Distribution rule of the in situ stress state and its influence on the permeability of a coal reservoir in the southern
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ORIGINAL PAPER
Distribution rule of the in situ stress state and its influence on the permeability of a coal reservoir in the southern Qinshui Basin, China Chenlin Wang 1 & Xiaodong Zhang 1,2 Received: 9 July 2018 / Accepted: 24 September 2018 # Saudi Society for Geosciences 2018
Abstract The recent development of the coalbed methane (CBM) industry has a significant role in advancing hydraulic fracturing theory and technology. However, further development requires a better understanding of how fractures influence reservoir permeability. In situ stress data from 54 CBM wells in the southern Qinshui Basin, China, were obtained by the injection/falloff test method to analyse the effect of in situ stress on the permeability of the CBM reservoir. The types of in situ stress states were classified, and the coal reservoir permeability under different in situ stress states was analysed. The results indicate that the maximum horizontal principal stress (σH), minimum horizontal principal stress (σh) and vertical principal stress (σv) all have positive linear relationships with the coal seam burial depth. Three in situ stress states were observed from the shallow to deep regions of the CBM reservoir in the study area: σH > σh > σv, σH > σv > σh and σv > σH > σh, which account for 9, 76 and 15% of the test wells, respectively. Coal reservoir permeability decreases with increasing horizontal principal stress, whereas it first decreases with increasing σv, then increases and finally decreases. The variation in permeability with σv is due to the conversion of the in situ stress states. Coal reservoir permeability has obvious differences under different in situ stress states. The permeability is the largest when σv > σH > σh, followed by σH > σh > σv and smallest when σH > σv > σh. The permeability differences are caused by the fracture propagation shape of the rock strata under different in situ stress states. Keywords In situ stress state . Coal reservoir . CBM . Permeability . Fracture propagation shape
Introduction The porosities and fractures of coal seams influence coalbed methane (CBM) accumulation zones and migration channels, therefore highlighting the importance of understanding these structural features to optimise CBM exploitation (Fan et al. 2010; Gerami et al. 2016; Mostaghimi et al. 2017). Previous studies have shown that the development of coal rock porosities and fractures is controlled by the coalification process and the geological evolution of the tectonic stress field, with
* Chenlin Wang [email protected] 1
School of Energy Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, Henan Province, China
2
Collaborative Innovation Center of Coalbed Methane (Shale Gas) in Central Plains Economic Zone, Jiaozuo 454000, Henan Province, China
the extension direction of the natural fractures primarily influenced by the modern tectonic stress state (Shen et al. 2014; Matsumoto et al. 2015; Guo et al. 2016). In situ stresses consist of tectonic and gravitational stresses. The tectonic stresses, whi
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