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Brittleness evaluation of the inter-salt shale oil reservoir in Jianghan Basin in China
Marine and Petroleum Geology 102 (2019) 109–115
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Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo
Research paper
Brittleness evaluation of the inter-salt shale oil reservoir in Jianghan Basin in China
T
X. Fana,b,∗, J.Z. Sua,b, X. Changa,c, Z.W. Huanga,b, T. Zhoua,b, Y.T. Guoa,c, S.Q. Wud a
State Energy Center for Shale Oil Research and Development, China SINOPEC Exploration & Production Research Institute, Beijing, China c Institute of Rock and Soil Mechanics, China Academy of Sciences, Wuhan, China d Research Institute of Exploration and Development, Jianghan Oil Field, SINOPEC, Qianjiang, China b
A B S T R A C T
The inter-salt shale in the Qianjiang formation of Jianghan Basin in China are characterized by multiple bedding planes, low rock strength, complicated mineral compositions and high heterogeneity of rock mechanics. The brittleness evaluation of the inter-salt shale has not been investigated. Core samples of the inter-salt shale from the fourth member of the Qianjiang Formation were tested by using CT scan, X-ray diffraction, tri-axial rock test at the in-situ condition. An integrated investigation was conducted on the brittleness evaluation of inter-salt shale from the perspective of core characteristics, brittle mineral components, elastic parameters and the complete stress–strain characteristics of rocks. The results indicate that the inter-salt shale exist with bedding planes and glauberite, which leads to high anisotropy and heterogeneity. The argillaceous dolomite is a good candidate for fracturing due to high brittle mineral content. The elastic parameters of the inter-salt shale at the in-situ conditions are quite different with that of the gas shale because of the difference in the mineral composition, the bedding plane structure and the fluid type stored in the pore space. The brittleness index of the inter-salt shale can be calculated through a modified Rickman model. The transformation model of the dynamic-static mechanic parameters has been built based on the logging data and the laboratory data in order to obtain the brittleness index profile along the wellbore. The post-peak stress-strain relations indicates the rock has high residual strength at confined stress state, which explains that the rock is damaged through shear failure along the bedding plane and fails to generate complex fractures. This study would provide theoretical basis for fracability evaluation model used in the inter-salt shale oil reservoir.
1. Introduction Shale oil refers to the oil that exists in a variety of forms in the organic shale, such as free oil, adsorbed oil or dissolved oil (Jia et al., 2012). The United States is the earliest and also the most successful country in the development of the shale oil. According to EIA (EIA, 2015), there are 59 billion barrels of technology recoverable resource in the US. In present, the shale oil production is about 4.3 million barrels per day, accounting for 47% of the total US crude oil production. The major shale oil plays in the US include Bakken, Eagle Ford, Monterey and Niobrara, which are all formed in the marine sedimentary environment. The shale oil resources in China are formed in the continental deposition, mainly distributed in the Songliao Basin, Bohai Bay Basin, Ordos Basin, Jianghan Basin and the western Junggar Basin. EIA predicted that there are 32 billion barrels of technology recoverable resource in China (EIA, 2013). The Qianjiang Formation in the Jianghan Basin developed 193 rhythm layers with a cumulative thickness of more than 2000 m, of which the Qian 3 and Qian 4 are the best potential layers with shale oil. The oil is generated and stored in the intersalt shales due to the separation of the upper and lower salt rock.
∗
The brittle mineral content and the elastic rock parameters are the two methods widely used in the brittleness evaluation of marine gas shale both in US and China. Jarvie (Jarvie et al., 2007) proposed that the mass ratio of quartz minerals in the core to all minerals should be used as an index to evaluate the rock brittleness. Wang (Wang and Gale, 2009) believed that the brittleness index should be the sum of the total mass ratio of quartz and dolomite minerals. Due to the complex mineral compositions of China's marine gas-bearing shale, scholars (Zhao et al., 2014) proposed that the mass ratio of quartz, feldspar, calcite and dolomite to the sum of quartz, feldspar, calcite, dolomite and clay mineral can represent the shale brittleness. In the Longmaxi Formation shale in the southern Sichuan Basin, there are high clay mineral content (13.4%–66.1%,with average of 34.7%) and carbonate mineral content (14.6%–80.0%, with average of 35.9%), and relatively lower quartz content (5.2%–41.4%, with average of 21.0%). The contents of brittle minerals of the Longmaxi Formation shale range from 26.9% to 86.6%, with an average of 62.5% suggesting it has good brittleness. The statistics of the Barnett shale in the Fort Worth Basin show that the Young's modulus ranges from 10 GPa to 80 GPa while the Poisson's ratio ranges from 0.1 to 0.4. Rickman (Rickman et al., 2008) proposed a brittle
Corresponding author. 31 Fuxue Rd., Haidian District, Beijing, China. E-mail address: [email protected] (X. Fan).
https://doi.org/10.1016/j.marpetgeo.2018.12.013 Received 1 June 2018; Received in revised form 28 November 2018; Accepted 6 December 2018 Available online 08 December 2018 0264-8172/ © 2018 Elsevier Ltd. All rights reserved.
Marine and Petroleum Geology 102 (2019) 109–115
X. Fan et al.
is 0.022 MPa/m, which means that the in-situ temperature is 95–100 °C and the in-situ stress is 55–57 MPa. The detail of the core samples is shown in Table 1.
index using the dimensionless Young's modulus and Poisson's ratio for Barnett shale in the Fort Worth Basin. The Rickman model must be corrected to evaluate the shale brittleness because of the variation in rock mechanics in different regions. For the Longmaxi shale in Sichuan Basin, the Young's modulus varies from 8 GPa to 56 GPa and the Poisson's ratio changes from 0.1 to 0.36 (Yuan et al., 2013). Consequently a brittleness index model has to be modified for the Longmaxi shale. The Young's modulus and Poisson's ratio of the rock are obtained under the assumption of elastic deformation, while the rock at the downhole condition usually has plastic deformation and large residual strength at the high confining pressure. Therefore, the Rickman model fails to evaluate the rock brittleness when the rock has high residual strength after the stress peak in the stress-strain curve. It is necessary to comprehensively evaluate the brittleness by using the full stress-strain curve by the tri-axial compression test. Some complicated models have been proposed to evaluate the brittleness from the perspective of strain energy at the pre-peak and the post-peak in the stress-strain curve (Li et al., 2012; Zhou et al., 2014; Hou et al., 2016; Munoz et al., 2016). The inter-salt shale in the Qianjiang formation of Jianghan Basin in China are characterized by multiple bedding planes, low rock strength, complicated mineral compositions and high heterogeneity of rock mechanics compared with the gas shale. The brittleness evaluation of the inter-salt shale has not been investigated. Core samples of inter-salt shale from the Q4 Lower Layer of the Qianjiang Formation are tested by using CT scan, X-ray diffraction, tri-axial rock test at the in-situ conditions. An integrated investigation is conducted on the brittleness evaluation of inter-salt shale. This study would provide theoretical basis for fracability evaluation model of the inter-salt shale oil reservoir.
3.1. Microscopic structures The cross plot between the porosity and permeability of the intersalt shale of the Q4 Lower Layer is shown in Fig. 3. Three points in the upper left of Fig. 3 demonstrate the micro fractures exist in the cores of the inter-salt shale. The average porosity of the matrix is 18.8% while the average permeability is 0.33 mD. The matrix of the inter-salt shale with median porosity and low permeability illustrates that the connectivity between pore spaces is very poor. It is proved by the fact that the pore volume is plugged because of the presence of salt minerals labeled by the red box in Fig. 4. The median pore-throat radius is 21 nm and 219 nm for the dolomitic claystone and argillaceous dolomite respectively. 3.2. Mineral composition X-ray diffraction (XRD) is commonly used to detect the mineral composition of rocks. The mineral composition data of the core samples shown in Table 2 indicates that the clay minerals are mainly made up by illite, while the non-clay minerals consist of quartz, calcite, feldspar, dolomite, pytire, etc. For the argillaceous dolomite, it is featured by low content of clay minerals, high content of dolomite and quartz. For the dolomitic claystone, it has low content of quartz, high content of clay and calcite. The salt rock is mainly made up by NaCl, containing a small amount of glauberite and gypsum. Based on the mineral composition of the inter-salt shale, a brittle mineral content evaluation model is introduced as follows:
2. Geological description The inter-salt shale oil is mainly formed in the dolomitic shale where the lake basin can accommodate the largest space and the supply of organic material is insufficient. It is a continuously distributed layered reservoir with multiple rhythms and large area, as shown in Fig. 1. In the longitudinal direction, the inter-salt shale is mainly composed of five lithologies: argillaceous dolomite, dolomitic claystone, limeclaystone, glauberite-filling dolomitic claystone and salt. Fig. 2 shows the comprehensive strata log diagram of Q3 Layer of the Qianjiang Formation, which has the same lithology with Q4 Lower Layer.
Bm =
wquartz + wdolomite + wfeldspar + wcalcite wtotal
× 100%
(1)
The brittle mineral contents of the argillaceous dolomite are larger than that of the dolomitic claystone, which implies that the argillaceous dolomite is a good candidate for fracturing. However, the mechanical parameters must be taken into consideration to further investigate the shale brittleness. 3.3. Bedding plane Micron CT scan technology is used to observe the micro structure of the inter-salt shale. Fig. 5 shows the structure of standard core column with several types of lithology. The dolomitic claystones, i.e. 6–1 and 7–3, are characterized by the bedding planes with aperture of 0.042–0.72 mm. The effect of bedding plane on mechanical properties will be discussed in the next part. The argillaceous dolomites, i.e. 3–1 and 7–2, are more homogeneous and are not rich in bedding planes. Some of the argillaceous dolomites are filled with glauberite particles (white spots in 2-2 and 7–1) which accounts for 20% of the total volume.
3. Core characteristics The core samples obtained from the downhole are all from the Q4 Lower Layer of Qianjiang formation. The slope of the formation is about 75–80° which implies the bedding plane is almost parallel with the core axial. The temperature gradient is 3.3 °C/100m and the stress gradient
4. Brittleness index 4.1. Brittleness evaluation by laboratory data In order to get the mechanical parameters at the in-situ conditions, all of the inter-salt shale cores are saturated with the low viscosity crude oil. The reservoir temperature (95 °C) and confining pressure (10 MPa, 30 MPa, 50 MPa) are applied in the tri-axial compression experiment. The Young's modulus and Poisson's ratio of the core samples are plotted in Fig. 6, in which the elastic parameters of gas shale from Longmaxi formation in Sichuan basin are compared. According to the experimental results, the Young's modulus of the Q4 Lower Layer of inter-salt shale is between 1 and 13 GPa and the Poisson's ratio is
Fig. 1. Stratigraphic sequence of Qianjiang Formation. 110
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X. Fan et al.
Fig. 2. Comprehensive strata log diagram of Q3 Layer of the Qianjiang Formation.
between 0.11 and 0.48. Generally the inter-salt oil shale has much lower Young's modulus (8 GPa) and larger Poisson's ratio (0.28), while the gas shale has much larger Young's modulus (24 GPa) and lower Poisson's ratio (0.18). It can be inferred that the brittleness of the oil shale is less than that of the gas shale according to Rickman equation (Rickman et al., 2008). The elastic parameters from Q3 Layer to Q4 Lower Layer can be obtained using logging data in section 4.2. The calculated static Young's modulus is within the scope of 0.79–24.6 GPa while the Poisson's ratio lies between 0.28 and 0.38. Overall, a modified brittleness index model suitable for the shallow inter-salt shale is presented as follows:
Table 1 Detail of core samples obtained from the 4th formation of the Qianjiang basin. No.
Lithology
Depth, m
Rhythm
2–2 3–1 3–2 6–1 6–2 6–3 7–1 7–2 7–3
Glauberiteous Claystone Argillaceous Dolomite Salt Rock Dolomitic Claystone Argillaceous Dolomite Salt Rock GlauberiteousClaystone Argillaceous Dolomite Dolomitic Claystone
2108.00–2125.06 2311.64–2311.77 2319.10–2319.28 2505.06–2505.20 2506.62–2506.68 2512.22–2512.42 2612.90–2613.02 2610.80–2610.93 2598.26–2598.46
5 7 7 12 12 12 14 14 14
En =
E − 0.79 56 − 0.79
(2)
νn =
0.48 − ν 0.48 − 0.11
(3)
Bi =
En + νn 2
(4)
The brittleness indexes of the entire 24 specimens have been calculated by Eq. (2) to Eq. (4) and the results are shown in Fig. 7. The average brittleness index of the dolomitic claystone is 33.7%, while this value goes up to 35.7% for the argillaceous dolomite. From the viewpoint of brittleness index, it is recommended to set the argillaceous dolomite as the target section for fracturing. The mechanical properties are greatly affected by the bedding planes. It can be concluded from Table 3 that the cores parallel to the bedding plane have much lower modulus and larger Poisson's ratio, which weakens the rock brittleness index compared with the cores perpendicular to the bedding plane. The brittleness index has been calculated for the orthogonal cores in Table 3.
Fig. 3. Cross plot between porosity and permeability of the inter-salt shale of the Q4 Lower Layer. 111
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X. Fan et al.
G (3DTS 2 − 4DTC 2) (DTS 2 − DTC 2)
Ed =
0.5(DTS 2
(7)
2DTC 2)
− (DTS 2 − DTC 2)
μd =
(8)
where ρ is the bulk density, kg/m ; DTS is shear wave slowness, μs/ft; DTC is the compression wave slowness, μs/ft; G is the bulk modulus, GPa; Ed is the dynamic Young's modulus, GPa; μd is the dynamic Poisson's ratio. The relationship between the dynamic elastic parameters and the static elastic parameters acquired in the laboratory can be built to predict the static mechanical parameters along the target zone. For the specific specimen at the high level of confining pressure, Fig. 9 shows the correlation relationship of Young's modulus and Poisson's ratio. The static Young's modulus can be expressed by: 3
(9)
Es = 0.47Ed − 5.85 where Ed represents dynamic Young's modulus. The static Poisson's ratio can be expressed by:
μs = 0.94μd + 0.07
(10)
where μd represents dynamic Poisson's ratio. The brittleness profile of the target zone can be calculated according to Eqs. (2)–(10). There is a good consistency between the brittleness index and the brittle mineral content acquired by XRD test. It is concluded that the section from 2605 m to 2610 m in Fig. 10 should be chosen for fracturing. 5. Brittleness index based on full stress-strain curve With the increase of confining pressure, the rock failure is controlled by shearing. The residual strength is obviously increased and the plastic transformation dominates after stress-peak in the stress-strain curve. The brittleness index, based on the average of the dimensionless Young's modulus and Poisson's ratio, cannot reveal the characteristics of the rock failure after the stress peak. Tarasov and Potvin (2013) considered brittleness as the rock capability to self-sustaining macroscopic failure under external loads. It is proposed that the ratio between the post-peak rupture energy and the withdrawn elastic energy should be used as the brittleness evaluation index at high confining pressure.
Fig. 4. SEM images of cores from Q4 Lower Layer.
4.2. Brittleness evaluation by logging data The regression of compression wave slowness (DTC) and shear wave slowness (DTS) can be got through the full wave acoustic logging in the field. Fig. 8 presents the cross plot of DTC and DTS of the inter-salt shale interval in Qianjiang formation. The regression model could be expressed as follows:
K=
ρ DTS 2
2EM σB2 − σC2
=
M−E M
(11)
where E represents the elastic modulus and M is the post-peak modulus. Stress-strain curves of different types of shale are obtained at 50 MPa confining pressure in Fig. 11. All the specimens have high level of residual strength after the peak stress, and the failure mode is shear. Some of the inter-salt shale, such as 6–2 and 7–1 in Fig. 11, almost have no stress peak at the time of rupture and act like plastic material. The brittleness index K, shown in Table 4, can be calculated by Eq. (11) at high confining pressure. The lower the K value is, the more brittle the rock is. The results indicate that the inter-salt shale is semibrittle at the high level of confining pressure and it is less brittle than
The dynamic elastic modulus and Poisson's ratio can be calculated by wave slowness:
G = 304.82
σB2 − σC2 (M − E )
2E
(5)
DTS = 2.22DTC − 29.9
dWe − dWa =− dWe
(6)
Table 2 Mineral composition and brittle mineral content of the core samples. Lithology
Dolomitic Claystone Argillaceous Dolomite
No
6–1 7–3 3–1 6–2 7–2
Brittle Minerals
Clay
Salt Minerals
Brittle Mineral Content%
Dolomite %
Calcite %
Quartz %
Feldspar %
Illite %
Kaolinite %
Salt %
Gypsum %
Glauberite %
9.8 4.4 42.2 67.6 26.1
16.9 35.6 10.1 8.9 2.2
3.0 4.1 7.7 10.1 8.2
29.4 24.5 21.5 – 24.8
28.3 21.7 8.4 10.5 15.8
/ / / / /
0.7 2.3 / 2.7 3.5
/ 1.5 / / 16.1
/ 1.9 / / /
112
59.1 68.6 81.5 86.7 61.3
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Fig. 5. μm CT scans of standard core column with several types of lithology.
Fig. 6. Cross plot of Young's modulus and Poisson's ratio of typical shales. Fig. 8. Regression of DTC and DTS of the inter-salt shale interval.
the gas shale according to the scale of brittleness indices (Tarasov and Potvin, 2013). For the inter-salt shale with low Young's modulus (E < 10 GPa), the energy stored before the peak stress is too small to sustain the fracture propagation, and therefore it's much more ductile. Fig. 12 presents the rupture geometry after failure at the tri-axial compression test under high confining pressure. The crack surface intersects with the bedding plane when the K value of the specimen in Table 4 is smaller, such as 2–27 and 6–1. The angle between the crack surface and bedding plane becomes smaller, and it even decreases to zero when the K value of the specimen is larger, such as 6–2 and 7–1. 6. Discussions
Fig. 7. Brittleness index statistics of the entire core samples.
The elastic parameters of the inter-salt shale at the in-situ conditions are quite different with that of the gas shale. The main reasons for this difference mainly lie in that:
Table 3 The effect of bedding plane on mechanical properties and brittleness index. No.
7–1 7–1 7–3 7–3
Core Direction Compared with Bedding Plane
Confining Pressure (MPa)
Young's Modulus (GPa)
Poisson's Ratio
Brittleness Index
Parallel Perpendicular Parallel Perpendicular
30 30 0 0
1.87 3.52 1.06 2.2
0.45 0.22 0.21 0.17
0.05 0.38 0.37 0.43
(1) The quartz mineral content directly determines the strength of rock. Fig. 13 gives the pie chart of mineral compositions of the entire Q4 Lower Layer. The average quartz content of the inter-salt shale in the Q4 Lower Layer is only 5.8%, which is much lower than that (with average of 21.0%) of the Longmaxi gas shale in Sichuan Basin. On the other hand, the salt minerals containing glauberite, gypsum and salt account for 18.4% weakening the strength of the core samples. 113
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Fig. 9. Cross plot between dynamic and static elastic parameters.
(2) As we can see from Fig. 5, the bedding planes in the core samples of the inter-salt shale are quite developed. The results of the direct shear test, beyond the scope of this paper, indicate that the cohesive force of the bedding plane of the inter-salt is only 0.7–2.9 MPa while this value reaches 4.5–9.9 MPa for the Longmaxi gas shale. (3) The fluid type stored in the pore space of shale is another factor influencing the mechanical property. Low viscosity crude oil is saturated in the inter-salt shale at the downhole condition. The experimental results show that the Young's modulus reduces as much as 50% if the cores are saturated with oil. Although the brittleness index of the inter-salt shale is not as high as that of the gas shale, it could be improved by hydraulic fracturing technology. The fracturing fluid having lower viscosity, e.g. slick water and super critical CO2, tends to penetrate into the pores more easily than that having higher viscosity, and thus could generate much more complex fracture network (Ishida et al., 2012; Zhou et al., 2016). The laboratory results indicate that the Young's modulus is increased by 8.9% while the Poisson's ratio is decreased by 10.9% after the inter-salt shale is saturated with the super critical CO2, and therefore the brittleness index is improved by 36.5%.
Fig. 10. Profile of brittleness index based on well logging data.
7. Conclusions The dolomitic claystone is rich in bedding planes leading to high anisotropy. The argillaceous dolomite is a good candidate for fracturing due to high brittle mineral content. The elastic parameters of the inter-salt shale at the in-situ conditions are quite different with that of the gas shale because of the difference in the mineral composition, bedding plane structure and fluid type stored in the pore space. The brittleness index of the inter-salt shale can be calculated through a modified Rickman model. The transformation model of the dynamic-static mechanic parameters has been built based on the well logging data and the laboratory data. The mechanical brittleness index profile along the wellbore can be obtained and it can be used to select the target zone for fracturing. The inter-salt shale is semi-brittle at the high level of confining pressure according to the full stress-strain curve theory. For the intersalt shale with low Young's modulus (E < 10 GPa), the energy stored before the peak stress is too small to sustain the fracture propagation,
Fig. 11. Stress-strain curves of different types of shales.
Table 4 Brittleness evaluation based on energy evolution. Specimen
Confining Pressure(MPa)
Elastic Modulus,E(GPa)
Post-peak Modulus,M(GPa)
K
Shale from Longmaxi Formation 2-27 Dolomitic Claystone 6-1 Argillaceous Dolomite 3-1 Dolomitic Claystone 7-1 Argillaceous Dolomite 6-2
50 50 50 50 50
19.88 12.6 12.8 4.5 7.2
−10.0 −4.66 −4.02 −0.24 −0.11
2.99 4.05 4.18 19.75 66.45
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Fig. 12. Rupture geometry of the inter-salt shale after the triaxial compression test. Projections to 2040. https://www.eia.gov/outlooks/archive/aeo15/. U.S. Energy Information Administration, April 2013. Annual Energy Outlook 2013 with Projections to 2040. https://www.eia.gov/outlooks/archive/aeo13/. Hou, Z.K., Yang, C.H., Wang, L., Xu, F., 2016. Evaluation method of shale brittleness based on indoor experiments. J. Northeast. Univ., Nat. Sci. 37 (10), 1496–1500. Ishida, T., Niwa, T., Aoyagi, K., Yamakawa, A., Chen, Y., Fukahori, D., Murata, S., Chen, Q., Nakayama, Y., 2012. AE monitoring of hydraulic fracturing laboratory experiment with supercritical and liquid state CO2. In: Rock Engineering and Technology for Sustainable Underground Construction Eurock 2012-the 2012 ISRM International Symposium. 28-30 May, Stockholm, Sweden. Jarvie, D.M., Hill, R.J., Ruble, T.E., Pollastro, R.M., 2007. Unconventional shale-gas systems: the Mississippian Barnett shale of North-central Texas as one model for thermogenic shale-gas assessment. AAPG Bull. 91 (4), 475–499. Jia, C.Z., Zheng, M., Zhang, Y.F., 2012. Unconventional hydrocarbon resources in China and the prospect of exploration and development. Petrol. Explor. Dev. 39 (2), 129–136. Li, Q.H., Chen, M., Jin, Y., Wang, F.P., Hou, B., Zhang, B.W., 2012. Indoor evaluation method for shale brittleness and improvement. Chin. J. Rock Mech. Eng. 31 (8), 1680–1685. Munoz, H., Taheri, A., Chanda, E.K., 2016. Fracture energy-based brittleness index development and brittleness quantification by pre-peak strength parameters in rock uniaxial compression. Rock Mech. Rock Eng. 49 (12), 4587–4606. Rickman, R., Mullen, M.J., Petre, J.E., Grieser, W.V., Kundert, D., 2008. A practical use of shale petrophysics for stimulation design optimization: all shale plays are not clones of the Barnett shale. In: SPE Annual Technical Conference and Exhibition. 21-24 September, Denver, Colorado, USA, SPE 115258. Tarasov, B., Potvin, Y., 2013. Universal criteria for rock brittleness estimation under triaxial compression. Int. J. Rock Mech. Min. Sci. 59, 57–69. Wang, F., Gale, J., 2009. Screening criteria for shale-gas systems. GCAGS Trans 59, 779–793. Yuan, J.L., Deng, J.G., Zhang, D.Y., Li, D.H., Yan, W., Chen, C.G., Cheng, L.J., Chen, Z.J., 2013. Fracability evaluation of shale-gas reservoirs. Acta Petrol. Sin. 34 (3), 523–527. Zhou, H., Meng, F.X., Zhang, C.Q., Xu, R.C., Lu, J.J., 2014. Quantitative evaluation of rock brittleness based on stress-strain curve. Chin. J. Rock Mech. Eng. 33 (6), 1114–1122. Zhou, T., Zhang, S.C., Yang, L., Ma, X.F., Zou, Y.S., Lin, H., 2016. Experimental investigation on fracture surface strength softening induced by fracturing fluid imbibition and its impacts on flow conductivity in shale reservoirs. J. Nat. Gas Sci. Eng. 36, 893–905. Zhao, P., Li, X.Q., Sun, J., L, S.N., Fu, T.Y., Su, G.P., Tian, X.W., 2014. Study on mineral composition and brittleness characteristics of shale gas reservoirs from the lower Paleozoic in the Southern Sichuan Basin. Geosci. 28 (2), 396–403.
Fig. 13. Pie chart of mineral compositions of the Q4 Lower Layer.
and therefore it is much more ductile and fails to generate complex fractures. However, the ultra-low viscosity fracturing fluid is beneficial to improving the brittleness index of the inter-salt shale. Acknowledgement Thanks to the support from National Research Council of Science and Technology Major Project of the Ministry of Science and Technology of China (2017ZX05049-003-002), and the fund from State Energy Center for Shale Oil Research and Development (G5800-16-ZSKFNY008). References U.S. Energy Information Administration, April 2015. Annual Energy Outlook 2015 with
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Research paper
Brittleness evaluation of the inter-salt shale oil reservoir in Jianghan Basin in China
T
X. Fana,b,∗, J.Z. Sua,b, X. Changa,c, Z.W. Huanga,b, T. Zhoua,b, Y.T. Guoa,c, S.Q. Wud a
State Energy Center for Shale Oil Research and Development, China SINOPEC Exploration & Production Research Institute, Beijing, China c Institute of Rock and Soil Mechanics, China Academy of Sciences, Wuhan, China d Research Institute of Exploration and Development, Jianghan Oil Field, SINOPEC, Qianjiang, China b
A B S T R A C T
The inter-salt shale in the Qianjiang formation of Jianghan Basin in China are characterized by multiple bedding planes, low rock strength, complicated mineral compositions and high heterogeneity of rock mechanics. The brittleness evaluation of the inter-salt shale has not been investigated. Core samples of the inter-salt shale from the fourth member of the Qianjiang Formation were tested by using CT scan, X-ray diffraction, tri-axial rock test at the in-situ condition. An integrated investigation was conducted on the brittleness evaluation of inter-salt shale from the perspective of core characteristics, brittle mineral components, elastic parameters and the complete stress–strain characteristics of rocks. The results indicate that the inter-salt shale exist with bedding planes and glauberite, which leads to high anisotropy and heterogeneity. The argillaceous dolomite is a good candidate for fracturing due to high brittle mineral content. The elastic parameters of the inter-salt shale at the in-situ conditions are quite different with that of the gas shale because of the difference in the mineral composition, the bedding plane structure and the fluid type stored in the pore space. The brittleness index of the inter-salt shale can be calculated through a modified Rickman model. The transformation model of the dynamic-static mechanic parameters has been built based on the logging data and the laboratory data in order to obtain the brittleness index profile along the wellbore. The post-peak stress-strain relations indicates the rock has high residual strength at confined stress state, which explains that the rock is damaged through shear failure along the bedding plane and fails to generate complex fractures. This study would provide theoretical basis for fracability evaluation model used in the inter-salt shale oil reservoir.
1. Introduction Shale oil refers to the oil that exists in a variety of forms in the organic shale, such as free oil, adsorbed oil or dissolved oil (Jia et al., 2012). The United States is the earliest and also the most successful country in the development of the shale oil. According to EIA (EIA, 2015), there are 59 billion barrels of technology recoverable resource in the US. In present, the shale oil production is about 4.3 million barrels per day, accounting for 47% of the total US crude oil production. The major shale oil plays in the US include Bakken, Eagle Ford, Monterey and Niobrara, which are all formed in the marine sedimentary environment. The shale oil resources in China are formed in the continental deposition, mainly distributed in the Songliao Basin, Bohai Bay Basin, Ordos Basin, Jianghan Basin and the western Junggar Basin. EIA predicted that there are 32 billion barrels of technology recoverable resource in China (EIA, 2013). The Qianjiang Formation in the Jianghan Basin developed 193 rhythm layers with a cumulative thickness of more than 2000 m, of which the Qian 3 and Qian 4 are the best potential layers with shale oil. The oil is generated and stored in the intersalt shales due to the separation of the upper and lower salt rock.
∗
The brittle mineral content and the elastic rock parameters are the two methods widely used in the brittleness evaluation of marine gas shale both in US and China. Jarvie (Jarvie et al., 2007) proposed that the mass ratio of quartz minerals in the core to all minerals should be used as an index to evaluate the rock brittleness. Wang (Wang and Gale, 2009) believed that the brittleness index should be the sum of the total mass ratio of quartz and dolomite minerals. Due to the complex mineral compositions of China's marine gas-bearing shale, scholars (Zhao et al., 2014) proposed that the mass ratio of quartz, feldspar, calcite and dolomite to the sum of quartz, feldspar, calcite, dolomite and clay mineral can represent the shale brittleness. In the Longmaxi Formation shale in the southern Sichuan Basin, there are high clay mineral content (13.4%–66.1%,with average of 34.7%) and carbonate mineral content (14.6%–80.0%, with average of 35.9%), and relatively lower quartz content (5.2%–41.4%, with average of 21.0%). The contents of brittle minerals of the Longmaxi Formation shale range from 26.9% to 86.6%, with an average of 62.5% suggesting it has good brittleness. The statistics of the Barnett shale in the Fort Worth Basin show that the Young's modulus ranges from 10 GPa to 80 GPa while the Poisson's ratio ranges from 0.1 to 0.4. Rickman (Rickman et al., 2008) proposed a brittle
Corresponding author. 31 Fuxue Rd., Haidian District, Beijing, China. E-mail address: [email protected] (X. Fan).
https://doi.org/10.1016/j.marpetgeo.2018.12.013 Received 1 June 2018; Received in revised form 28 November 2018; Accepted 6 December 2018 Available online 08 December 2018 0264-8172/ © 2018 Elsevier Ltd. All rights reserved.
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is 0.022 MPa/m, which means that the in-situ temperature is 95–100 °C and the in-situ stress is 55–57 MPa. The detail of the core samples is shown in Table 1.
index using the dimensionless Young's modulus and Poisson's ratio for Barnett shale in the Fort Worth Basin. The Rickman model must be corrected to evaluate the shale brittleness because of the variation in rock mechanics in different regions. For the Longmaxi shale in Sichuan Basin, the Young's modulus varies from 8 GPa to 56 GPa and the Poisson's ratio changes from 0.1 to 0.36 (Yuan et al., 2013). Consequently a brittleness index model has to be modified for the Longmaxi shale. The Young's modulus and Poisson's ratio of the rock are obtained under the assumption of elastic deformation, while the rock at the downhole condition usually has plastic deformation and large residual strength at the high confining pressure. Therefore, the Rickman model fails to evaluate the rock brittleness when the rock has high residual strength after the stress peak in the stress-strain curve. It is necessary to comprehensively evaluate the brittleness by using the full stress-strain curve by the tri-axial compression test. Some complicated models have been proposed to evaluate the brittleness from the perspective of strain energy at the pre-peak and the post-peak in the stress-strain curve (Li et al., 2012; Zhou et al., 2014; Hou et al., 2016; Munoz et al., 2016). The inter-salt shale in the Qianjiang formation of Jianghan Basin in China are characterized by multiple bedding planes, low rock strength, complicated mineral compositions and high heterogeneity of rock mechanics compared with the gas shale. The brittleness evaluation of the inter-salt shale has not been investigated. Core samples of inter-salt shale from the Q4 Lower Layer of the Qianjiang Formation are tested by using CT scan, X-ray diffraction, tri-axial rock test at the in-situ conditions. An integrated investigation is conducted on the brittleness evaluation of inter-salt shale. This study would provide theoretical basis for fracability evaluation model of the inter-salt shale oil reservoir.
3.1. Microscopic structures The cross plot between the porosity and permeability of the intersalt shale of the Q4 Lower Layer is shown in Fig. 3. Three points in the upper left of Fig. 3 demonstrate the micro fractures exist in the cores of the inter-salt shale. The average porosity of the matrix is 18.8% while the average permeability is 0.33 mD. The matrix of the inter-salt shale with median porosity and low permeability illustrates that the connectivity between pore spaces is very poor. It is proved by the fact that the pore volume is plugged because of the presence of salt minerals labeled by the red box in Fig. 4. The median pore-throat radius is 21 nm and 219 nm for the dolomitic claystone and argillaceous dolomite respectively. 3.2. Mineral composition X-ray diffraction (XRD) is commonly used to detect the mineral composition of rocks. The mineral composition data of the core samples shown in Table 2 indicates that the clay minerals are mainly made up by illite, while the non-clay minerals consist of quartz, calcite, feldspar, dolomite, pytire, etc. For the argillaceous dolomite, it is featured by low content of clay minerals, high content of dolomite and quartz. For the dolomitic claystone, it has low content of quartz, high content of clay and calcite. The salt rock is mainly made up by NaCl, containing a small amount of glauberite and gypsum. Based on the mineral composition of the inter-salt shale, a brittle mineral content evaluation model is introduced as follows:
2. Geological description The inter-salt shale oil is mainly formed in the dolomitic shale where the lake basin can accommodate the largest space and the supply of organic material is insufficient. It is a continuously distributed layered reservoir with multiple rhythms and large area, as shown in Fig. 1. In the longitudinal direction, the inter-salt shale is mainly composed of five lithologies: argillaceous dolomite, dolomitic claystone, limeclaystone, glauberite-filling dolomitic claystone and salt. Fig. 2 shows the comprehensive strata log diagram of Q3 Layer of the Qianjiang Formation, which has the same lithology with Q4 Lower Layer.
Bm =
wquartz + wdolomite + wfeldspar + wcalcite wtotal
× 100%
(1)
The brittle mineral contents of the argillaceous dolomite are larger than that of the dolomitic claystone, which implies that the argillaceous dolomite is a good candidate for fracturing. However, the mechanical parameters must be taken into consideration to further investigate the shale brittleness. 3.3. Bedding plane Micron CT scan technology is used to observe the micro structure of the inter-salt shale. Fig. 5 shows the structure of standard core column with several types of lithology. The dolomitic claystones, i.e. 6–1 and 7–3, are characterized by the bedding planes with aperture of 0.042–0.72 mm. The effect of bedding plane on mechanical properties will be discussed in the next part. The argillaceous dolomites, i.e. 3–1 and 7–2, are more homogeneous and are not rich in bedding planes. Some of the argillaceous dolomites are filled with glauberite particles (white spots in 2-2 and 7–1) which accounts for 20% of the total volume.
3. Core characteristics The core samples obtained from the downhole are all from the Q4 Lower Layer of Qianjiang formation. The slope of the formation is about 75–80° which implies the bedding plane is almost parallel with the core axial. The temperature gradient is 3.3 °C/100m and the stress gradient
4. Brittleness index 4.1. Brittleness evaluation by laboratory data In order to get the mechanical parameters at the in-situ conditions, all of the inter-salt shale cores are saturated with the low viscosity crude oil. The reservoir temperature (95 °C) and confining pressure (10 MPa, 30 MPa, 50 MPa) are applied in the tri-axial compression experiment. The Young's modulus and Poisson's ratio of the core samples are plotted in Fig. 6, in which the elastic parameters of gas shale from Longmaxi formation in Sichuan basin are compared. According to the experimental results, the Young's modulus of the Q4 Lower Layer of inter-salt shale is between 1 and 13 GPa and the Poisson's ratio is
Fig. 1. Stratigraphic sequence of Qianjiang Formation. 110
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Fig. 2. Comprehensive strata log diagram of Q3 Layer of the Qianjiang Formation.
between 0.11 and 0.48. Generally the inter-salt oil shale has much lower Young's modulus (8 GPa) and larger Poisson's ratio (0.28), while the gas shale has much larger Young's modulus (24 GPa) and lower Poisson's ratio (0.18). It can be inferred that the brittleness of the oil shale is less than that of the gas shale according to Rickman equation (Rickman et al., 2008). The elastic parameters from Q3 Layer to Q4 Lower Layer can be obtained using logging data in section 4.2. The calculated static Young's modulus is within the scope of 0.79–24.6 GPa while the Poisson's ratio lies between 0.28 and 0.38. Overall, a modified brittleness index model suitable for the shallow inter-salt shale is presented as follows:
Table 1 Detail of core samples obtained from the 4th formation of the Qianjiang basin. No.
Lithology
Depth, m
Rhythm
2–2 3–1 3–2 6–1 6–2 6–3 7–1 7–2 7–3
Glauberiteous Claystone Argillaceous Dolomite Salt Rock Dolomitic Claystone Argillaceous Dolomite Salt Rock GlauberiteousClaystone Argillaceous Dolomite Dolomitic Claystone
2108.00–2125.06 2311.64–2311.77 2319.10–2319.28 2505.06–2505.20 2506.62–2506.68 2512.22–2512.42 2612.90–2613.02 2610.80–2610.93 2598.26–2598.46
5 7 7 12 12 12 14 14 14
En =
E − 0.79 56 − 0.79
(2)
νn =
0.48 − ν 0.48 − 0.11
(3)
Bi =
En + νn 2
(4)
The brittleness indexes of the entire 24 specimens have been calculated by Eq. (2) to Eq. (4) and the results are shown in Fig. 7. The average brittleness index of the dolomitic claystone is 33.7%, while this value goes up to 35.7% for the argillaceous dolomite. From the viewpoint of brittleness index, it is recommended to set the argillaceous dolomite as the target section for fracturing. The mechanical properties are greatly affected by the bedding planes. It can be concluded from Table 3 that the cores parallel to the bedding plane have much lower modulus and larger Poisson's ratio, which weakens the rock brittleness index compared with the cores perpendicular to the bedding plane. The brittleness index has been calculated for the orthogonal cores in Table 3.
Fig. 3. Cross plot between porosity and permeability of the inter-salt shale of the Q4 Lower Layer. 111
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G (3DTS 2 − 4DTC 2) (DTS 2 − DTC 2)
Ed =
0.5(DTS 2
(7)
2DTC 2)
− (DTS 2 − DTC 2)
μd =
(8)
where ρ is the bulk density, kg/m ; DTS is shear wave slowness, μs/ft; DTC is the compression wave slowness, μs/ft; G is the bulk modulus, GPa; Ed is the dynamic Young's modulus, GPa; μd is the dynamic Poisson's ratio. The relationship between the dynamic elastic parameters and the static elastic parameters acquired in the laboratory can be built to predict the static mechanical parameters along the target zone. For the specific specimen at the high level of confining pressure, Fig. 9 shows the correlation relationship of Young's modulus and Poisson's ratio. The static Young's modulus can be expressed by: 3
(9)
Es = 0.47Ed − 5.85 where Ed represents dynamic Young's modulus. The static Poisson's ratio can be expressed by:
μs = 0.94μd + 0.07
(10)
where μd represents dynamic Poisson's ratio. The brittleness profile of the target zone can be calculated according to Eqs. (2)–(10). There is a good consistency between the brittleness index and the brittle mineral content acquired by XRD test. It is concluded that the section from 2605 m to 2610 m in Fig. 10 should be chosen for fracturing. 5. Brittleness index based on full stress-strain curve With the increase of confining pressure, the rock failure is controlled by shearing. The residual strength is obviously increased and the plastic transformation dominates after stress-peak in the stress-strain curve. The brittleness index, based on the average of the dimensionless Young's modulus and Poisson's ratio, cannot reveal the characteristics of the rock failure after the stress peak. Tarasov and Potvin (2013) considered brittleness as the rock capability to self-sustaining macroscopic failure under external loads. It is proposed that the ratio between the post-peak rupture energy and the withdrawn elastic energy should be used as the brittleness evaluation index at high confining pressure.
Fig. 4. SEM images of cores from Q4 Lower Layer.
4.2. Brittleness evaluation by logging data The regression of compression wave slowness (DTC) and shear wave slowness (DTS) can be got through the full wave acoustic logging in the field. Fig. 8 presents the cross plot of DTC and DTS of the inter-salt shale interval in Qianjiang formation. The regression model could be expressed as follows:
K=
ρ DTS 2
2EM σB2 − σC2
=
M−E M
(11)
where E represents the elastic modulus and M is the post-peak modulus. Stress-strain curves of different types of shale are obtained at 50 MPa confining pressure in Fig. 11. All the specimens have high level of residual strength after the peak stress, and the failure mode is shear. Some of the inter-salt shale, such as 6–2 and 7–1 in Fig. 11, almost have no stress peak at the time of rupture and act like plastic material. The brittleness index K, shown in Table 4, can be calculated by Eq. (11) at high confining pressure. The lower the K value is, the more brittle the rock is. The results indicate that the inter-salt shale is semibrittle at the high level of confining pressure and it is less brittle than
The dynamic elastic modulus and Poisson's ratio can be calculated by wave slowness:
G = 304.82
σB2 − σC2 (M − E )
2E
(5)
DTS = 2.22DTC − 29.9
dWe − dWa =− dWe
(6)
Table 2 Mineral composition and brittle mineral content of the core samples. Lithology
Dolomitic Claystone Argillaceous Dolomite
No
6–1 7–3 3–1 6–2 7–2
Brittle Minerals
Clay
Salt Minerals
Brittle Mineral Content%
Dolomite %
Calcite %
Quartz %
Feldspar %
Illite %
Kaolinite %
Salt %
Gypsum %
Glauberite %
9.8 4.4 42.2 67.6 26.1
16.9 35.6 10.1 8.9 2.2
3.0 4.1 7.7 10.1 8.2
29.4 24.5 21.5 – 24.8
28.3 21.7 8.4 10.5 15.8
/ / / / /
0.7 2.3 / 2.7 3.5
/ 1.5 / / 16.1
/ 1.9 / / /
112
59.1 68.6 81.5 86.7 61.3
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Fig. 5. μm CT scans of standard core column with several types of lithology.
Fig. 6. Cross plot of Young's modulus and Poisson's ratio of typical shales. Fig. 8. Regression of DTC and DTS of the inter-salt shale interval.
the gas shale according to the scale of brittleness indices (Tarasov and Potvin, 2013). For the inter-salt shale with low Young's modulus (E < 10 GPa), the energy stored before the peak stress is too small to sustain the fracture propagation, and therefore it's much more ductile. Fig. 12 presents the rupture geometry after failure at the tri-axial compression test under high confining pressure. The crack surface intersects with the bedding plane when the K value of the specimen in Table 4 is smaller, such as 2–27 and 6–1. The angle between the crack surface and bedding plane becomes smaller, and it even decreases to zero when the K value of the specimen is larger, such as 6–2 and 7–1. 6. Discussions
Fig. 7. Brittleness index statistics of the entire core samples.
The elastic parameters of the inter-salt shale at the in-situ conditions are quite different with that of the gas shale. The main reasons for this difference mainly lie in that:
Table 3 The effect of bedding plane on mechanical properties and brittleness index. No.
7–1 7–1 7–3 7–3
Core Direction Compared with Bedding Plane
Confining Pressure (MPa)
Young's Modulus (GPa)
Poisson's Ratio
Brittleness Index
Parallel Perpendicular Parallel Perpendicular
30 30 0 0
1.87 3.52 1.06 2.2
0.45 0.22 0.21 0.17
0.05 0.38 0.37 0.43
(1) The quartz mineral content directly determines the strength of rock. Fig. 13 gives the pie chart of mineral compositions of the entire Q4 Lower Layer. The average quartz content of the inter-salt shale in the Q4 Lower Layer is only 5.8%, which is much lower than that (with average of 21.0%) of the Longmaxi gas shale in Sichuan Basin. On the other hand, the salt minerals containing glauberite, gypsum and salt account for 18.4% weakening the strength of the core samples. 113
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Fig. 9. Cross plot between dynamic and static elastic parameters.
(2) As we can see from Fig. 5, the bedding planes in the core samples of the inter-salt shale are quite developed. The results of the direct shear test, beyond the scope of this paper, indicate that the cohesive force of the bedding plane of the inter-salt is only 0.7–2.9 MPa while this value reaches 4.5–9.9 MPa for the Longmaxi gas shale. (3) The fluid type stored in the pore space of shale is another factor influencing the mechanical property. Low viscosity crude oil is saturated in the inter-salt shale at the downhole condition. The experimental results show that the Young's modulus reduces as much as 50% if the cores are saturated with oil. Although the brittleness index of the inter-salt shale is not as high as that of the gas shale, it could be improved by hydraulic fracturing technology. The fracturing fluid having lower viscosity, e.g. slick water and super critical CO2, tends to penetrate into the pores more easily than that having higher viscosity, and thus could generate much more complex fracture network (Ishida et al., 2012; Zhou et al., 2016). The laboratory results indicate that the Young's modulus is increased by 8.9% while the Poisson's ratio is decreased by 10.9% after the inter-salt shale is saturated with the super critical CO2, and therefore the brittleness index is improved by 36.5%.
Fig. 10. Profile of brittleness index based on well logging data.
7. Conclusions The dolomitic claystone is rich in bedding planes leading to high anisotropy. The argillaceous dolomite is a good candidate for fracturing due to high brittle mineral content. The elastic parameters of the inter-salt shale at the in-situ conditions are quite different with that of the gas shale because of the difference in the mineral composition, bedding plane structure and fluid type stored in the pore space. The brittleness index of the inter-salt shale can be calculated through a modified Rickman model. The transformation model of the dynamic-static mechanic parameters has been built based on the well logging data and the laboratory data. The mechanical brittleness index profile along the wellbore can be obtained and it can be used to select the target zone for fracturing. The inter-salt shale is semi-brittle at the high level of confining pressure according to the full stress-strain curve theory. For the intersalt shale with low Young's modulus (E < 10 GPa), the energy stored before the peak stress is too small to sustain the fracture propagation,
Fig. 11. Stress-strain curves of different types of shales.
Table 4 Brittleness evaluation based on energy evolution. Specimen
Confining Pressure(MPa)
Elastic Modulus,E(GPa)
Post-peak Modulus,M(GPa)
K
Shale from Longmaxi Formation 2-27 Dolomitic Claystone 6-1 Argillaceous Dolomite 3-1 Dolomitic Claystone 7-1 Argillaceous Dolomite 6-2
50 50 50 50 50
19.88 12.6 12.8 4.5 7.2
−10.0 −4.66 −4.02 −0.24 −0.11
2.99 4.05 4.18 19.75 66.45
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Fig. 12. Rupture geometry of the inter-salt shale after the triaxial compression test. Projections to 2040. https://www.eia.gov/outlooks/archive/aeo15/. U.S. Energy Information Administration, April 2013. Annual Energy Outlook 2013 with Projections to 2040. https://www.eia.gov/outlooks/archive/aeo13/. Hou, Z.K., Yang, C.H., Wang, L., Xu, F., 2016. Evaluation method of shale brittleness based on indoor experiments. J. Northeast. Univ., Nat. Sci. 37 (10), 1496–1500. Ishida, T., Niwa, T., Aoyagi, K., Yamakawa, A., Chen, Y., Fukahori, D., Murata, S., Chen, Q., Nakayama, Y., 2012. AE monitoring of hydraulic fracturing laboratory experiment with supercritical and liquid state CO2. In: Rock Engineering and Technology for Sustainable Underground Construction Eurock 2012-the 2012 ISRM International Symposium. 28-30 May, Stockholm, Sweden. Jarvie, D.M., Hill, R.J., Ruble, T.E., Pollastro, R.M., 2007. Unconventional shale-gas systems: the Mississippian Barnett shale of North-central Texas as one model for thermogenic shale-gas assessment. AAPG Bull. 91 (4), 475–499. Jia, C.Z., Zheng, M., Zhang, Y.F., 2012. Unconventional hydrocarbon resources in China and the prospect of exploration and development. Petrol. Explor. Dev. 39 (2), 129–136. Li, Q.H., Chen, M., Jin, Y., Wang, F.P., Hou, B., Zhang, B.W., 2012. Indoor evaluation method for shale brittleness and improvement. Chin. J. Rock Mech. Eng. 31 (8), 1680–1685. Munoz, H., Taheri, A., Chanda, E.K., 2016. Fracture energy-based brittleness index development and brittleness quantification by pre-peak strength parameters in rock uniaxial compression. Rock Mech. Rock Eng. 49 (12), 4587–4606. Rickman, R., Mullen, M.J., Petre, J.E., Grieser, W.V., Kundert, D., 2008. A practical use of shale petrophysics for stimulation design optimization: all shale plays are not clones of the Barnett shale. In: SPE Annual Technical Conference and Exhibition. 21-24 September, Denver, Colorado, USA, SPE 115258. Tarasov, B., Potvin, Y., 2013. Universal criteria for rock brittleness estimation under triaxial compression. Int. J. Rock Mech. Min. Sci. 59, 57–69. Wang, F., Gale, J., 2009. Screening criteria for shale-gas systems. GCAGS Trans 59, 779–793. Yuan, J.L., Deng, J.G., Zhang, D.Y., Li, D.H., Yan, W., Chen, C.G., Cheng, L.J., Chen, Z.J., 2013. Fracability evaluation of shale-gas reservoirs. Acta Petrol. Sin. 34 (3), 523–527. Zhou, H., Meng, F.X., Zhang, C.Q., Xu, R.C., Lu, J.J., 2014. Quantitative evaluation of rock brittleness based on stress-strain curve. Chin. J. Rock Mech. Eng. 33 (6), 1114–1122. Zhou, T., Zhang, S.C., Yang, L., Ma, X.F., Zou, Y.S., Lin, H., 2016. Experimental investigation on fracture surface strength softening induced by fracturing fluid imbibition and its impacts on flow conductivity in shale reservoirs. J. Nat. Gas Sci. Eng. 36, 893–905. Zhao, P., Li, X.Q., Sun, J., L, S.N., Fu, T.Y., Su, G.P., Tian, X.W., 2014. Study on mineral composition and brittleness characteristics of shale gas reservoirs from the lower Paleozoic in the Southern Sichuan Basin. Geosci. 28 (2), 396–403.
Fig. 13. Pie chart of mineral compositions of the Q4 Lower Layer.
and therefore it is much more ductile and fails to generate complex fractures. However, the ultra-low viscosity fracturing fluid is beneficial to improving the brittleness index of the inter-salt shale. Acknowledgement Thanks to the support from National Research Council of Science and Technology Major Project of the Ministry of Science and Technology of China (2017ZX05049-003-002), and the fund from State Energy Center for Shale Oil Research and Development (G5800-16-ZSKFNY008). References U.S. Energy Information Administration, April 2015. Annual Energy Outlook 2015 with
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