eurock 2016_bohloli et al

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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

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