Influence of mechanical contrast between the matrix and gravel on fracture propagation of glutenite

Abstract

Glutenite is composed of gravel, matrix and their cementing surface in meso-scale. The fracture propagation in it is therefore different from the fine-grained rocks such as shale and sandstone. In order to study the influence of mechanical contrast between the matrix and gravel on fracture propagation and mechanical properties of glutenite, three-point bending tests on semi-circular samples were performed. The results of three-point bending indicate that not all glutenite was enhanced by gravel on fracture toughness. Therefore, we classified the glutenite into gravel-enhanced and gravel-weakened. In gravel-enhanced glutenite, gravel increases the fracture toughness of the glutenite, and the fracture is characterized by deflection when it hits the gravel. In gravel-enhanced glutenite, the fracture is single and tortuous when gravel content is less than 35 % and bifurcate and tortuous when gravel content is more than 35 %. As for the gravel-weakened glutenite, gravel reduces fracture toughness of glutenite, and the fracture penetrates the gravel when it hits gravel, which leads to a single and flat fracture. The enhancing or weakening effect of gravel is caused by the mechanical contrast between the matrix and gravel. In gravel-enhanced glutenite, the hardness of gravel is higher than that of the matrix, and the gravel prevents fracture from propagating. While in gravel-weakened glutenite, there are large amounts of weak gravels whose hardness is lower than that of the matrix. Instead of preventing the fracture from propagating, the weak gravels are penetrated. Gravel content and support pattern also have an important effect on the mechanical properties of glutenite. When gravel content is less than 35 %, the gravel is not in contact with each other, the supporting phase is the matrix, and the mechanical properties of glutenite are therefore mainly controlled by the matrix. When gravel content rises above 35 %, gravel begins to contact with each other, and the contacted gravel forms a skeleton bearing the external load. In this case, the supporting phase changes from the matrix to gravel, resulting in a drastic variation of elastic modulus of glutenite as gravel content increases. According to gravel content and the hardness contrast between the matrix and gravel, glutenite was subdivided into four types. The main controlling factors of fracture propagation of the four types were clarified. This classification is of great significance in guiding hydraulic fracturing for different glutenite reservoirs.