Mathematical Modelling of a Fault Slip Induced by Water Injection

Transcription

1 Mathematical Modelling of a Fault Slip Induced by Water Injection T. S. Nguyen, 1 J.Rutqvist 2 and Y. Gugliemi 2 1 Canadian Nuclear Safety Commission 2 Lawrence Berkeley National Laboratory ComGeo IV Symposium Assisi, Italy May 2 4, 2018 e-doc

2 Content Rationale for fault injection experiments and modelling Mont Terri fault injection experiments Mathematical model for secondary fault injection Model results Conclusions and future work 2

3 Need to Better Understand Fault Slip Mechanisms Induced seismicity due to waste water injection (petroleum industry), CO 2 storage and other activities For geological disposal of radioactive waste, potential re-activation of a nearby fault can be caused by several factors such as pore pressure increase due to radiogenic heat or water infiltration after future glaciation-deglaciation cycles 3

4 The CNSC Involvement in Fault Slip Modelling The Canadian Nuclear Safety Commission (CNSC) is Canada s nuclear regulator The CNSC performs independent research on safety aspects related to the deep geological disposal of radioactive waste In this example of research, the CNSC collaborates with other researchers on the modelling of fault slip experiments at the Mont Terri underground research facility This research will allow a better understanding of fault slip mechanisms and how they might impact the long-term safety of deep geological repositories 4

5 Fault Slip Tests at Mont Terri 5

6 Secondary Fault Injection Experimental Set-up 6

7 Secondary Fault Injection FE Model σ y =6 MPa; p=0.5 MPa North σ z =7 MPa; p=0.5 MPa σ x =3.2 MPa; p=0.5 MPa Injection Fault Fault plane dip 65 o, strike N45 o E Roller; p=0.5 MPa 7

8 Rock Matrix Model Rock matrix modelled as isotropic poro-elastic medium: bulk modulus 5.9 GPa, shear modulus 2.3 Gpa permeability m2 8

9 Fault Mechanical Properties Fault modelled as transversely isotropic poroelastoplastic medium: young moduli: 15 Gpa (perpendicular to fault) 60 Gpa (in fault plane) shear modulus: 4GPa Mohr-Coulomb yield criterion with non-associated flow rule: friction angle 22 o, dilation angle 17 o 9

10 Fault Permeability Model Fault permeability: s: fracture spacing b h : hydraulic aperture of each fracture bh = bhi + bhe + A bhp where Db he elastic fracture opening; Db hp plastic opening; A damage enhancing factor 10

11 Displacement at Injection Point 70 Displacement (microm) Time (seconds) Injection pressure disp_north _experimental disp_west_experimental 11

12 Injection Flow Rate Flow rate (L/min) Flow_measured flow_model Time (seconds) 12

13 Pressure at Monitoring Point P3 70 Pressure (bar) Time (seconds) injection point monitoring point P3_model Injection P3 13

14 Stresses Along Fault P3 normal effective stress Injection point normal effective stress P3 shear stress Injection point shear stress 14

15 Fault Failure 15

16 Fault Permeability 16

17 Fault Opening and Pressure 17

18 Normal and Shear Displacement at Injection Point 120 D i s p l a c e m e n t ( µ m ) Time (seconds) Normal Shear 18

19 Conclusions Simulation of fault slip test using poro-elastoplastic framework Cause of fault slip and induced seismicity: pore pressure increase With increasing injection pressure: the fault permeability first increases imperceptibly at high injection pressure, shear failure develops and propagates, resulting in permeability increase by a few orders of magnitude around the injection point and a sharp increase in the injection flow 19

20 Conclusions (2) Basic mechanisms seem to be sufficiently captured with poroelastoplastic framework Difficulty resides in characterization of fault properties: heterogeneity, scale effects, anisotropy, spatial variability, permeability relationship with stress and strain Future work: modelling of injection in major fault different permeability functions, directionally-dependent plasticity modelling of seismic events triggered by fault slip scoping analysis: effects of radiogenic heat from a waste repository on nearby fault 20

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