ABSTRACT:

Rock layering is pervasive in the Shanxi formation of Ordos Basin. In this area, coal, shale and limestone rock are developed vertically and alternatively and high heterogeneity between layers has a great effect on the fracture propagation. In this study, the true tri-axial experiments were conducted with the combination of coal, shale and limestone cement sheets. The effect of in-situ stress condition, bedding planes and cleat systems on the fracture height propagation was mainly studied. The results showed that fracture morphologies could be classified into four patterns: arresting, diverting, branching and crossing. Furthermore, hydraulic fractures tended to cross the weak plane and then connected with the adjacent layer under small vertical stress difference and large horizontal stress difference. In addition, Complex natural fracture network contributed to fracture turning and branching and horizontal fracture distribution. However, it could dissipate the hydraulic energy and limit the fracture height growth. The characteristic of pumping curve pressure "increasing-dropping-increasing-fluctuation" indicated the interface crossing. The results give a deep understanding of hydraulic fracture propagation in coalbed and shale gas reservoir and also provide practical advice to field fracturing operation.

1. Introduction

Generally, coalbed and shale interbedded formation have complicated layers of different lithologic combinations, accompanied by varying in-situ stress, bedding planes and small cleats. Many scholars have studied the fracture propagation in the layered reservoir through laboratory experiment. Hydraulic fracture interactions with multiple bedding planes during hydraulic fracture treatment generated T-shape, kinks, offsets and ledges along the bedding planes (Fisher and Warpinski, 2012) as the result of fracture crossing, arresting, blunting, and diversion at the bedding planes. Liu et al. (2016) studied the influence of different well inclination, well azimuth and perforation parameters on the initiation and propagation of hydraulic fractures in layered strata. Xing et al. (2018) investigated the influence of interface cementing strength, stress difference on the vertical propagation behavior of the fracture. Tan et al. (2017) summarized the fracture geometries in sandstone coal interbedding and studied the effect of geological and engineering factor. Olsen et al (2003) indicated that when the hydraulic fracture meets the joint in the coal seam, leading to complex fracture or limiting the growth of fracture. De pater et al. (2005) considered the displacement and the viscosity of fracturing fluid to predict the possibility of natural fracture opening with experimental method. Four typical fracture geometries were summarized by Gu and Siebrits (2006), according to the fracture propagation modes of hydraulic fracture and interface. Ouchi et al. (2017) studied quantitatively the vertical propagation behavior of hydraulic fractures with different interface obliquity by using of fluid solid coupling hydraulic fracturing model. Goldstein and Osipenko et al (2015) investigated the influence of interface friction on the fracture propagation. They found that slip distance increases with the dip angle of the interface increasing. Zhang and Jeffrey (2007) established numerical model of height growth in layered media with displacement discontinuity method and finite difference method. They studied that the coupling effect of rock deformation, fluid flow, and interface friction and sliding on fracture vertical propagation. Zhao et al. (2015) built the mathematical model of vertical propagation of hydraulic fracture based on the theory of linear elastic fracture mechanics.

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