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Energies 2017, 10(3), 292; doi:10.3390/en10030292

Mechanical Behavior and Permeability Evolution of Reconstituted Coal Samples under Various Unloading Confining Pressures—Implications for Wellbore Stability Analysis

1
State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400030, China
2
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China
3
College of Petroleum Engineering, Southwest Petroleum University, Chengdu 610500, China
4
Quanzhou Institute of Equipment Manufacturing, Haixi Institutes, Chinese Academy of Sciences, Quanzhou 362200, China
5
Landslide Research, Faculty of Civil Geo and Environmental Engineering, Technische Universität München, Munich 80333, Germany
*
Author to whom correspondence should be addressed.
Academic Editor: Mofazzal Hossain
Received: 14 December 2016 / Accepted: 28 February 2017 / Published: 2 March 2017
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Abstract

Low pressure, low permeability, and low saturation of Chinese coal-bed methane (CBM) reservoirs make underbalanced drilling (UBD) widely used for mining CBM in China. In this study, mechanical behavior and permeability of coal rock were investigated under different degrees of unloading confining pressure (UCP)-reloading axial stress (RAS) by a triaxial experimental apparatus. These tests revealed that: (1) peak deviatoric stress of coal rock in UCP-RAS is lower than that in a conventional triaxial compression (CTC) test, and the peak deviatoric stress linearly relates the degree of unloading confining pressure. The deformation modulus of coal in UCP-RAS is lower than that in CTC, while the lateral expansion ratio is larger than that in CTC; (2) higher UCP leads to a faster increase of permeability during RAS until the failure of coal; (3) the cohesion and internal friction angle tested by UCP-RAS are lower by 4.57% and 15.18% than those tested by CTC. In addition, a field case (Zhaozhuang well, Qinshui Basin, China) of a well collapse problem validates the higher probability of wellbore collapse due to the increase of equivalent collapse fluid density, which is calculated by using coal rock parameters tested by UCP-RAS rather than by CTC. View Full-Text
Keywords: coal; coalbed methane; permeability; underbalanced drilling; unloading confining pressure coal; coalbed methane; permeability; underbalanced drilling; unloading confining pressure
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Zhang, Q.; Fan, X.; Liang, Y.; Li, M.; Li, G.; Ma, T.; Nie, W. Mechanical Behavior and Permeability Evolution of Reconstituted Coal Samples under Various Unloading Confining Pressures—Implications for Wellbore Stability Analysis. Energies 2017, 10, 292.

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