Evolution of Shale Microstructure under Microwave Irradiation

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Evolution of shale microstructure under microwave irradiation stimulation Guozhong Hu, Chao Sun, Jinxin Huang, Guang Xu, and Jieqi Zhu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03299 • Publication Date (Web): 20 Oct 2018 Downloaded from http://pubs.acs.org on October 27, 2018

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Evolution of shale microstructure under microwave irradiation stimulation Guozhong Hu a, *, Chao Sun a, Jinxin Huang b, Guang Xu b, Jieqi Zhu a a

School of Mines, Key Laboratory of Deep Coal Resource Mining, Ministry of Education of China, China

University of Mining and Technology, 221116 Xuzhou, Jiangsu, China b

Department of Mining Engineering and Metallurgical Engineering, Western Australian School of Mines,

Curtin University, 6430 Kalgoorlie, WA, Australia ABSTRACT As the low permeability of shale gas reservoirs limit it’s the large-scale commercial development, the microwave irradiation stimulation is proposed as a new approach to increase the shale gas reservoir’s production. In this study, in order to investigate the microwave’s effect on shale pore structure, experiments were performed using nuclear magnetic resonance to compare the pore size distribution of shale samples before and after microwave treatment. The results showed that the numbers as well as sizes of mesopores and macropores in shale samples increased, while the number of micropores decreased after the microwave irradiation. This is benefit to enhance shale gas flow in reservoirs. In addition, the moisture within the shale samples increased its ability to absorb microwave energy, leading to a positive effect on the increase of breathable pores and microfractures. When the accompanying thermal stress and vapour pressure generated inside the shale is larger than the cementation strength between the shale mineral particles, the shale cracks and forms microcracks until it is destroyed. The experimental results demonstrated that the microwave stimulation is effective in expanding breathable pores and inducing fissures in shale samples; it may therefore have applications in enhancing shale gas reservoirs. KEYWORDS shale, pore structure, microwave irradiation stimulation, microwave-induced fracturing, shale gas

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1.

INTRODUCTION Shale gas is an unconventional natural gas trapped within shale formations. Owing to its advantages

of efficiency and environmental friendliness, it has been exploited and developed as a major strategic replacement energy resource.1 According to a report of the World Resources Institute, China holds 36.1 trillion cubic metres of untapped shale gas, which is approximately eight times larger than that of conventional natural gas reserves; therefore, it has large potentials for commercial development.2 Unlike conventional natural gas reservoirs, the permeability of shale gas reservoirs is very low,3-6 which limit its large-scale commercial development. The most commonly used method to address this challenge is hydraulic fracturing.7 However, the extensive use of hydraulic fracturing led to various environmental problems, including a large consumption of fresh water resources and intensified the water supply stress, contamination of surface- and ground-water, and possible induction of earthquakes, etc.8,9 In order to avoid these environmental impacts during production, some new stimulation technologies were proposed to replace hydraulic fracturing, such as nitrogen foam fracturing,5 nitrogen gas injection,10,11 hightemperature combustion technology12 and supercritical CO2 fracturing technology.13,14 However, these technologies have difficulties in injected gas transport and storage, and the treatment costs are potentially high. Therefore, the search for a new, environmentally friendly, and efficient shale gas production technology has become a key issue in the global shale gas development research. In recent years, with its unique heating characteristics, the microwave irradiation technology has been successfully used in the exploitation of heavy oil15,16 and underground oil shale.17,18 As the microwave irradiation technology is effective, economical, and convenient, it was proposed as a new method of unconventional oil and gas development with large development prospects. Previous studies suggested that microwave radiation is feasible and effective in the reformation of coalbed methane reservoirs and increase of their permeability.19-21 As shale gas reservoirs are very similar to coalbed methane reservoirs in terms of pore structure and reservoir properties, microwave irradiation technology seems to have great potential in stimulating shale gas reservoirs. A fundamental factor in the microwave-induced fracturing is that, owing to the heterogeneous distribution of the moisture and minerals within the shale sample, the microwave


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irradiation leads to non-uniform heating and thermal expansion, and consequently to thermal fractures.22,23 In addition, the vapour pressure owing to the moisture evaporation contributes to fissure formation.6 Laboratory experiments have been performed to verify the microwave irradiation stimulation technology’s feasibility. Kumar at al. reported observations of fracture generation and aperture enhancement in coal with microwave exposure under isotropic stress conditions.24 Hu et al. indicated that microwave irradiation provides positive effects by changing the methane adsorption and improving the methane diffusion and gas penetrability properties of coal samples.19 Li et al. proposed an exploratory study on the improvement of coal porosity and permeability through microwave treatment.20 Hong et al. evaluated the effect of microwave heating on the fractal dimension of coal cores by a nuclear-magneticresonance (NMR) experiment.25 Wang et al. investigated the effects of microwave heating on the petrophysical properties of sandstone samples, including porosity, permeability, structure and pore size distribution of tight sandstones.26 Although microwave’s effects on coal and sandstone samples have been investigated preliminarily, only a few studies focused on shale samples. The pore structure variations such as quantity, scale, and connectivity of the shale samples under microwave irradiation are not yet fully understood. The aim of this study was to investigate the evolution of the shale pore structure under microwave irradiation. For this purpose, NMR was employed to analyse the pore size distribution of shale samples before and after microwave treatment. In addition, the surface microtopographies of a shale slice were observed using scanning electron microscopy (SEM) before and after microwave treatment. Based on the experimental results, the mechanism of microwave irradiation stimulation on shale samples was presented and potential field application was proposed.

2.

EXPERIMENTAL

2.1 Sample preparation For the microwave irradiation tests, shale samples were drilled from the same rock from Qianjiang Basin in China and processed into standard cylinders (diameter: 25mm; height: 50mm). The physical


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properties of the shale samples are shown in Table 1. To ensure there are no original fissures on the sample surfaces, the samples were carefully examined, and 12 perfect samples were selected for the P-wave test. According to the P-wave test results, the rock samples with similar wave velocity were numbered and attributed to the same group, with a total of 3 groups and 4 samples in each group. Three other small shale slices with observation areas of approximately 1 cm2 were prepared for SEM tests. The 2g shale powder was prepared for X-ray diffraction (XRD) tests. Table 1. Physical property of the shale sample Rock type

Porosity (%)

Density (g/cm3)

Compressive strength (MPa)

Tensile strength (MPa)

Shale

5.41

2.74

91.86

3.59

2.2 NMR detection 2.2.1 NMR test principle Commonly used methods to investigate pore structures of rock samples include computed tomography (CT), SEM, mercury injection capillary pressure (MICP), low-temperature nitrogen adsorption, and NMR.19,26 However, the resolution of CT is too low to observe details of microstructures, while MICP and N2 adsorption can significantly damage the samples.26 With the advantages of a wide pore-size range, effectiveness and non-destructiveness measurement, NMR is used to characterise the pore size distributions of the shale samples in this study. As hydrogen contains magnetic moments, pores filled with water are detected by the NMR relaxation signals. The relative amplitudes of fluids within the pore structures were revealed in the transverse relaxation-time (T2) spectrum, which indirectly reflects the pore size distribution of the measured sample.27 T2 is defined as: 1

𝑇2 ≈ 𝜌(𝐴/𝑉)𝑝𝑜𝑟𝑒

(1)

where 𝜌 is the surface relaxation intensity (mm/ms), A represent the surface area of the pore, and V denotes the pore volume. If we suppose that the pores are spherical, the relation between T2 and radius of the pores is:


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1

𝑇2 ≈ 𝜌(3/𝑟 )𝑝𝑜𝑟𝑒

(2)

where r is the radius of the pores (mm). The size of pores increases with the T2. According to their sizes, the pores can be divided into micropores (T2 < 10 ms), mesopores (10 ms < T2 < 100 ms), and macropores (100 ms < T2). From the T2 distribution curve,28 the amplitude of the signal corresponds to the number of pores; that is, a larger amplitude, implies more pores. The T2 spectrum area is proportional to the volume of the pores (a larger area, implies a larger volume of the pores). Therefore, if the pore structure changes, the distribution characteristics of the T2 curve changes accordingly. 2.2.2 The test system Magnet box

Radio frequency

Monitor

Spectrometer Figure 1. NMR test system.

In the experiment, an NMR system with a main frequency of 12 MHz (MicroMR12-025V, Suzhou Niumag Analytical Instrument Corporation) was used. This system (shown in Figure 1) consists of NMR magnets, NMR test and analysis software, and electronic control system. In the test, the magnetic field strength was 0.28 T, the probe diameter was 25.4 mm, and the temperature in the control crate was 35 °C. 2.2.3 The test procedures The shale samples were placed into a vacuum-drying oven with a temperature of 70 °C. After 24 h of vacuum drying, the shale samples were placed in a vacuum-pressure saturating device until each sample was fully saturated. An NMR test was then performed on these saturated samples. After the first NMR detection, the shale samples will be treated as different saturabilities according to the test plan, and subjected to microwave irradiation. After microwave irradiation, all the samples will be


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saturated again, and performed an NMR test one more time. 2.3 Microwave irradiation The microwave irradiation of the shale samples was performed in a microwave oven, as shown in Figure 2. The oven had a continuously adjustable power in the range of 0 to 2.0 kW and fixed frequency of 2.45 GHz. In this case, all the shale samples were treated under microwave irradiation at 2.0 kW for 30s.

(A) Principle structure graph

(B) Prototype

Figure 2. Microwave oven for experiment.

In order to investigate the effect of the microwave irradiation on the saturated shale, samples with various saturabilities were prepared before microwave treatment. As all the shale samples were fully saturated after the NMR test, they were dried according to the procedure described in Table 2. The shale samples in Group I were completely dried with a vacuum-drying oven, those in Group II remained saturated, while those in Group III were dried for 1, 2, 4 and 8 h. Table 2. Drying time of saturated shale sample Group I

Number Drying time (h)

Group II

Group III

1#

2#

3#

4#

5#

6#

7#

8#

9#

10#

11#

12#

24

24

24

24

0

0

0

0

1

2

4

8


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2.4 SEM tests SEM is a useful method for microscopic morphology observations in scientific research. Through a proper adjustment of the magnification, the distribution of mineral particles and microcracks in shale samples can be observed.28 In this study, the SEM tests were performed with a field-emission environmental SEM system (QuantaTM 250, FEI Corporation, USA). This instrument operates at an accelerating voltage in the range of 0.2–30 kV and magnification in the range of 6 to 1,000,000. Before and after microwave irradiation, SEM was used to measure the surface microtopographies of the shale slices. The results of the two SEM experiments were compared and analysed to infer the effect of the microwave irradiation on the microstructures of the shale samples. 2.5 XRD tests X-ray diffraction (XRD) analysis was carried out on shale powder < 325 mesh using a Bruker D8 ADVANCE diffractometer (Co Kα-radiation, 45 kv, 35 mA).

3.

RESULTS AND DISCUSSION

3.1 Mineral composition and initial pore structure characteristics of the shale samples 3000

2500

Q Intensity (a.u.)

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2000

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Q-quartz; F-feldspar; C-calcite; Py-pyrite; M-muscovite; I-illite Figure 3. XRD result of shale powder.

Mineral composition of the shale show that the shale is composed by the quartz, feldspar, calcite,


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muscovite, illite and pyrite, as shown in Figure 3. In order to investigate the initial pore structures of the shale samples, NMR tests were performed for the saturated conditions of the samples before microwave treatment. The samples had similar pore size distributions; a typical result is shown in Figure 4. As can be seen, there are two peaks in the T2 spectrum of the shale sample. The size of most of the pores is approximately 2 nm; therefore, they can be classified as micropores. Besides, there is a certain amount of mesopores, while the number of macropores is negligible. 1000

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600

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Figure 4. NMR T2 spectrum of shale sample 3#.

Shale is a type of special porous medium and it exhibits many characteristics related to micropores, including porosity, pore diameter distribution, and pore volume.29 Micropores, as an important place for gas adsorption, have a large specific surface area and strong adsorption capacities for gas, whereas mesopores and macropores are the main channels for gas diffusion and permeation.30 Shale gas reservoir fracturing technology is used mainly to increase flowing channels of gas in reservoirs, by increasing the numbers of mesopores, macropores, and microfractures. As a result, we define mesopores and macropores as breathable pores in this paper. 3.2 Effects of the microwave irradiation on the pore structures of the dried shales As mentioned above, the shale samples in Group I were thoroughly dried before the microwave treatment. The T2 spectra of these shale samples before and after the microwave irradiation were similar to


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each other, as shown in Figure 5. 1000

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Figure 5. NMR T2 spectra of dried samples before and after microwave irradiation.

In order to quantify the evolution of shale microstructure under microwave irradiation stimulation, the surface integrals of the T2 spectra of the dried shale samples were calculated, as shown in Table 3; and the increment (∆𝑆) of T2 spectrum area proportion for macropores and breathable pores & microfractures proportion after microwave irradiation is defined as ∆𝑆 = 𝑆𝑎 ― 𝑆𝑏

(3)

where Sb, Sa is the T2 spectrum area proportion of pores before and after microwave irradiation. As shown in Table 3, the proportion of the first peak is reduced by approximately 1.075% (on average), which indicates that the number of micropores slightly decreased after the microwave irradiation. The increase of the second-peak area suggests that there is a significant increase (from 18.37% to 36.11%) in the number of mesopores.


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Table 3. The T2 spectrum area of dried samples before and after microwave irradiation Sample 1# 2# 3# 4#

Total area Before 37746 40169 38980 39603

after 36590 39509 38512 39374

Micropores Sb 0.964 0.951 0.951 0.952

Sa 0.951 0.941 0.942 0.943

∆S -1.35% -1.05% -0.95% -0.95%

Breathable pores and microfractures Sb Sa ∆S 0.036 0.049 36.11% 0.049 0.059 20.41% 0.049 0.058 18.37% 0.048 0.057 18.75%

In addition, the second peak shifts to larger transverse relaxation time, which corresponds to larger pore sizes. Considering the high compactness, there were hardly any macropores (relaxation time larger than 100 ms) in the shale sample before microwave irradiation. However, the maximum transverse relaxation time reveals that macropores appeared after microwave irradiation. As shown in Figure 5, the maximum transverse relaxation time of the sample 4# significantly increased (from 83.09 ms to 382.74 ms). Sample 1# (from 77.52 ms to 126.03 ms), sample 2# (from 89.07 ms to 102.34 ms), and sample 3# (from 89.07 ms to 144.81 ms) also showed an increasing trend in the maximum transverse relaxation time. It can be concluded that the micropores and mesopores expanded and merged into larger pores and fractures upon the microwave treatment, leading to the increase of the numbers and sizes of mesopores and macropores. Table 3 shows an average decrease of 1.20% in the spectrum area, which may be attributed to the following factors. Some of the micropores and mesopores expanded and connected with each other. In other words, many small pores combined and became a larger pore. The combination of small pores significantly increases the permeability of the shale sample; however, a decreasing trend of the spectrum area is observed in the NMR tests. It should be mentioned that there were new visible fissures on the surfaces of the shale samples after the microwave irradiation; no significant difference in the T2 spectrum was observed when T2>1,000, as the number of new fissures was too small to be noticed compared to the number of micropores and mesopores. In addition, a small part of the shale sample (less than 1%) fell out during the microwave treatment, leading to the decrease in the total pore number. Given the above, the variation tendency of micropores, mesopores and macropores is approximately


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consistent, although the amplitude and area changes of T2 is not exactly same in different samples. This is caused by the samples heterogeneity as the mineral composition and structure is different between samples. 3.3 Effects of the microwave irradiation on the pore structures of the saturated shale samples The saturated shale samples (Group II) were treated under microwave irradiation at 2.0 kW for 30 s. However, two of them broke along the bedding plane and were in a brittle failure state; therefore, they could not be employed for the further NMR testing. When the shale is in a microwave field, microwave energy is absorbed by the shale generating power loss, resulting in thermal effect of the microwave to the shale. A larger power loss of the microwave implies a stronger thermal effect to the shale. The power loss of the microwave in a unit volume of the shale is:32 '' P  2 f  0  eff E2

(4)

where P is the power density of the microwave absorption by the shale sample (W/cm3), f is microwave frequency (Hz), ε0 is the dielectric constant in vacuum (8.854×10−14 F/cm), ε′′eff is the dielectric loss factor, and E is the electric field strength (V/cm).

(a) 1#

(b) 3#

(c) 7#

(d) 8#

Figure 6. IRT pictures of shale samples under microwave radiation at 2.0 kW for 30 s.

As the dielectric constant of water in shale is significantly larger than that of the minerals in the shale, the increase of the moisture content increases the dielectric constant of the saturated shale samples. As shown in eq 4, the power loss of the microwave is proportional to the dielectric constant. Therefore, a larger saturability of the shale corresponds to a larger shale expansion deformation and thermal stress. When the thermal stress generated inside the shale is larger than the cementation strength between the shale mineral


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particles, the shale cracks forming microcracks until it is destroyed. The infrared temperature (IRT) photography results of the other two samples showed that their average temperatures were significantly higher than those of the dried shale samples, as shown in Figure 6. This suggests that the saturated shale samples have a better ability to absorb microwave energy. The moisture within the shale sample evaporates, and the accompanying vapour pressure crushes the sample upon the thermal stress. 1000

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Figure 7. NMR measurements of fully saturated samples before and after microwave irradiation. 1000


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Figure 8. NMR T2 spectra of partially saturated samples before and after microwave irradiation. Table 4. The T2 spectrum area of saturated samples before and after microwave irradiation Sample 7# 8# 9# 10# 11# 12#

Total area Before 38435 41995 37164 39503 42454 37812

After 36407 35735 32061 39004 39219 32606

Micropores Sb 0.947 0.975 0.948 0.951 0.955 0.945

Sa 0.934 0.957 0.92 0.932 0.939 0.924

∆S -1.37% -1.85% -2.95% -2.00% -1.68% -2.22%

Breathable pores and microfractures Sb Sa ∆S 0.053 0.066 24.53% 0.025 0.043 72.00% 0.052 0.08 53.85% 0.049 0.068 38.78% 0.045 0.061 35.56% 0.055 0.076 38.18%

As shown in Figures. 7 and 8, the proportion of micropores of the saturated samples slightly decreased after the microwave irradiation, while the variation of proportion is generally larger than that for the dried samples. Table 4 shows that the proportions of breathable pores and microfractures of the saturated samples increased significantly more than those for the dried samples. Compared with the T2-spectrum-area changes of the dried samples after the microwave irradiation (with respect to that before the irradiation) (as shown in Table 3), the variation rates of the proportions of the saturated samples' breathable pores and microfractures significantly increased (from 24.53% to 72.00%), while the variation rates of the dried samples increased slightly (from 18.37% to 36.11%). This indicated that the saturated samples achieved a better improvement of the proportion of breathable pores and microfractures. This can be explained as two micropores or micropores and macropores connect with each other leading to the increase of the number of larger-size pores. The main change in the pore structure within the shale is the increase of the number of macropores and microfractures. The main form of damage of the shale at this time is the development and expansion of macropores and microfractures. As shown in Figure 8, the number of micropores decreased, while the numbers and sizes of mesopores and macropores increased significantly after the microwave treatment, as shown in Table 4. It is worth noting that a third peak, whose relaxation time is approximately 310.787 ms, appears in the T2 spectrum of the sample 12#. Even though its amplitude is small compared to that of the micropores, the pore structures


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consisting of thousands of micropores and mesopores have a significant effect on the porosity. Moreover, it has been reported that a slight increase in porosity leads to a large improvement in permeability.26 Therefore, the permeability of the sample 12# should be significantly increased after the microwave treatment. In summary, the moisture content within the shale sample has positive effect on the microwaveinduced fracturing. The moisture within the sample increases its ability to absorb microwave energy; the accompanying thermal stress and vapour pressure help the microwave-induced fracturing. When the thermal stress in the shale is larger than the bonding strength between the shale mineral grains, the shale samples crack generating microfractures until they are destroyed. 3.4 Mechanism of microwave irradiation on shale pore structure Due to wave-particle duality, the microwave has the characteristics of penetration, reflection, and absorption as an electromagnetic wave. When the shale sample is under microwave irradiation, the internal part and surface of the shale can absorb microwave energy to generate heat, yielding the integral heating effect. As the minerals within shale have different microwave-absorption abilities, the heating rates of the minerals within the shale differ, as shown in Figure 6. Therefore, apparent temperature differences appear at the interface of different minerals, leading to differences in the mineral particle deformations, and consequently to thermal stress. The effect of the microwave irradiation on the shale pore structure is mainly reflected in two aspects. First, the microwave heating changes the mechanical properties of the shale. The rapid temperature increase of the shale causes rapid expansion and deformation of the shale mineral particles and their cements; therefore, the shale will be more vulnerable to brittle failure. When the thermal stress generated at the interface of different minerals exceeds the cementation strength, crack damage emerges in the shale, as shown in Figure 9. Second, the moisture within the shale samples increases its ability to absorb microwave energy; the accompanying vapour pressure help the microwave fracturing. As revealed in Figure 9A, the surface of the shale sample before the microwave treatment is basically smooth and flat. On the contrary, several horizontal and vertical fissures appear after the microwave irradiation, as shown in Figure 9B.


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(A)

(B) Crack Crack

Figure 9. The images of shale sample surface before and after microwave irradiation. (A)

(B)

(C)

(D)

(E)

(F)


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Figure 10. SEM images of shale samples before (A, C, E) and after (B, D, F) microwave irradiation (× 1000).

In order to investigate the more-detailed variations on the shale samples’ surfaces, the surface microtopographies of a shale slice before and after the microwave treatment were observed with SEM. The results can be qualitatively analyzed to deduce the influence of microwave irradiation on the microstructures of the shale samples. As shown in Figures. 10A, 10C, and 10E, there are many voids on the surface of the original shale sample, while visible fissures can hardly be observed. There are also many small particles accumulated on the surface, helpful to fine pore networks.33 However, owing to the thermal stress and vapour pressure, cracks and fissures appear upon the microwave treatment, as shown in Figures. 10B, 10D, and 10F. In addition, the number and size of the accumulated particles exhibited a decreasing trend, caused by the thermal decomposition of minerals within small holes. Consequently, the number of fine pore networks decreases, while a fissure system develops.

4.

POTENTIAL APPLICATIONS OF THE MICROWAVE IRRADIATION FOR SHALE GAS STIMULATION Shale gas reservoir fracturing is the main method in shale gas development. With the progress of the

shale gas development, many countries imposed stricter requirements on the fracturing technology. It should be able to avoid damages to the shale gas reservoir, reduce the consumption of water resources, and decrease the negative effect to the environment. In addition, the fractured volume of shale gas reservoirs has attracted a large interest.34 In recent years, scholars have proposed several new technical methods exploiting unconventional natural gas, such as the ultrasonic vibrating method and the heat injection method.35, 36 The ultrasonic vibrating method is to weaken the adhesion of the methane on the shale pores by utilizing the mechanical vibration effect and the thermal effect of ultrasonic wave, so as to promote the desorption and diffusion of methane. The heat injection into coal seams can stimulate coalbed methane reservoirs to enhance the recovery of coalbed methane, and the porosity and permeability of the coal matrix were greatly increased by both temperature enhanced gas desorption and thermal fracturing. However, the energy efficiency of


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microwave irradiation is higher. As shale is a poor conductor of heat, the above mentioned method of heating is relatively inefficiency. The microwave irradiation is a kind of electromagnetic radiation energy. It is far more intensive than the ultrasonic vibrating method. Microwave irradiation has the feature of integrity heating on the dielectric material, which heat up internal and external of material simultaneously.19 The key technology in shale gas reservoirs fracturing is to induce opening of natural fractures, generate new artificial fractures, make artificial fractures connect with more natural fractures, and form complex fracture networks in reservoirs.28 According to the experimental results, the numbers and sizes of mesopores, macropores, and microfractures in the shale significantly increased after the microwave irradiation. In addition, macroscopic fractures appeared on the surface of the shale, which demonstrates that a complex fracture network is formed within the shale reservoir. This is very helpful in the improvement of the permeability of the stratum near the shale gas well. Therefore, the microwave irradiation stimulation technology has several advantages; it does not cause damage to the reservoir, reduces the consumption of water resources, avoids groundwater pollution, and increases the fracturing volume of the reservoir. Therefore, it is expected to become one of the important technologies in the efficient development of shale gas. If combine the hydraulic fracturing technology with the microwave irradiation method, the efficiency of fracturing will improve drastically and the negative effect of hydraulic fracturing can be decreased. As shown in Figure11, hydraulic fracturing can form the main fractures at first, however, the number of micro cracks is not enough for the seepage of methane. With the using of microwave irradiation, the water lock effect will be reduced and the number of micro cracks will increase, which will benefit desorption and migration of methane a lot.


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Figure 11. Fractures formed by hydraulic fracturing and microwave irradiation.

The rock formation near the wellbore will absorb microwave energy and the temperature will increase under microwave irradiation. As the temperature continues to rise, polar molecular materials such as water and organic matter in the rock formation will gradually separate out, migrate, decompose and volatilize. And then the dielectric constant of the rock formation near the wellbore gradually decreases to a certain extent, so that the microwave can propagate to the farther rock formation. In addition, the increase of breathable pores and microfractures induced by microwave irradiation is nondirective and some microfractures will close under the real ground stress, however, there are still many microfractures that will be reserved, such as the ones paralleling to the maximum principal stress. In general, there are two methods to apply microwave in a shale gas reservoir: (1) the microwave transmitter is set deep down in the well and microwave is generated to directly irradiate the reservoir; (2) the microwave transmitter is set on the ground and a wave guide is used to transmit the microwave down to the reservoir. As the gas-guide tube is too small for the microwave transmitter, it is not economical to drill another well for the microwave transmitter. In addition, laboratory experiments have been performed to irradiate samples through wave-guides and antennas. Therefore, it is feasible to generate microwaves on the ground and transmit them to the destination with a wave-guide. Based on the proposed theory, the onsite microwave system was designed, as shown in Figure 12.


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Figure 12. Design of on-site microwave irradiation stimulation system for shale gas reservoir.

The microwave irradiation method can be implemented on-site according to the following procedures. First, a shale gas well is drilled and hydraulic fracturing is implemented before the microwave irradiation. The microwave, generated by the microwave transmitter on the drilling rigs, is then transmitted to the horizontal well through the wave-guide and antenna. The microwave will penetrate the production casing through the holes formed by hydraulic fracturing perforation technology. According to the microwave heating effects on the shale gas reservoir, the energising time and power of the microwave are adjusted through a control panel on the ground. Finally, the shale gas extraction is started with the production casing.

5.

CONCLUSION 

Based on NMR results, the pore size distributions of the shale samples before and after the microwave treatment were compared and analysed. The numbers and sizes of mesopores and macropores in the shale samples increased, while the numbers of micropores slightly decreased after the microwave treatment.



In addition, the moisture increased the shale sample’s ability to absorb microwave energy, and the accompanying thermal stress and vapour pressure helped the microwave-induced fracturing.


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When the thermal stress generated inside the shale was larger than the cementation strength between the shale mineral particles, the shale cracked forming micro-cracks until it was destroyed. 

The comparison of the surface microtopographies of a shale slice before and after the microwave treatment revealed that the number of surface accumulated particles decreased, and that cracks and fissures appeared upon the microwave treatment.



The effect of the microwave irradiation on the shale pore structure was mainly reflected in two aspects. First, the microwave heating effect changed the mechanical properties of the shale sample. The rapid temperature increase of the shale caused rapid expansion and deformation of the shale mineral particles and their cements, leading to the increase in brittleness; consequently, the shale will be more vulnerable to brittle failure. Second, the moisture within the shale samples increased its ability to absorb microwave energy; the accompanying thermal stress and vapour pressure helped the microwave-induced fracturing.

In conclusion, the microwave irradiation has a significant effect as it increases the pore size and induces fissures in the shale samples. The presented results reveal its large potentials for applications in enhancement of the shale gas reservoirs. This study provided valuable insights into the effect of microwave irradiation on shale.



AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]. ORCID Guozhong Hu: 0000-0003-0752-3667 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the ﬁnal version of the manuscript.


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Notes The authors declare no competing ﬁnancial interest.



ACKNOWLEDGMENTS

This work was supported by the Petrochemical Joint Funds of National Natural Science Foundation of China and China National Petroleum Corporation (Grant number U1762105); the National Natural Science Foundation of China (Grant number 51774279); and the Fundamental Research Funds for the Central Universities (Grant number 2015XKZD04).



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Evolution of Shale Microstructure under Microwave Irradiation

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