Microfracture characterization of shale constrained by mineralogy and bedding

The characterizations of microfractures within shale, and their associations with bedding, mineralogy and loading, are important for shale gas exploration and related engineering safety. Research on this topic has been limited due to technological constraints; however, advances in scanning electron microscope (SEM) techniques allow for highly detailed observations of microfractures. In this study, Longmaxi Shale samples were obtained from the locality of Shizhu, China. The samples were cored parallel and perpendicular to the bedding, and SEM observations were performed in specimens before and after compression triaxial testing. SEM images were used for qualitative and quantitative characterization of the microfractures. These tests provided some important insights into the effects of bedding, mineralogy and loading on the characteristics of microfractures. It was observed that the size of the microfractures follows a power law distribution regardless of loading. The parameters of the power law suggest that microfractures associated with brittle minerals increase in complexity after triaxial compression testing. The characteristics of quartz and carbonates show a similar changing pattern, while clay and feldspar show the reverse due to crystal foliation. Microfracture orientations tend to be parallel to subparallel to bedding for all samples, regardless of loading and loading direction. The transition from tensile failure to shearing failure dominates the failure patterns for all minerals under external loading. The mineral's strength and crystal foliation appear to dominate the microfracture mechanisms. Microfractures through carbonates showed complex networks due to the weak grains, and those through quartz tended to develop evenly, indicating that these minerals are prone to forming fracture networks. Microfractures through clays were predominantly parallel and subparallel to the bedding, and those through feldspars showed a tendency towards foliation, indicating that stress direction has crucial effects on these minerals. These results provide microscale information on shale fracturing processes, which can benefit current efforts for hydraulic fracturing enhancement.

Automated summary

The characteristics of microfractures within shale are important for shale gas exploration and engineering safety, but research in this area has been constrained by technology. Advances in SEM techniques have allowed for detailed observations of microfractures, providing insights into the effects of bedding, mineralogy, and loading on these features.