Reservoir characteristics and brittleness evaluation of Wufeng Formation-Longmaxi Formation shale in Yichang area, Western Hubei Province

Based on the data of X-ray diffraction (XRD) whole rock analysis, X-ray diffraction clay mineral analysis, physical analysis, gas-bearing test and geophysical logging of the Wufeng Formation-Longmaxi Formation shale in Yichang area, the development characteristics and brittleness characteristics of the shale reservoirs in the area are studied and evaluated comprehensively. The results show that the shale of Wufeng Formation-Longmaxi Formation in Yichang area is mainly composed of quartz, clay minerals, feldspar, dolomite, calcite and pyrite, and has the characteristics of high silica, and low clay in the bottom and low silica and high clay in the upper layer. The shale porosity mainly ranges from 1.01% to 1.77%, with an average of 1.29%, permeability mainly ranges from 0.001×10^-3 μm² to 0.843×10^-3 μm², average 0.234×10^-3 μm², belonging to low porosity and ultra-low permeability reservoir. The shale gas content in Lower Wufeng Formation-Longmaxi Formation whose thickness can reach 20 m is generally greater than 1 m³/t. The results of brittleness analysis by brittle mineral content method show that the brittleness index of shale varies from 39.6% to 89.6%, with an average of 55.4%. The brittleness index of shale at the bottom of 10 m is more than 60%, which is beneficial to the formation of complex fracture network system after reservoir fracturing. The results of brittleness analysis by elastic parameter method show that the Young′s modulus of the Wufeng Formation-Longmaxi Formation shale is mainly between 25.2 GPa and 67.5 GPa, with an average of 38.9 GPa; the Poisson′s ratio is mainly between 0.177 and 0.327, with an average of 0.297, and the brittleness index is mainly between 8.5% and 97.3%, with an average of 33.2%. The 13 m shale at the bottom has typical high Young′s modulus, low Poisson′s ratio and high brittleness index. The comprehensive evaluation results of shale brittleness indicate that the 9 m shale of the bottom of Wufeng Formation-Longmaxi Formation has the relative optimum Young′s modulus and Poisson′s ratio. Its brittleness belongs to medium and high degree. The 9 m shale has strong fracture making ability and good fracturing ability. The whole shale is divided into 4 sections of modifiable reservoir. The shale section of 2 714.4-2 723.4 m at the bottom has the characteristics of relatively minimum Poisson′s ratio, best gas content, best brittleness and high organic matter content, which constitutes the optimal reconstruction section in the study area.