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Experimental and Numerical Investigation of the Effect of Bedding Layer Orientation on Fracture Toughness of Shale Rocks

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Abstract

Fracture toughness is a critical parameter responsible for fracture initiation and propagation. Fracture toughness in shale gas reservoir, however, is highly variable because of its anisotropy and spatial variation of clay content. In this paper, a series of laboratory and numerical experiments are carried out to estimate shale fracture toughness at different bedding plane orientations with respect to loading direction (angle of bedding layer with respect to Cracked Chevron notched Brazilian disc (CCNBD) test samples) and in different brine solutions. The CCNBD test was conducted on 12 cylindrical samples for fracture toughness. Shale samples were prepared at four different angles (0°, 30°, 45° and 90°) relative to the bedding plane. The prepared specimens were saturated in potassium chloride (KCL) solutions of different concentrations. The laboratory results of toughness have shown to be highly variable with respect to both bedding plane and brine concentration and that the sample at the angle of 90° in 4% KCL concentration exhibited the highest fracture toughness. Numerical simulations based on extended finite element method (XFEM) were also carried out to simulate fracture evolution and propagation in CCNBD samples with different bedding planes. The results have shown that the bedding layers caused the fracture path to deflect. The deviation from straight crack path is caused by mixed mode fracture initiation and propagation instead of tension mode. The mixed mode fracture propagation behavior was verified by analyzing fracture propagation path using both laboratory experiments and numerical simulation.

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Abbreviations

a :

Crack length

a 0 :

Initial chevron notched crack length

α0 :

Initial chevron notched crack length divide Specimen radius

a 1 :

Final chevron notched crack length

α1 :

Final chevron notched crack length divide Specimen radius

αB :

Specimen thickness divide Specimen radius

\(a_{1j}\) :

The node strengthening freedom associated with Heaviside function

\(a_{2j}\) :

The node strengthening freedom associated with Heaviside function

B :

Specimen thickness

CCNBD:

Cracked chevron-notched Brazilian disc

D :

Specimen diameter

F max :

The maximum axial load at failure

G C :

Total mixed-mode fracture energy

G C I :

The normal fracture energy

G C II :

The shear fracture energy

G S :

The amount of mode II and mode III critical strain energy release rate

G T :

The total critical strain energy release rate

H(x):

The step function

K IC :

Mode I fracture toughness

N i :

The node shape function

N j :

The node Heaviside enrichment function

N m :

The node crack tip enrichment function

R :

Specimen radius

R s :

Saw radius

P :

Load on specimen

t :

Notch width

Y*:

Minimum value of the dimensionless stress intensity factor

α:

The exponent

\(u_{i}\) :

The nodal displacement

\(u_{{\text{m}}}^{{{\text{tip}}}}\) :

The crack tip asymptotic displacement field

\(v_{i}\) :

The nodal displacement

\(v_{{\text{m}}}^{{{\text{tip}}}}\) :

The crack tip asymptotic displacement field

σ0max :

The allowable maximum principal stress

Ω:

The entire calculation region

ΩT :

The crack tip unit area

ΩU :

The unit area that crack completely goes through

\(u\) :

Parameter depends on α0 and αB

ν:

Parameter depends on α0 and αB

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Authors and Affiliations

  1. School of Minerals and Energy Resources Engineering, University of New South Wales, Sydney, Australia

    Yu Suo, Zhixi Chen, Sheikh S. Rahman & Huifang Song

Authors
  1. Yu Suo
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  2. Zhixi Chen
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  3. Sheikh S. Rahman
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  4. Huifang Song
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Corresponding author

Correspondence to Sheikh S. Rahman.

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The authors declare that the received funds do not lead to any conflicts of interest regarding the publication of this manuscript.

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Suo, Y., Chen, Z., Rahman, S.S. et al. Experimental and Numerical Investigation of the Effect of Bedding Layer Orientation on Fracture Toughness of Shale Rocks. Rock Mech Rock Eng 53, 3625–3635 (2020). https://doi.org/10.1007/s00603-020-02131-1

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  • DOI: https://doi.org/10.1007/s00603-020-02131-1

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