Self-propped fracture is an important gas flow channel in the hydraulic fracturing process of shale. It is important to analyze the deformation characteristics of self-propped fracture under closure stress. In this paper, a fracture simulation model is established based on the surface data of self-propped fracture obtained from the shale in southern China. Finite element method is used to simulate the deformation process of the fracture under different stresses. The results show that surface roughness has a main influence on the deformation of fracture under low stress. As the stress increases, Young's modulus gradually becomes the main influence factor. The research results can provide a new idea for the deformation study of self-propped fractures in shale.

1. INTRODUCTION

Hydraulic fracturing has become a key means to ensure the efficient development of shale reservoirs in southern China. Due to the influence of shear stress, shear dislocation occurs on the rough surfaces of self-propped fracture, so it can't completely close under closure stress of formation. This feature of self-propped fracture makes it becomes an important gas flow channel. Especially, due to the complex internal structure, strong heterogeneity and high closure stress of shale reservoir, the flow process of shale gas in self-propped fractures is complicated. Therefore, the mechanical behavior of self-propped fracture under different closure stresses is of great significance for the development of shale reservoirs.

The surface morphology of self-propped fracture has an important influence on its deformation characteristics under closure stress. Barton (1977) analyzed the rough surface morphology of a large number of rocks and proposed the use of JRC to describe the roughness. The JRC value was divided into ten intervals by establishing a standard joint profile. Lee (1990) used the fractal dimension to analyze the standard joint profile, and proposed an empirical equation for the conversion between JRC and fractal dimension. Malagon (2006) conducted experiments on three rock samples of different lithological and obtained the height characteristics of rock surface roughness. On this basis, the characterization of surface morphology was achieved by the fracture surface three-dimensional image obtained. Rodrigues (2011) used a three-dimensional laser profilometer to obtain the surface parameters of the fractures after acid etching, and established a cloud point database of the fracture surface using kriging interpolation. Based on this, the height change of the asperities on the fracture surface under different closure stresses was studied. Briggs (2017) used a laser profilometer to scan the fracture surface in the shale fracture conductivity test. Based on this, the influence of rock properties on the fracture conductivity is studied among fractures with different surface morphology. Lu (2017) used a three-dimensional imaging system to scan a large number of rock samples, and proposed the use of horizontal and vertical tortuosity ratios to describe the surface morphology of acid-etching fracture. Meanwhile, the fracture surface morphology was classified based on this parameter. Scholars mainly obtain the surface morphology of fractures by searching for some characteristic parameters or using digital characterization methods. However, for shale self-propped fracture, due to the influence of closure stress, it is difficult to directly obtain the surface morphology after compression. Therefore, it is necessary to study the deformation process of the fracture under closure stress to obtain an accurate fracture morphology.

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