Cyclic Hydraulic Fracturing of Cubic Granite Samples Under Triaxial Stress State With Acoustic Emission, Injectivity and Fracture Measurements
- L. Zhuang (Korea Institute of Civil Engineering and Building Technology) | K. Y. Kim (Korea Institute of Civil Engineering and Building Technology) | S. G. Jung (University of Science and Technology) | M. Diaz (University of Science and Technology) | K.-B. Min (Seoul National University) | S. Park (Seoul National University) | A. Zang (Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences) | O. Stephansson (Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences) | G. Zimmermann (Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences) | J. S. Yoon (Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences)
- Document ID
- American Rock Mechanics Association
- 52nd U.S. Rock Mechanics/Geomechanics Symposium, 17-20 June, Seattle, Washington
- Publication Date
- Document Type
- Conference Paper
- 2018. American Rock Mechanics Association
- 5 in the last 30 days
- 118 since 2007
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ABSTRACT: Hydraulic fracturing tests on cubic granite samples with a side length of 50 mm were conducted under a true tri-axial test equipment combined with acoustic emission (AE) monitoring. The applied three principal stresses are 20 MPa (σv), 20 MPa (σH) and 10 MPa (σh), respectively. Experimental results show that cyclic hydraulic fracturing (CHF) can help reduce breakdown pressure as well as induced seismicity, compared to the conventional hydraulic fracturing. However, injectivity enhancement effect is less pronounced. Re-injection tests were conducted on the fractured samples by applying multiple incremental injection rates step-by-step. Propagation of existing fractures and creation of new fractures were confirmed, without inducing larger AE compared to continuous injection. Moreover, based on computed tomography (CT) imaging, fractures along grain boundaries are more frequently observed in CHF, particularly at large number of cycles and re-injection tests with multiple injection rates. Fractures cutting across mineral grains dominate as a result of conventional HF.
Cyclic hydraulic fracturing (CHF) with water injection is suggested to reduce induced seismicity and mitigate larger magnitude events compared to conventional HF with continuous injection (Zang et al., 2013). Laboratory experimental results on core-scale granite cylinders show that the hydraulic breakdown pressure (BP) by cyclic injection is generally 20% less than the one by continuous injection (Zhuang et al., 2016, 2017). Also, maximum amplitude of induced acoustic emission (AE) is reduced by an average of ca. 20%. However, the permeability enhancement effect by the cyclic injection is not manifested compared to the continuous injection (Zhuang et al., 2017). Induced vertical tensile fractures are predominant in these hydraulic fracturing experiments due to the applied stress condition whereas the Enhanced Geothermal System (EGS) expects to generate shear factures in the intact rock mass or along pre-existing discontinuities under in situ stress condition in the field.
Patel et al. (2017) performed CHF tests considering horizontal differential stress on Tennessee sandstone (6% porosity) samples with diameter of 101.6 mm and height of 139.7 mm. They reported that cyclic injection reduces BP by 16% in dry samples while it does not have any effect on saturated samples. They found that the observed process zone generated by cyclic injection is twice that induced by conventional fracturing. Hydraulic fracturing by cyclic injection increases the fracture permeability compared to conventional hydraulic fracturing by a factor of three to ten. These findings show that CHF has advantages in terms of BP reduction and permeability enhancement for sandstone.
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