1. Outline
Background
Theoretical concept: potential of microseismicity
Experimental method: direct shear box
Results
Conclusions

2. Background
Location od Svalbard
Surface seismic array
(Courtesy of NORSAR)

3. Background
Geological sequence
and well tests in
borehole Dh6
Injection
zone
Bohloli et al. (2014)

4. Background
θ
t
'
s
t
Change in fluid pressure
σ1-Pfσ3-Pf
2θ
σ3σ3
σn-Pf
σ1
σ1
[MPa]
Critical
orientation
The magnitude of pressure
change that can initiate slip of
fractures in wellbore Dh6,
depth of 428 m.
Parameters Unit Values
In-situ vertical stress, σv [MPa] 9.6
In-situ minimum horizontal stress, σh [MPa] 13.0
In-situ maximum horizontal stress, σH [MPa] 23.5
Pore pressure, Pp [MPa] 4.3
Friction coefficient, μ [-] 0.6

5. Definition of the research problem
1. Injection test at Longyerabyen pilot site has likely
resulted in failure of formation
2. There was a microseismic monitoring array
present at surface but did not record any signal
3. Is there any potential for non-seismic (aseismic)
failure?
We utilize laboratory direct shear test to examine the
concept of seismic/aseismic slip.

6. Theoretical concept:
Velocity stepping in direct shear test (DST) to analyse Seismic risk
The concept from: Dieterich (1978) Figure modified after
increase of friction coef. (vel. strengthening)
decrease of friction coef. (vel. weakening)
Increase of shear vel.

7. Direct shear box rig at NGI
Investigation of velocity strengthening/weakening σv
σH
Shear box
Max. normal force 450 kN
Max. shear force 250 kN
Max. shear displac. 50 mm
Max. pore pressure 20 bar
Fluid flow in fracture plane with/without conf. pressure
Testing on intact or pre-fractured specimens

8. Sample
preparation for DST

9. A pre-fractured Rurikfjellet shale
tested in DST

10. Experimental results:Velocity-stepping Direct Shear Test

11. Friction coefficient vs shear velocity
Friction coefficient of LYB shale sample increases with increasing velocity
Slip on bedding-parallel fractures may be non-sesimic

12. Mineralogy and seismicity
Mineralogy can give useful
information on the potential
of seismicity or aseismicity of
rocks.

13. Conclusions
Running direct shear box tests with different velocities can give
useful information that might be linked to the risk for seismic
hazards
Results of this study imply non-seismic slip of Svalbard shale at
the tested interval and are consistent with the field
observations
Test result are also in agreement with implications from the
mineral composition of shale.

14. Thank you for your attention!
Sponsors:

15. Backup slides follow

16. Area correction factor
contact area changes proportional to displacement

17. s
t
σ1σ3
Critical injection
pressure, Pc
σ2
Mohr circle representation of stress state and
critical injection pressure.
SS
RF
NF
Pp_insitu = 4.3 Mpa
(Assumed as hydrostatic)
Bound to cause
wellbore
breakout (if any)
Background

18. Stress-dependent permeability of fractures
Radial flow from a hole drilled at the centre of plug
to fracture plane is used to back-calculate fracture
permeability; kf (Esaki et al., 1995):
f
f
f
ehgp
RRQ
k


2
)ln( 12
