Experimental Investigation of Mechanical Properties and Failure Behavior of Fluid-Saturated Hot Dry Rocks
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Original Paper
Experimental Investigation of Mechanical Properties and Failure Behavior of Fluid-Saturated Hot Dry Rocks Daobing Wang ,1,3 Xiaobing Bian,2,3 Hao Qin,1 Dongliang Sun,1 and Bo Yu1,3 Received 31 August 2020; accepted 22 September 2020
Thousands of cubic meters of fluid are continuously injected for a long term to create complex fracture patterns in hydraulic fracturing of hot dry rocks. However, the physics and mechanics behind the interaction of fluid–rock are not fully understood at present. To reveal the related damage mechanisms of saturated rock samples such as damage initiation and evolution at various alternative stress levels, a series of in-house laboratory tests were performed on a TAW-series triaxial rock mechanics testing system, combined with ultrasound measurement and acoustic emission (AE) monitoring. After saturation with nanoemulsion and distilled water, ultrasound velocity of longitudinal wave was increased by 40%. Saturation weakens these mechanical parameters such as the crack damage stress ratio, fracture toughness and cohesive strength under different stress conditions. Fluid-saturated rock sample has higher AE hit rate than dry rock sample. Meanwhile, many step-like jumps appeared on the curve of cumulative AE events. Failure envelop, b-value and frequency spectrum were analyzed out to compare the mechanical difference between fluid-saturated and dry rock samples. The experimental results demonstrate that the saturation increased the pore pressure in rocks and further promoted crack propagation in hydraulic fracturing. Moreover, nano-emulsion liquid is more advantageous than distilled water for enhancing fracture complexity. This investigation provides for better understanding of the mechanisms of complex fracture formation in deep geothermal reservoirs. KEY WORDS: Hot dry rock, Rock mechanics, Experimental study, Acoustic emission, Hydraulic fracturing.
INTRODUCTION Hot dry rock is a kind of igneous or metamorphic rock with a temperature higher than 200 °C. Burial depth of hot dry rocks is normally between 3
1
School of Mechanical Engineering, Beijing Key Laboratory of Pipeline Critical Technology and Equipment for Deep Water Oil and Gas Development, Beijing Institute of Petrochemical Technology, Beijing 102617, China. 2 Sinopec Research Institute of Petroleum Engineering, Beijing 100101, China. 3 To whom correspondence should be addressed; e-mail: [email protected], [email protected], [email protected]
and 10 km, with little or no fluid inside (Wan et al. 2005; Yang et al. 2019). The thermal energy stored in hot dry rock in China is equivalent to about 856 trillion tons of standard coal energy (Gong et al. 2020), which accounts for about 16% of the worldÕs energy resources. Thus, hot dry rock is expected to become a strategic alternative energy source. Hydraulic fracturing treatment is a key technology to extract deep-subsurface geothermal energy (Jupe et al. 1992; Wang et al. 2019a, b, c; Zhang et al. 2019; Wang et al. 2020a, b, c, d). Generally, a large volume of f
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