Experimental Investigation of the Transformation of Oil Shale with

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Experimental investigation of the transformation of oil shale with fracturing fluids under microwave heating in the presence of nanoparticles Zhaozhong Yang, Jingyi Zhu, Xiaogang Li, Dan Luo, Shuangyu Qi, and Min Jia Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00908 • Publication Date (Web): 31 Aug 2017 Downloaded from http://pubs.acs.org on September 3, 2017

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Experimental investigation of the transformation of oil shale with fracturing fluids under microwave heating in the presence of nanoparticles Zhaozhong Yang†, Jingyi Zhu*,†, Xiaogang Li†, Dan Luo†,‡, Shuangyu Qi†, Min Jia† †State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, Sichuan 610500, China. ‡Department of Physics and Texas Center for Superconductivity, University of Houston, Houston, TX 77204. KEYWORDS: Oil shale; In situ exploitation; Microwave heating; Hydraulic fracturing; Nanoparticles.

ABSTRACT: Traditional methods of exploiting oil shale such as mining or in situ electric heating, cause environmental pollution, and have huge energy losses, and high costs. These problems can be solved by combining microwave heating with hydraulic fracturing for in situ exploitation of oil shale. In this study, an experimental microwave apparatus was manufactured for laboratory experiments. Different weight proportions of iron oxide nanoparticles (0.1 wt%, 0.5 wt%, and 1 wt%), microwave output power (600 W, 800 W, and 1000 W), and ultimate reaction temperatures (550 °C, 750 °C, and 950 °C) were taken into account in the design of an

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orthogonal experiment. Temperature distributions were influenced by microwave power as well as by the concentration of iron oxide nanoparticles. The iron oxide nanoparticles facilitated a noticeable rise in the temperature of the oil shale in a short time. The experimental results confirmed the advantages of microwave heating, compared to conventional heating, in terms of temperature increases and improved yields of higher quality oil. Specifically, the oil collected under microwave irradiation contained more saturation and aromatics, and less sulfur and nitrogen, than that obtained by conventional heating. The highest oil yield and the best oil quality were obtained with the parameters of output power of 800 W, ultimate reaction temperature of 950 °C, and iron oxide nanoparticles at 0.1 wt%. Our findings contribute to the application of microwave technology to unconventional resources, and field tests at small scale should be supported.

INTRODUCTION In recent years, unconventional resources have attached more attentions because they can provide sufficient hydrocarbons to balance the deficit of conventional resources. Oil shale, which is widely distributed worldwide but concentrated mainly in the United States, Brazil and China, is a valuable potential source of liquid hydrocarbons and fuel products.1 Pyrolyzing kerogen can crack the oil shale and yield a number of shale oils that can be used as materials for petrochemical industries. This thermal method called pyrolysis is presently utilized to convert many materials into useful products.2-4 A number of oil shale processing techniques have been proposed and developed, and these can be classified into three main categories. Mining and aboveground retorting cause heavy pollution and have a low recovery rate, but in situ exploitation techniques are more efficient and


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environmentally-friendly for developing oil shale that is in a deeply buried in a thich layer.5-9 Among these techniques, microwave heating10-12 is a promising, cost effective, and environmentally friendly in situ technology for production of higher quality oils that meet pipeline and refinery specifications. It is well established that microwaves can penetrate a formation and generate heat throughout its entire volume, irrespective of reservoir geometry, heterogeneity, or properties.13 This unique feature makes it possible to apply a microwave field to heat the surface and interior of a reservoir, thus saving energy and reducing processing time. Additionally, the use of microwave absorbents has the advantage of selective heating, which provides a faster heating process and a more uniform temperature profile compared to conventional heating.14 Microwave pyrolysis is a thermal process that is used to transform materials such as plastic wastes, oil wastes, sewage sludge, and oil shale into useful fuels and products.15 Several researchers have investigated the feasibility of using microwave pyrolysis to transform oil shale into high-quality oil, and it has been shown that the method enhances oil recovery and also produces oil with lower N and S content.16 In addition, a zeolite-based catalyst has been used during microwave pyrolysis of oil shale to crack high-molecular-weight compounds into lighter products.17 Significant enhancements in heavy oil recovery can also be achieved by microwave heating, as numerical stimulation analysis has confirmed.18 Although microwave heating has potential for in situ exploitation and pyrolysis of oil shale resources, a key problem is the low porosity and permeability of shale rocks19, which makes it difficult to extract oil and gas from the reservoir under microwave irradiation. Consequently, it is imperative to improve seepage channels to guarantee sufficient space for oil and gas to flow into production wells. In general, hydraulic fracturing could meet these requirements, and its


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successful application has proven its potential for unlocking production from shale formations.20,21 During the fracturing treatment, if fluid is pumped into a well faster than it can escape into the formation, pressure inevitably increases and fractures eventually form. When the pumping rate is maintained at a high level, the newly created fractures continue to propagate and grow. Once seepage channels are well established, there is sufficient space for oil and gas to flow and finally be produced effectively.22,23 However, oil shale and heavy oil have low dielectric constants under formation conditions and therefore exhibit weak absorption under microwave irradiation24, causing drawbacks when microwaves are employed in the heating process. A solution proposed by some researchers25,26 for improving the absorption efficiency involves changing the dielectric properties of the material by adding a microwave absorbent, such as carbon or metal oxide nanoparticles. Active carbon can be used as an effective microwave absorbent to achieve the high temperature needed for the cracking of frying oil into desirable biofuel.27 It has also been concluded that metal nanoparticles not only can help raise the temperature of oil but also can promote its cracking and vaporization under microwave irradiation.28 Kewen et al. successfully used a carbon nanocatalyst with microwave heating to upgrade heavy crude oil into lighter oil.29 From these conclusions, it is feasible that carbon or metal oxide nanoparticles may increase the efficiency of heating and promote the transformation of oil shale under microwave irradiation. Technical success at the laboratory scale and in numerical simulations leads to the next critical question, namely, how nanoparticles can be injected and transported under formation conditions. This study focuses on this challenge, and considers the direct addition of iron oxide nanoparticles to fracturing fluid which is then pumped to create fractures.30-32 This procedure certainly has some advantages. Firstly, the main role played by the iron oxide nanoparticles is


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improving the heating efficiency of microwaves. Secondly, this unique fluid avoids the procedure of mixing an iron oxide nanoparticle suspension with fracturing fluids. After fracturing fluid has been pumped into a formation, the aqueous phase will evaporate because of the high temperature; the production of water vapor will further augment the formation pressure and form additional mini-fractures. Finally, it will not be necessary to perform a flowback operation because the fracturing fluids are entirely evaporated. This paper reports on microwave heating technology for in situ exploitation of oil shale after hydraulic fracturing. Keeping in mind the current shortcomings of laboratory-scale experimental setups,33,34 we devised a novel microwave apparatus and prepared iron oxide nanoparticles to add to the base fluid of a foam fracturing fluid. Under these circumstances, the transformation of oil shale in Guangdong (China) under microwave irradiation was investigated. The operational parameters, such as microwave output power, ultimate reaction temperature, and the addition of iron oxide nanoparticles, were analyzed at a frequency of 2450 MHz and optimized with regard to the yield and quality of oil samples. MATERIALS AND METHODS Preparation and characterization of magnetic nanoparticle dispersions. Ferrous chloride tetrahydrate, ferric chloride hexahydrate, citric acid, ammonia, and deionized water were obtained from Chengdu Kelong Co., Ltd., China (99% analytical reagents) and used to synthesize iron oxide nanoparticles using a combination of chemical and ultrasonication procedures.35,36 Then the iron oxide nanoparticles in the dispersions were stabilized by deionized water.


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Because the iron oxide nanoparticles suspension was added to the base fluid of a foam fracturing fluid, it was important to assess its pore size and stability. A pore size that is too large would block formation and cause additional damage, and instability could have a negative effect on the performance of the fracturing fluid. Cryo-scanning electron microscopy (cryo-SEM) was conducted to observe pore size and particle dispersion. A small drop of the dispersion was mounted on an aluminum stub with a diameter of 0.5 cm. The sample was transferred to a cryopreparation chamber and frozen by liquid nitrogen at a temperature of −90 °C under a high vacuum. After being sputter-coated with Au, the sample was placed in the vacuum chamber of a Quanta 450 scanning electron microscope (FEI, USA). The particle size distribution and zeta potential were measured by an analyzer (Zeta PALS 190 Plus, Brookhaven, USA). Specifically, iron oxide nanoparticles (pH = 7.9) were divided into two dispersions, sonicated for 20 minutes, and then individually transferred into disposable cells. One of the disposable cells was used for the particle size distribution experiment, and the other was used for the measurement of zeta potential. According to the DLVO theory, a system can be regarded as stable if the electrostatic repulsion dominates the attractive van der Waals forces. Particles containing many negative or positive charges result in high zeta potential, thereby resulting in interparticle repulsion and the attainment of stability of the whole system. Magnetization measurements were performed with a vibrating sample magnetometer (VSM; 730T, Lake Shore, USA). A magnetization curve was recorded for the iron oxide nanoparticles at room temperature with an applied magnetic field ranging from −15000 Oe to 15000 Oe. Experimental apparatus and operating conditions. Based on in situ microwave heating, the experimental apparatus represented in Figure 1 should simulate formation conditions to the


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greatest extent possible. The homogeneity of the microwave field is guaranteed by the use of multiple microwave sources, which means that more than two microwave sources can continuously emit waves of a frequency of 2.45 GHz with a maximum output power of 1200 W. A low-angle inclined plane is installed on the waveguide. This inclined plane allows a portion of the microwaves to be transported in the oblique direction so that waves of direct and oblique directions can irradiate the material more widely and a greater spread of radiation can be achieved in the resonant cavity. In detail, an armored thermocouple assembled from thermocouple wires, insulating materials, and a protecting metal pipe is used as a temperature sensor with a maximum temperature of 1000 °C. When the reaction temperature arrives at a set point, it can be kept static automatically by means of a proportional–integral–derivative controlling mode. During the reaction, all recorded data can be displayed on a screen, which enables operators to design heating parameters and collect data. Several details are taken into account to ensure safety. To prevent leakage of microwaves, there is a chock on the apparatus door to form wave impedance. In addition, a pressure gauge installed on the thermocouple is used to monitor the reaction pressure in the event that gas cannot be discharged in time. The sample container, which is made from quartz glass (diameter of 69 mm, height of 105 mm), has two ports on its top. One of these, on the left, is used as an outlet for gases, and the other, on the right, provides access to a thermometer. The thermometer, which contacts the samples directly, measures the reaction temperature, and the gas emerging from the left port is collected. At the bottom of the container, there are several small holes (diameter of 0.25 cm) for gas effluent. All ports top and bottom connect with condenser tubes in which alcohol is


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circulated for cooling gas to collect liquid products. In addition, all of the experiments were conducted under ambient-air atmosphere. To study the effect of water under microwave irradiation, a 100 g sample of oil shale was heated with and without 5 mL of water at a microwave output power of 600 W. Temperature data were recorded to determine whether the system could reach the transformation temperature. To enhance the microwave heating efficiency, iron oxide nanoparticles were added into the base fluid of the foam fracturing fluid and mixed thoroughly with the sample. Foam fracturing fluids are attracting much attention because of their small liquid content, low cost, and low damage. Typically, the base fluid of a foam fracturing fluid consists of deionized water, foam agents (surfactants), and foam stabilizer. However, it is necessary to note that because of the iron oxide nanoparticles, the base fluid in this study contained no foam stabilizer, which may cause extra damage to permeability. On the other hand, different microwave output powers and different reaction temperatures correspond to differences in thermal reaction of oil shale. The high temperatures attained when using microwave absorbents under microwave irradiation could be advantageous in the decomposition of nitrides, sulfides, and the cracking of large compounds into lighter products, and this method has been proven to be improved approach for upgrading to oil with desirable properties. 37 Thus, different doses of iron oxide nanoparticles (0.1 wt%, 0.5 wt%, and 1 wt%), microwave output powers (600 W, 800 W, and 1000 W), and ultimate reaction temperatures (550 °C, 750 °C, and 950 °C) were used to design an orthogonal experiment with three factors and three levels. All iron oxide nanoparticles were added to 5 mL of base fluid (in practice, deionized water), and each sample was selected to have a weight of 100 g. The total heating period was 30 minutes. If the oil shale reached the set point in less than 30 minutes, the


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temperature in the chamber would remain unaltered. Under these conditions, optimization of the orthogonal combination with regard to liquid yield and quality was accomplished. To be able to compare microwave heating with conventional heating, two groups of oil shale samples were placed in an electrical oven and heated to 550 °C and 750 °C, respectively. The liquid product was collected in the same way as in the microwave experiments, but there was no port at the bottom of the conventional electric oven. All recoveries were determined on a weight basis. Sample characterization and analysis. The oil shale chosen for this study originated from Maoming, Guangdong Province, China. A master batch sample was prepared by grinding and sieveing to a particle size range of 3–8 mm with a standard sieves. The characteristics of this sample are given in Table 1. Particularly, a Fischer assay was conducted under anoxic conditions, and all experimental data were analyzed on an as-received basis. The oil shale and each group of liquid samples obtained by microwave heating and conventional heating were investigated by Fourier transform infrared (FTIR) spectroscopy (Nicolet 6700, Thermo Scientific, USA) to analyze functional groups of organic matter qualitatively using the KBr tableting method. Furthermore, the four fractions of the oil samples were analyzed by thin-layer chromatography and flame ionization detection (IATROSCAN MK6s, IATROSCAN, Japan), and the contents of carbon, hydrogen, nitrogen, and sulfur of the oils were determined with an elemental analyzer (Vario EL-III, Elementar, Germany). These evaluations are not only important for comparing the two heating methods, but are also important for assessing the feasibility of developing microwave techniques as improved sustainable methods for exploiting oil shale and producing useful petroleum products.


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THEORETICAL SCHEME OF IN SITU MICROWAVE HEATING OF OIL SHALE As a rapid, highly efficient, and environmentally friendly method, microwave heating has great potential for in situ development of oil shale. Because oil shale reservoirs have low porosity and low permeability and are also weak microwave absorbents, fracturing fluids containing iron oxide nanoparticles are used to improve seepage channels and heating efficiency. As shown in Figure 2a, a horizontal well is used for hydraulic fracturing. It is important to realize that the transmission distance of microwaves at frequencies of 915 MHz or 2450 MHz is limited under reservoir conditions. Therefore, several vertical wells within a certain distance are drilled into the oil shale formation (Figure 2a). An antenna is installed at all well bottoms to emit microwaves directly (Figure 2b), and under the microwave irradiation, oil and gas are yielded in the same wells. Additionally, in the area of the pay zone, the casings material should be nonmetallic.38 In general, the procedure of the in situ exploitation of oil shale includes several steps: 1) Fracturing fluids containing iron xide nanoparticles are pumped into the formation and fractures are formed during the fracturing treatment. 2) Antennae are installed in the production wells to heat the whole oil shale formation. 3) Once the transformation temperature is reached, oil and gas are yielded in the production wells. As the oil and gas are extracted completely near the microwave generator, the rock becomes so transparent to microwaves that microwave irradiation can penetrate to greater depths. Flowback of fracturing fluids is unnecessary because water evaporates during the heating. RESULTS AND DISCUSSION


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Properties of the iron oxide nanoparticles. Images of the iron oxide nanoparticles were acquired by cryo-SEM (Figure 3). Discrete particles are visible and there is no extensive aggregation. The particle size distribution of the iron oxide nanoparticles, with a median diameter of 81.8 nm, is shown in Figure 4. The result indicated that the iron oxide nanoparticles prepared in the lab meet the requirements of nanometer-level size and no serious aggregation. Iron oxide nanoparticles have to overcome an energy barrier of electrostatic repulsion to approach closely and form agglomerates. An improvement in zeta potential is well in agreement with a reduction of aggregation.39 Typically, when the zeta potential decreases to less than −20 mV, significant aggregation will occur.40 In this study, electrophoretic measurements were conducted three times and the average value of the zeta potential was −33.87 mV, indicating that the suspension remained stable and that particle aggregation hardly occurred.41 The magnetic properties of the iron oxide nanoparticles, including hysteresis loops, remanence, and coercivity, were investigated by vibrating sample magnetometer. Figure 5 shows the magnetization of the iron oxide nanoparticles versus the applied field at room temperature by cycling the field between −15000 Oe and 15000 Oe. There is a magnetic hysteresis loop in the top-left corner of Figure 5, so the iron oxide nanoparticles are ferromagnetic. Additionally, the saturation magnetization corresponding to the maximum value of the curve in the first quadrant at the Y axis is 53 emu/g, which shows that the iron oxide nanoparticles are prone to becoming magnetized. The remanence corresponding to the intersection of the positive direction of the Y axis and the magnetic hysteresis loop is 2.95 emu/g, and the coercivity corresponding to the intersection of the negative direction of the X axis and magnetic hysteresis loop is 33.5 Oe. With decrease of particle size, especially to the nanometer scale, the number of atoms on the surface is increasing because of the high specific surface area, which thus contributes to the activity


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enhancement of the iron oxide nanoparticles. As a result, accelerated movements of atoms and electrons promote magnetization, thereby converting electromagnetic energy into heat energy under microwave irradiation. Finally, the microwave absorption of the iron oxide nanoparticles is enhanced. It can be seen that the iron oxide nanoparticles in the dispersions prepared at the laboratory scale had high stability and microwave-absorbing characteristics. However, it is necessary to adopt an ultrasonic technique to maintain the performance of the iron oxide nanoparticles before adding them base fluid of a foam fracturing fluid. Effect of fracturing fluid on the temperature of oil shale under microwave irradiation. Unconventional reservoirs such as shale oil, coal seam gas, and tight oil reservoirs have low porosity and low permeability, which causes great difficulties for their exploitation. Hydraulic fracturing is a major technology that plays an important role in achieving commercial development of unconventional resources.42 In this study, considering their wide application in tight shale reservoirs, foam fracturing fluids were used as the working fluids for the stimulation of oil shale during fracturing treatment. Apart from improving seepage channels, fracturing fluids containing iron oxide nanoparticles were also regarded as being microwave absorbent under microwave irradiation. It should be noted that the iron oxide nanoparticles were mixed with the base fluid of the fracturing fluid. Because surfactants decompose easily at high temperatures,43 the iron oxide nanoparticles were actually added to deionized water to simplify the experimental procedure. However, the properties of foam fracturing fluids containing iron oxide nanoparticles are beyond


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the scope of this article. The two main factors, namely, deionized water and iron oxide nanoparticles, will be discussed under microwave irradiation. Alexandra et al.44 reported that the natural resonance frequencies of water molecules are similar to microwave frequencies, which leads to the absorption of microwave energy by water. Figure 6 shows the difference in heating rate between three experiments. Within 20 minutes, the temperature of oil shale with no water could not reach 270 °C, which is far below the pyrolysis temperature of oil shale. However, a different sample with water reached 320 °C quickly when above the boiling point of water. Although water enhanced the heating rate, there was a slowing trend over time. Therefore, 0.5 wt% iron oxide nanoparticles within 5 mL of base fluid was employed to further increase the efficiency of the heating. As shown in Figure 6, there is a striking upward trend in temperature promoted by the iron oxide nanoparticles under the same experimental conditions. The results of the VSM experiments prove that the iron oxide nanoparticles are prone to magnetization under a magnetic field. Moreover, their nanoscale size contributes to improving the activity of the particles. Thus, because of the rapid movements of atoms as well as the electronic effects, it is not surprising that electromagnetic energy can be quickly converted into thermal energy. Effect of experimental parameters on the transformation of oil shale under microwave irradiation. Oil shale can be effectively and rapidly transformed into shale oil with the help of iron oxide nanoparticles under microwave irradiation. However, the effects of the concentration of iron oxide nanoparticles, microwave output power, and ultimate reaction temperature on the transformation of oil shale are also important. The orthogonal experiment was designed in accordance with these three factors to determine the best combination for oil yield and quality.


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The nine experiments are summarized in Table 2, and the temperature curves for each experiment are shown in Figure 7. For the different output powers, the general trends in these figures are slightly different. In particular, at an output power of 600 W, groups 2 and 3 could not reach the ultimate set temperature of 750 °C and 950 °C, respectively, in 30 minutes. However, it was observed that the temperature of the oil shale increased rapidly and the time taken to reach the set point decreased as the output power was increased. There are two regions in the temperature curves: an initial heating part and a subsequent stabilization part. Hascakir et al. 45 demonstrated that maintaining the reaction temperature for an extended time is important for recovery from oil shale, and this was the reason that the oil shale was heated for a while at the set temperature. The difference in the heating rate for different concentrations of iron oxide nanoparticles is less obvious when the output power is higher. An addition of 0.1 wt% iron oxide nanoparticles is sufficient for oil shale to be transformed rapidly into oil and gas when the output power exceeds 800 W because higher power promotes the movement of molecules and enhances energy conversion. Although it is beyond the scope of this paper, it is necessary to evaluate the performance of fracturing fluids when different amounts of iron oxide nanoparticles are added to them. With regard to oil production, as shown in Figure 8, groups 10 and 11 under conventional heating provided relatively low yields. For microwave heating, the yield of liquid from the samples initially increased with output power. However, at an output power of 1000 W, the yield decreased. This phenomenon may be explained by the fact that gases could not be discharged in time because of the more rapid increase of temperature in the cavity. Therefore, gas trapped by


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char products easily undergoes a second thermal cracking, which leads to a decrease in liquid production. For the ultimate reaction temperature, the results were also different. The recovery from oil shale increased with ultimate reaction temperature except when the output power was 1000 W. This phenomenon shows that higher temperatures contribute, to some extent, to the complete transformation of oil shale under microwave irradiation. Last but not least, we observed an interesting phenomenon during the experiments. When oil and gas were both discharged from the port at the top and the several small holes at the bottom of the container, more gas was discharged from the top port while more oil was collected from the bottom holes. This means that oil extracted from oil shale preferentially flows downward. However, there is usually one port at the top or on the side of a conventional electric heating furnace, which may result in the loss of liquid products. As a result, the highest oil yield was obtained by group 6, followed by group 5. In the FTIR spectra (Figure 9), it is obvious that the bands of aliphatic moieties (symmetric stretching vibrations of CH2 groups at 2920 cm-1 and asymmetric stretching vibrations of CH2 groups at 2850 cm-1) shown in Figure 9(a) disappear in Figure 9(b). This result indicates that the majority of organic matter in the raw material had been transformed into oil and gas. However, there are weak absorption peaks in the band at around 1620 cm-1 in Figure 9(b), which are assigned to C=C stretching vibrations of aromatic groups, demonstrating that an almost negligible amount of organic matter remained in the spent shale. As the reaction temperature increases, the peak height decreases, and some peaks almost disappear at the temperature of 950 °C, which provides evidence in support of the above conclusion. In addition, the intense absorption band at around 1100 cm-1 shows that the decomposition of minerals occurs mainly at higher temperatures.


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Comparison between microwave heating and conventional heating. Neto et al. proved that the yields of products containing the >C6 and C6–C10 fractions under microwave irradiation increased in the presence of a zeolite. We may reasonably draw a similar conclusion in the presence of fracturing fluids. Nine groups of oil samples obtained by microwave heating and two groups of oil samples obtained by conventional heating were compared with regard to liquid oil yield and oil quality. FTIR spectra can be used to assess the presence of different mineral and organic groups;46 therefore, oil samples were initially analyzed by FTIR spectra. Figure 10 shows the FTIR spectra of the 11 groups of liquid products from shale oil. Obviously, no matter which type of heating method was used, all of the spectra are fairly similar. The intense band at around 2800–3100 cm1

is attributed to vibrations of organic matter. It should be noted that the asymmetric stretching

vibrations of CH3 groups are found at 2954 cm-1 instead of at 2960 cm-1, indicating the presence of some long-chain aliphatic moieties. Moreover, the absorption bands of aliphatic moieties (2925 cm-1 and 2854 cm-1) are more intense than those of crude shale, which illustrates that the lengths of the long aliphatic chains in the oil samples are less than those in the oil shale. FTIR analysis is considered to be a qualitative approach for characterizing chemical groups in liquid products. Although the band at 900 cm-1 may have arisen from the vibrations of S-OH groups and the band around 600–830 cm-1 may correspond to the stretching vibrations of C-S bonds, no conclusion could be reached about the sulfur content under microwave and conventional heating. In order to characterize the quality of the liquid products further, it was necessary to employ other analytical methods. The results for four oil fractions are shown in Figure 11. It can be seen


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that the four fractions exhibit similar behavior in the nine sample groups under microwave irradiation. However, the saturation and aromatics percentages for the conventional heating groups are significantly lower than those of the other groups, as shown in Figure 12. This phenomenon is attributed to the fragmentation of larger molecules, such as long-chain, branchedchain, and heterocyclic compounds, and the formation of small-molecule fractions because of the microwave irradiation.47,48 As a result, this may provide strong proof that microwave heating is more advantageous than conventional heating with respect to oil quality. Focusing on the nine sample groups from the microwave experiments, different factors gave different results. The greatest sum of saturation and aromatics is found in group 6 (800 W, 950 °C, 0.1 wt% iron oxide nanoparticles), followed by that in group 3 (600 W, 950 °C,1 wt% iron oxide nanoparticles), both of which were approximately 50%. The percentages of saturation and aromatics corresponding to the microwave output power of 1000 W are relatively low. This result may be explained by the fact that the output powers of 600 W and 800 W favored the resonance of molecules in the liquid samples; therefore, the oil was upgraded when the other experimental conditions were suitable. Oil containing nitrides and sulfides may potentially cause harm. For example, nitrides, which originate mainly from heterocyclic compounds, easily form resins and pose a threat to oil storage, transportation, and further processing, whereas sulfides are likely to corrode metal equipment. Therefore, it is imperative to evaluate oil quality by investigating the contents of nitrogen and sulfur. Nicholas et al.49 found that it is possible to perform hydrodesulfurization reactions using microwave heating in conjunction with iron powder as a catalyst. Figure 13 shows the contents of N and S and the H/C ratio of the 11 oil samples. The results showed that


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under microwave irradiation, the N content was reduced by a maximum of 31.1% and the S content was reduced by a maximum of 42.4% compared to the oil obtained by conventional heating. In addition, the H/C ratio was increased by a maximum of 11%. All of these results reflect the superiority of microwave heating compared to conventional heating. Moreover, the oil samples contain less N and S and have greater H/C ratio at the output powers of 800 W and 1000 W with a reaction temperature of 950 °C, which means that higher output power facilitates the removal of N and S. In detail, the highest yield and quality of oil was found in group 6. Although the temperature of the current in situ exploitation method is hampered by the limitations of material of the tube and underground equipment, the in situ reaction temperature of oil shale could be improved with the development of materials’ technology. CONCLUSIONS A method of in situ exploitation of oil shale combining microwave technology and hydraulic fracturing was proposed. Microwave irradiation is applied for thermal cracking of kerogen into oil, while hydraulic fracturing improves seepage channels in the shale matrix and guarantees effective production of the oil. Based on the above-described in situ exploitation method, an experimental microwave heating apparatus was reformed to simulate formation conditions to the greatest extent possible and then used to conduct thermal decomposition of oil shale under microwave irradiation. Iron oxide nanoparticles prepared at the laboratory scale can be used as an effective microwave absorbent to convert microwave irradiation into heat energy. Results have demonstrated that such iron oxide nanoparticles can meet the requirements of enhancing the effect of microwave heating and achieving high temperatures for the transformation of oil shale.


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In this study, after an iron oxide nanoparticle suspension was mixed with the base fluid of a foam fracturing fluid, the transformation of oil shale under microwave irradiation was investigated. It was concluded that microwave irradiation is advantageous because of its shorter process time, lower energy usage, and higher oil production and quality compared with conventional heating. Regarding oil quality, microwave irradiation can both crack the chemical bonds in large molecules and convert them into light hydrocarbons and also reduce the contents of nitrogen and sulfur, thereby demonstrating favorable potential for upgrading oil into petrochemical products with high saturation and aromatics compositions. In summary, it was established that combining microwave heating with hydraulic fracturing offers a number of advantages and shows excellent potential for exploiting oil shale. However, it was also found that this practice possesses challenges and uncertainties, so more investigations are required. Moreover, we should point out that we did not consider the influence of iron oxide nanoparticles on the performance of foam fracturing fluids and did not take into account the analysis of gases. Notwithstanding its limitations, this study clearly indicates that oil shale can be effectively transformed by microwave heating after hydraulic fracturing. FIGURES. Graphics were uploaded separately. TABLES. Table 1. Characteristics of Maoming oil shale Proximate analysis Moisture Volatile matter

Proportion (wt%) 0.64 75.5


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Ash Fixed carbon Ultimate analysis C H N S X-ray diffraction analysis Total clay Quartz Potash feldspar Plagioclase Calcite Pyrite Siderite Fischer assay Oil yield Water yield Semi-coke Gas + loss

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18.66 5.2 Proportion (wt%) 11.9 2.76 0.41 0.7 Proportion (wt%) 41.2 40.1 5.1 3.1 2.0 4.6 3.9 Proportion (wt%) 10.52 3.51 75.82 10.15

Table 2. Factors considered in the orthogonal microwave experiment on oil shale Factor Group

Output power (W)

Ultimate reaction temperature (°C)

1 2 3 4 5 6 7 8 9

600 600 600 800 800 800 1000 1000 1000

550 750 950 550 750 950 550 750 950

Concentration of iron oxide nanoparticles (%) 0.1 0.5 1 0.5 1 0.1 1 0.1 0.5

ASSOCIATED CONTENT Supporting Information. The steps of preparing iron oxide nanoparticles (PDF)


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Table 1. Oil production of eleven groups of liquid products (PDF) Table 2. Four fractions analysis of eleven groups of liquid products (PDF) Table 3. Results for element analysis(C, H, N, S) of oil shale samples (PDF) AUTHOR INFORMATION Corresponding Author *Tel.: +86 028 83032050. E-mail: [email protected] Author Contributions Zhaozhong Yang†, Dan Luo†‡ and Jingyi Zhu† conceived and designed the study. Jingyi Zhu† and Shuangyu Qi† performed the experiments. Jingyi Zhu† wrote the paper. Zhaozhong Yang†, Jingyi Zhu†, Xiaogang Li† and Min jia† reviewed and edited the manuscript. All authors read and approval the manuscript. Funding Sources The foundation called Qihang(431) Project at Southwest Petroleum University provides financial support for the manufacture of microwave setup and the analysis of oil shale. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by Qihang Foundation of Southwest Petroleum University(431). We sincerely thank other researchers in state key laboratory of oil and gas reservoir geology and exploitation at Southwest Petroleum University for the experimental study.


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We also grateful for Microwave Creative Technology Co,. Ltd(Qingdao, China) to provide technical guidance for experimental apparatus. ABBREVIATIONS wt, weight percent cryo-SEM, cryo-scanning electron microscopy

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(25) Greff, J.; Babadagli, T. Catalytic effects of nano-size metal ions in breaking asphaltene molecules during thermal recovery of heavy-oil. SPE Annual Technical Conference and Exhibition. Society of Petroleum Engineers, Denver, Colorado, October 30- November, 2011. (26) Al-Farsi, H.; Pourafshary, P.; Al-Maamari, R. S. Application of Nanoparticles to Improve the Performance of Microwave Assisted Gravity Drainage (MWAGD) as a Thermal Oil Recovery Method. SPE EOR Conference at Oil and Gas West Asia, Muscat, Oman, March 2123, 2016. (27) Lam, S. S.; et al. Pyrolysis using microwave absorbents as reaction bed: An improved approach to transform used frying oil into biofuel product with desirable properties. J. Cleaner Prod. 2017, 147: 263-272. (28) Greff, J.; Babadagli, T. Use of nano-metal particles as catalyst under electromagnetic heating for in-situ heavy oil recovery. Journal of Petroleum Science and Engineering 2013, 112(3), 258-265. (29) Li, K.; et al. Application of carbon nanocatalysts in upgrading heavy crude oil assisted with microwave heating. Nano Lett. 2014, 14(6), 3002-3008. (30) Lv, Q.; et al. Study of nanoparticle–surfactant-stabilized foam as a fracturing fluid. Industrial & Engineering Chemistry Research 2015, 54(38), 9468-9477. (31) Li, Y.; et al. An experimental study on application of nanoparticles in unconventional gas reservoir CO2 fracturing. Journal of Petroleum Science and Engineering, 2015, 133(9), 238-244. (32) Qajar, A.; et al. Modeling fracture propagation and cleanup for dry nanoparticle-stabilizedfoam fracturing fluids. Journal of Petroleum Science and Engineering 2016, 146(10), 210-221.


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Figure 1. Schematic diagram of the microwave setup used in this study: (1) wave generator; (2) sample holder made from quartz glass; (3) digital display board; (4) thermocouple; (5) pressure gauge; (6) upper liquid receptor; (7) lower liquid receptor; (8) condensation loop device; (9) lower tube; (10) upper tube. 123x179mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Figure 2. Conceptual design of microwave heating for in situ exploitation of oil shale: a) well design for in situ exploitation; b) production well with microwave device. 48x22mm (300 x 300 DPI)


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Figure 3. Cryo-SEM image of iron oxide nanoparticles in dispersion stabilized by deionized water. 77x71mm (300 x 300 DPI)


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Figure 4. Curve of particle size distribution in dispersion determined by dynamic light scattering. 84x58mm (300 x 300 DPI)


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Figure 5. Magnetization curve of iron oxide nanoparticles between −15000 Oe and 15000 Oe at room temperature. 84x58mm (300 x 300 DPI)


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Figure 6. Temperature curves of three groups of oil shale samples (without water and nanoparticles, with 5 mL water, and with 5 mL water and 0.5 wt% nanoparticles) under microwave irradiation at 600 W. 84x58mm (300 x 300 DPI)


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Figure 7. Temperature curves of the nine sample groups of different microwave output powers, ultimate pyrolysis temperatures, and concentrations of iron oxide nanoparticles. 59x41mm (300 x 300 DPI)


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Figure 8. Oil production from oil shale (Groups 1 to 9 were under microwave irradiation and groups 10 to 11 were under conventional heating) 221x170mm (300 x 300 DPI)


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Figure 9. FTIR spectra of oil shale and spent shale: a) oil shale originating from Maoming; b) spent shale obtained by microwave heating at 800 W at different pyrolysis temperatures. 84x64mm (300 x 300 DPI)


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Figure 10. FTIR spectra of oil samples: a) 11 groups of oil samples, nine groups obtained by microwave irradiation and two groups obtained by conventional heating, respectively; b) oil samples obtained at a microwave output power of 600 W and a pyrolysis temperature of 550 °C with the addition of 0.1% nanoparticles; c) oil samples obtained at a pyrolysis temperature of 550 °C by conventional methods. 84x64mm (300 x 300 DPI)


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Figure 11. Percentages of four fractions in oil samples (Groups 1 to 9 were obtained under microwave irradiation and groups 10 and 11 were obtained under conventional heating). 84x58mm (300 x 300 DPI)


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Figure 12. Comparison of percentages of four fractions in the oil samples: a) total percentages of saturation and aromatics in all 11 groups; b) total percentages of resin and asphaltene in all 11 groups. 84x64mm (300 x 300 DPI)


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Figure 13. Ultimate analysis of oil samples: a) histogram of results for contents of nitrogen and sulfur; b) histogram of results for H/C ratio. 84x64mm (300 x 300 DPI)


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Experimental Investigation of the Transformation of Oil Shale with

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