6. Fracturing

Question 1

What is the distinction between Mode I, II, III, IV fractures?

Answer 1

Mode I, Extensional Fractures, displacement normal to the fracture walls in an opening sense

Mode II, III, Shear Fractures, displacement parallel to the fracture walls - the two modes distinguish two orthogonal slip directions

Mode IV, Contractional Fractures, displacement normal to the fracture walls in a closing sense (anticracks)

Question 2

What is the distinction between an extension fracture, a shear fracture, a stylolite, and a hybrid fracture?

Answer 2

Extension Fractures: open parallel to the direction of minimum compressive stress (σ₃)

Shear Fractures: form at an angle of less than 45 degrees to σ₁ and ideally contain σ₂

Stylolites: contractional, irregular surfaces perpendicular to the direction of maximum compressive stress σ₁

Hybrid Fractures: mixed mode, combination of other types (usually extention + shear OR compression + shear)

Question 3

If I were to draw a pair of conjugate shear fractures could you indicate the orientation of σ₁ and σ₃?

Answer 3

Question 4

If I were to draw a tension gash array, could you indicate the direction of σ₁, σ₃, the sense of shear, the expected orientation of a pressure solution cleavage?

Answer 4

Question 5

calculate σ_n and τ on a fracture plane using a Mohr diagram, and be able to show the effect that increasing pore fluid pressure has on the location of the Mohr circle

Answer 5

increasing pore fluid pressure shifts the Mohr circle left without changing its diametre

Question 6

What is a failure envelope on a Mohr diagram? Sketch the expected orientation of fractures with respect to σ₁ and σ₃ for Mohr circles that touch the failure envelope on a Mohr diagram in different places.

Answer 6

plotting the normal stress (σ_n) and shear stress (τ ) on the fracture plane at failure for sample deformed at different confining pressures, defines a failure slope for a mohr diagram

Question 7

Be able to infer sense of shear from wing cracks and horsetailing at the tip of a shear fracture

Answer 7

Question 8

What is hydraulic fracturing?

Answer 8

when fluid pressure builds up sufficiently to generate extension fractures

• minerals can then precipitate making veins (solubility decreases with fluid pressure)