An improved evaluation method for the brittleness index of shale and its application — A case study from the southern north China basin

The brittleness index (BI) is an important indicator used to characterize the brittleness and fracability of shale reservoirs. The BI calculation methods based on the brittle mineral and mechanical parameters content of shale are widely used, but there are still some limitations, such as the separation of minerals and mechanics, the inconsistent viewpoints regarding brittle minerals of different scholars, and the equivalent treatments of the brittleness of each mineral. By taking the Permian Shanxi and Taiyuan Formation in the Southern North China Basin (SNCB) as an example, this paper improved the BI calculation method, then examined and applied it. The results show that the weight coefficients of brittleness contributions of various minerals are different because of their marked differences in mechanical properties. By multiplying and summing the mineral contents and their weight coefficients, an improved evaluation method for a shale's BI is proposed. The new BI has more reasonable physical significance and avoids the equivalent treatment of various minerals on the rock brittleness. Applications indicate that when compared with the original mineral BI, most of the new BI decrease, which reduces the artificial optimization of the shale brittleness. The new BI of limestone reduces significantly, which reveals that the brittleness and fracability of limestone should be less than those of shale and sandstone. The roof and floor limestone of shale will prevent induced fractures from propagating across the interfaces between the limestone and shale, and can thus be used as an effective barrier bedding to help the fracturing and formation of the complex fractures in shale sections. The improved BI method is able to evaluate shale brittleness and selecting a favorable fracturing shale section more accurately.