SAFOD Stress

How do plate motions stress SAF ?

  1. Plot borehole breakouts & tensile cracks w/Z downhole
  1. Correlate with fault strike (orientation)

Borehole shape ∆ with depth

  1. Cylindrical borehole initially
  1. Rapidly under lotsa stress

homogeneous stress state equal diff σ1, σ2, σ3

  1. Shmax disturbs stress field

∴ Resolve stress orientation by msmt crack orientation

Borehole breakouts

= Elongated intervals with non-circular cross sections

2 diametrically opposed elongated zones

whose long axes share common direction

Formation

Spalling of borehole walls

Due to stress concentration in SHmin orientation (min horiz stress)

Cylindrical bore hole --> lens shaped

Tensile
Cracks

Stress reduction occurs in direction SHmax

∴ Tensile fractures form parallel to SHmax

  1. Tensile cracks from parallel to Shmax

5. Borehole breakouts parallel with SHmin

Orientation SHmax vs Shear Fracture

via hydraulic fracture

If SAF simple shear (theory)

30° between shear fracture & SHmax (σ1)

n.b. opening angle btwn conjugate fractures ~60° thus could be 30° either side of SHmax

Angle observed btwn SAF & SHmax

(from msmt tensile crack orientation vs SAF strike)

some places ~30°

other places - much > 30° (up to 70°)

Implications
high angle

High angle btwn local SAF strike & SHmax

yet SAF constantly creeping...

∴ Decomposed stress vector onto SAF
comprises high normal stress component

V compressive - not ideal for sliding

Measuring SHmax

Orientation tensile fractures

Invert earthquake focal mechanisms

SAFOD Stress Orientation

SHmax trends ~ NE-SW according to equake focal mech

Shmax at depth in pilot hole (0.8-2.2km)

Shmax ~50+/- 17° from SAF strike

Borehole breakouts

SHmax trends ~ NE-SW according to equake focal mech

Evidence
SHmax = σ1

∴ SAF accomodates strike slip motion only

SAF= tanspressive but folds either side accomodate compression

(since both σ1 and σ3 are horizontal for strike slip faults)

In strike slip faults, max principal stress = max horiz stress

Shear failure occurs when:

Shear stress along possible fault overcomes:

  1. Cohesive strength of rock
  1. Resistance along that plane (once shear plane formed)

Friction coefficient

Gradient of coulomb criterion in Mohr space

according to coulomb

Descriptor of frictional strength of rock

Ratio of shear & normal forces required for slide

Forces required either to

  1. initiate slide μs
  1. maintain continual slide μd

SAF Friction Coefficient

Friction coefficient μ determined for SAF & host rocks

How? Deformation of core rock material retrieved from SAFOD drill hole

via shear tests for different slip velocities

Lockner et al 2011

Fault rocks 3 times weaker vs damage zone rocks

Damage zone
μ ~ 0.6

adjacent to S & Centrally deforming zones

Fault gouge
μ ~ 0.2

actively deforming sedimentary rocks

however damage = accumulating in rock

Host rocks are stronger still