant suspended on the solution surface. It can be seen from Figure 5b,d that the suspension effect of GRC proppant in guar gum solution at 0.1 wt% and 0.2 wt% concentrations is excellent. Weighing the mass of uncoated proppant suspended on the surface of guar gum solution in Figure 5c,e, it is found that the amount of suspended proppant at 0.2 wt% is about 1.3 times that at 0.1 wt%. The suspension effect of proppants in fracturing fluid can directly reflect their migration distance in fractures of shale reservoirs. Fluid can transport proppants with high suspension ability to the fracture tip.
The settlement time of the proppant in the deionized water and guar gum solution is shown in Figure 5f. This time is how long it takes for the single-particle proppant to settle in a 100 mL measuring cylinder. The settlement time of uncoated proppant or GRC proppant extended as the concentration of guar gum solution increased, but the settlement time of proppant increased more significantly after treatment. The GRC proppant has a 22.75% increase in settlement time at 0.25 wt% concentration compared with uncoated proppant. The settlement effect of GRC proppant at 0.2 wt% concentration was 3.8 times higher than that at 0% (deionized water).

3.4. Electrical Conductivity

The electrically conductive property of proppant is also very outstanding for improving the in-situ modification efficiency of middle and low maturity shale oil, which can be employed as an electrically conductive material to form resistive heating element. After the heat enters the formation, the shale oil reservoir is in-situ modified to obtain high-quality oil with good fluidity. We added graphene into the surface of the resin-coated proppant to improve the electrical conductivity of the proppant, and the 10 mm × 10 mm × 1 mm proppant block was prepared for electrical conductivity testing. As shown in Figure 5g, the current flowing through uncoated proppants is almost negligible, while GRC proppants have almost constant electrical conductivity under different voltages. The electrical conductivity of the GRC proppant at 10 V is 0.1259 S/m, which is four orders of magnitude away from the uncoated proppant. As the electric time progresses, the GRC proppants that migrated and settled in the fracture have the potential to generate heat to accelerate the in-situ modification of shale oil. The conductivity of proppants allows them to be used as conductive materials in the electric heating method of in-situ modification of shale oil.

3.5. Adhesion Properties

The adhesion force results of uncoated and GRC proppants tested by atomic force microscopy (AFM) are shown in Figure 6. Figure 6a describes the diagrammatic sketch of the adhesion force test. The van der Waals force is influenced by attractive force and repulsive force. The adhesion force between proppant and object surface is completely due to van der Waals force. We tested the adhesion force by contacting the proppant with the probe in the AFM. The relationship between adhesion force and load force is shown in Figure 6b, and the load force increased from 1 μN to 3 μN every 0.5 μN, while the contact time was fixed at 1 s. With the increase of the load force, the adhesion force of the uncoated proppant was about 35–55 nN which is hardly changed, while the adhesion force of the GRC proppant increased positively by 84.31%, 58.71%, 76.66%, 58.35%, and 40.47%, respectively, compared with the uncoated proppant. The results show that the coating improves the surface adhesion of proppant. The relationship between adhesion force and contact time is shown in Figure 5c, and the contact time increased from 1 s to 3 s every 0.5 s, while the load force was fixed at 1 μN. With the increase in contact time, the adhesion force of uncoated and GRC proppant was almost unchanged, but that of GRC proppant increased by about 63.13% compared with uncoated proppant. The adhesion force between proppants and the shale surface in the air and fracturing fluid intuitionally shows the adhesion behavior of proppant to the shale fracture wall. The increased adhesion force makes the proppant more easily adhere to the fracture after a long-term migration with the fracturing fluid and renders the proppants aggregate to improve the ability to support the fracture.

3.6. Liquid Conductivity

Figure 6d indicated changes in proppant conductivity were measured under different closing pressures from 5 Mpa to 40 Mpa under experimental conditions with proppant sand placement concentration, and fracturing fluid displacement of 5 kg/m2 and 3 mL/min, respectively. Under the selected range of closure pressure, while the conductivity of two proppants is both decreased with the increase of closure pressure, the conductivity of GRC proppant is always higher than uncoated proppant, especially at the effect of lower closure pressure is more apparent. The conductivity of GRC proppant under 20 Mpa is 33.39% higher than that of uncoated proppant. As discussed above, the surface coating reduced the roughness of ceramic proppants, which can increase liquid flowability. Besides, the coating material is a hydrophobic polymer. It can provide a coating with excellent shear strength and make the liquid flow better, therefore improving the liquid conductivity ability [41]. Conductivity is one of the most critical properties of proppant, which directly determines the migration and production efficiency of shale oil.

4. Conclusions

In this paper, we reported a graphene and resin coated proppant with electrically conductive properties for in-situ modification of shale oil. The experimental strategy mentioned in the article would be transferable across shales. Compared with uncoated ceramic proppant, the GRC proppant has average sphericity greater than 0.7 and a low broken rate. The adhesion force of the GRC proppant is also enhanced, making it easier to adhere to the fracture surface. The liquid conductivity and suspension properties of the GRC proppant are 33.39% and 54.58% higher than that of the uncoated proppant. Ultimately, the GRC proppant has almost four orders of magnitude increase in electrical conductivity, which can further improve the in-situ modification efficiency of middle and low-maturity shale oil.

Author Contributions

Conceptualization, S.C., F.L. and Y.Z.; methodology, S.C. and F.L.; validation, Q.X., Y.L. and Y.J.; formal analysis, X.L.; investigation, S.L.; resources, L.W.; data curation, Y.Z.; writing—original draft preparation, S.C. and X.L; writing—review and editing, S.L. and L.W.; visualization, F.L.; supervision, Y.J.; project administration, Y.L.; funding acquisition, Q.X. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Key Research and Development Program of China, grant number 2020YFC1808102; Basic Research Program on Deep Petroleum Resource Accumulation and Key Engineering Technologies, grant number U19B6003; Science Foundation of China University of Petroleum, Beijing, grant number 2462019BJRC007; and Science Foundation of China University of Petroleum, Beijing, grant number 2462019QNXZ02.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

GRCGraphene and resin coated
ICPIn-Situ-Conversion
SEMScanning electron microscope
EDXEnergy-dispersive X-ray spectroscopy
AFMAtomic force microscopy
CAContact angle

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Figure 1. Schematic diagram of electric heating in-situ modification technology and electrically conductive proppants.
Figure 1. Schematic diagram of electric heating in-situ modification technology and electrically conductive proppants.
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Figure 2. SEM images of uncoated (ac) and GRC (df) proppants.
Figure 2. SEM images of uncoated (ac) and GRC (df) proppants.
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Figure 3. EDX mapping of uncoated (ae) and GRC (fj) proppants.
Figure 3. EDX mapping of uncoated (ae) and GRC (fj) proppants.
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Figure 4. (a,b) Photographs of 0.2 wt% guar gum solution on the surface of uncoated proppants and GRC proppants. (c) Histogram of the contact angle between proppants and liquid. (d) The contact angle of uncoated proppants and deionized water. (e) The contact angle of GRC proppants and deionized water. (f) The contact angle of uncoated proppants and 0.2 wt% guar gum solution. (g) The contact angle of GRC proppants and 0.2 wt% guar gum solution.
Figure 4. (a,b) Photographs of 0.2 wt% guar gum solution on the surface of uncoated proppants and GRC proppants. (c) Histogram of the contact angle between proppants and liquid. (d) The contact angle of uncoated proppants and deionized water. (e) The contact angle of GRC proppants and deionized water. (f) The contact angle of uncoated proppants and 0.2 wt% guar gum solution. (g) The contact angle of GRC proppants and 0.2 wt% guar gum solution.
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Figure 5. (a) Mass percentage of proppant suspended on different guar gum solutions. (b,c) GRC and uncoated proppant suspended on the surface of guar gum solution at 0.1 wt%. (d,e) GRC and uncoated proppant suspended on the surface of guar gum solution at 0.2 wt%. (f) Settlement time of uncoated proppant and GRC proppants in guar gum solution with different mass concentrations. (g) Electrical conductivity of uncoated proppant and GRC proppant under different voltage.
Figure 5. (a) Mass percentage of proppant suspended on different guar gum solutions. (b,c) GRC and uncoated proppant suspended on the surface of guar gum solution at 0.1 wt%. (d,e) GRC and uncoated proppant suspended on the surface of guar gum solution at 0.2 wt%. (f) Settlement time of uncoated proppant and GRC proppants in guar gum solution with different mass concentrations. (g) Electrical conductivity of uncoated proppant and GRC proppant under different voltage.
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Figure 6. (a) Diagrammatic sketch of the adhesion force test. (b) Adhesion performance of the uncoated and GRC proppants at different load forces. (c) Adhesion performance of the uncoated and GRC proppants at different contact times. (d) Liquid conductivity of uncoated proppant and GRC proppants under different closure pressure.
Figure 6. (a) Diagrammatic sketch of the adhesion force test. (b) Adhesion performance of the uncoated and GRC proppants at different load forces. (c) Adhesion performance of the uncoated and GRC proppants at different contact times. (d) Liquid conductivity of uncoated proppant and GRC proppants under different closure pressure.
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Chen, S.; Liu, F.; Zhou, Y.; Lan, X.; Li, S.; Wang, L.; Xu, Q.; Li, Y.; Jin, Y. Graphene and Resin Coated Proppant with Electrically Conductive Properties for In-Situ Modification of Shale Oil. Energies 2022, 15, 5599. https://doi.org/10.3390/en15155599

AMA Style

Chen S, Liu F, Zhou Y, Lan X, Li S, Wang L, Xu Q, Li Y, Jin Y. Graphene and Resin Coated Proppant with Electrically Conductive Properties for In-Situ Modification of Shale Oil. Energies. 2022; 15(15):5599. https://doi.org/10.3390/en15155599

Chicago/Turabian Style

Chen, Siyuan, Fanghui Liu, Yang Zhou, Xiuping Lan, Shouzhen Li, Lulu Wang, Quan Xu, Yeqing Li, and Yan Jin. 2022. "Graphene and Resin Coated Proppant with Electrically Conductive Properties for In-Situ Modification of Shale Oil" Energies 15, no. 15: 5599. https://doi.org/10.3390/en15155599

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