SPE-181369-MS Macroscale Mechanical and

SPE-181369-MS Macroscale Mechanical and Microscale Structure Changes in Chinese Wufeng Shale with Supercritical Carbon Dioxide Fracturing

Qing-You Liu, Southwest Petroleum University and Xihua University; Lei Tao, Hai-Yan Zhu**(Corresponding Author), Southwest Petroleum University; Zheng-Dong Lei, RIPED-Beijing, PetroChina; Shu Jiang, China University of Petroleum (East China) and University of Utah

Abstract Waterless fracturing for shale gas exploitation using supercritical carbon dioxide (scCO2) is both effective and environment-friendly, so it has become a popular 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 above 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 macro mechanical changes of Wufeng shale immersed in scCO2 or N2 for 10 hours, triaxial tests with controlled coring angles were done. 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 impact the microstructures. However, the SEM images for the microstructures 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 kinds of minerals were broken into small particles and left with

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some 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.

Keywords: supercritical carbon dioxide; Chinese Wufeng shale; multiscale characteristics; triaxial compression experiment; microstructure

1. 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 hydraulic fracturing techniques for releasing shale gas require thousands of tons of water, and it is difficult and costly to deal with the polluting flow-back fluid. Large-scale hydraulic fracturing may generate micro-seismic events that can trigger stronger geological movements (Bao et al. 2016). Many Chinese shale gas fields are in forest and mountain 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 supercritical carbon dioxide fracturing. Because supercritical CO2 (scCO2) fluid has a density similar to liquids, low viscosity, a high diffusivity similar to that of gases, no hydration with clay, and is non-toxic, it has been widely used as the working fluid in wells to carry cuttings and enhance recovery (Barati et al. 2014, Allawzi et al. 2011, Brunner, 2015). The scCO2 is environmental-friendly, because (1) there are no drawbacks or additives to pollute the water and soil, (2) in regions where there is a water-shortage, the CO2 may be more available than water, (3) the CO2 can be obtained from the exhaust of fossil fuel power plants or some factories, such as bio-refineries, ethylene production plants, so there is no need to capture it from the air, and (4) 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 et al. 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

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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 the China’s Jilin oilfield, 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 carbon dioxide 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 et al. (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 in shale was altered significantly by CO2. Many researchers (Busch et al. 2009, Alemu et al. 2011, Emberley et al. 2005, Lin et al. 2008, 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 K+, Mg2+, and Ca2+ 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 Kollé (2000) found that scCO2 jets could reduce the fracturing pressure and improve the shale gas penetration 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

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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 mentioned above 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 carried out triaxial compressive strength tests on the samples exposed to scCO2, and we obtained SEM images and did XRD analysis of the mineral components.

2. Experimental Process 2.1 Materials Our Wufeng shale samples shown in Fig. 1(a) 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%) (Jiao et al. 2014, Zou et al. 2010). The Wufeng shale (Upper Ordovician Formation) outcrop is composed of grey-dark carbonaceous mud shale rocks. Wufeng Formation has an abundance of silicified graptolite and radiolarian fossils on a well-developed horizontal bedding, which makes the rocks highly brittle (Guo et al. 2014). After we removed the weathered surfaces, we collected the shale outcrop samples in sizes larger than 500 mm ×500 mm ×500 mm. Bedding planes is a significant feature of shale, so the samples were cored at the coring angles (β) of 0°, 15°, 30°, 45°, 60°, 75°, and 90°(Zhu et al. 2014a, Zhu et al. 2014b). See Fig. 1(b). The angle β is defined as the angle between core axis and the normal direction of the bedding plane. In order 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 the 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. 2(a). Row 1 is the experimental group that will be tested with CO2 injection, and the Row 2 is the control group that will be tested with N2 injection. Due to the effect of gas

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pressure on porosity, we could not conduct the same triaxial compression tests on the control group without any gas injection; so nitrogen, N2, which is commonly available in the lab and that 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 about 5 mm from each sample. See Fig. 2(b).

Normal direction of bedding plane

Core axis

Profiles of bedding planes

Outcrop sample

Graptolite on the bedding planes

Bedding planes

(a) (b) Figure 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, Zhu et al. 2014b).

Samples with scCO2

Slices with scCO2

Samples with N2 Slices with N2

20 mm

20 mm (a)

(b)

Figure 2. Partially prepared shale samples. (a) The seven samples in the first row were used for the Group 1, which were to be exposed to scCO2, and the seven samples in the second row were used for the Group 2, which were to be

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exposed to N2. (b) The thin slices were used as controls for the SEM and XRD experiments done before the triaxial tests.

2.2 Experimental apparatus and method To conduct our experiments, we used the TAW-1000 Deep-water Pore Pressure Servo Experimental System (Fig. 3) developed by the China University of Petroleum (Beijing). Figs. 3(a) and 3(b) show the apparatus used to create the necessary conditions for scCO2 (above 31.8 °C and 7.29 MPa). The improved primary components and workflow of the experimental system are described below (Ranjith et al. 2011).

(a)

(b) 1 10

2 3 11 18 5

(c) 1 12

15

13

1 6

14 17 11

1. top plate; 2. confining pressure barrel; 3. heating ring; 4. assembled sample with sensors; 5. data acquisition and 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;4 13. radial displacement transducer; 14. gas outlet (the gas inlet is invisible behind the top plate); 15. heat shrinkable tubing; 16. the sample wrapped with heat shrinkable tubing; 17. axial displacement transducer; 18. bottom plate.

1 8

Figure 3. (a) Schematic diagram 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. 3(c). 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. 3(c). (3) Tightly seal the confining pressure barrel with a sealing ring. (4) Set the

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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. Table 1. The experimental parameters. NO.

Group name

Pconf. (MPa)

Temperature (°C)

Ppore (MPa)

Gas in pores

Number of tests

Group 1

scCO2 Group

20

40

10

scCO2

7

Group 2

Nitrogen Control Group

20

40

10

N2

7

Number of SEM images 7 (before test) 7 (after test) 7 (before test) 7 (after test)

Number of XRD trials 7 (before test) 7 (after test) 7 (before test) 7 (after test)

After getting the fresh test portions of each sample (Fig. 4), we determined the mineralogy and petrology of the 14 samples using X-ray diffraction (XRD) and scanning electron microscopy (SEM). To reduce the variations due to heterogeneity, we tested three XRD and SEM points for each sample. Using Kaszuba’s test method (Kaszuba et al. 2011), samples for whole rock XRD analysis were ground, and particles of less than 45 μm in diameter were sifted through a 325 mesh sieve and analyzed from 2° to 70° 2θ 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 to the Group 1 results. We tested the seven coring angles (0°, 15°, 30°, 45°, 60°, 75°, and 90°) in each Group. Control test point (before) Test point 1 (after) Bedding planes Test point 2 (after) Test point 3 (after) Figure 4. The XRD and SEM test points for each sample (slice) before and after the triaxial compression tests.

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3. Results and Discussions 3.1 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. In order to determine the microscopic structure of Wufeng shale, SEM images for the samples were obtained with a HITACHITM3030. It is difficult to find the same area of the broken samples to do 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 get 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 treated 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. 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 micro-holes and microfractures with sizes from 0.1 μm to 5 μm. Fig. 6 shows that for the control group (Group 2), the microscopic structure after the triaxial compression test seen in Figs. 6(e, f, g, h) is similar to the original structure shown in Figs. 6(a, b, c, d). Tight bedding planes and undamaged minerals were found in Group 1 before testing and Group 2 before and after testing. The hard and brittle minerals (such as quartz and feldspar) have smooth surfaces, sharp edges, and angular fragments. See Figs. 5(c), 5(d), 6(c), and 6(d). The brittle minerals are cemented tightly with clay minerals, as shown in Figs. 6(b), 6(c), 5(b), and 5(c), and the natural microscopic fractures are not severely affected by the axial load (Fig. 6). The microstructure in the control group is almost the same as that in the original shale before the experiments, which shows that the triaxial test with N2 almost does not affect the microstructures of shale. Figs. 7(a) and 7(b) show that after the scCO2 treatment, the surface of the sample were corroded and some mineral were dissolved probably, which left some corrosion holes. For further studies, maybe we can conduct some other experiments to monitor the changes in porosity and permeability of the scCO2-treated shale samples. And the mass of sample increased, because of the adsorption of CO2. Figs. 7(c) and 7(d) show that the microscopic structures changed, which is consistent with Fig. 5.

(a)

Sample 1 in Group 1 Before scCO2 treatment

(b) Sample

2 in Group 1 Before scCO2 treatment

Tight cementation

Tight bedding plane

Sample 1 in Group 1 After scCO2 treatment

(f)

Damaged bedding plane


Corrosion fractures 30 μm

Undamaged mineral surfaces and natural fractures

(j)


(g)

Minerals broken into small grains

Eroded minerals with rough and uneven surfaces


Corrosion voids and fractures 20 μm

(d) Sample

4 in Group 1 Before scCO2 treatment

Undamaged calcite and tight cementation with the clay 20 μm

20 μm


20 μm

100 μm

(i)

Sample 3 in Group 1 Before scCO2 treatment

100 μm

100 μm

(e)

(c)

(h)

Corrosion voids and fractures 20 μm

20 μm

(k) Sample

3 in Group 1 After scCO2 treatment

Eroded minerals with rough and uneven surfaces 20 μm


(l)


Minerals broken into small grains

20 μm

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

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(a)

(e)

10

Sample 1 in Group 2 Before N2 treatment

(b)


Tight bedding plane

Tight cementation plane

100 μm

100 μm

Sample 1 in Group 2 After N2 treatment

Undamaged bedding plane

(f)


Tight cementation

100 μm

30 μm

(c)


(d)

Framboidal pyrite and Undamaged natural fractures

Tight cementation 20 μm

20 μm

(g)


Undamaged minerals 20 μm


(h)


Undamaged mineral surfaces 20 μm

Figure 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.

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(a)

Before scCO2 treatment

(b)

After scCO2 treatment

Dissolve holes 5 mm

5 mm

(c)

Before scCO2 treatment

(d)

Undamaged calcite and tight cementation

Figure 7. The macro and microscopic structures of a shale sample before and after scCO2 treated.

After scCO2 treatment

Eroded minerals with rough and uneven surfaces

The shale microstructure of the scCO2-treated sample (Group 1) is significantly altered (Fig. 5). Due to the lower viscosity and higher diffusion coefficient of the scCO2 fluid, it easily moves into the micropores and micro-fractures, causing the fractures to expand and generating some long fractures, which increases the probability that the micro-pores and micro-fractures linked up. See Figs. 5(e, h, i, j). This causes the sheets to separate (delaminate). See Fig. 5(l). The findings are similar to those found by previous researchers (Jiang et al., 2016; Yin et al., 2016). The minerals are broken into lots of 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. 3.2 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 done 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.

Table 2. Mineralogical analysis results of the XRD experiments. (*the total content of pyrite, hematite, and siderite.) Sample NO. 1-1 1-2 1-3 1-4 1-5 1-6 1-7 2-1 2-2 2-3 2-4

State of sample Before scCO2 After scCO2 Before scCO2 After scCO2 Before scCO2 After scCO2 Before scCO2 After scCO2 Before scCO2 After scCO2 Before scCO2 After scCO2 Before scCO2 After scCO2 Before N2 After N2 Before N2 After N2 Before N2 After N2 Before N2

Mass fraction of mineral (%) Quartz 21.8 30.9 22.8 28.2

Feldspar 0.8 0.6 0.7 0.6

Calcite 19.4 14.6 23.2 19.2

Dolomite 35.9 27.0 29.0 23.6

Pyrite* 5.4 2.6 3.8 2.3

Total clay 16.7 24.3 20.5 26.1

14.9 23.1 30.8 36.7

1.2 0.8 1.5 1.4

22.8 19.9 24.6 21.4

36.0 28.9 20.7 16.1

4.5 2.4 6.3 3.1

20.6 24.9 16.1 21.3

26.0 43.1 39.0 42.1 37.4 43.8 25.6 23.9 26.3 24.8 21.1 20.1 30.8

1.9 1.1 2.4 1.3 2.1 1.2 1.2 1.4 1.1 1.7 1.6 2.4 1.5

22.1 17.1 21.9 17.3 29.4 25 19.3 17.9 21.6 24.2 25.6 27.2 24.6

29.0 22.8 23.5 19.3 19.9 16.2 26.9 27.4 25.7 23.9 22.7 21.4 20.7

3.8 3.2 4.2 3.1 1.2 0.7 3.6 4.1 3.8 3.8 4.2 3.6 6.3

17.2 12.7 9.0 16.9 10.0 13.1 23.4 25.3 21.5 21.6 24.8 25.3 16.1

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2-5 2-6 2-7 2-8

13

After N2 Before N2 After N2 Before N2 After N2 Before N2 After N2 Before scCO2 After scCO2

27.2 25.2 24.0 26.1 25.4 26.9 25.6 28.1

1.4 1.1 1.3 1.6 1.8 1.2 1.4 1.1

25.3 24.7 23.9 24.2 24.2 22.6 23.5 19.6

21.8 22.6 21.8 21.1 21.1 22.9 23.1 40.7

5.8 3.8 4.1 4.6 4.6 3.1 2.8 0

18.5 22.6 24.9 22.4 22.9 23.3 23.6 10.5

33.7

1.0

15.3

39.9

0

10.1

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

Two-theta (deg) Figure 8. Typical XRD spectra of shale before and after scCO2 treatment. (A-quartz; B-calcite; C-dolomite)

Little differences (less than 3.0%) were found in the mineral components before and after the N2-treated shale; but when the scCO2-treated, the amounts of calcite, dolomite and pyrite decreased, maybe leading to the formation of corrosion voids and grooves as seen in Figs. 5(h, i, j). The concentration of calcite decreased an average of about 4.1% with a maximum of 4.8% and a minimum of 2.9%. The concentration of the dolomite decreased 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 this data is 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). See Fig. 6(c). 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 contents 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.

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According to Wu et al.’s tests, the moisture content of Silurian shale is about 1.18% to 1.68％ (Wu et al. 2015), and Jiang’s results showed that crystalline water was released from clay minerals in the shale after CO2 treatment (Jiang et al. 2016). The reactions between scCO2 and water in the micro-pores in shale can produce H+ under high temperature and pressure. Shale minerals and organic matter will react with the H+ (Brunner, 2015, Barati et al. 2014, Gupta et al. 2005, Du et al. 2013, Tian et al. 2014). Calcite and dolomite minerals dissolve due to the following chemical changes (Tian et al. 2014): CaCO3  calcite   H   Ca 2  HCO3

(1)

CaMg  CO3 2  dolomite   2H   Ca 2  Mg 2  2HCO3

(2)

According to the research done by Tian et al. (2014), the dissolved ions will bind to Ca2+, Mg2+, and Fe2+ to product magnesite and ankerite. Mg2  HCO3  MgCO3  magnesite   H 

(3)

Ca 2  0.3Mg 2  0.7Fe 2  2HCO3  CaMg 0.3Fe 0.7  CO3 2  ankerite   2H 

(4)

Although the Ca2+ and Mg2+ may not stay dissolved after the scCO2 treatment (Yin et al.2016), the process of dissolving and recrystallizing probable makes the micro-pores more smooth, 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 micro-holes and micro-fractures seen in Figs. 5(e, h, i, j). The mineral components in the N2 control group change little. In conclusion, according to the previous researchers’ finding 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. But due to the uncertainty of the XRD test method, the changes in mineral constituents is not very clear, so further studies of the chemical reaction mechanisms for the interaction of scCO2 and carbonate or clay minerals needs to be done.

3.3 The changes of rock mechanics on macro-scale The relationship between the average triaxial compressive strength and the coring angle (β) is shown by the curves in Fig. 9. The triaxial compressive strength increases to a maximum value when the β is 15°, it then decreases to a minimum value when the β is 60°, and it increases when β is from 60°to 90°. The trend is consistent with a previous study (Chen et al. 2015). Although the average for the triaxial compressive strength of the scCO2 experimental group (291.8 MPa) is lower than that of the nitrogen control group (330.4 MPa), there is a clear variation in compressive

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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’s compressive strength is consistent with that found in Zheng’s experimental results (Zheng et al. 2015). In their experiments, the triaxial

Triaxial compressive strength / MPa

compressive strength of sandstone exposed to a CO2–NaCl solution decreased by 7% to 15%.

400

Group 1: scCO2 injection Group 2: N2 injection

350

300

250

200

150 0

15

30

45

60

75

90

Coring angle /°

Figure 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.

For the angles of 45°and 60°, the variation of triaxial compressive strengths is similar for Groups 1 and 2, but the scCO2-treated samples collapses 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. 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 0º, 30º, 60º, and 75º. See Figs. 10(a, c, d, e). From the stress peak point, the stress of the scCO2 experimental group drops more sharply than for the other group.

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450

400

400

400

350

350

200

Group 1: scCO2

300

300 250 200 150

Group 2: N2

100 50

-0.75

-0.50

(Axial strain) 0 0.00

-0.25

0.25

0.50

0.75

1.00

(Radial strain)

1.25

0 -1.00 -0.75 -0.50 -0.25 0.00

Strain 

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0 -1.00 -0.75 -0.50 -0.25 0.00

1.50

100

350

200

0.25

0.50

0.75

1.00

150 100

strain

(d) Two groups stress-strain curves for 60°

-0.50

-0.25

1.50

Group 2: N2

200 150 100

(Radial strain)


1.25

50

(Radial strain) -0.75

1.00

250

50


0.75

Group 1: scCO2

300

250

50

0.50

(c) Two groups stress-strain curves for 30°

Axial stress / MPa

150

(Radial strain)

(Axial strain) 0.25

Strain

Group 2: N2

300

Group 2: N2

Axial stress / MPa

Axial stress / MPa

1.00

Group 1: scCO2

200

-0.25

0.75

350

Group 1: scCO2

-0.50

0.50

Group 2: N2

(Radial strain)

(Axial strain) 0.25

Group 1: scCO2

100 50

(b) Two groups stress-strain curves for 15°

300

-0.75

150

Strain 

(a) Two groups stress-strain curves for 0°

250

200

Group 2: N2

50

(Radial strain) -1.00

Group 1: scCO2

100

250

Axial stress / MPa

250

150

350

Axial stress / MPa

Axial stress / MPa

300

0.25

0.50

0.75

1.00

1.25

Strain

(e) Two groups stress-strain curves for 75°

-0.75

-0.50

-0.25


0.25

0.50

0.75

1.00

1.25

1.50

Strain

(f) Two groups stress-strain curves for 90°

Figure 10. Stress-strain curves for the seven coring angles with the confining pressure of 20 MPa. The scCO2 reduced the triaxial strength of Wufeng shale by 3.82% to 19.38%.

Changes in mechanical properties such as Young’s modulus, due to chemical reaction. There may be some volumetric alteration of minerals (e.g., calcite dissolution) 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 a scCO2treated specimen varies from 11.5% to 26.5%. This is probably due to the chemical reaction between scCO2 and the minerals of shale, creating micro-cracks and micro-pores, 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 a scCO2-treated sample is higher than that of a N2-treated sample. Therefore, the changes in Young’s modulus may be caused by the changes in the volume of minerals (e.g.,

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calcite dissolution) and the propagation of microfractures in shale. Table 3 The Young’s modulus of specimens treated with N2 and scCO2 in the triaxial compressive experiments.

Coring angles of specimens 0 15 30 45 60 75 90

Young’s modulus/ GPa After scCO2 treatment After N2 treatment 32.4 36.6 28.6 38.9 29.6 35.7 30.9 28.9 37.3 34.1 29.9 34.2 35.7 32.9

Changes after scCO2 treated (EscCO2-EN2)/ EN2 -11.5% -26.5% -17.1% 6.9% 9.4% -12.6% 8.5%

3.4 The failure modes changes of the samples on macro-scale 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 direction, bedding plane direction, and fracture morphology. The combination of scCO2 treated 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).

fracture

fracture

fracture

fracture fracture

bedding planes

bedding planes

bedding planes bedding planes bedding planes

scCO2-treated 0° (Group 1)

scCO2-treated 15° (Group 1) fracture


bedding planes

scCO2-treated 45° (Group 1) bedding planes

bedding planes


fracture


fracture


bedding planes fracture

fracture

fracture bedding planes

N2-treated 0° (Group 2)

bedding planes


bedding planes

bedding planes




fracture


bedding planes


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

(a) 0°

(b) 15°

(c) 60°

(d) 75°

Figure 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° to 15°), more fractures were found in the scCO2-treated samples than that for 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. See Fig. 12(b). The scCO2 group had a higher strain value prior to the peak stress than for the nitrogen control group. See (Fig. 10(b). 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 (30ºto 60º), the fractures propagated across 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. 12(c) 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 60ºcoring angle. When the coring angles are higher (75ºto 90º), some fractures that are due to 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. See Fig. 12(d). Consequently, fragmentation at high coring angles is more intensive than those at small and moderate angles. To better understand the fracture initiation or growth associated with CO2 fracturing, it is necessary to study the effect of 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

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of fracture propagation. We prepared standard disc specimens with a thickness of 15 mm and a diameter of 100 mm for the fracture toughness test before and after scCO2 treatment. Due to the material shortage of the Wufeng shale, we only conducted two tests for the change in fracture toughness. See Figs. 13(a) and 13(b). The center-fractured Brazilian disk specimens of Wufeng shale were loaded with no confining pressure until the fractures extended over the entire specimens. The experimental data is shown in Figs. 13(c) and 13(d).

(a) The specimen after Brazilian disk splitting test

(b) The specimen after Brazilian disk splitting test

(without CO2-treatment)

(after CO2-treatment)

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

(d) The axial load vs. displacement curve of the scCO2treated test

Figure 13 Fracture toughness of the shale disk samples for the control and scCO 2-treated tests

The mode I fracture toughness was calculated using the following formula, and the results are shown in Table 4.

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K IC 

P a NI RB π

(5)

where KIC is the fracture toughness of mode I (MPa·m0.5), P is the axial load (kN), a is the half-length of a man-made fracture (m), R is the radius of the disk specimens (m), B is the thickness of the disk specimens (m), and NI is dimensionless stress intensity factor of mode I. Table 4 Fracture toughness test results of the Wufeng shale under the scCO 2 and control conditions. Sample NO.

Test condition

1 2

control scCO2

Size (φR×B) /mm φ99×15.3 φ99×15.6

Diameter of hole /mm 30.4 30.2

Half-length of prefabricated fracture /mm 2.9 3.0

Axial load P/kN 8.27 5.39

Fracture toughness of mode I KIC /MPa·m0.5 1.102 0.725

As shown in Figs. 13(a) and 13(b), the CO2-treated specimens have more fractures after the Brazilian disk splitting tests than the control group, and Table 4, Figs.13(c) and 13(d) 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 scCO2 could make the fractures of shale propagate and extend more easily.

4. Conclusions There is a variation in compressive strength with changes in the coring angle: the strength increases firstly from 0°to 15°of coring angle, and decreases from 30°to 60°, then increases secondly from 60°to 90°. Supercritical carbon dioxide (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, SEM, and XRD tests show that calcite and dolomite concentrations seem to decrease due to scCO2 treatment. Due to 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,

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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. Maybe scCO2 can be used as prepad fluid for produce more fractures before injecting the conventional fracturing fluid with proppant, which should also promote the backflow rate.

Acknowledgements

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 (No. 51604232, No. 41728004). This work was also supported by Research Project of Key Laboratory of Fluid and Power Machinery (Xihua University). The authors sincerely thank senior technician Yinghua Zhang, Dr. Qiang Tan, Dr. Wei Yan of China University of Petroleum, Beijing, who provided enthusiastic help during the experiments. The authors also sincerely thank the editors and the reviewers for their efforts in this article.

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