Tunnel and Underground Space (터널과지하공간)

Korean Society for Rock Mechanics and Rock Engineering (한국암반공학회)
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Experimental Study of Breakdown Pressure, Acoustic Emission, and Crack Morphology in Liquid CO2 Fracturing

액체 이산화탄소 파쇄법의 파쇄 압력, 음향 방출, 균열 형상에 관한 실험적 연구

  • Ha, Seong Jun (School of Civil and Environmental Engineering, Yonsei University) ;
  • Yun, Tae Sup (School of Civil and Environmental Engineering, Yonsei University)
  • 하성준 (연세대학교 공과대학 건설환경공학과) ;
  • 윤태섭 (연세대학교 공과대학 건설환경공학과)
  • Received : 2019.05.17
  • Accepted : 2019.06.27
  • Published : 2019.06.30
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Abstract

The fracturing by liquid carbon dioxide ($LCO_2$) as a fracking fluid has been an alternative to mitigate the environmental issues often caused by the conventional hydraulic fracking since it facilitates the fluid permeation owing to its low viscosity. This study presents how $LCO_2$ injection influences the breakdown pressure, acoustic emission, and fracture morphology. Three fracturing fluids such as $LCO_2$, water, and oil are injected with different pressurization rate to the synthetic and porous mortar specimens. Also, the shale which has been a major target formation in conventional fracking practices is also tested to examine the failure characteristics. The results show that $LCO_2$ injection induces more tortuous and undulated fractures, and particularly the larger fractures are developed in cases of shale specimen. On the other hand, the relationship between the fracturing fluids and the breakdown pressure shows opposite tendency in the tests of mortar and shale specimens.

액체 이산화탄소 파쇄법은 기존 수압 파쇄법에서 물 사용으로 발생하는 환경 문제를 완화시키기 위한 차세대 해결책으로 제안되어 왔으며, 액체 이산화탄소의 낮은 점성도를 이용하여 암석 공극 내 유체 주입을 수월하게 할 수 있다. 본 연구에서는 액체 이산화탄소의 공극 내 주입이 파쇄 과정 중에 발생하는 파쇄 압력, 음향 방출, 균열 형상에 어떻게 영향을 미치는지에 대해 초점을 맞추었다. 이를 위해 점성도가 다른 액체 이산화탄소, 물, 오일을 파쇄 유체로 사용하여 주입 속도를 다르게 하며 인공적으로 제작한 다공성 모르타르 시편을 대상으로 실내실험을 수행하였다. 또한 기존 수압 파쇄법의 주 대상 암종인 셰일 시편의 실험에서 액체 이산화탄소 파쇄법에 의한 셰일의 파괴 특징들을 분석하였다. 실험 결과 이산화탄소 주입 시 균열이 더 비틀린 물결 형상을 띄었으며 특히, 셰일 시편에서는 그 균열 부피가 물 주입에 비해 더 발달하였다. 반면, 파쇄 유체와 파쇄 압력의 관계는 두 시편의 실험에서 반대의 경향을 보였다.

Keywords

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Fig. 1. Schematic illustration of the fracturing experiment system

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Fig. 2. Evolution of borehole pressure, accumulative AE count, permeated volume during (a) LCO2, (b) water, and (c) oil injections at a pressurization rate of 0.2 MPa/sec (Ha et al., 2018)

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Fig. 4. Breakdown pressure with permeated volume normalized by pore volume (Ha et al., 2018)

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Fig. 5. Evolution of borehole pressure and AE count rate during LCO2, water, and oil injections

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Fig. 6. Total AE count emitted during crack extension period at different pressurization rates (Ha et al., 2018)

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Fig. 7. The difference between the breakdown pressure (Pb) and the fracture initiation pressure (Pi) and the ratio of Pi to Pb at different pressurization rates (Ha et al., 2018)

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Fig. 8. 16-bit gray scaled image in 2D and reconstructed fracture in 3D for mortar specimens fractured by (a) LCO2, (b) water, and (c) oil injections (Ha et al., 2018)

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Fig. 9. 16-bit gray scaled image in 2D and reconstructed fracture in 3D for fractured shale specimens of 4 cases (a) SL1, (b) SL2, (c) SW1, and (d) SW2. The specimens of SL1 and SL2 are fractured by LCO2 injection and other specimens of SW1 and SW2 are fractured by water injection

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Fig. 10. Breakdown pressure and Fracture volume in the tests using mortar and shale specimens

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Fig. 11. Fluid efficiency (fracture volume / injected fluid volume) of each fracturing fluid in the tests using mortar and shale specimens

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Fig. 3. (a) Breakdown pressure and (b) permeated volume normalized by pore volume at different pressurization rates (Ha et al., 2018)

Table 1. Properties of mortar and shale specimen

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Table 2. Breakdown pressure and Injected volume measured in fracturing tests using shale specimens

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