Microfracture types of marine shale reservoir of Sichuan Basin and its influence on reservoir property

The study analyzed the core samples from the Longmaxi Formation to investigate the types of fractures and their effects on reservoir properties by means of argon ion polishing-field emission scanning electron microscopy observation, experimental measurement of the porosity and permeability with overburden pressure, CT scanning and reconstruction, image mosaic, image grayscale recognition and threshold binarization, with a view to clarify the influence of microfractures on the physical properties of subsurface shale reservoir in Fuling shale gas field, Sichuan Basin. The results show that the microfractures for the shale samples can be divided into 6 types, including bedding fracture, particle edge fracture, dissolution fracture, diageneticcontraction fracture, abnormal pressure fracture and structural fracture. Microfractures can function to improve reservoir porosity and play an important role in increasing conductivity in shale reservoirs. Microfractures may form a net of fractures, connecting the various pores throughout the shale reservoir. Among them, particle edge fracturesare the main channels for vertical connection, while bedding fractures are mainly for lateral connection, both of which can be combined to form the best fracture network channels for shale reservoir. Effective fracture network can be composed by particle edge fractures in themselves, dissolution fractures can form fracture network in local area, improving the shale reservoir connectivity. The microfractures will exert a great influence on the permeability of shale reservoirs: the average permeability of microfracture samples is 62.9 times the mean permeability of microfracture-free shale samples. Under normal formation conditions, the microfractures in the shale reservoir should be open when they are shallow at a depth of 3, 500 meters. Given the abnormal high pressure, sedimentation, tectonism and other rock mineral characteristics of shale reservoir fluid, the depth of the shale microfracture opening can be increased appropriately. © 2019, OIL & GAS GEOLOGY Editorial Board. All right reserved.