Influence of Bedding and Mineral Composition on Mechanical Properties and Its Implication for Hydraulic Fracturing of Shale Oil Reservoirs1

摘要

The premise of hydraulic fracturing is to have an accurate and detailed understanding of the rock mechanical properties and fracture propagation law of shale reservoirs. In this paper,a comprehensive evaluation of the mechanical properties of the shale oil reservoir in the south of Songliao Basin is carried out. Based on the experiments and the in-situ stress analysis, the fracture propagation law of three types of shale reservoirs is obtained,and the suggestions for fracturing are put forward. The results have shown that the fracture propagation of pure shale and low mature reservoir is easy to open along the bedding plane under compression loading,which is greatly influenced by the bedding. Sand-bearing shale is slightly better,the fractures of which are not easy to open along the bedding plane. The mechanical experimental results show that all the samples have the characteristics of low compressive strength,low Young’s modulus and strong anisotropy,indicating that the shale oil reservoir is certain plastic,which is related to its high clay mineral content and controlled by the bedding development. Compared with pure shale and low mature shale,the sandbearing shale has less clay content and less developed bedding,which maybe the main reason for its slightly better brittleness. Overall,the expansion of hydraulic fracture is controlled by in-situ stress and bedding. Because of the development of bedding,it is easy to form horizontal fractures. Thus it is not suitable for horizontal well fracturing.Because of the high content of clay minerals,the applicability of conventional slick hydraulic fracturing fluid is poor. It is suggested to use vertical well or directional well to carry out volume fracturing. In this way,the effect of bedding can be effectively used to open and connect the bedding and form a larger fracture network.