eurock 2016_bohloli et al
TRANSCRIPT
Outline
Background
Theoretical concept: potential of microseismicity
Experimental method: direct shear box
Results
Conclusions
BackgroundLocation od Svalbard
Surface seismic array
(Courtesy of NORSAR)
Background
Geological sequence and well tests in borehole Dh6
Injection zone
Bohloli et al. (2014)
Background
θt
's
t
Change in fluid pressure
σ1-Pfσ3-Pf2θ
σ3σ3σn-Pf
σ1
σ1
90
60
30
0
330
300
270
240
210
180
150
120
Stereonets (poles to planes): Dh6, z= 428
2
4
6
8
10
12
14
16
18
[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
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.
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.
Direct shear box rig at NGI Investigation of velocity strengthening/weakening
σv
σH
Shear box
Max. normal force 450 kNMax. shear force 250 kNMax. shear displac. 50 mmMax. pore pressure 20 barFluid flow in fracture plane with/without conf. pressureTesting on intact or pre-fractured specimens
Sample preparation for DST
A pre-fractured Rurikfjellet shale tested in DST
Experimental results:Velocity-stepping Direct Shear Test
Friction coefficient vs shear velocity
Friction coefficient of LYB shale sample increases with increasing velocitySlip on bedding-parallel fractures may be non-sesimic
Mineralogy and seismicity
Mineralogy can give useful information on the potential of seismicity or aseismicity of rocks.
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.
Thank you for your attention!
Sponsors:
Backup slides follow
Area correction factorcontact area changes proportional to displacement
s
t
σ1σ3
Critical injection pressure, Pc
σ2
Mohr circle representation of stress state and critical injection pressure.
Minimum horizontal stress, Shmin [MPa]0 5 10 15 20 25 30
Max
imum
hor
izon
tal s
tress
, SH
max
[MP
a]
0
5
10
15
20
25
30
Sv SHmax,low bound
SHmax,upper bound
SS
RF
NF
Pp_insitu = 4.3 Mpa(Assumed as hydrostatic)
Bound to cause wellbore breakout (if any)
Background
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
ff ehgp
RRQk
2)ln( 12