Tunnel and Underground Space (터널과지하공간)

Korean Society for Rock Mechanics and Rock Engineering (한국암반공학회)
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Numerical modelling of Fault Reactivation Experiment at Mont Terri Underground Research Laboratory in Switzerland: DECOVALEX-2019 TASK B (Step 2)

스위스 Mont Terri 지하연구시설 단층 내 유체 주입시험 모델링: 국제공동연구 DECOVALEX-2019 Task B(Step 2)

  • 박정욱 (한국지질자원연구원 지질환경연구본부) ;
  • ;
  • ;
  • ;
  • 박의섭 (한국지질자원연구원 지질환경연구본부)
  • Received : 2019.06.18
  • Accepted : 2019.06.26
  • Published : 2019.06.30
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Abstract

We simulated the fault reactivation experiment conducted at 'Main Fault' intersecting the low permeability clay formations of Mont Terri Underground Research Laboratory in Switzerland using TOUGH-FLAC simulator. The fluid flow along a fault was modelled with solid elements and governed by Darcy's law with the cubic law in TOUGH2, whereas the mechanical behavior of a single fault was represented by creating interface elements between two separating rock blocks in FLAC3D. We formulate the hydro-mechanical coupling relation of hydraulic aperture to consider the elastic fracture opening and failure-induced dilation for reproducing the abrupt changes in injection flow rate and monitoring pressure at fracture opening pressure. A parametric study was conducted to examine the effects of in-situ stress condition and fault deformation and strength parameters and to find the optimal parameter set to reproduce the field observations. In the best matching simulation, the fracture opening pressure and variations of injection flow rate and monitoring pressure showed good agreement with field experiment results, which suggests the capability of the numerical model to reasonably capture the fracture opening and propagation process. The model overestimated the fault displacement in shear direction and the range of reactivated zone, which was attributed to the progressive shear failures along the fault at high injection pressure. In the field experiment results, however, fracture tensile opening seems the dominant mechanism affecting the hydraulic aperture increase.

본 연구에서는 TOUGH-FLAC 연동해석기법을 이용하여 Mont Terri 지하연구시설에서 수행된 단층 내 물 주입시험을 수치적으로 모델링하고, 단층의 재활성과 수리역학적 거동 특성을 살펴보았다. TOUGH2 해석에서는 단층을 Darcy의 법칙과 삼승법칙(Cubic law)을 따르는 연속체 요소로 모델링하였으며, FLAC3D 해석에서는 미끄러짐과 개폐가 허용되는 불연속 인터페이스 요소를 통해 모사하였다. 현장에서 획득한 단층의 균열개방압력(fracture opening pressure), 주입율, 모니터링 압력, 변위 곡선 등을 바탕으로, 단층의 탄성적 변형과 파괴에 의한 수직팽창 특성을 반영할 수 있는 수리간극모델과 수리역학 커플링 관계를 해석모델에 반영하였다. 한편, 현지응력 조건, 단층의 강도 및 변형 특성에 따른 파라미터 해석을 실시하여 각 입력변수가 해석 결과에 미치는 영향을 분석하였으며, 이를 통해 현장시험 결과를 가장 잘 재현할 수 있는 파라미터 조합을 선정하였다. 해석 결과, 균열개방압력에서 단층의 주입율과 모니터링 압력이 크게 증가하는 현상을 합리적으로 재현할 수 있었다. 하지만, 동일한 입력 변수 조건에서 단층의 전단변위와 파괴영역의 범위는 현장시험 결과에 비해 과대평가되는 결과를 보였다. 이는 해석모델에서는 고압의 주입조건에서 단층의 지속적인 전단파괴가 유도되는 반면, 현장에서는 수리간극의 변화가 전단 미끄러짐보다는 인장력에 의한 단층면의 개방(tensile opening)에 크게 의존하는 것으로 추정되기 때문이다.

Keywords

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Fig. 1. Geological profile along the Mont Russelin and Mont Terri tunnels (Bossart et al., 2017)

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Fig. 2. Mont Terri 'Main Fault' reactivation experiment (after Guglielmi et al., 2014, Guglielmin et al., 2017): (a) SIMFIP test

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Fig. 3. Injection pressure scheme

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Fig. 4. Field experimental results selected for Task B step 2 modelling: (a) Injection and monitoring pressures and injection flow rate; (b) relative displacement of upper anchor to lower anchor (in vertical, northern and western directions)

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Fig. 5. FLAC3D model: (a) rotation of coordinate system for representing in-situ stress condition; (b) numerical mesh in rotated coordinate system

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Fig. 6. Initial conditions of normal stress and shear stress acting on fault

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Fig. 7. Variations of Injection flow rate obtained from field experiment and numerical model (best matching case)

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Fig. 8. Variations of injection pressure and monitoring pressures obtained from field experiment and numerical model (best matching case)

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Fig. 9. Variations of Injection flow rate obtained from field experiment and numerical model (best matching case)

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Fig. 10. Displacement vector at upper and lower anchors: (a) 420 s and (b) 453 s of water injection (best matching case)

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Fig. 11. Fault slip zone and shear displacement in meters simulated at 453 s of injection (best matching case)

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Fig. 12. Variations of injection flow rate with different normal stiffness and creation aperture magnitude: (a) kn= 70 GPa, hc=28 μm; (b) Kn= 55 GPa, hc= 40 μm

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Fig. 13. Effects of fault dip angle on anchor displacement: (a) dip angle of 60°; (b) dip angle of 70°

Table 1. Parametric study to match field data

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