Effects of Gravel Size and Content on the Mechanical Properties of Conglomerate

The conglomerate reservoir in Mahu sag, located in the northwest margin of Junggar basin, China, is of low porosity and permeability. Hydraulic fracturing is necessary to improve oil production efficiency. However, the influence of strong reservoir heterogeneity and large difference in gravel content on mechanical properties still needs study. In this study, uniaxial compression tests were conducted on the conglomerate in which the gravel diameter ranges from 2 to 26 mm. In these tests, a large number of micro-fractures were found to be generated evenly around gravels. Additionally, we found that as the gravel content increased, the uniaxial compressive strength and elastic modulus of conglomerate decreased, and the plasticity characteristics increased. The experimental results show that the gravel content is the key factor determining the mechanical properties of the conglomerate. When gravel content is less than the critical content (~ 14.61–31.72%), at which the brittle–plastic transition of rocks exists, the macro-mechanical properties are mainly influenced by the cementing material. When gravel content is greater than the critical gravel content (~ 78.50%), failure is determined by the Hertz contact stress among the gravels. When gravel content is between (~ 14.61–31.72%) and (~ 78.50%), Orowan additional stress is the main factor determining the mechanical mechanism of rock failure. This failure is closely related to gravel content and cementation strength between the gravel and matrix.