Abstract
Tight sandstone reservoirs are an important petroleum resources in recent years. Hydraulic fracturing is widely used to stimulate development of tight sandstone oil reservoirs by creating underground fractures, but the low flowback rate of fracturing fluid leads to the water blocking damage and low oil recovery of tight sandstone oil reservoirs compared with those of conventional oil reservoirs. The object of this study is to experimentally investigate the possibility of improving flowback efficiency and oil recovery efficiency through achievement of the supercritical water condition. Self-developed reaction system is used to conduct hydraulic fracturing for tight sandstone samples under both regular and supercritical conditions. While comparing the oil recovery factor and flowback efficiency of the regular and supercritical water hydraulic fracturing, mechanisms behind these results are explored through examination of the change in oil components, the change in rock minerals and the change in pore-fracture distribution. Results show that the dynamic viscosity of the crude oil after the supercritical water hydraulic fracturing is significantly lower than that before hydraulic fracturing, with a decrease of 2.88 mPa·s under ambient condition and a decrease of 0.39 mPa·s under in situ condition. Lighter oil components occupy more percentage of the totoal oil components in the recovered oil from supercritical water hydraulic fracturing than that in the oil recovered from regular hydraulic fracturing, with an average increase of 16% for the oil components of molecular weight from 100 to 200. Heavier oil components of molecular weight larger than 300 have an average decrease of 15.5% after the supercritical water hydraulic fracturing. This indicate the visbreaking of the crude oil under the supercritical water condition. Oil recovery after supercritical water hydraulic fracturing is always higher than that after regular hydraulic fracturing, and the ultimate oil recovery after supercritical water hydraulic fracturing is 66.5% compared with 60% of regular hydraulic fracturing. Fracturing fluid after the supercritical water condition flows much quicker and smoothly than that after the regular hydraulic fracturing, and the ultimate flow back factor of the fracturing fluid is 63% after the supercritical water hydraulic fracturing compared with that of 49% after the regular hydraulic fracturing. Increase in percentage of larger pores/fractures after the supercritical water hydraulic fracturing is more significant than that after regular hydraulic fracturing. The percentage of interstratified illite-montmorillonite decreases an average of 15.2%, while that of kaolinite increase an average of 14.3% in the rock samples after supercritical water hydraulic fracturing compared with the original rock samples. This will benefit the recovery process when oil and water flows together into the well bore after the hydraulic fracturing.
