Study of micromechanical characteristics and failure mechanism of conglomerate reservoirs

Abstract

Mahu Sag, located in Junggar Basin, China, is the largest conglomerate oil reservoir in the world (in this paper, conglomerate generally refers to rocks containing gravel, including pebbly sandstone, sand conglomerate, and conglomerate). The reservoir has the characteristics of low porosity and low permeability and needs hydraulic fracturing. The micromechanical properties and rock fracture characteristics are studied to provide a theoretical basis for hydraulic fracturing treatment. In this study, the mechanical properties, micromechanical properties, fracture characteristics, and micromechanical mechanism of conglomerate are studied experimentally. Assuming that the gravel size is constant, conglomerate with the same cementation type has the following rules: When the gravel content (GC) 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 GC is 40%–65%, the GC and cementation strength are the key factors of rock fracture, resulting in the fracture network around the gravel. When the GC is ≥ 65%, the rock fracture is controlled by Hertz contact stress. The GC is the key factor. Surrounding gravel and passing 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.