Brittleness evaluation of the Lower Silurian marine shale reservoirs: A case study of Longmaxi shale in Fenggang block, Southern China
The brittleness of shale determines the fracturability of shale reservoir and has a great influence on the exploration and development of shale gas. Therefore, prediction of brittleness and evaluation of fracturability of shale are very important in finding favorable areas for shale gas. We used the mineral composition, rock mechanics experiment, logging evaluation, two-dimensional seismic data inversion and fracture development degree to evaluation and analysis the vertical and plane brittleness characteristics and main controlling factors of the Longmaxi shale. In addition, we established the fracturability index (FI) of shale based on the brittleness index, critical strain energy release rate and fracture toughness. The results indicate that the brittle mineral content of Longmaxi shale in Fenggang block is between 69% and 90%, shale samples are prone to brittle deformation by microscopic observation. Brittle deformation has a positive effect on the porosity and percolation ability of shale; the fractures in the upper and lower parts of Longmaxi shale are relatively developed, and the degree of core fracture development is consistent with rock brittleness. The brittleness distribution of two-dimensional seismic data inversion is in good agreement with the brittleness predicted by well logging; shale reservoirs with low fracture toughness, low critical strain energy release rate and high brittleness index have high fracturability. Therefore, the research on shale brittleness and fracturability of Lower Silurian Longmaxi shale is expected to have important guiding significance for shale gas exploration and development in Southern China.