Macroscale Mechanical and Microscale Structural

J181369 DOI: 10.2118/181369-PA Date: 7-December-17

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Macroscale Mechanical and Microscale Structural Changes in Chinese Wufeng Shale With Supercritical Carbon Dioxide Fracturing Qing-You Liu, Lei Tao, and Hai-Yan Zhu, Southwest Petroleum University and State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation; Zheng-Dong Lei, PetroChina; Shu Jiang, China University of Petroleum (East China) and University of Utah; and John David McLennan, University of Utah

Summary Waterless fracturing for shale-gas exploitation using supercritical carbon dioxide (scCO2) is both effective and environmentally friendly, and has become an extensive research topic. Previous researchers have focused on the chemical and physical properties and microstructure of sandstone, carbonate, and shale caprock, rather than on the properties of shale-gas formations. The macroscale mechanical properties and microscale fracture characteristics of Wufeng Shale exposed to scCO2 (at greater than 31.8 C and 7.29 MPa) are still not well-understood. To study the macroscale and microscale changes of shale subjected to scCO2, we obtained Chinese Wufeng Shale crops (Upper Ordovician Formation) from Yibin, Sichuan Basin, China. The shale samples were divided into two groups. The first group was exposed to scCO2, and the second group was exposed to nitrogen (N2). Scanning-electron-microscope (SEM) and X-ray-diffraction (XRD) images were taken to study the original microstructure and mineral content of the shale. To study the macroscale mechanical changes of Wufeng Shale immersed in scCO2 or N2 for 10 hours, triaxial tests with controlled coring angles were conducted. SEM and XRD images were taken after the triaxial tests. In the SEM images, tight bedding planes and undamaged minerals (with sharp edges and smooth surfaces) were found in N2-treated samples both before and after testing, indicating that exposure to N2 did not affect the microstructures. However, the SEM images for the microstructure scCO2-treated samples before and after testing were quite different. The bedding planes were damaged, which left some connected microfractures and corrosion holes, and some mineral types were broken into small particles and left with uneven mineral surfaces. This shows that scCO2 can change rock microstructures and make some minerals (e.g., calcite) fracture more easily. The complex microscale fractures and the decrease in strength for scCO2-treated shale aid the seepage and gathering of gas, enhancing shale-gas recovery. Knowledge of the multiscale physical and chemical changes of shale exposed to scCO2 is not only essential for scCO2 fracturing, but it is also important for scCO2 jets used to break rock and for the geological storage of CO2. Introduction China has the world’s most abundant shale-gas resources, but it is costly to exploit them because Chinese shale is deeply buried, is very compact, has properties that vary greatly with direction (anisotropy), and has a complex structure (Zou et al. 2015). Current hydraulicfracturing techniques for releasing shale gas require thousands of tons of water, and it is difficult and costly to deal with the polluting flowback fluid. Large-scale hydraulic fracturing may generate microseismic events that can trigger stronger geological movements (Bao and Eaton 2016). Many Chinese shale-gas fields are in forested and mountainous regions where there is a shortage of water, so the cost of hydraulic fracturing is huge. Because of these issues, it is important to seek new waterless techniques, such as scCO2 fracturing. Because scCO2 fluid has a density similar to liquids, low viscosity, a high diffusivity similar to that of gases, no hydration with clay, and is nontoxic, it has been widely used as the working fluid in wells to carry cuttings and enhance recovery (Allawzi et al. 2011; Barati and Liang 2014; Brunner 2015). The scCO2 is environmentally friendly because there are no drawbacks or additives to pollute the water and soil; in regions where there is a water shortage, the CO2 may be more available than water; the CO2 can be obtained from the exhaust of fossil-fuel power plants or some factories, such as biorefineries and ethylene production plants, so there is no need to capture it from the air; and after the scCO2 injection, some carbon will be sequestrated (Middleton et al. 2015). Because the use of scCO2 jet fracturing can increase shale permeability, enhance the recovery of hydrocarbons by replacing or displacing them (Palmer and Sito 2013; Sun et al. 2016), and lead to carbon geological sequestration (Busch et al. 2008), there is great interest in this new environmentally friendly fracturing method in China and other countries. The final cost of CO2-promoted shale gas will depend on the source of CO2. Sometimes the cost of the CO2 will be higher than for water, but if the CO2 source is from a power plant or factory, the cost can be much lower. For example, if there are coal-fired power plants next to the shale-gas field, the cost of CO2 could be minimal (Guo et al. 2015; Middleton et al. 2015). At present, some field tests have been successfully implemented with satisfactory results (Meng et al. 2016). Based on the CO2 fracturing-field experience in China’s Jilin oil field, carrying 10.5 m3 of proppant into a shale-gas formation required 290 m3 of liquefied CO2 (Ministry of Land and Resources of China 2014). The experimental studies of geological storage of carbon mainly relate to scCO2 jet breaking of rock and the physical and chemical interactions between the scCO2 and rock. Therefore, an understanding of the multiscale changes of shale exposed to scCO2 is important for the development of CO2-sequestration methods as well as for the recovery of shale gas. Previous research can be divided into the following three categories. Microscale Physical Changes. Okamoto et al. (2005) and Vialle and Vanorio (2011) found that the distribution of the pores and throats in sandstone and carbonate rocks was changed by scCO2. Chiquet et al. (2007) indicated that the wettability of quartz and mica C 2017 Society of Petroleum Engineers Copyright V

This paper (SPE 181369) was accepted for presentation at the SPE Annual Technical Conference and Exhibition, Dubai, 26–28 September 2016, and revised for publication. Original manuscript received for review 10 July 2016. Revised manuscript received for review 28 August 2017. Paper peer approved 30 August 2017.

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in shale was altered significantly by CO2. Many researchers (Emberley et al. 2005; Lin et al. 2008; Busch et al. 2009; Alemu et al. 2011; Liu et al. 2012) found that scCO2 eroded the surfaces of minerals (such as quartz, feldspar, some of the carbonate rocks, and illite), which damaged the original pore structure and left irregular etched marks on these minerals’ crystal surfaces. Wollenweber et al. (2010) found that repeated CO2 treatments can increase the permeability of shale caprock, and Jiang et al. (2016) found that the specific surface area and porosity of the shale increased after being treated with scCO2. Microscale Chemical Changes. Lahann et al. (2013) found that the concentrations of Kþ, Mg2þ, and Ca2þ in the filtrate for the reaction of shale and CO2/brine were significantly higher than in the control case. Many researchers, such as Xu et al. (2005), Angeli et al. (2009), and Allawzi et al. (2011) found that shale-organic-matter debris and kerogen decomposed when exposed to high-pressure scCO2. Yin et al. (2016) found that the organic matter and some mineral components (such as montmorillonite, kaolinite, and calcite) in the shale decreased because these substances dissolved in the scCO2. Macroscale Mechanical Changes. Kolle´ (2000) found that scCO2 jets could reduce the fracturing pressure and improve the shale-gaspenetration rate. Du et al. (2012) found that scCO2 can reduce the threshold pressure of rock breaking. Wang et al. (2015) found that scCO2 jets created grid-like fractures on the end faces of shale cores in scCO2 jet experiments. Zheng et al. (2015) found a decrease of 7 to 15% in the triaxial compressive strength of sandstone in CO2/brine/rock systems. The researchers discussed in this Introduction section mainly focused on the chemical and physical properties and rock microstructure of sandstone, carbonate rock, and shale caprock, rather than on the properties of shale-gas formations. Because of the special characteristics of Chinese shale and the complexities of the interactions between shale and scCO2, the changes of shale mechanical properties are still unclear, and these are the keys to guide scCO2 fracturing. For our research, we chose an outcrop of Wufeng Shale and prepared cylindrical samples with seven different angles with respect to the axis through the cylinders (0, 15, 30, 45, 60, 75, and 90 ). We systematically studied the mechanical characteristics and fracture properties on both the microscale and macroscale levels. We then performed triaxial-compressive-strength tests on the samples exposed to scCO2, and we obtained SEM images and performed XRD analysis of the mineral components. Experimental Process. Materials. Our Wufeng Shale samples shown in Fig. 1a were taken from the Yibing shale gas field, Sichuan Province, China. The shale has an average of 2.94% organic matter (with a maximum of 8.75%), a high thermal maturity (with a range of Ro from 1.88 to 4.36%), and an average porosity of 4.83% (with a range from 2.43 to 15.72%) (Zou et al. 2010; Jiao et al. 2014). The Wufeng Shale (Upper Ordovician Formation) outcrop is composed of dark-gray carbonaceous mud-shale rocks. The Wufeng Formation has an abundance of silicified graptolite and radiolarian fossils on a well-developed horizontal bedding, which makes the rocks highly brittle (Guo and Zhang 2014). Normal direction of bedding plane

Core axis

β

Outcrop sample

Profiles of bedding planes

Graptolite on the bedding planes

(a)

Bedding planes

(b)

Fig. 1—The shale-outcrop sample and the coring method. (a) The origin shale-outcrop sample and the identifiable bedding planes. (b) The coring of shale samples at seven different angles (Zhu et al. 2014a, b).

After we removed the weathered surfaces, we collected the shale-outcrop samples in sizes larger than 500 500 500 mm. Bedding planes are a significant feature of shale, so the samples were cored at the coring angles (b) of 0, 15, 30, 45, 60, 75, and 90 (Zhu et al. 2014a, b) (Fig. 1b). The angle b is defined as the angle between the core axis and the normal direction of the bedding plane. To improve the accuracy of the experiment and reduce the mineralogical differences among the shale samples, we drilled these samples from the same shale outcrop. We cut the samples into standard cylinders with a length of 50 mm and a diameter of 25 mm (with errors less than 0.5 mm), and we polished the two ends to keep them smooth, parallel to each other, and perpendicular to each cylinder’s axis (with errors less than 0.02 mm). The samples are shown in Fig. 2a. Row 1 is the experimental group that will be tested with CO2 injection, and Row 2 is the control group that will be tested with N2 injection. Because of the effect of gas pressure on porosity, we could not conduct the same triaxial compression tests on the control group without any gas injection; therefore N2, which is commonly available in the laboratory and does not react with the minerals in rock, was used as the test gas for the control group. To compare the microstructure and the mineral content of the experimental group and the control group, we cut a slice with a thickness of approximately 5 mm from each sample (Fig. 2b). Experimental Apparatus and Method. To conduct our experiments, we used the TAW-1000 Deepwater Pore Pressure Servo Experimental System (Fig. 3) developed by the China University of Petroleum, Beijing. Figs. 3a and 3b show the apparatus used to create the necessary conditions for scCO2 (greater than 31.8 C and 7.29 MPa). The improved primary components and work flow of the experimental system are described here (Ranjith and Perera 2011). 2

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Samples with scCO2

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Slices with scCO2

Samples with N2

Slices with N2

20 mm

20 mm (a)

(b)

Fig. 2—Partially prepared shale samples. (a) The seven samples in the first row were used for Group 1, which were to be exposed to scCO2, and the seven samples in the second row were used for Group 2, which were to be exposed to N2. (b) The thin slices were used as controls for the SEM and XRD experiments performed before the triaxial tests.

1

(a)

(b)

2

1 10

3

2 3 11

4

18 5

5

6

7

8

9

10

(c) 1 12

15

13

16

14 17 11

1. Top plate; 2. Confining-pressure barrel; 3. Heating ring; 4. Assembled sample with sensors; 5. Data-acquisition/control system; 6. Gas-outlet device; 7. Confining unit; 8. Air compressors; 9. Liquefied-gas booster; 10. CO2 or N2 bottle; 11. Seal ring; 12. Temperature sensor; 13. Radial displacement transducer; 14. Gas outlet (the gas inlet is invisible behind the top plate); 15. Heatshrinkable tubing; 16. The sample wrapped with heat-shrinkable tubing; 17. Axial displacement transducer; 18. Bottom plate.

18

Fig. 3—(a) Schematic of the experimental system, (b) the main components of the experimental system, and (c) the sample set up between the top and the bottom plates in the confining-pressure barrel.

The sample to be tested was placed in the triaxial apparatus shown in Fig. 3c. The experimental steps were as follows. 1. Place the sample wrapped in heat-shrinkable tubing on the bottom plate. 2. Install the axial displacement transducer, radial displacement transducer, and temperature sensors on the sample, as shown in Fig. 3c. 3. Tightly seal the confining-pressure barrel with a sealing ring. 4. Set the confining pressure (Pconf.), temperature, and gas pressure to the experimental parameters. 5. Continue the confining pressure, temperature, and gas pressure for 10 hours, which ensures that the shale pores are filled with the scCO2 or N2 and keeps the pore pressure (Ppore) constant. 6. Start the axial load with the constant displacement of 0.04 mm/min. 7. Continue the test until the sample collapsed and failed to bear the axial stress. 8. After the triaxial compression experiments, broken samples are obtained and used for the SEM and XRD experiments. The experimental parameters for the two groups of triaxial compression tests are described in Table 1. After obtaining the fresh test portions of each sample (Fig. 4), we determined the mineralogy and petrology of the 14 samples using XRD and SEM. To reduce the variations caused by heterogeneity, we tested three XRD and SEM points for each sample. Using the Kaszuba et al. (2011) test method, samples for the whole-rock XRD analysis were ground, and particles smaller than 45 mm in diameter were sifted through a 325-mesh sieve and analyzed from 2 to 70 2h using a Rigaku MiniFlex II powder diffractometer. The seven samples in Group 1 were injected with scCO2, and the seven samples in Group 2 were injected with N2. The Group 2 samples act as a control group for comparison with the Group 1 results. We tested the seven coring angles (0, 15, 30, 45, 60, 75, and 90 ) in each group. 2017 SPE Journal



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No.

Group Name

Pconf (MPa)

Temperature (°C)

Ppore (MPa)

Gas in Pores

No. of Tests

No. of SEM Images

No. of XRD Trials

Group 1

scCO2 group

20

40

10

scCO2

7

7 (before test)

7 (before test)

Group 2

Nitrogen control group

20

40

10

N2

7

7 (after test)

7 (after test)

7 (before test)

7 (before test)

7 (after test)

7 (after test)

Table 1—The experimental parameters.

Control test point (before) Test point 1 (after) Bedding planes Test point 2 (after) Test point 3 (after) Fig. 4—The XRD and SEM test points for each sample (slice) before and after the triaxial compression tests.

Results and Discussion The Changes in Microscopic Structure. An understanding of the microscopic structure of shale is essential for shale-gas exploration and exploitation (Chen et al. 2013). The microscopic structure not only influences the amount of shale-gas storage and migration, but also influences the shale macroscopic properties, such as the shale mechanical strength, failure modes, and fracture-network generation during hydraulic fracturing. To determine the microscopic structure of Wufeng Shale, SEM images for the samples were obtained with a Hitachi-TM3030. It is difficult to find the same area of the broken samples to perform the XRD test before and after the triaxial compression experiments, but the samples used for the comparison were collected from the same cylinder. We used the same experimental procedures and conditions for each test to reduce random errors. To obtain reliable results, we collected a large number of SEM pictures at different magnifications and in different places in each sample. Several representative pictures of microscopic structures for the sample before and after gas treatment are shown in Figs. 5 and 6, and the typical macroscopic and microscopic structure pictures of the shale before and after scCO2 treatment are shown in Fig. 7.

(a) Sample 1 in Group 1

before scCO2 treatment

Tight bedding plane 100 µm

(e) Sample 1 in Group 1 after scCO2 treatment

Damaged bedding plane

(b) Sample 2 in Group 1 before scCO2 treatment

Tight cementation 100 µm

(f) Sample 2 in Group 1 after scCO2 treatment

Minerals broken into small grains 20 µm

100 µm

(i)

Sample 1 in Group 1 after scCO2 treatment

(j) Sample 2 in Group 1

after scCO2 treatment

Corrosion voids and fractures

Corrosion fractures 30 µm

20 µm

(c) Sample 3 in Group 1 before scCO2 treatment

Undamaged mineral surfaces and natural fractures 20 µm

(g)


Eroded minerals with rough and uneven surfaces 20 µm

(k) Sample 3 in Group 1 after scCO2 treatment

Eroded minerals with rough and uneven surfaces 20 µm

(d) Sample 4 in Group 1 before scCO2 treatment

Undamaged calcite and tight cementation with the clay 20 µm

(h)


Corrosion voids and fractures 20 µm Sample 4 in Group 1 (l) S a after scCO2 treatment

Minerals broken into small grains 20 µm

Fig. 5—The typical microscopic structures from the scCO2-treated samples (before and after the triaxial compression test). After the scCO2 treatment, the microstructure of shale had changed. For example, there were damaged bedding planes, eroded minerals, corrosion voids, and fractures. 4



(a) Sample 1 in Group 2

(b)

before N2 treatment

Sample 2 in Group 2 before N2 treatment

Tight cementation plane

Tight bedding plane 100 µm

(e)

Sample 1 in Group 2 after N2 treatment

100 µm

(f)


Tight cementation

Undamaged bedding plane

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(c) Sample 3 in Group 2

Sample 4 in Group 2 before N2 treatment

(d)

before N2 treatment

Framboidal pyrite and undamaged natural fractures 20 µm

Tight cementation 20 µm

(g) Sample 3 in Group 2

(h)

after N2 treatment

Undamaged minerals 30 µm

100 µm

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Undamaged mineral surfaces 20 µm

20 µm

Fig. 6—The typical microscopic structures of the control N2-treated samples (before and after the triaxial compression test). There is little change of the shale microstructure after the triaxial compression tests with N2 injection.

Before scCO2 treatment

(a)

After scCO2 treatment

(b)

Dissolve holes 5 mm

5 mm (c)

Before scCO2 treatment

(d)

Eroded minerals with rough and uneven surfaces

Undamaged calcite and tight cementation

TM3030

2016/11/10

N

D6.8 ×8.0k

After scCO2 treatment

10 µm

TM3030

2016/11/14

N

D7.8 ×8.0k

10 µm

Fig. 7—The macro/microscopic structures of a shale sample before and after scCO2 treatment.

Previous research (Jiao et al. 2014; Wang et al. 2014) shows that the Wufeng Shale is composed of quartz, dolomite, clay minerals, and organic matter, and that there is a small number of microholes and microfractures with sizes from 0.1 to 5 mm. Fig. 6 shows that for the control group (Group 2), the microscopic structure after the triaxial compression test seen in Figs. 6e, 6f, 6g, and 6h is similar to the original structure shown in Figs. 6a, 6b, 6c, and 6d. Tight bedding planes and undamaged minerals were found in Group 1 before testing and in Group 2 before and after testing. The hard and brittle minerals (such as quartz and feldspar) have smooth surfaces, sharp edges, and angular fragments (Figs. 5c, 5d, 6c, and 6d). The brittle minerals are cemented tightly with clay minerals (Figs. 5b, 5c, 6b, and 6c), and the natural microscopic fractures are not severely affected by the axial load (Fig. 6). The microstructure in the control group is nearly the same as that in the original shale before the experiments, which shows that the triaxial test with N2 has only a negligible effect on the microstructures of shale. Figs. 7a and 7b show that after the scCO2 treatment, the surface of the sample was corroded and some mineral was probably dissolved, leaving some corrosion holes. For further studies, we may conduct some other experiments to monitor the changes in porosity and permeability of the scCO2-treated shale samples. The mass of sample increased because of the adsorption of CO2. Figs. 7c and 7d show that the microscopic structures changed, which is consistent with Fig. 5. The shale microstructure of the scCO2-treated sample (Group 1) is significantly altered (Fig. 5). Because of the lower viscosity and higher diffusion coefficient of the scCO2 fluid, it easily moves into the micropores and microfractures, causing the fractures to expand and generating some long fractures, which increases the probability that the micropores and microfractures linked up (Figs. 5e, 5h, 5i, and 5j). This causes the sheets to separate (delaminate) (Fig. 5l). The findings are similar to those found by previous researchers (Jiang 2017 SPE Journal



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et al. 2016; Yin et al. 2016). The minerals are broken into many small grains, which left some uneven mineral surfaces (Fig. 5). These results show that scCO2 can change the microstructure of shale and make some minerals (e.g., calcite) break into small grains more easily. Therefore, the scCO2 can help shale collapse completely and reduce the threshold pressure during scCO2 jet fracturing. The Probable Mineralogical Changes. Shale in the Wufeng-Longmaxi Formation is primarily composed of clay minerals and quartz, with a little plagioclase, potassium feldspar, calcite, dolomite, and pyrite (Liang et al. 2012). An analysis of the components of Wufeng Shale minerals was performed using XRD tests, and the mass fractions of minerals are shown in Table 2. The typical XRD spectra before and after scCO2 treatment are shown in Fig. 8. Mass Fraction of Mineral (%) Sample No.

State of Sample

Quartz

Feldspar

Calcite

Dolomite

Pyrite

Total Clay

1-1

Before scCO2

21.8

0.8

19.4

35.9

5.4

16.7

After scCO2

30.9

0.6

14.6

27.0

2.6

24.3

Before scCO2

22.8

0.7

23.2

29.0

3.8

20.5

After scCO2

28.2

0.6

19.2

23.6

2.3

26.1

Before scCO2

14.9

1.2

22.8

36.0

4.5

20.6

After scCO2

23.1

0.8

19.9

28.9

2.4

24.9

1-4

Before scCO2

30.8

1.5

24.6

20.7

6.3

16.1

After scCO2

36.7

1.4

21.4

16.1

3.1

21.3

1-5

Before scCO2

26.0

1.9

22.1

29.0

3.8

17.2

After scCO2

43.1

1.1

17.1

22.8

3.2

12.7

Before scCO2

39.0

2.4

21.9

23.5

4.2

9.0

After scCO2

42.1

1.3

17.3

19.3

3.1

16.9

Before scCO2

37.4

2.1

29.4

19.9

1.2

10.0

After scCO2

43.8

1.2

25

16.2

0.7

13.1

Before N2

25.6

1.2

19.3

26.9

3.6

23.4

After N2

23.9

1.4

17.9

27.4

4.1

25.3

2-2

Before N2

26.3

1.1

21.6

25.7

3.8

21.5

After N2

24.8

1.7

24.2

23.9

3.8

21.6

2-3

Before N2

21.1

1.6

25.6

22.7

4.2

24.8

After N2

20.1

2.4

27.2

21.4

3.6

25.3

Before N2

30.8

1.5

24.6

20.7

6.3

16.1

After N2

27.2

1.4

25.3

21.8

5.8

18.5

Before N2

25.2

1.1

24.7

22.6

3.8

22.6

After N2

24.0

1.3

23.9

21.8

4.1

24.9

Before N2

26.1

1.6

24.2

21.1

4.6

22.4

After N2

25.4

1.8

24.2

21.1

4.6

22.9

2-7

Before N2

26.9

1.2

22.6

22.9

3.1

23.3

After N2

25.6

1.4

23.5

23.1

2.8

23.6

2-8

Before scCO2

28.1

1.1

19.6

40.7

0

10.5

After scCO2

33.7

1.0

15.3

39.9

0

10.1

1-2 1-3

1-6 1-7 2-1

2-4 2-5 2-6

Table 2—Mineralogical analysis results of the XRD experiments. The pyrite measurement includes the total content of pyrite, hematite, and siderite.

Few differences (less than 3.0%) were found in the mineral components before and after the N2-treated shale, but with the scCO2 treatment, the amounts of calcite, dolomite, and pyrite decreased, possibly leading to the formation of corrosion voids and grooves (Figs. 5h, 5i, and 5j). The concentration of calcite decreased by an average of approximately 4.1%, with a maximum of 4.8% and a minimum of 2.9%. The concentration of the dolomite decreased by an average of 5.7% with a maximum of 8.9% and a minimum of 3.7%. The pyrite concentration decreased from 1 to 2%, but these data are less reliable because the framboidal pyrite is usually distributed unevenly in the shale, and pyrite does not transform into new minerals at low temperatures (Chen et al. 2015; Huang et al. 2015) (Fig. 6c). Because there is little reaction between the quartz and CO2 in the short term, as other minerals decrease, the relative concentration of quartz increases. In Fig. 8, we find some changes in mineral content between the untreated- and scCO2-treated-shale samples. After the scCO2 treatment, dolomite and calcite content decreased. More likely, the main reason for these changes is the dissolution of minerals in scCO2. According to Wu et al. (2015), the moisture content of Silurian shale is approximately 1.18 to 1.68%, and the Jiang et al. (2016) results showed that crystalline water was released from clay minerals in the shale after CO2 treatment. The reactions between scCO2 and water in the micropores in shale can produce Hþ under high temperature and pressure. Shale minerals and organic matter will react with the Hþ (Gupta et al. 2005; Du et al. 2013; Barati and Liang 2014; Tian et al. 2014; Brunner 2015). Calcite and dolomite minerals dissolve because of the following chemical changes (Tian et al. 2014): 6



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CaCO3 ðcalciteÞ þ Hþ ! Ca2þ þ HCO 3 ; . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ð1Þ CaMgðCO3 Þ2 ðdolomiteÞ þ 2Hþ ! Ca2þ þ Mg2þ þ 2HCO 3 : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ð2Þ

Untreated scCO2 treated

A

Intensity

C

B

A

A C CB C

B C

15

20

25

30

35

A, B A C AC

40

45

2θ (°) Fig. 8—Typical XRD spectra of shale before and after scCO2 treatment: A 5 quartz, B 5 calcite, C 5 dolomite.

According to the research performed by Tian et al. (2014), the dissolved ions will bind to Ca2þ, Mg2þ, and Fe2þ to produce magnesite and ankerite: þ Mg2þ þ HCO 3 ! MgCO3 ðmagnesiteÞ þ H ; . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ð3Þ þ Ca2þ þ 0:3Mg2þ þ 0:7Fe2þ þ 2HCO 3 ! CaMg0:3 Fe0:7 ðCO3 Þ2 ðankeriteÞ þ 2H : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ð4Þ

Although the Ca2þ and Mg2þ may not stay dissolved after the scCO2 treatment (Yin et al. 2016), the process of dissolving and recrystallizing probably makes the micropores smoother, which favors the flow of the shale gas. Yin et al. (2016) indicated that some organic matter and mineral components, such as montmorillonite, kaolinite, and calcite, were dissolved by scCO2. Contact between the shale and scCO2 may lead to the decomposition of organic matter, calcite, dolomite, and illite, generating the connective corrosion microholes and microfractures seen in Figs. 5e, 5h, 5i, and 5j). The mineral components in the N2 control group change little. In conclusion, according to the previous researchers’ findings and the SEM and XRD results in this paper, we primarily confirmed that the calcite and dolomite concentrations in shale decrease when treated with scCO2. However, because of the uncertainty of the XRD test method, the changes in mineral constituents are not very clear, so further studies of the chemical-reaction mechanisms for the interaction of scCO2 and carbonate or clay minerals need to be performed.

Triaxial Compressive Strength (MPa)

The Changes of Rock Mechanics on Macroscale. The relationship between the average triaxial compressive strength and the coring angle (b) is shown by the curves in Fig. 9. The triaxial compressive strength increases to a maximum value when b is 15 , then decreases to a minimum value when b is 60 , and increases when b is from 60 to 90 . The trend is consistent with a previous study (Chen et al. 2015).

Group 1: scCO 2 injection

400

Group 2: N 2 injection 350

300

250

200

150 0

15

30

45

60

75

90

Coring Angle (°) Fig. 9—The relationship between triaxial compressive strength and coring angles. The compressive strength decreases by 3.82 to 19.38% of its original value for the scCO2 group. 2017 SPE Journal



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Although the average for the triaxial compressive strength of the scCO2 experimental group (291.8 MPa) is lower than that of the N2 control group (330.4 MPa), there is a clear variation in compressive strength with changes in the coring angle. The shale-compressive strength for Group 1 decreases by 3.82 to 19.38% of its original value. This reduction of the Wufeng Shale compressive strength is consistent with that found in the Zheng et al. (2015) experimental results. In their experiments, the triaxial compressive strength of sandstone exposed to a CO2/sodium chloride solution decreased by 7 to 15%. For the 45 and 60 angles, the variation of triaxial compressive strengths is similar for Groups 1 and 2, but the scCO2-treated samples collapse at lower axial stresses, and the peak triaxial strength for the Group 1 sample at a 60 core angle is significantly lower (at less than 250 MPa). Fig. 10 shows the stress/strain curves for the different coring angles. There is no obvious stage on the curves that indicates compaction of the fissures and pores. Before the stress-peak point, the curves are approximately linear, indicating that the shale is dense and homogeneous. As the stress increases, the stress curves start to deviate from a straight line. After reaching the peak, the shale failed with an obvious sound of brittle fracture. For the post-peak section of the curve, the stress decreases rapidly in a linear fashion to the lowest stress point without any residual stresses.

450

400 350

200

Group 1: scCO2

150

300

300 250 200 150

Group 2: N2

100

Group 1: scCO2

100

50

Group 2: N2

50

(Radial strain)

(Axial strain) 0 0.00

0.25

0.50

0.75

1.00

(Radial strain)

1.25

0 –1.00 –0.75 –0.50 –0.25 0.00

Strain (%)

(a) The two groups' stress/strain curves for 0°

(Radial strain)

0 –1.00 –0.75 –0.50 –0.25 0.00

1.50

350

Group 2: N2

100

250 200 150 100

50

0.25

0.50

0.75

Strain (%)

1.00

–0.50

–0.25

1.50

200 150 100

(Radial strain)


0.25

0.50

0.75

1.00

Strain (%)

(d) The two groups' stress/strain curves for 60°

1.25

50

(Radial strain) –0.75

1.00

250

50


0.75

Group 2: N2

300

Axial Stress (MPa)

300

150

0.50

Group 1: scCO2

Group 1: scCO2

200

(Axial strain) 0.25

(c) The two groups' stress/strain curves for 30°

Group 2: N2

(Radial strain)

Group 2: N2

(b) The two groups' stress/strain curves for 15°

Axial Stress (MPa)

Axial Stress (MPa)

1.25

350

250

–0.25

1.00

Group 1: scCO2

100

Strain (%)

Group 1: scCO2

–0.50

0.75

150

Strain (%)

300

–0.75

0.50

200

50

(Axial strain) 0.25

250

Axial Stress (MPa)

250

350

350

Axial Stress (MPa)

Axial Stress (MPa)

300

–1.00 –0.75 –0.50 –0.25

400

400

(e) The two groups' stress/strain curves for 75°

1.25

–0.75 –0.50 –0.25


0.25

0.50

0.75

1.00

1.25

1.50

Strain (%)

(f) The two groups' stress/strain curves for 90°

Fig. 10—Stress/strain curves for the seven coring angles with confining pressure of 20 MPa. The scCO2 reduced the triaxial strength of the Wufeng Shale by 3.82 to 19.38%.

A comparison of the stress/strain curves with different coring angles in Fig. 10 shows that the stresses of scCO2-treated shale enter into the failure stage at a smaller yield strain than for the control group. At the same time, the peak stresses of the scCO2-treated shale are lower than those in the control group, especially with the coring angles of 0, 30, 60, and 75 (Figs. 10a, 10c, 10d, and 10e). From the stress-peak point, the stress of the scCO2 experimental group drops more sharply than for the other group. Changes in mechanical properties such as Young’s modulus are caused by the chemical reaction. There may be some volumetric alteration of minerals (e.g., calcite dissolution) that would cause changes in Young’s modulus. Table 3 shows the Young’s modulus that can be calculated from the stress/strain curves. This study showed that the Young’s modulus for the CO2-treated specimens was reduced, except for the coring angles of 45, 60, and 90 . When the coring angle varies from 0 to 30 , the decline of the Young’s modulus of an scCO2-treated specimen varies from 11.5 to 26.5%. This is probably because of the chemical reaction between scCO2 and the minerals of shale, creating microcracks and micropores and causing the deformations of scCO2-treated specimens to be larger than for the N2-treated samples. This causes the Young’s modulus to decrease. When the coring angle is large (45, 60, and 90 ), shear slippage causes the axial strain value to vary, so the Young’s modulus of an scCO2-treated sample is higher than that of an N2-treated sample. Therefore, the changes in Young’s modulus may be caused by the changes in the volume of minerals (e.g., calcite dissolution) and the propagation of microfractures in shale. The Failure-Mode Changes of the Samples on Macroscale. Rock-failure modes are determined by many factors, including mineralogical character, internal microscopic structure, test-air conditions, load direction, and bedding-plane direction. The failure modes vary with respect to splitting failure and shear failure. Fig. 11 shows the shale-failure modes for different experimental conditions (N2- or scCO2-treated) and different coring angles. Fig. 12 shows a series of typical fracture-surface morphologies of typical splitting modes. We studied the relationship among loading 8



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direction, bedding-plane direction, and fracture morphology. The combination of the scCO2 treatment and the bedding-plane direction caused a diversity of failure modes, and the main findings are consistent with a previous study (Chen et al. 2015). Young’s Modulus (GPa)

Changes After scCO2 Treatment

Coring Angles of Specimens (°)

After scCO2 Treatment

After N2 Treatment

(EsCO − EN2 ) / EN2

0

32.4

36.6

–11.5%

15

28.6

38.9

–26.5%

30

29.6

35.7

–17.1%

45

30.9

28.9

6.9%

60

37.3

34.1

9.4%

75

29.9

34.2

–12.6%

90

35.7

32.9

8.5%

2

Table 3—The Young’s modulus of specimens treated with N2 and scCO2 in the triaxial compressive experiments.

Fracture

Fracture

Fracture

Fracture Fracture

Bedding planes

Bedding planes

Bedding planes

Bedding planes Bedding planes

scCO2-treated 0° (Group 1)


Bedding planes

Bedding planes




Fracture

Bedding planes

Fracture

Fracture

Fracture Fracture



Bedding planes Fracture

Fracture

Fracture Bedding planes

N2-treated 0° (Group 2)

Bedding planes N2-treated 15° (Group 2)

Bedding planes

Bedding planes




Fracture


Bedding planes N2-treated 90° (Group 2)

Fig. 11—The shale-fracture modes from the triaxial compression tests for Groups 1 and 2.

(a) 0°

(b) 15°

(c) 60°

(d) 75°

Fig. 12—Fracture-surface morphology of typical splitting modes with different coring angles. The left sample in each pair is scCO2-treated, and the right sample in each pair is N2-treated.

Because of the reaction between scCO2 and shale, when the coring angles of the samples were low (0–15 ), more fractures were found in the scCO2-treated samples than in the control group. The failure modes of the shale samples were both splitting and shear modes (Fig. 11). Such failure modes lead to rough fracture surfaces with jagged edges. Similar failure modes were also observed in the N2-treated samples, but the scCO2-treated samples had more fractures with the 15 coring angle (Fig. 12b). The scCO2 group had a 2017 SPE Journal



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higher strain value before the peak stress than for the nitrogen control group (Fig. 10b). This implies that shale in scCO2 experienced a longer compression time before failure. This provides more time for interactions between the broken fractures and the scCO2, resulting in more-complex fractures. When the coring angles of samples are moderate (30–60 ), the fractures propagate more easily along the bedding planes. The failure modes follow the shear modes (because of the slipping), forming one or two smooth shear surfaces along the bedding plane. Fig. 12c shows some graptolites on the ruptured surfaces. Because of the constant confining pressure and higher coring angle, the splitting-failure modes observed at lower coring angles are not seen. On the other hand, weak bedding planes slip under the axial load, forming smooth shear fractures. As a result, the triaxial compressive strength of the shale is the lowest at the 60 coring angle. When the coring angles are higher (75–90 ), some fractures that are caused by tensile damage were found in the samples. Because the direction of the axial compression loading is nearly parallel to the shale bedding planes at high coring angles, the failure modes mainly follow the splitting mode (Fig. 12d). Consequently, fragmentation at high coring angles is more intensive than that at small and moderate angles. To better understand the fracture creation or growth associated with CO2 fracturing, it is necessary to study the effects of a geochemical reaction caused by CO2/brine/rock minerals on fracture properties. According to Kemeny (1991) and Fan et al. (2014), fracture toughness can be used to assess the likelihood of fracture propagation. We prepared standard disk specimens with a thickness of 15 mm and a diameter of 100 mm for the fracture-toughness test before and after scCO2 treatment. Because of the material shortage of the Wufeng Shale, we conducted only two tests for the change in fracture toughness (Figs. 13a and 13b). The center-fractured Brazilian disk specimens of Wufeng Shale were loaded with no confining pressure until the fractures extended over the entire specimen. The experimental data are shown in Figs. 13c and 13d.

(a) The specimen after Brazilian disk-splitting test (without CO2 treatment)

(b) The specimen after Brazilian disk-splitting test (after CO2 treatment)

9

9

8

8 7

6

Axial Load (kN)

Axial Load (kN)

7

5 4 3 2 1

6 5 4 3 2 1

0

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Displacement (mm)

(c) The axial load vs. displacement curve for the control test (without CO2 treatment)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Displacement (mm)

(d) The axial load vs. displacement curve of the scCO2-treated test

Fig. 13—Fracture toughness of the shale-disk samples for the control and scCO2-treated tests.

The Mode I fracture toughness was calculated using the following formula, and the results are shown in Table 4: pffiffiffi P a pffiffiffi NI ; . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ð5Þ KIC ¼ RB p 10



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where KIC is the fracture toughness of Mode I (MPam0.5), P is the axial load (kN), a is the half-length of a manmade fracture (m), R is the radius of the disk specimens (m), B is the thickness of the disk specimens (m), and NI is the dimensionless stress-intensity factor of Mode I.

Sample No.

Test Condition

Size, ϕ R×B (mm)

Diameter of Hole (mm)

Half-Length of Prefabricated Fracture (mm)

Axial Load P (kN)

Fracture Toughness of 0.5 Mode I KIC (MPa·m )

1

Control

ϕ 99×15.3

30.4

2.9

8.27

1.102

2

scCO2

ϕ 99×15.6

30.2

3.0

5.39

0.725

Table 4—Fracture-toughness-test results of the Wufeng Shale under the scCO2 and control conditions.

As shown in Figs. 13a and 13b, the CO2-treated specimens have more fractures after the Brazilian disk-splitting tests than the control group, and Table 4 and Figs. 13c and 13d show that the failure load of the CO2-treated specimens (5.39 kN) is lower than that of the control samples (8.27 kN). The fracture toughness of CO2-treated specimens is much lower than that of the control specimens, which suggests that scCO2 could make the fractures of shale propagate and extend more easily. Conclusions There is a variation in compressive strength with changes in the coring angle: The strength increases first from 0 to 15 of coring angle, decreases from 30 to 60 , and then increases again from 60 to 90 . The scCO2 reduces the triaxial compressive strength, amplitude of variation of the compressive strength with changes in the angle, and fracture toughness of Wufeng Shale. The shale microscopic structures and bedding planes, which are essential for the migration of gases, are damaged by the scCO2, leaving many microscopic fractures, small mineral debris, and uneven mineral surfaces. On both the macroscale and microscale levels, after scCO2 treatment, complex chemical and physical changes in shale minerals may lead to favorable conditions for the seepage and gathering of shale gas. The experimental results of triaxial compression tests, SEM images, and XRD tests show that calcite and dolomite concentrations seem to decrease because of scCO2 treatment. Because of the uncertainty of the XRD test method, the changes in mineral constituents are not very clear, so the mechanism of the chemical reaction between scCO2 and carbonate or clay minerals requires further study. A method for predicting the spatial distribution of some minerals after scCO2 treatment will be proposed on the theoretical studies of CO2 fracturing and carbon sequestration. The findings of this paper will provide basic experimental parameters for analyzing scCO2 fracturing, scCO2 jet breaking of rock, and carbon sequestration. The findings will also provide experimental verification for numerical engineering simulations of scCO2-treated rock. For fracture treatment, this study shows that in scCO2 fracturing, more-uniform fractures form than for conventional fracturing techniques, and the necessary pressure for fracturing shale-gas reservoirs is reduced. Perhaps scCO2 can be used as a prepad fluid to produce more fractures before injecting the conventional fracturing fluid with proppant, which should also promote the flowback rate. Nomenclature a ¼ half-length of a manmade fracture of the disk specimen in the fracture-toughness test, m B ¼ thickness of the disk specimens in the fracture-toughness test, m EN2 ¼ Young’s modulus of the specimen after N2 treatment, GPa EscCO2 ¼ Young’s modulus of the specimen after scCO2 treatment, GPa KIC ¼ fracture toughness of Mode I, MPam0.5 NI ¼ stress-intensity factor of Mode I, dimensionless P ¼ axial load on the disk specimen in the fracture-toughness test, kN Pconf ¼ confining pressure of the triaxial tests, MPa Ppore ¼ pore pressure of the triaxial tests, MPa R ¼ radius of the disk specimen in the fracture-toughness test, m b ¼ coring angle between core axis and the normal direction of the bedding plane, degrees 2h ¼ experimental angle in the rock XRD analysis, degrees Acknowledgments This work was funded by the National Key Basic Research Program of China (973 Program, No. 2014CB239205) and the National Natural Science Foundation of China (Nos. 51604232 and 41728004). 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Wollenweber, J., Alles, S., Busch, A. et al. 2010. Experimental Investigation of the CO2 Sealing Efficiency of caprocks. Int. J. Greenh. Gas Contr. 4 (2): 231–241. https://doi.org/10.1016/j.ijggc.2010.01.003. Wu, Y., Fan, T., Jiang, S. et al. 2015. Methane Adsorption Capacities of the Lower Paleozoic Marine Shales in the Yangtze Platform, South China. Energ. Fuel. 29 (7): 4160–4167. https://doi.org/10.1021/acs.energyfuels.5b00286. Xu, T., Apps, J. A., and Pruess, K. 2005. Mineral Sequestration of Carbon Dioxide in a Sandstone–Shale System. Chem. Geol. 217 (3–4): 295–318. https://doi.org/10.1016/j.chemgeo.2004.12.015. Yin, H., Zhou, J., Jiang, Y. et al. 2016. Physical and Structural Changes in Shale Associated with Supercritical CO2 Exposure. Fuel 184 (15 November): 289–303. https://doi.org/10.1016/j.fuel.2016.07.028. Zheng, H., Feng, X., and Pan, P. 2015. Experimental Investigation of Sandstone Properties Under CO2–NaCl Solution-Rock Interactions. Int. J. Greenh. Gas Contr. 37 (June): 451–470. https://doi.org/10.1016/j.ijggc.2015.04.005. Zhu, H. Y., Deng, J. G., Liu, S. J. et al. 2014a. Hydraulic Fracturing Experiments of Highly Deviated Well with Oriented Perforation Technique. Geomech. Eng. 6 (2): 153–172. https://doi.org/10.12989/gae.2014.6.2.153. Zhu, H. Y., Guo, J., Zhao, X. et al. 2014b. Hydraulic Fracture Initiation Pressure of Anisotropic Shale Gas Reservoirs. Geomech. Eng. 7 (4): 403–430. https://doi.org/10.12989/gae.2014.7.4.403. Zou, C., Dong, D., Wang, S. et al. 2010. Geological Characteristics, Formation Mechanism and Resource Potential of Shale Gas in China. Petrol. Explor. Dev. 37 (6): 641–653. https://doi.org/10.1016/S1876-3804(11)60001-3. Zou, C., Dong, D., Wang, Y. et al. 2015. Shale Gas in China: Characteristics, Challenges and Prospects (I). Petrol. Explor. Dev. 42 (6): 753–767. https:// doi.org/10.1016/S1876-3804(15)30072-0. Qing-You Liu is a professor at the State Key Laboratory of Oil and Gas Reservoir and Exploitation, Southwest Petroleum University, China, and Xihua University, China. His research interests include rock mechanics, dynamic design method of drill bit, design theory and method of polycrystalline-diamond-compact bit, and drillstring/bit/rock/system kinetic models. Liu has authored orcoauthored more than 160 academic papers. He holds a PhD degree in mechanical engineering from Southwest Petroleum University. Lei Tao is currently a PhD degree student in the Department of Oil and Natural Gas Engineering at Southwest Petroleum University. His research interests include petroleum-related rock mechanics and applications of scCO2 in petroleum engineering, especially performing laboratory experiments and numerical simulations. Tao holds a bachelor’s degree from Southwest Petroleum University. Hai-Yan Zhu (corresponding author) is currently an associate professor in the Department of Oil and Natural Gas Engineering at Southwest Petroleum University, Chengdu, China. Previously, he worked for the Jianghan Oilfield Company of China Petroleum & Chemical Corporation (SINOPEC) for 1 year from 2009–2010. Zhu’s research interests are petroleum-related rock mechanics, including laboratory experiments and numerical simulation of wellbore drilling, reservoir stimulation, sand production, 4D geostress evolution, and rock breaking. He has authored or coauthored more than 20 academic papers. Zhu holds a PhD degree in petroleum-related rock mechanics and engineering from the China University of Petroleum, Beijing. Zheng-Dong Lei is currently a senior engineer in the PetroChina Research Institute of Petroleum Exploration and Development, Beijing, China. His research interests are fracture characterization and unconventional-reservoir modeling, Lei has authored or coauthored more than 20 academic papers. He holds a PhD degree in oil/gasfield-development engineering from the China University of Petroleum, Beijing. Shu Jiang is a senior research scientist and coordinator of China Program Development at the Energy and Geoscience Institute at the University of Utah, where he is also a research associate professor of petroleum engineering in the College of Engineering. In addition, Jiang is an adjunct professor at the China University of Petroleum, East China, and at Qingdao University and China University of Geosciences at Wuhan. Previously, he worked for CNOOC and the University of Colorado. Jiang has extensive experience in geology and engineering for conventional and unconventional reservoirs. He has authored or coauthored more than 70 peer-reviewed papers in journals and books. Jiang holds a PhD degree in petroleum geology from China University of Geosciences at Wuhan. He is a certified petroleum geologist and an active member of the American Association of Petroleum Geologists (AAPG), the Society of Exploration Geophysicists, the International Association of Sedimentologists and the Geological Society of America. Jiang also serves as an advisory member of the AAPG Shale Gas and Liquids Committee and has convened and chaired many international meetings. He serves as the deputy associate editor for Interpretation and as an associate editor for Petroleum Science. John David McLennan (co-corresponding author) is currently an associate professor in the Department of Chemical Engineering at the University of Utah. He has been a senior research scientist at the Energy & Geoscience Institute and a research professor in the Department of Chemical Engineering at the University of Utah since January 2008. McLennan has 30 years of experience in geomechanics with petroleum service and technology companies. He worked for Dowell Schlumberger for 9 years in their Denver, Tulsa, and Houston facilities. Later, with TerraTek in Salt Lake City, Utah, USA, Advantek International in Houston, and ASRC Energy Services in Anchorage, McLennan worked on projects concerned with coalbed methane recovery, rock mechanical properties determinations, produced-water and drill-cuttings reinjection, and casing-design issues related to compaction. Recent work has focused on optimized gas production from shales and unconsolidated formations, fluid-rock interactions, geothermal energy recovery, in-situ microbial generation of natural gas and high-temperature rock testing. McLennan holds a PhD degree in civil engineering from the University of Toronto.

2017 SPE Journal
