Abstract
High brittleness is the prerequisite for the creation of a hydraulic fracture network, which is crucial for shale reservoir development. The test evaluation methods are restricted in the field by lack of continuous brittleness profiles. Mineralogical evaluation cannot incorporate the external factors like stress state. The most widely used method is mechanical evaluation method based on statistical results of Young’s modulus and Poisson’s ratio. This method cannot explain the brittle tensile failure mechanism of shale and contradiction between high brittleness and barriers of high strength rock. With these methods, brittle evaluation and interval selection errors will happen in the application. Starting from the mechanism of rock failure and considering the effect of rock mechanical properties on brittleness, a new method for rock brittleness evaluation has been established. Firstly, the failure surface features were analyzed and the dominant mechanisms in complex failure were identified with triaxial test and scanning electron microscopy test. Secondly, a stress model for shale reservoirs with thin sand and shale interbedded was set up based on elasticity and the factors affecting the stress distribution and failure modes have been analyzed. Eventually, based on experiments and theoretical recognition, the brittleness index for shale was proposed by integrating Young’s modulus, Poisson’s ratio and fracture toughness. Moreover, a method for acquiring brittleness index from well logs was developed from which brittleness profiles for shale formations in Sichuan basin were obtained. Failure features analysis demonstrates that tensile failure dominates during rock brittle failure. It was observed that compression stress on rock is only relevant to mechanical parameters and independent of space location under both internal and external stresses. Finally, comparison with the results from Rickman’s method and microseismic monitoring demonstrates that the method developed in this work possesses more satisfactory results of brittleness evaluation and interval selection in field application.
Similar content being viewed by others
Abbreviations
- B:
-
Dimensionless index
- B a :
-
Intermediate value
- B amax :
-
Maximum intermediate values in a reservoir block
- B amin :
-
Minimum intermediate values in a reservoir block
- BI:
-
Brittleness index
- E i :
-
Young’s modulus of a layer i (MPa)
- E :
-
Young’s modulus of a layer (MPa)
- E max :
-
Maximum reference values of Young’s modulus (MPa)
- E min :
-
Minimum reference values of Young’s modulus (MPa)
- E d :
-
Dynamic elastic modulus (MPa)
- G i :
-
Shear modulus of layer i (MPa)
- h i :
-
The thickness of layer i (m)
- K IC :
-
Fracture toughness (\({\text{MPa}}/\sqrt m\))
- K 0IC :
-
Fracture toughness at zero confining pressure (\({\text{MPa}}/\sqrt m\))
- m j :
-
Average-weighted thickness of m ji (j = 1, 2, 3, 4)
- P c :
-
Confining pressure (MPa)
- S t :
-
Tensile strength (MPa)
- S c :
-
Compression strength (MPa)
- V P :
-
Velocity of P wave (m/s)
- V S :
-
Velocity of S wave (m/s)
- V cl :
-
Clay content
- α :
-
Influence coefficients
- β :
-
Influence coefficients
- v i :
-
Poisson’s ratio of layer i
- v :
-
Poisson’s ratio
- v max :
-
Maximum reference values of Poisson’s ratio
- v min :
-
Minimum reference values of Poisson’s ratio
- μ d :
-
Dynamic Poisson’s ratio
- σ 11 :
-
Far field stress in x directions (MPa)
- σ 22 :
-
Far field stress in z directions (MPa)
- σ 12 :
-
Far field shear stress (MPa)
- σ 11.b :
-
Initial stress in x directions (MPa)
- σ 22.b :
-
Initial stress in z directions (MPa)
- σ12.b :
-
Initial shear stress (MPa)
- σ11.i :
-
Stress in x directions of layer i (MPa)
- σ22.i :
-
Stress in z directions of layer i (MPa)
- σ12.i :
-
Shear stress of layer i (MPa)
- Δε 11 :
-
Strain increment in x directions
- Δε 22 :
-
Strain increment in z directions
- Δε 12 :
-
Shear strain increment
- Δσ 11.i :
-
Stress increment in x directions of layer i (MPa)
- Δσ 33.i :
-
Stress increment in y directions of layer i (MPa)
- Δσ 22.i :
-
Stress increment in z directions of layer i (MPa)
- Δσ 12.i :
-
Shear stress increment of layer i (MPa)
References
Altindag R (2003) Correlation of specific energy with rock brittleness concepts on rock cutting. J S Afr Inst Min Metall 103(3):163–171
Amann F, Thoeny R, Button EA (2011) Insight into the brittle failure behavior of clay shales in unconfined and confined compression. In: ARMA-11-536, 45th US rock mechanics/geomechanics symposium, San Francisco, California
Bernander S (1978) Brittle failures in normally consolidated soils, vol 8–9. Väg-& Vattenbyggaren, pp 49–52
Bishop AW (1967) Progressive failure with special reference to the mechanism causing it. In: Proceedings of the geotechnical Conference, vol 2. Oslo, pp 142–150
Bourne SJ (2003) Contrast of elastic properties between rock layers as a mechanism for the initiation and orientation of tensile failure under uniform remote compression. J Geophys Res: Solid Earth 108(B8):2395
Copur H, Bilgin N, Tuncdemir H, Balci C (2003) A set of indices based on indentation tests for assessment of rock cutting performance and rock properties. J S Afr Inst Min Metall 103(9):589–599
Guo TK, Zhang SC, Ge HK (2013) A new method for evaluating ability of forming fracture network in shale reservoir. Rock Soil Mech 34(4):947–954
Hajiabdolmajid V, Kaiser P (2003) Brittleness of rock and stability assessment in hard rock tunneling. Tunn Undergr Space Technol 18(1):35–48
Hammes U, Krause M, Mutti M (2013) Unconventional reservoir potential of the upper Permian Zechstein Group: a slope to basin sequence stratigraphic and sedimentological evaluation of carbonates and organic-rich mudrocks, Northern Germany. Environ Earth Sci 70(8):3797–3816
Honda H, Sanada Y (1956) Hardness of coal. Fuel 35(4):451–461
Hucka V, Das B (1974) Brittleness determination of rocks by different methods. Int J Rock Mech Min Sci Geomech Abstr 11(10):389–392
Jarvie DM, Hill RJ, Ruble TE, Pollastro RM (2007) Unconventional shale-gas systems: The Mississippian Barnett Shale of north-central Texas as one model for thermogenic shale-gas assessment. Am Assoc Pet Geol Bull 91:475–499
Jin Y, Chen M, Zhang XD (2001) Determination of fracture toughness for deep well rock with geophysical logging data. Chin J Rock Mech Eng 20(4):454–456 (in Chinese)
Jin X, Shah SN, Roegiers J-C, Zhang B (2014) Fracability evaluation in shale reservoirs-an integrated petrophysics and geomechanics approach. In: SPE 168589, SPE hydraulic fracturing technology conference, The Woodlands, Texas, USA
Kerschke DI, Schulz HM (2013) The shale gas potential of Tournaisian, Visean, and Namurian black shales in North Germany: baseline parameters in a geological context. Environ Earth Sci 70(8):3817–3837
Kissinger A, Helmig R, Ebigbo A, Class H, Lange T, Sauter M, Heitfeld M, Klünker J, Jahnke W (2013) Hydraulic fracturing in unconventional gas reservoirs—risks in the geological system, Part 2. Environ Earth Sci 70(8):3855–3873
Lange T, Sauter M, Heitfeld M, Schetelig K, Jahnke W, Kissinger A, Helmig R, Ebigbo A, Class H (2013) Hydraulic fracturing in unconventional gas reservoirs—risks in the geological system, Part 1. Environ Earth Sci 70(8):3839–3853
Lawn BR, Marshall DB (1979) Hardness, toughness, and brittleness: an indentation analysis. J Am Ceram Soc 62(7–8):347–350
Li QH, Chen M, Wang FP, Jin Y, Li ZM (2012) Influences of engineering factors on shale gas productivity: a case study from the Haynesville shale gas reservoir in North America. Nat Gas Ind 32(4):54–59 (in Chinese)
Liu XJ, Liu TY, Liu SQ (2006) Principle and engineering application of logging [M]. Petroleum Industry Press, pp 118–119, 148–149 (in Chinese)
Mayerhofer M, Lolon E, Warpinski N et al (2010) What is stimulated reservoir volume? SPE Prod Oper 25(1):89–98
Nagel NB, Gil I, Sanchez NM, Damjanac B (2011) Simulating hydraulic fracturing in real fractured rocks-overcoming the limits of pseudo 3D models. In: SPE-140480, SPE hydraulic fracturing technology conference, The Woodlands, Texas, USA
Paterson MS, Wong T (2005) Experimental rock deformation: the brittle field [M]. Springer, Berlin
Quinn JB, Quinn G (1997) Indentation brittleness of ceramics: a fresh approach. J Mater Sci 32(16):4331–4346
Rickman R, Mullen MJ, Petre JE, Grieser WV, 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-115258, SPE annual technical conference and exhibition, Denver, Colorado, USA
Timoshenko SP, Goodier JN (1970) Theory of elasticity. McGraw-Hill, New York, p 567
Wong T, Baud P (2012) The brittle-ductile transition in porous rock: a review [J]. J Struct Geol 44:25–53
Yagiz S (2006) Overview on geo-mechanical assessments of Denizli travertines in Turkey. In: 284, Proceedings of 10th International Association of Engineering Geologists Congress, Engineering Geology for Tomorrows Cities
Acknowledgments
The authors thank the National Natural Science Foundation of China (51374178) and National Science and Technology Major Project of China (2011ZX05002-004-007HZ) for their financial support.
Author information
Authors and Affiliations
Corresponding author
Rights and permissions
About this article
Cite this article
Guo, JC., Zhao, ZH., He, SG. et al. A new method for shale brittleness evaluation. Environ Earth Sci 73, 5855–5865 (2015). https://doi.org/10.1007/s12665-015-4268-z
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s12665-015-4268-z