Study on the influence of lithofacies types on the formation and evolution of fracture: A case study of Longmaxi Formation in Northwest Guizhou

Abstract

The change of sedimentary environment leads to the gradation of rock facies, which controls the formation and evolution of fracture. It is of great engineering value for the fine evaluation of the brittleness of the sweet spot layer to reveal the development rule and related mechanism of fractures in different shale facies. In this paper, by means of X-ray diffraction experiment, polarized light microscope, rock mechanics experiment and numerical simulation, the different shale lithofacies types are classified, and the mechanical properties and failure modes of different lithofacies types are discussed in depth. Finally, the dynamic visualization process of fracture formation and evolution in two different lithofacies superimposed geological body models is given. The results show that the organic-rich shale interval of Longmaxi Formation mainly develops three types of lithofacies, such as siliceous shale, calcareous-siliceous mixed shale, and clayey-siliceous mixed shale. Siliceous shale has high stress drop rate and elastic energy conversion efficiency when its failure, and has strong fracture development ability and brittleness as a whole, the longitudinal and transverse fractures are developed and intersect and connect with each other. Calcareous-siliceous mixed shale ranks second in fracture expansion ability, while clay siliceous mixed shale has the weakest fracture expansion ability. The clay-siliceous mixed shale interlayer has a strong stress barrier effect on the propagation of pore water pressure and principal stress. The ability to propagate fractures across layers is weakened with a sharp decrease in stress. The calcareous-siliceous shale has a weak stress barrier effect, and the fracture has a strong ability to propagate across layers. It is expected that the relevant research results will provide some reference and guidance for the research of sweet spot layer optimization, horizontal well geological guidance and fracturing construction parameter adjustment.