Application of supercritical water conditions to improve the flowback of fracturing fluid in ultra-low-permeability sandstone formations

Hydraulic fracturing is an important approach for stimulating the production of ultra-low-permeability oil reservoirs; however, the low flowback efficiency of the fracturing fluid (FF) may cause contamination of the groundwater and reduce oil recovery. This study experimentally investigates the possibility of improving the flowback efficiency of water-based FF and oil production through the achievement of supercritical water (SCW) conditions after the injection of FF in ultra-low-permeability oil reservoirs. Possible mechanisms are explored through measurement of the capillary pressure, visbreaking of oil, pore-fracture distribution, and mineral components. The results show that the FF flowback efficiency and oil recovery after the SCW condition treatment are an average of 10.2% and 19% higher than those after regular hydraulic fracturing (RHF), respectively. The lower capillary pressure after the SCW condition treatment indicates a lower flow resistance. Furthermore, the increase in the proportion of large pores ranges from 6.6% to 9.0% for the fracturing process after the SCW condition treatment, whereas the increase in large pores after RHF ranges from 2.8% to 4.5%. The increases in the porosity of the core samples after the SCW condition treatment are 0.87%, 1.08%, and 1.43%, which are markedly higher than those after RHF (0.33%, 0.24%, and 0.26%, respectively). The percentage of kaolinite in the rock samples increases, whereas that of interstratified illite-montmorillonite decreases after the SCW condition treatment, thus increasing the stability of the minerals in water. The reduced dynamic viscosity and lighter components of the recovered oil from the SCW condition treatment reveal the occurrence of visbreaking of the crude oil.