Evolution of Pore and Fracture Structure of Oil Shale under High


Sep 6, 2017 - Using X-ray microcomputed tomography (μCT) scan technology, Zhao ..... of fracture, so porosity, TNF, and MAF show a slight decreasing ...
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Evolution of Pore and Fracture Structure of Oil Shale under High Temperature and High Pressure Yide Geng, Weiguo Liang, Jian Liu, Mengtao Cao, and Zhiqin Kang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01071 • Publication Date (Web): 06 Sep 2017 Downloaded from http://pubs.acs.org on September 8, 2017

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Evolution of Pore and Fracture Structure of Oil Shale under High Temperature and High Pressure Yide Geng,†,‡ Weiguo Liang,*,†,‡ Jian Liu,†,‡ Mengtao Cao,§and Zhiqin Kang†,‡ †



College of Mining Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, China Key Laboratory of In-situ Property Improving Mining of Ministry of Education, Taiyuan University

of Technology, Taiyuan, Shanxi 030024, China §

Department of Petroleum Engineering, College of Engineering and Applied Science, University of

Wyoming, Laramie 82071, USA

* Corresponding Author:

E-mail address: [email protected]

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ABSTRACT In order to study the coupled effect of the temperature and pressure on pyrolysis characteristics and pore and fracture structures of oil shale, a total of 25 groups of pyrolytic reaction experiments have been conducted on 14 mm long and 7 mm in diameter cylindrical oil shale specimens under different temperature, pressure conditions ranging from 100-600℃ and 0.1-15 MPa. Further, both X-ray micro computed tomography (μCT) and mercury intrusion porosimetry (MIP) have been used to comprehensively investigate the network structure, inter-connectivity and evolution of pore and fractures. The results show that the temperature significantly affects the pyrolysis characteristics of oil shale. With rising temperature, both the mass loss and the porosity increase gradually, number and the maximum aperture of fractures also increase, and the pyrolytic degree intensifies progressively. The increase is most significant from 300 to 500 ℃. The maximum mass loss ratio is 20.84 %, the largest porosity is 13.52 times larger than that under the room temperature, and the total number and the maximum aperture of the fractures are 813 and 0.383 mm, respectively. Moreover, the pressure has a significant effect on the pore and fracture structures of oil shale. As the pressure increases, both the pore volume and the fracture distributions firstly decreased and then increased. With the continuous increase of pressure, the porosity and the total number of fractures reach a maximum at the pressure of 15 MPa. Under the coupled effect of temperature and pressure, with both the temperature and pressure increasing, the pores and fractures in the oil shale specimens developed increasingly. Furthermore, using the micro-CT scan technology, 2

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the distribution laws and the connectivity characteristics of the pores and fractures have been investigated. The connected fractures appears when the temperature reaches 300 ℃, and further extend along the bedding plane or pass through it at 600 ℃.

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1. INTRODUCTION Oil shale belongs to one of the unconventional oil and gas resources. As the solid skeleton of oil shale contains kerogen, so shale oil and mixing hydrocarbon gas products can be obtained from its thermal decomposition. Therefore, oil shale is an alternate energy for petroleum [1-5]. In terms of the amount of shale oil, there are approximately 4.8 trillion barrels oil shale reserves on the earth, while the reserves in China are around 354 billion barrels, accounting for 7 % of the world’s reserves

[6]

.

Further, oil shale in China is mainly distributed in Daqing, Huadian, Nongan, Xinjiang, Fushun, and Maoming [6-7]. Exploitation of oil shale can be aboveground (ex-situ) or underground (in-situ)

[8]

.

For the ex-situ exploitation, oil shale is extracted to the surface through opencast or underground methods, and oil shale is retorted by horizontal or vertical retort under [9-11]

anaerobic conditions at low-temperature

. On the other hand, for the in situ

exploitation, oil and gas products are collected by heating oil shale seam, with different heating methods and at different heating rates (e.g. ICP technology ExxonMobil’s Electrofrac technology

[13]

, Chevron’s CRUSH technology

[14]

[12]

,

and

in-situ injecting superheated steam exploitation technology of Taiyuan University of Technology

[15]

). Presently, although the in-situ underground exploitation technology

of oil shale is still in the research and development stage, oil gas can nevertheless be obtained without exploiting the mineral products to the surface. This has the merits of simple technological process, low cost, high exploitation rate and less occupation of land. So, it will become the main method for exploitation and utilization of oil shale in the future. Inside oil shale, distribution and connectivity of pore-fracture channels are vital for the pyrolysis products to discharge. Utilizing various testing methods, researchers 4

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investigated the pore and fracture structures of oil shale before and after pyrolysis. Using the micro-CT scan technology, Zhao and Kang et al. investigated the structure and permeability rules of oil shales from Yanan, Daqing and Fushun which had been subjected to 20-600 ℃ pyrolysis. The results showed that the pyrolysis significantly affected the pore structure, and a large number of fractures emerged when the temperature was higher than 350℃

[16-17]

. Also using the micro-CT scan technology,

Tiwari et al. investigated the pore structures of oil shale before and after pyrolysis, and used Lattice Boltzmann simulation to calculate the permeability. The results showed that a larger fracture channel was generated after pyrolysis and the level of porosity was related to the distribution of kerogen

[18]

. Using the N2 adsorption

method, Han et al. examined the pore evolution mechanism of oil shale and pore structure of oil shale ash after Gonglangtou oil shale was subjected to heating at 850 ℃. The results showed that under higher heating rate, the pore volume and specific surface area first decreased, then increased and finally decreased again with rising temperature [19-20]. Also using the N2 adsorption method, Sun et al. investigated the porosity characteristics of oil shale specimens when the water under different pressures flowed through the heating devices at various temperatures. Its total organic carbon (TOC) content was measured so as to simulate the pyrolytic process of oil shale under true hydrous geological conditions. The results showed that the migration of products along the developmental pore structure caused the pore size and porosity to be larger, and the thermal transformation accelerated the hydrocarbon migration [21]. Bai et al. utilized various methods to conduct 100-800℃ pyrolysis on Huadian oil shale and investigated the thermal effects and variations of the physical and chemical characteristics. The output of chemical products was closely associated with the variations of the pore structure and temperature, and 350-500℃ was the main 5

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temperature for kerogen decomposition [22]. Using the micro-CT scan technology, Saif et al. dynamically monitored the pyrolytic process of the oil shale. The results showed that there were many unconnected pore and fractures at 390 ℃ . However, at 390-400℃, the number of interconnected pores and porosity dramatically increased. Further, the increase in pore size from 300 to 500℃ was observed using 3-D imaging [23-24]

.

The past pyrolysis and retorting experiments of oil shale mostly took the temperature and heating rate into account, but without considering the effect of the external pressure on the kerogen pyrolysis. This study uses the μCT and MIP technologies, to investigate and analyze the characteristics of oil shale pyrolysis and evolution of pore and fracture structures under the coupled effect of temperature and pressure, from room temperature to higher temperatures and from unconstrained to higher pressure.

2. EXPERIMENTAL METHODS 2.1 Oil shale samples Oil shale samples were collected from the Eastern Fushun Opencast Mine, Liaoning province, China. The colors of the samples were dark brown. The samples were processed into 14 mm long and 7 mm in diameter cylindrical specimens. In order to reduce the impact of the heterogeneity and individual differences of the specimens, all specimens were drilled from one oil shale sample. The average mass of the specimen was around 1.31 g, with the axial directions of the specimens parallel to the horizontal bedding plane of the oil shale. The basic physical parameters of the oil shale specimens are shown in Table 1. High ash is the most significant feature of Fushun oil shale. 6

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2.2 Experiment procedure The experiment used WYF-I type high-temperature and high-pressure pyrolysis device developed by Taiyuan University of Technology in China. The device is heated by four electrical heating rods, which are installed outside the vessel and can reach a maximum temperature of 600 ℃. Further, nitrogen generated by a gas booster pump can apply pressure to the specimens continuously under high temperature. The maximum pressure it can reach is 20 MPa. The value of pressure refers to the gas pressure inside the vessel. Fig. 1 shows a schematic diagram of the pyrolysis device. The temperature range for this experiment is from 100 to 600 ℃ at a step of 100 ℃. The pressure range is from 0.1 to 15 MPa at a step of 5 MPa. 0.1 MPa is the value of one standard atmosphere; it means there is no external gas pressure affecting the specimens inside the vessel. All the specimens have been divided into 24 groups. There is one additional group which has been selected to be tested under 20℃ room temperature, and the unconstrained condition. Prior to the experiment, all the specimens were cleaned and weighed. Through heating at the rate of 2℃/min, when the temperature and pressure reached the specified values, they were kept for 12 h. In the course of the experiment, pressure was always maintained at the specified value by adjusting the booster pump and the exhaust valve. After the experiment, the heating rod was turned off and the device was cooled naturally to room temperature. The specimens were then taken out and weighed again. The mass loss was then calculated. Thereafter, the micro-CT scan and MIP test were 7

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used. 2.3 Mercury Intrusion Porosimetry (MIP) In this experiment, POREMASTER-33 type mercury intrusion porosimetry manufactured by American Quantachrome Ins. was used. The range of pore diameters which the porosimetry can measure is between 0.007-1000 µm, within two pressure ranges, i.e. low pressure 0.2-50 psi, and high pressure 20-33000 psi. The pore volumes under the corresponding pore scales can be determined by measuring and recording the amount of mercury that entered the pores under various injection pressures, following Washburn equation [25], as follows: Pr  2 cos

(1)

where r is the pore radius (μm), P is the absolute intrusion pressure (MPa), γ is the mercury surface tension (N/m), and θ is the contact angle between mercury and the pore surface. In this experiment, γ has been set at 480 N/m, and θ at 140°. Eq. (1) can be simplified to: Pr  0.735

(2)

According to Eq. (2), the pore diameter distributions under various injection pressures can be calculated. Consequently, the data under various conditions, including porosity, average pore diameter (RA), specific surface area (SSA), total pore volume (TPV) and pore size distributions (PSDS) can be obtained. 2.4 X-ray Micro-Computed Tomography (μCT) Using the micro-CT scan system (μCT225kVFCB) at Taiyuan University of Technology, 360°micro-CT scan of all the 25 groups of oil shale specimens have 8

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been carried out [26-27]. The scans have been conducted in accordance to 400 frames, superimposed frame rate is 2fps. There is a total of 1500 scanned layers. In this experiment, the scanning voltage was 70 kV, electric current was 90 μA, magnification was 47.255 times, and the lowest resolution was 4.106 μm. For the middle part of the scanned specimens, the most middle layer of the specimens, i.e. the 750th layer, has been used for the fractures calculation.

3. RESULTS AND DISCUSSIONS 3.1 Pyrolysis of specimens Fig. 2 shows the photographs of the surfaces and the end faces of the oil shale specimens undergoing pyrolysis under different temperatures between 20-600℃ at 0.1 MPa. It can be seen that the color of the specimens’ surfaces are darker at higher temperatures. The color darkens from dark brown to black. Between 300-600℃, there are connected fractures on the surfaces, and the aperture of the fractures is larger at higher temperatures, which is due to the high-temperature carbonification. On the 600 ℃ pyrolysis specimens’ surface, there are dropwise residuals formed as shown in the red ellipse in Fig. 2(g), due to the sublimation of shale oil. The mass loss ratio refers to the rate of mass loss. Fig. 3 shows the mass loss ratio of oil shale specimens after pyrolysis at different temperatures and pressures. At the same pressure, at higher temperature, the mass loss ratio is higher. From 100 to 300℃, the mass loss ratio increases from 0.14% to 2.84%. The mass loss is relatively small at this temperature range, namely the stage of water evaporation. From 300 to 500℃, the mass loss is much higher, the mass loss ratio increases from 2% to 17.72%, namely 9

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the stage of kerogen pyrolysis. From 500 to 600 ℃, the mass loss ratio increases from 13.31% to 20.84%, namely the stage of inorganic minerals decomposition.. With pyrolysis at 600 ℃, the mass loss of the oil shale specimens is at the maximum, i.e. 0.26 g at 10 MPa. The maximum mass loss ratio is 20.84 %. Using a thermogravimetry (TG), earlier researchers analyzed the mass loss ratio of oil shale. Bai et al. divided the mass loss curve of Huadian oil shale into three segments. From 300℃ to 500 ℃, the mass of the solid residual increased rapidly [22, 28]

. Sun et al. found that from 300℃ to 400 ℃, the mass loss of the Huadian oil shale

was greatest and was more than 40 %

[4]

. Syed et al. investigated the mass loss

regularities at different heating rates. He found that the mass of residual char decreased exponentially

[29]

. Wang et al. researched TG curve of North Korea at four

heating rates. He found that a sharp decrease of mass from 400℃ to 600℃

[30]

. All

these results are consistent with the large mass loss of the oil shale from 300℃ to 500 ℃ in this experiment. 3.2 Pore structure analysis by MIP 3.2.1 Analysis of porosity Porosity is a main parameter to measure the development degree of pores in porous materials. Using the MIP test, this study has measured the porosity of oil shale specimens at different temperatures and pressures. The main pore characteristic parameters of the 25 groups of specimens are shown in Table 2. It can be seen that the TPV increases with rising temperature. The highest rate of increase is from 300 to 500 ℃. Further, the largest TPV is 0.289 cm3/g in Sample E1. Both RA and SSA 10

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increase with rising temperature, and the largest RA is 91.22 nm in Sample E4, and the largest SSA is 20.698 m2/g in Sample F1. The porosity of oil shales after pyrolysis are different because they come from different regions and measured by different methods. Nonetheless, all the results show that the porosity of oil shale increases with rising temperature. Yang et al. used the MIP test to measure the porosity of Daqing oil shale, and the maximum porosity was 34.64% at 600 ℃

[31]

. Bai et al. also used MIP test to measure the porosity of

Huadian oil shale after pyrolysis, and found that the porosity increased with rising temperature, and the maximum porosity was 59% [22]. Sun et al. measured the porosity of Huadian oil shale, and found that the porosity increased continuously from 300 to 600℃, which was due to the local chemical reaction. The maximum porosity was more than 50% at 600℃

[4]

.

As shown in Fig. 4, the variation of porosity with temperature has been analyzed in three temperature ranges, i.e. the low temperature range (100-300℃), medium temperature range (300-500 ℃ ), and high temperature range (500-600 ℃ ). The variation of porosity with pressure has been analyzed in two pressure ranges, i.e. the low pressure range (0.1-5 MPa), and high pressure range (5-15 MPa). In the low temperature range, the porosity increases slowly from 3.35% to 11.76%. In the middle temperature range, the porosity increases sharply from 22.16% to 44.43%. In the high temperature range, the increase in porosity is insignificant, from 44.43% to 45.84%. At constant pressure, the porosity increases with rising temperature. The variation of the porosity is an S-shaped curve, i.e. the increases in 11

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porosity are small at the low and high temperatures, and large at the medium temperature. The maximum increase is at 600℃. In the low pressure range, the porosity varies from high to low. Further, the variations are different at different temperatures. In the high pressure range, the porosity increases continuously with increasing pressure. Under constant temperature, the porosity firstly decreases and then increases with increasing pressure. The maximum increase is at 15 MPa. Since pyrolysis of kerogen only occurs when the temperature is higher than 300℃, hence Samples C1, D2, E3 and F4 have been selected to compare with Sample O which has the unconstrained condition so as to investigate the coupled effect of temperature and pressure on the pore structure. the porosity of Sample C1 is 3.08 times higher than that of Sample O due to higher temperature. With the continuous increase of temperature and pressure, the porosities of Samples D2 and E3 and F4 are increased continuously. The porosity of Sample F4 is 45.84%, and is highest under all of 25 conditions, which is 13.52 times higher as that of Sample O. Hence, the porosity of oil shale gradually increases with increases in both temperature and pressure. 3.2.2 Pore size distributions by MIP Fig. 5 shows the differential pore size distributions (PSDs) obtained by the MIP, at 0.1, 5, 10 and 15 MPa. The values of the pore diameter corresponding to the peak of the curve reflect the main range of the pore size distribution. Between 20-300℃, the peak value of pore diameter (PPD) is around 10 nm. Between 400-600℃, the PPD increases rapidly with rising temperature, with the biggest pore diameter is 305.9 nm 12

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of Sample E1 at 500℃ and 0.1 MPa. An rise in temperature causes the pore diameter to be bigger, e.g. 34-77 nm at 400 ℃, 54-306 nm at 500 ℃, and 85-128 nm at 400 ℃. Under different pressures, the PSDs show certain differences, with PPD increases with rising temperature at 0.1 and 15 MPa, and the PPD at 500 ℃ are higher than those at 600 ℃ at 5 and 10 MPa. In this study, according to Hodot’s classification method [32], the pore structure has been divided into four classifications: i.e. micropore (< 0.01μm), transition-pore (0.01~0.1μm), mesopore (0.1~1μm) and macropore (1~100μm). The pore volumes under different pore diameters have been accumulated to obtain the distribution diagram of the four types of pore volume percentages under different temperatures and pressures, as shown in Fig. 6. For each temperature, there are four pore structure distributions. From left to right, the distributions are for the pressures 0.1, 5, 10, and 15 MPa, respectively. Micropore mainly existed in the non-pyrolysis stage which means the temperature was under 300℃, and the average volume occupied is 32.6%. A large amount of micropores were expanded and interconnected to form larger pores in the pyrolysis stage when the temperature was greater than 300℃. As a result, the micropore’s proportion decreased sharply to 2.1% by average. Transition-Pore and mesopore were playing a dominant role in the pore structure of oil shale. The volume proportion of mesopore is an important parameter to measure the degree of pyrolysis and the permeability of oil shale. In the non-pyrolysis stage, the transition-pore occupying 57.4% on average. Mesopore was formed above 100℃ 13

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and only occupies 4.8% of the total volume. Both TPV and porosity increased rapidly in the pyrolysis stage. The volume proportion of transition-pore firstly increased and then decreased; it reached the maximum with 88.5% of sample D3 at 400℃. The mesopore’s proportion increased continuously, from 17.2% to 44.5% at 400-600℃. Macropore mainly refers to the larger visible pores. There are two parts in the macropore. One is the big pores formed by the break and collapse of the thinner pore walls due to the very high mercury intrusion pressure, and the other is the new pore-fracture channels produced by pyrolysis. Between 100-300 ℃, macropore is composed of the first part, with a maximum total volume of 0.0016 cm3. But the TPV is smaller, which accounts for 7.3% of the total volume. At 400-600℃, second part occupies a leading position, with a total volume of 0.0084 cm3. However, compared with the low temperature conditions, the increase in the TPV is more significant, but the proportion decreases to 3.1%. In particular, the proportion of transition-pore and mesopore are different under some conditions due to the individual differences of the specimen and the external gas pressure. 3.3 Fracture analysis by CT Since the fracture distributions are complex, in order to extract the fractures easily and to conduct the analysis, in this study, the software MATLAB (MathWorks) has been used to reprocess the rebuilt images. The interconnected pores are called a pore cluster, and if a cluster reaches a certain scale, it becomes a fracture and might be equivalent to an ellipse. The ellipse’s length along the long axis is equivalent to the 14

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length of the pore cluster, namely the length of fracture (L), and the ellipse’s length along the short axis is equivalent to the aperture of this pore cluster, namely the aperture of fracture (A). Firstly, the function Imadjust was used to adjust the brightness of the gray image, then the function Imfilter was used to filter and enhance the contrast of the image, so the initial image was formed. Subsequently, the initial image was disposed with binarization and color inversion, and the function Regionprops was used to extract the length and aperture of the fracture. Finally, classification, comparison and analysis have been carried out. When the single fracture extends along the bedding plane, the aperture at different positions is different. So, the lengths and apertures extracted by Matlab are the longest length and widest aperture of a single fracture. 3.3.1 Fracture distributions Using the micro-CT scan technology, the distributions of fractures under different temperatures and pressures can be clearly observed. Fig. 7 shows the 750th layer’s CT scanned cross-section graph belonging to some oil shale specimens. In the images, different gray levels represent materials of different densities. The white dots are the inorganic minerals with higher density, and the gray parts are the organic matters with lower density, and the black parts are the pore and fractures [15,17,26]. From Fig. 7, it can be seen that the inside of the oil shale shows heterogeneity, where the different mineral crystal particles distribute. The bedding is very developed, and there are a large number of defects including the crystal grain boundary or voids inside the oil shale. With rising temperature, due to the thermal stress, some defects firstly expands 15

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and cracks, and then they are interconnected with bedding and extend continuously along the bedding plane to form some closed fractures, which are not connected with the surface of the specimen. When they extend to the surface, the connected fractures are finally formed, which penetrate the upper and lower ends and connect the inside and outside of specimens. According to the values of A and L and the A/L ratios, the fractures with L > 100 μm have been divided into three types, i.e. flat-microfracture, microfracture and fracture, as shown in Table 3. Fig. 8 is an extract from the CT cross-section graph and shows the fracture distributions of the oil shale after pyrolysis at 0.1 MPa and from 100 to 600 ℃. Combining Fig. 7 with Fig. 8, the distributions of oil shale fractures under different temperatures and pressures have been analyzed. At 20℃, the specimen is dense inside. The flat-microfracture appears at 100℃, microfracture appears at 200℃. Fractures formed at 300℃. From 300 to 400℃, some closed fractures are generated inside the specimens, and connected fracture is formed at the surface of the specimen. Between 500 to 600℃, the number of closed fractures increased, and length and aperture of the connected fracture increased significantly. All fractures extended along the bedding plane. From 0.1 to 5 MPa, the breaking degree was weakened. With the continuous increase in pressure, the number of closed fractures and the length and aperture of connected fractures all increased from 5 to 15MPa. Specifically, as shown in Fig. 7(1), there is a serious thermal break due to the heterogeneity of the samples at specimen C2. 16

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The specimens with increases in both the temperature and pressure have been selected, i.e. Specimens C1, D2, E3 and F4, as shown in Table 2. The corresponding scanned images are shown in Figs. 7(d), 7(i), 7(n), and 7(s). At 400 ℃ and 5MPa(Specimen D2), the numbers of the fractures are the smallest and the length of the connected fracture is the shortest. With the increase of temperature and pressure, the more the number of fractures, the further developed fractures. Coupling effect of temperature and pressure promoted the expansion and extension of the fractures. From Figs. 7(p)-7(s), it can be seen that the connected fractures at 600 ℃ all have the ‘turn’ phenomena, i.e. the tips of fractures turn from the bedding plane to the neighboring bedding plane and then extended continuously. This is different from the law that governs how the fractures expand along a single bedding plane under different temperatures. Due to the non-uniformity distributions of the inorganic mineral particles and temperatures, the order of pyrolysis and the development degree of fracture are different in different parts when high temperature have lasted for a long time. As a result, the mechanical properties in different places are significantly different and there is anisotropy. For the fractures that connect the inside and outside of the specimens, they are the direct links of the tips and fractures to the external environment. As the temperature increases, the thermal stress accelerates the generation and development of the internal fractures. Therefore, the phenomena passing through different bedding plane occur after the fractures are closed and extend inside the specimens, which connect to the connected fractures. In particular, there are the ‘turn’ phenomena in Specimen E4, as shown in Fig. 6(k). For the specimens with 17

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heterogeneity, this phenomenon also can occur when there is sufficient pyrolysis. 3.3.2 Analysis of the total number and the maximum aperture of fractures The total number of fractures and the aperture are the two important parameters to measure the development of fractures. Fig. 9 shows the distributions of the total number of fractures (TNF) under different temperatures and pressures. For each temperature, from left to right, the distribution is for the pressures 0.1, 5, 10, and 15 MPa. Fig. 10 shows the variations of the maximum aperture of the fractures (MAF). As the temperature rises, a large number of pores expanded and broke to interconnect and form new fracture channels, causing the TNF to increase exponentially. The increase in the number of flat-microfracture is the most significant, the average number of microfracture at 600℃ is 687 and is 113 times higher than 100℃. Also, the average MAF increase continuously from 0.01 mm at 100 ℃ to 0.29mm at 600 ℃. The rule that defines the effect of pressure on the total number of fractures is the same as that of its effect on porosity. The TNF first reduced, but then increased significantly from 5-15 MPa. The effect of increasing pressure on the maximum aperture of the fracture is more complex. Between 300-400℃, the MAF increases slightly with increasing pressure. Between 500-600 ℃, with increasing pressure, the MAF decreases from 0.1 to 5 MPa. However, it increases from 5 to 15 MPa. In particular, the fracture aperture of Sample D4 is very large due to the differences in the specimens. Under the coupled effect of temperature and pressure, from Sample C1 to Sample F4, 18

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the numbers of three types of fractures all increase, and the MAF increases continuously. The TNF of Sample F4 is 1.13 times that of Sample F1, and the MAF of Sample F4 is 1.09 times of that of Sample F1. The maximum of TNF is 813 and the maximum of MAF is 0.38mm at 600℃ and 15MPa of specimen F4. 3.4 Evolution mechanism of pore and fracture structure during pyrolysis In this study, the influences of temperature, pressure and the coupling effect of temperature and pressure on the interaction between pore and fracture structures and its evolution rules have been taken into account. The following three conditions are used to explain and analyze preliminarily. ① Same pressure and different temperatures In the low temperature range, internal free water and adsorbed water evaporate with the emission of some volatile components and absorption gases

[22]

, resulting in

release and connection with parts of pore space. As a result, the pore volume increases and some closed fracture forms, and the first connected fracture with small aperture appears at 300℃. In the medium temperature range, with the temperature reaching the pyrolysis temperature of kerogen, the solid kerogen is decomposed into liquid shale oil and hydrocarbon gas

[19]

, which release and discharge through the pore and fracture

channels. In addition, pores will expand and crack under the effect of thermal stress resulting in the formation of new larger pores. The original larger pores will generate fracture due to the collapse of the pore walls and the connection of adjacent pores, which makes the pore and fracture channels increase. Therefore, the porosity 19

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increases significantly, and the TNF and MAF also increases sharply. In the high temperature range, the pyrolysis reaction of kerogen inside the oil shale is basically completed

[33]

. But a series of physical and chemical interactions also can

happen, which include the dehydration of some crystalline water and decomposition of the residual inorganic minerals, and some fixed carbon undergoes coking carbonization to form shale semicocke

[15, 33-34]

. Although the growth of pore volume

slows down, the porosity and TNF and MAF continue to increase because thermal cracking intensifies under high temperature. ② Same temperature and different pressures In the unconstrained state, the specimen is not affected by external pressure, and the pore volume is free to expand. However, when the specimen is applied by some certain external pressure, some of pores will close and it will produce irreversible plastic compaction deformation resulting in the decrease of pore volume and porosity [35]

.

When the temperature is lower than 300℃, rise of temperature will only promote the evaporation of water and emission of volatile components, so the value of external pressure has little effect on the volume of pore and fractures. On the contrary, when the temperature is higher than 300℃, pyrolysis reaction is enhanced with the rise of temperature, and a large number of new fractures are formed due to the thermal cracking[36], which results in the increase of the pore and fractures’ volume. At the pyrolysis stage with low pressure, the oil shale specimen mainly produces plastic compaction deformation, and lower external pressure has a smaller effect on 20

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the formation of fracture, so porosity and TNF and MAF shows a slight decreasing trend. At the pyrolysis stage with high pressure, higher external pressure further promote the expansion and extension of the fracture caused by thermal cracking, and change pore and fracture structures thoroughly. Its change is much greater than the influence of the enhanced plastic compression effect on the specimen with the increase of external pressure. Overall, pore volume and the fracture distributions present a gradually increasing trend. ③ Coupling effect of temperature and pressure The coupling effect of temperature and pressure plays a positive role in promoting on pyrolysis and thermal cracking of oil shale specimens. With the increase of temperature and pressure, the degree of kerogen pyrolysis and thermal cracking are enhanced, and a large number of pore and fracture channels are generated. In addition, high external pressure further promotes the expansion and extension of fractures. Consequently, the total pore volume of the specimen increases, porosity increases, also the total number and the maximum aperture of fracture increases.

4. CONCLUSIONS Oil shale has changed from a dense, low porosity material at room temperature to a material in which the pore and fractures are highly developed after the pyrolysis under high temperature and high pressure. More than 0.26 g is lost and the mass of the remaining residual decreases fastest from 300 to 500 ℃. Porosity and pore size distributions are the main parameters to describe the pore structure. The total number and the maximum aperture of fractures are reflecting the 21

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development degree of fractures. After high temperature and high pressure pyrolysis, the pore and fracture structures have undergone a fundamentally irreversible change. The effects of temperature on the pore and fracture structures of oil shale are very significant. With rising temperature, the pore and fracture’s volume expansion and increase, and a large number of newborn pore and fracture channels are generated. All parameters are all increasing with the rise of temperature. Pressure has inhibited the evolution of the pore and fracture structures to some certain extent. By the combined effect with plastic compaction deformation and thermal cracking, with increasing external pressure, porosity and the development degree of fractures decrease firstly and then increase. 5MPa is an important inflection point. Under the coupled effect of temperature and pressure, the pore volume, porosity and the peak value of pore diameter increase gradually. Both the total number and the maximum aperture of fractures increase significantly. At 600℃ and 15 MPa, all parameters reaches the maximum. High temperature and high pressure can better promote the pyrolysis and thermal cracking of oil shale, to improve its permeability and produce more shale oil and gas.

ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (Grants No. 51604182, 51225404 and U1261102).

REFERENCES (1) Fletcher, T. H.; Gillis, R.; Adams, J.; Hall, T.; Mayne, C. L.; Solum, M. S.; 22

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Pugmire, R. J. Characterization of Macromolecular Structure Elements from a Green River Oil Shale, II. Characterization of Pyrolysis Products by13C NMR, GC/MS, and FTIR. Energy & Fuels 2014, 28(5), 2959-2970. (2) Solum, M. S.; Mayne, C. L.; Orendt, A. M.; Pugmire, R. J.; Adams, J.; Fletcher, T. H. Characterization of Macromolecular Structure Elements from a Green River Oil Shale, I. Extracts. Energy & Fuels 2014, 28(1), 453-465. (3) Hillier, J. L.; Fletcher, T. H.; Solum, M. S.; Pugmire, R. J. Characterization of Macromolecular Structure of Pyrolysis Products from a Colorado Green River Oil Shale. Industrial & Engineering Chemistry Research 2013, 52(44), 15522-15532. (4) Sun, Y.; Bai, F.; Liu, B.; Liu, Y.; Guo, M.; Guo, W.; Wang, Q.; Lü, X.; Yang, F.; Yang, Y. Characterization of the oil shale products derived via topochemical reaction method. Fuel 2014, 115, 338-346. (5) Dyni, J. R. Geology and resources of some world oil-shale deposits. Oil Shale 2003, 20(3), 193-252. (6) World Energy Council. World Energy Resources: 2013 Survey, 2013. http://www.worldenergy.org/publications/2013/world-energy-resources-2013-survey/ (accessed April 13, 2017). (7) Li, S.Y. The developments of Chinese oil shale activities. Oil Shale 2012, 29(2), 101-102. (8) Soone, J.; Doilov, S. Sustainable utilization of oil shale resources and comparison of contemporary technologies used for oil shale processing. Oil Shale 2003, 20(3S), 311-323. 23

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(9) Jiang, X. M.; Han, X. X.; Cui, Z. G. New technology for the comprehensive utilization of Chinese oil shale resources. Energy 2007, 32(5), 772-777. (10) Jaber, J.O.; Probert, S.D. Exploitation of Jordanian oil-shales. Applied Energy 1997, 58(2), 161-175. (11) Qian, J. L.; Wang, J. Q.; Li, S. Y. Oil shale development in China. Oil Shale 2003, 20(3S), 356-359. (12) Vinegar, H. Shell’s in-situ conversion process. 26th Oil Shale Symposium, Colorado School of Mines, October 16–18, 2006; Colorado Energy Research Institute: Colorado, USA, 2006. (13) Symington, W.A. Olgaard, D.L. Otten, G.A. Phillips, T.C. Thomas, M.M. Yeakel, J. D. ExxonMobil’s Electrofrac Process for in situ oil shale conversion. 26th Oil Shale Symposium, Colorado School of Mines, October 16–18, 2006; Colorado Energy Research Institute: Colorado, USA, 2006. (14) Looney, M. D., Polzer, R., Yoshioka, K., Minnery, G. Chevron’s plans for rubblization of Green River Formation oil shale (GROS) for chemical conversion. 31st Oil Shale Symposium, Colorado School of Mines, October 17–19, 2011; Oil Shale Technology and Research: Colorado, USA, 2011. (15) Zhao, J.; Yang, D.; Kang, Z.; Feng, Z. A Micro-CT study of changes in the internal structure of Daqing and Yan’an oil shale at high temperatures. Oil Shale 2012, 29(4), 357-367. (16) Zhang, N.; He, M.; Zhang, B.; Qiao, F.; Sheng, H.; Hu, Q. Pore structure characteristics and permeability of deep sedimentary rocks determined by mercury 24

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intrusion porosimetry. Journal of Earth Science 2016, 27(4), 670-676. (17) Kang, Z, Yang, D, Zhao, Y, Hu, Y. Thermal cracking and corresponding permeability of Fushun oil shale. Oil Shale 2011, 28(2), 273-283. (18) Tiwari, P.; Deo, M.; Lin, C. L.; Miller, J. D. Characterization of oil shale pore structure before and after pyrolysis by using X-ray micro CT. Fuel 2013, 107, 547-554. (19) Han, X.; Jiang, X.; Yu, L.; Cui, Z. Change of Pore Structure of Oil Shale Particles during Combustion. Part 1. Evolution Mechanism. Energy & Fuels 2006, 20(6), 2408-2412. (20) Han, X.; Jiang, X.; Cui, Z. Change of Pore Structure of Oil Shale Particles during Combustion. 2. Pore Structure of Oil-Shale Ash. Energy & Fuels 2008, 22(2), 972-975. (21) Sun, L.; Tuo, J.; Zhang, M.; Wu, C.; Wang, Z.; Zheng, Y. Formation and development of the pore structure in Chang 7 member oil-shale from Ordos Basin during organic matter evolution induced by hydrous pyrolysis. Fuel 2015, 158, 549-557. (22) Bai, F.; Sun, Y.; Liu, Y.; Guo, M. Evaluation of the porous structure of Huadian oil shale during pyrolysis using multiple approaches. Fuel 2017, 187, 1-8. (23) Saif, T.; Lin, Q.; Singh, K.; Bijeljic, B.; Blunt, M. J. Dynamic imaging of oil shale pyrolysis using synchrotron X-ray microtomography. Geophysical Research Letters 2016, 43(13), 6799-6807. (24) Saif, T.; Lin, Q.; Bijeljic, B.; Blunt, M. J. Microstructural imaging and 25

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characterization of oil shale before and after pyrolysis. Fuel 2017, 197, 562-574. (25) Washburn, E. W. The Dynamics of Capillary Flow. Phys. Rev 1921, 17, 273-283. (26) Yu, Y.; Liang, W.; Hu, Y.; Meng, Q. Study of micro-pores development in lean coal with temperature. International Journal of Rock Mechanics and Mining Sciences 2012, 51, 91-96. (27) Xue-gui, S.; Xian-jie, D.; Hong-hu, Y.; Ben-kui, L. Research of the thermal stability of structure of resin anchoring material based on 3D CT. International Journal of Adhesion and Adhesives 2016, 68, 161-168. (28) Bai, F.; Guo, W.; Lü, X.; Liu, Y.; Guo, M.; Li, Q.; Sun, Y. Kinetic study on the pyrolysis behavior of Huadian oil shale via non-isothermal thermogravimetric data. Fuel 2015, 146, 111-118. (29) Syed, S.; Qudaih, R.; Talab, I.; Janajreh, I. Kinetics of pyrolysis and combustion of oil shale sample from thermogravimetric data. Fuel 2011, 90, (4), 1631-1637. (30) Wang, W.; Li, S.; Li, L.; Ma, Y.; Yue, C.; He, J. Pyrolysis characteristics of a North Korean oil shale. Petroleum Science 2014, 11(3), 432-438. (31) Yang, L.; Yang, D.; Zhao, J.; Liu, Z.; Kang, Z., Changes of Oil Shale Pore Structure and Permeability at Different Temperatures. Oil Shale 2016, 33(2), 101-110. (32) Hodot, B. B. Outburst of Coal and Coalbed Gas; China Industry Press: Beijing, China, 1966, 318. 26

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(33) Kang, Z.; Zhao, J.; Yang, D.; Zhao, Y.; Hu, Y., Study of the Evolution of Micron-scale Pore Structure in Oil Shale at Different Temperatures. Oil Shale 2017, 34(1), 42-54. (34) Sedman, A., Talviste, P., Kirsimäe, K. The study of hydration and carbonation reactions and corresponding changes in the physical properties of co-deposited oil shale ash and semicoke wastes in a small-scale field experiment. Oil Shale 2012, 29(3), 279–294. (35) Li, B.; Wong, R., Effect of Heating in Steam-Based-Oil-Recovery Process on Deformation of Shale: A Compositional Thermal Strain Model. Journal of Canadian Petroleum Technology 2015, 54(1), 26-35. (36) Pan, Y.; Wang, S.; Zhang, Y.; Yang, S., The Experimental Research of the Effect of Heating Temperature and Heating Time for Oil Shale Crack. Journal of the Chemical Society of Pakistan 2017, 39(2), 177-182.

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Table 1. Physical properties of specimens of Fushun oil shale Proximate analysis(wt.%, ad) Moisture

Ash

Volatiles

2.36

73.34

22.88

Ultimate analysis(wt.%, ad) Fixed

carbon 3.78

C

H

O

N

S

16.59

2.57

6.13

0.71

0.66

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Table 2. Main pore characteristics of oil shale specimens obtained by MIP analysis Effective

Average pore

porosity

size

(%)

(nm)

0.1

3.39

27.09

0.022

3.250

0.1

4.02

34.04

0.025

2.895

A2

5

3.35

23.61

0.020

3.401

A3

10

3.70

27.99

0.024

3.415

A4

15

4.31

29.61

0.028

3.472

0.1

7.18

29.65

0.042

5.661

B2

5

6.05

25.51

0.033

5.113

B3

10

6.74

24.81

0.034

5.477

B4

15

7.10

29.51

0.032

2.707

0.1

10.44

46.61

0.064

8.521

C2

5

8.04

44.57

0.048

4.281

C3

10

10.01

55.16

0.058

4.174

C4

15

11.76

53.95

0.067

4.938

0.1

24.64

43.37

0.134

12.369

D2

5

22.16

39.57

0.139

13.568

D3

10

26.51

32.95

0.157

18.129

Sample

Temperature

Pressure

number

(℃)

(MPa)

O

20

A1

100

B1

C1

D1

200

300

400

D4

Total pore volume (cm3/g)

Specific surface area (m2/g)

15

28.17

50.21

0.174

13.847

0.1

42.45

86.44

0.289

13.318

E2

5

39.05

44.26

0.200

18.008

E3

10

43.65

49.79

0.262

19.929

E4

15

44.43

91.22

0.279

11.499

0.1

43.14

53.50

0.250

20.698

F2

5

41.84

79.72

0.262

13.088

F3

10

44.79

77.13

0.276

14.298

F4

15

45.84

67.44

0.271

15.938

E1

F1

500

600

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Table 3. Classification of fractures Type

Ratio

Range

Flat-microfracture

A/L10μm and 1000.1

A>10μm and 2000.1

A>10μm and L>1000μm

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Pressure sensor Valve

Gas booster pump

Temperature sensor

Gas tank Oil shale sample

Figure1. Schematic diagram of pyrolysis device

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Heater

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Figure 2. Photographs of oil shale specimens at 0.1 MPa after pyrolysis

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100 95

Mass(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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90 85 0.1MPa 5MPa 10MPa 15MPa

80 75 100

200

300

400

500

600

Temperature(℃)

Figure 3. Mass ratio of oil shale specimens after pyrolysis at different temperatures and pressures

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Figure 4. Porosity of oil shale specimens at different temperatures and pressures

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0.35

20℃ 100℃ 200℃ 300℃ 400℃ 500℃ 600℃

0.35 0.30

100℃ 300℃ 500℃

0.08

dV/dlogD pore volume(cm3/g)

dV/dlogD pore volume(cm3/g)

0.40

0.06 0.04 0.02

0.25

0.00 1

10

100

1000

0.20 0.15 0.10 0.05

0.30

200℃ 400℃ 600℃

0.05 0.04 0.03

0.25

0.02 0.01

0.20

0.00 1

10

100

1000

0.15 0.10 0.05 0.00

0.00

1

10

100

1000

10000

100000

1

10

Pore Diameter(nm)

100

100℃ 300℃ 500℃

0.45

0.40

200℃ 400℃ 600℃

100℃ 300℃ 500℃

0.05 0.04

0.40

0.03

0.35

0.02 0.01

0.30

0.00 1

0.25

10000

100000

(b)

dV/dlogD pore volume(cm3/g)

0.50

1000

Pore Diameter(nm)

(a)

dV/dlogD pore volume(cm3/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10

100

1000

0.20 0.15 0.10 0.05

0.35

200℃ 400℃ 600℃

0.05 0.04 0.03

0.30

0.02 0.01

0.25

0.00 1

0.20

10

100

1000

0.15 0.10 0.05 0.00

0.00

1

10

100

1000

10000

100000

1

10

Pore Diameter(nm)

100

1000

10000

100000

Pore Diameter(nm)

(c)

(d)

Figure 5. Pore size distributions of oil shale specimens obtained by MIP at different temperatures and pressures. (a) 0.1 MPa (b) 5 MPa (c)10 MPa (d)15 MPa

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Micropore

100

Volumetric proportions(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Transition-Pore

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Mesopore

Macropore

80

60

40

20

0

20

100

200

300

400

500

600

Temperature(℃)

Figure 6. Volumetric proportions of four types of pore for oil shale specimens at different temperatures and pressures

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Figure 7. CT scan images of some oil shale specimens at different temperatures and pressures

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Figure 8. Fractures of oil shale specimens extracted from CT scan images at 0.1 MPa after pyrolysis

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900 Fracture Microfracture Flat-Microfracture

800

Number of fractures

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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700 600 500 400 300 200 100 0 100

200

300

400

500

600

Temperature(℃)

Figure 9. Number of three types of fractures of oil shale specimens at different temperatures and pressures

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0.40 0.1MPa 5MPa 10MPa 15MPa

0.35

Maximum aperture (mm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.30 0.25 0.20 0.15 0.10 0.05 0.00 100

200

300

400

500

600

Temperature(℃)

Figure 10. The maximum aperture of fractures of oil shale specimens at different temperatures and pressures

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