The Mechanical Properties and Fracture Characteristics of Shale Layered Samples from the Lucaogou Formation Considering Natural Crack and Mineral Distribution

Shale oil is one of the most promising alternative unconventional energies in the world, and recently the Lucaogou Formation showed significant exploration potential, becoming the primary target in northwestern China. This paper focuses on the mechanical properties and fracture characteristics of shale layered samples from the Lucaogou Formation, conducting uniaxial compressive tests with real-time micro-CT scanning, as well as mineral analysis after failure. It has been found that the mechanical and fracture features are both related to the composition, distribution, content and particle size of minerals, as well as natural fractures. The main crack tends to form in the weak mineral band, for example, calcite or clay band. Since the discontinuous stress usually forms at the interfaces of different minerals, the sample with several major minerals of close content is easier to break into a fractured zone, causing lower uniaxial compressive strength and elastic modulus, compared with the composition of only one dominant mineral. Also, the region will be more fractured after failure if the mineral particles there become smaller. Additionally, although natural cracks have a certain influence on the development of new fractures, not all of the natural ones will propagate into the final fracture network, some of them are just compacted and closed.