Dynamic Behavior of Salt Dissolution and Its Effect on Imbibition in Inter-Salt Oil Reservoir Due to Fracturing Fluid Injection

Abstract

Abstract Billions of tons in proven reserves for inter-salt shale oil has been recognized as crucial strategic resources. The fracturing fluids injection triggers unique salt dissolution chemical phenomena, which on the one hand improves the reservoir pore structure and on the other hand easily induces formation collapse. However, there is no research on triggering mechanism and controlling factors of salt dissolution, and effect of slat dissolution on extremely important imbibition function in inter-salt shale formation is lack of understanding. Herein, the complex mechanism of inter-salt shale dissolution reaction was revealed based on microcosmic pore structure network. The effect of salt dissolution on imbibition in inter-salt shale formation was assessed from various angles, which was conducted by online nuclear magnetic resonance (NMR), high-quality focused ion beam scanning electron microscopy (FIB-SEM), and CT technologies. The results indicate that high temperature improves slat dissolution reaction and enhances the seepage capacity by more than 60 times, which sharply exceeds the influence of fracturing fluid salinity, flow rate and pressure in inter-slat formation. Three kinds of pores for imbibition recovery contribution were 48.17%, 46.39%, and 5.44%, namely micropore, mesopore and macropore, respectively. Additionally, salt dissolution enlarged seepage channel so that spontaneous imbibition occurred in micropores, which was considered as conventionally unmovable areas. Besides, the salt dissolution effect mainly promoted oil discharge in the second stage of imbibition process, leading to 15.83% higher imbibition recovery. These results can furnish an in-depth understanding the nature of complicated mechanisms in inter-slat shale oil reservoirs with fracturing fluids injection.