EXPERIMENTAL STUDY ON MICROSTRUCTURE RESPONSE OF SHALE RESERVOIR UNDER STATIC ADSORPTION OF FRACTURING FLUID

During the process of hydraulic fracturing in shale gas reservoir, the fracturing parameters and reservoir protection will be affected by the interaction between fracturing fluid and shale due to the effects of ion adsorption, exchange and diffusion on the microstructure of shale. The changes of acoustic characteristics and microstructure of the siliceous shale of Lower Cambrian Niutitang Formation in northwestern Hunan were studied during static adsorption and diffusion of fracturing fluids with different ion concentrations and activities. And the response of macroscopic acoustic features and microstructures was revealed. The conclusions include the following. (1)Shale has the strongest adsorption capacity in a certain concentration of NaCl fracturing fluid, followed by SDBS fracturing fluid, and has the weakest adsorption ability in CTAB fracturing fluid. The higher the concentration of inorganic ions, the stronger the ability of diffusion and adsorption in shale. (2)Strong adsorption of ions and pore filling occurred in the static adsorption process, which resulted in the change of microporous structure and porosity. (3)As the ion concentration of the fracturing fluid with different surfactant, the number of shale micropores, the connectivity and the pore size distribution are all changed correspondingly. In the studied concentration range, when the concentration of NaCl salt ion is 15%,it is most beneficial to the protection of the reservoir, while a negative effect on the shale microscopic pore network occurs when it is combined with the surfactants.