Quantification of cracks and the evolution of permeability for reservoir rock under coupled THM: equipment development a
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ORIGINAL ARTICLE
Quantification of cracks and the evolution of permeability for reservoir rock under coupled THM: equipment development and experimental research Jie Huang . Zhenlong Song
. Zhiwei Liao . Wanchun Zhao . Dong Wang
Received: 21 July 2020 / Accepted: 1 October 2020 Springer Nature Switzerland AG 2020
Abstract This study designs and introduces an X-ray-compatible thermo-hydro-mechanical coupling triaxial device. The functional parameters of this device included a confining pressure, axial force, injected fluid pressure, and temperature up to 20 MPa, 400 kN, 10 MPa, and 100 C, respectively. Triaxial stress loading–unloading cycles and permeability experiments for coal specimens were carried out using this device. Following four pre-set cycles, the specimen was scanned to identify internal cracks, and the relationship between stress, strain, permeability, and crack evolution was analysed. The experimental results showed that new cracks were mainly caused by the expansion of initial cracks as the triaxial stress cycles. The crack area fraction (CAF) increased J. Huang Z. Song (&) Z. Liao D. Wang State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, China e-mail: [email protected] J. Huang Z. Song Z. Liao D. Wang School of Resources and Safety Engineering, Chongqing University, Chongqing 400044, China Z. Song Southern University of Science and Technology, Shenzhen 518055, Guangdong, China W. Zhao Institute of Unconventional Oil and Gas, Northeast Petroleum University, Daqing 163318, Heilongjiang, China
almost linearly with the stress cycle. With the increase in CAF, the permeability of coal had increased under both high and low deviatoric stress. However, the increment of permeability that was driven by the CAF increment had decreased and eventually became stable. During the initial cracking stage, the increase in permeability from the CAF increase under high deviator stress was always greater. This demonstrates that the contribution of new cracks to increasing permeability under different triaxial stresses was significantly different. The outcomes from this study may provide benefits for coalbed methane extraction and dynamic hazard prevention. Keywords Thermo-hydro-mechanical couplings X-ray Crack visualisation Multiaxial fatigue Coal
1 Highlights • The crack area fraction (CAF) increases almost linearly with an increase in the number of cycles. • The increased rate of permeability driven by the increment of CAF, decreases and eventually becomes stable as the triaxial stress loading– unloading cycle increases. • The contribution of newly generated cracks to permeability is sensitive to the stress state.
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Geomech. Geophys. Geo-energ. Geo-resour. (2020)6:63
2 Introduction Underground reservoirs are often in a state of thermohydro-mechanical (THM) coupling; its cracking behaviour is critical for the safety and efficiency of geological energy exploitations (Li et al. 2019a, b; Uchida et al. 2016; Xia
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