Mechanism of shear failure near fracture face during drainage process of CBM well
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ORIGINAL PAPER - PRODUCTION ENGINEERING
Mechanism of shear failure near fracture face during drainage process of CBM well Ping Xiong1,2 · Wang‑shui Hu1 · Hai‑xia Hu2 · Hailong Liu3,4 Received: 21 November 2017 / Accepted: 4 April 2018 © The Author(s) 2018
Abstract In this paper, whether the coal fines can be induced by shear failure during drainage process has been discussed in detail. By coupling with the percolation theory, the elasticity mechanics were used to construe the extra stresses in the formation surrounding with the hydraulic fracture. The safe window of the bottom hole pressure was also calculated from the failure envelope. The research shows that the formation pressure on the fracture surface of the coal seam is negatively related with the bottom hole pressure, and the induced stress is positively related with the bottom hole pressure during the drainage process of fractured CBM wells. The pore pressure around the fracture changed due to pore-elastic effects, which also caused a significant change of the in situ stresses. In order to avoid the breakout of the coal seam around hydraulic fracture during drainage process, the model of the reasonable bottom hole pressure is also built. Keywords Coalbed methane · Fracture · Induced stress · Shear failure
Introduction The coalbed methane (CBM) resources in China are abundant, and the exploration and development of CBM in China also have made great achievements; what is more, the CBM has achieved commercial exploitation and development. During the operation of CBM wells, coal fines are easily generated due to the rise of liquid level or pressure changes (Liu et al. 2012a, b; Zhang et al. 2009). The coal fines clog gas production channels of CBM wells, which can reduce the production of CBM wells even after they are put into operation. It is difficult to keep the production of CBM wells stable, and it also hard to maintain high yield, which affects the overall development of CBM benefits. Coal fines are among the key issues that restrict the highly efficient * Hailong Liu [email protected] 1
School of Earth Sciences, Yangtze University, Wuhan 430100, China
2
College of Engineering and Technology, Yangtze University, Jingzhou 434020, China
3
China Petrochemical Exploration and Development Research Institute, Beijing 100083, China
4
Key Laboratory of Marine Oil & Gas Reservoirs Production, Sinopec, Beijing 100083, China
production of CBM, because they cause blockages in reservoirs and accidents in wellbores. With the CBM producing gas, it also produces different extent of coal fines, especially hydraulic fractured well. Wei et al. (2013) points out that coal fines production results from unreasonable drainage operation parameters, such as unreasonable bottom hole pressure and drainage quantity (Wei et al. 2013). Some researchers (Chen et al. 2009; Marcinew and Hinkel 1990; Zhang et al. 2012) argue that coal fines unload and gather in the bottom of the well, which results in pipe blockage and getting stuck. The continuity of gas and water fl
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