Composed of gravel, matrix and the cementation surface, glutenite is different from fine-grained homogeneous reservoirs such as sandstone and shale in meso-structure. This unique structure inevitably leads to different fracture propagation characteristics. In order to study the fracture development in glutenite, three-point bending tests were conducted on two glutenites and one sandstone. During the tests, digital image correlation (DIC) and acoustic emission (AE) were used to monitor the fracture development. It was found that compared with the load-displacement curve, the load-CMOD (crack mouth opening distance) curve better reflects the fracture development. Fracture development in glutenite and sandstone are obviously different, and that in different glutenites are also different. In the weakly cemented glutenite, the fractures are deflected and mainly propagate around the gravel, forming tortuous and complex fractures. In strongly cemented glutenite, fractures tend to penetrate the gravel, forming straight fractures. In sandstone, the fractures are similar to those in the strongly cemented glutenite. AE occurs at a lower load ratio in weakly cemented glutenite. In contrast, in strongly cemented glutenite and sandstone, acoustic emission does not occur until the load rises to the peak load. In the weakly cemented glutenite, an extensive fracture process zone (FPZ) exists near the fracture tip, where microfractures appear and coalesce to form the main fracture. In contrast, the FPZ is smaller in glutenite of strong cementation and sandstone. The fracture development and size of FPZ are also related to fracture toughness. The lower the fracture toughness is, the lower the load ratio and the larger the FPZ are. Therefore, cementation between the gravel and matrix has an essential influence on fracture development and determines the development and morphology of fractures.
Glutenite reservoirs are widely distributed all over the world, especially in Junggar Basin, Xinjiang, China. It was reported that the geological reserves in Mahu Depression are as high as 1.76 billion tons. However, the glutenite reservoirs are dense and of low permeability (Zeng et al., 2010; Zhishuo et al., 2010). In order to effectively exploit glutenite reservoirs, hydraulic fracturing is widely used (Gong et al., 2008; Lu et al., 2013). However, the meso-structure of glutenite is different from that of fine-grained reservoirs such as sandstone and shale (Liu et al., 2020; ZHANG et al., 2020). The glutenite is composed of gravel, matrix and the cementation surface between the matrix and gravel. This unique structure inevitably leads to different fracture patterns (Liu et al., 2018). However, due to the inadequate understanding of fracture propagation in glutenite reservoirs, hydraulic fracturing in glutenite reservoirs still applies similar methods to the homogeneous reservoirs. Multi-stage fracturing was commonly used in horizontal wells, ignoring the differences between glutenite and homogeneous reservoirs (Guoxin et al., 2020).
