Study on micromechanical characteristics and failure mechanism of conglomerate reservoir

The tight conglomerate oil in Mahu sag, located in the northwest margin of Junggar basin, China, is the largest conglomerate oil reservoir in the world. The reservoir has the characteristics of low porosity, low permeability, different gravel content, different cementation types and heterogeneity. The micromechanical properties and rock fracture characteristics are studied to provide a theoretical basis for hydraulic fracturing development. In this study, the mechanical properties, micromechanical properties, fracture characteristics and micromechanical mechanism of conglomerate are studied experimentally. The conclusions are as follows: (1) conglomerate reservoir is a granular composite medium cemented by gravel and matrix. The key factors of conglomerate mechanical properties are gravel content, micromechanical properties of matrix and cementation strength of gravel and matrix; (2) With the increase of gravel volume content in conglomerate, the uniaxial compressive strength and elastic modulus of rock decrease, and the more complex the fracture network formed by fracture around gravel is; (3) According to different cementation types, the fracture characteristics are quite different. The higher the clay content in the matrix, the lower the cementation strength, and the easier the cementation surface of gravel and matrix to fracture and form fractures; (4) When the gravel content is ≤ 40%, the rock fracture is controlled by Orowan additional stress, and the strength of miscellaneous foundation is the key factor, forming a single fracture with high tortuosity; When the gravel content is 40-65%, the gravel content and cementation strength are the key factors of rock fracture, resulting in the fracture network around the gravel; When the gravel content is ≥ 65%, the rock fracture is controlled by Hertz contact stress. The gravel content is the key factor. Around gravel cracks and through gravel cracks are developed, resulting in complex fracture network, but the transformation scale of fracture network is small. Through this study, the fracture morphology and key factors of conglomerate fracture are explored, which can provide reference for hydraulic fracturing.