a b s t r a c tHydraulic fracturing is widely accepted and applied to improve the gas recovery in unconventional reservoirs. Unconventional reservoirs to be addressed here are with very low permeability, complicated geological settings and in-situ stress field etc. All of these make the hydraulic fracturing process a challenging task. In order to effectively and economically recover gas from such reservoirs, the initiation and propagation of hydraulic fracturing in the heterogeneous fractured/ porous media under such complicated conditions should be mastered. In this paper, some issues related to hydraulic fracturing have been reviewed, including the experimental study, field study and numerical simulation. Finally the existing problems that need to be solved on the subject of hydraulic fracturing have been proposed.Unconventional gas mainly includes shale gas, tight gas and coal seam gas. Shale gas is commonly in mudstone, shale and between them the interlayers of sandstone. Tight gas often has been stored in tight sandstone or sometimes limestone. Coal bed methane is contained within coal seams. Their common attribute is that the permeability of the matrix is very low, and the permeability often has been improved by artificial or natural fractures [55]. However, the differences between them are also significant. For example, the effective shale thickness for gas production should be more than 15 m while the height of coal is generally from 0.6 m to 5.0 m [68], as coal seams to be fractured may be multiple and thin, hydraulic fracturing in coal needs to be more accurately designed and controlled. Moreover, the Young's modulus of coal is smaller than shale and tight sandstone, the permeability of coal is more sensitive to stress due to the development of cleat system, and leakoff in coal may be more severe, which can significantly affect the fracturing result. Due to the complexity of unconventional reservoirs, it is challenging to predict the initiation and propagation of hydraulic fractures [39]. For example, the complex in situ stress state and distribution of rocks of varied attributes, which may change the profile of hydraulic fractures [38]; the existence of arbitrary pre-existing interfaces may diversify or arrest hydraulic fractures [93]; the temperature effect [75]; the fluid loss and transport of proppant; the competition between hydraulic fractures, and its recession and closure [4]. Thus, it is crucial to explore how hydraulic fracturing process will happen in complex geological settings.Firsthand materials of hydraulic fracturing come from in-door experiments, and field study. Laboratory study undergoes from small-scale rock samples with several cubic centimetres to large ones with one cubic metre or more. Since it is easy to control the stress conditions and make artificial structures within samples, hydraulic fracturing process with different stress field and rock structures can be conveniently studied. Especially in large scale experiments, it is possible to build a full size borehole, or to contr...

Please cite this article as: Li, Q., Xing, H., Numerical analysis of the material parameter effects on the initiation of hydraulic fracture in a near wellbore region, Journal of Natural Gas Science & Engineering (2015), doi: 10.1016/j.jngse.2015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Abstract: Hydraulic fracturing is recognized as a key method to enhance unconventional reservoirs, but it is also quite complicated and challenging due to so many affecting factors such as the formation material parameters, fluid behaviors and geological setting/conditions. However, in unconventional reservoirs, in the near wellbore region, the rock may consist of different materials, and the stress is redistributed during drilling and fracturing process, so the preferable hydraulic fracture path is hard to predict. This paper aims to study the effects of formation material parameters, including drilling effects, on the hydraulic fracturing process in unconventional reservoirs. First, hydraulic fracturing has been simulated in nine cases with different parameters specifically permeability, porosity, Young's modulus, and Poisson's ratio. Then hydraulic fracturing has been studied by a heterogeneous model with multiple zones and layers, and complex stress condition. It is found that under a constant hydraulic pressure boundary, when the values of permeability, porosity, Young's modulus, and Poisson's ratio are higher, hydraulic fracturing initiates earlier; with a heterogeneous distribution of materials, stress concentration is formed between different materials, and the tensile stress concentrated areas become the preferable paths of hydraulic fracturing. M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT

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