Experimental Investigation of Thermal Effect on Fracability Index of Geothermal Reservoirs
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Original Paper
Experimental Investigation of Thermal Effect on Fracability Index of Geothermal Reservoirs Daobing Wang ,1,3 Fujian Zhou,2,3 Yongcun Dong,1 Dongliang Sun,1 and Bo Yu1,3 Received 31 May 2020; accepted 12 August 2020
The thermal effect is not considered in the current fracability index model. In this paper, we investigated experimentally the formation capacity of fracture networks in hot dry rocks at different temperatures, by using rock mechanics triaxial testing system and acoustic emission monitoring. A new brittleness index with consideration of thermal effect is proposed on the basis of crack volumetric strain calculated at different stress levels, which captures the following two characteristics: (1) thermal micro-crack density in the crack closure stage and (2) crack propagation, nucleation and coalescence by external stress. Then, a comprehensive fracability index model is established, which integrates the thermal effect and fracture toughness at different temperatures and confining pressures. The fracability index calculated by the new model is consistent with acoustic emission characteristic, microstructure analysis and b-value analysis, which verify that the proposed model is reliable. The results show that both temperature and confining pressure have a significant effect of formation capacity of fracture networks in hot dry rocks. This study provides new insight into improving the thermal efficiency in geothermal reservoirs. KEY WORDS: Hot dry rock, Fracability, Thermal effect, Enhanced geothermal system, Rock mechanics, Fracture toughness.
INTRODUCTION In recent years, geothermal energy has been listed in the priority development region around the world because of the advantages of cleanliness, continual stability and wide distribution (Finsterle et al. 2013; Hou et al. 2018). Geothermal resources include mainly three types of energy resources (Wang et al. 2018; Zhou et al. 2018; Guo et al. 2020): 1
School of Mechanical Engineering, Beijing Key Laboratory of Pipeline Critical Technology and Equipment for Deep Water Oil & Gas Development, Beijing Institute of Petrochemical Technology, Beijing 102617, PeopleÕs Republic of China. 2 Institute of Unconventional Oil and Gas Science and Technology, China University of Petroleum, Beijing 102249, PeopleÕs Republic of China. 3 To whom correspondence should be addressed; e-mail: [email protected], [email protected], [email protected]
shallow geothermal resources, hydrothermal resources and hot dry rock geothermal resources. Hot dry rock is a kind of metamorphic rock or crystalline rock with temperature in the range of 150 to 650°C, which is normally buried deep underground 3– 10 km (Wei et al. 2015). On a conservative estimate, the energy contained in hot dry rocks within the earth crust is equivalent to 30 times the energy contained in all oil, natural gas and coal in the world (Wei et al. 2015). Hydraulic fracturing is the key technique behind the new enhanced geothermal systems that could effectively extract heat energy from hot dry rocks,
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