Abstract:

The brittleness determines the fracability of shale gas reservoir, which has a significant impact on shale gas exploration and development. Recently, China has achieved a breakthrough of the Lower Silurian Longmaxi shale in the Sichuan Basin and its surrounding areas. Compared with the Longmaxi shale, the Lower Cambrian Niutitang shale has a greater deposition thickness and wider distribution area, which is another significant stratum for Chinas shale gas. In this paper, the brittleness of Niutitang shale was evaluated and analyzed in the study area based on the data of core mechanical test, array sonic logging, mineral composition and fracture parameters. The results show that the relationship between brittleness and brittle mineral content is not a simple positive correlation. The brittleness and fracability of shale are determined by the content of brittle minerals and TOC (total organic carbon), the degrees of thermal evolution (diagenesis stage) and fracture development. Because of the positive correlation between quartz and TOC contents, when the TOC content is less than 6.5%, the increase in the brittleness caused by the increase in the quartz content is greater than the increase in the ductility caused by the increase in the TOC content; when the TOC content is greater than 6.5%, the ductility increases faster than the brittleness, which results in negative correlations of the brittleness with the TOC and quartz contents and low fracture density in the sections with high TOC content. This understanding will guide the exploration and development of shale gas and is very helpful for optimizing drilling and fracture stimulation intervals in southern China.

Key words: shale, brittleness, mineral composition, elastic parameter, Niutitang Formation, Lower Cambrian, Upper Yangtze region