Mechanism Studies on Removing Oil Film from Rock in Oil-Water-Rock Interface Using Surfactant-Based Chemical Flooding Systems
Authors: Zhu Youyi , Fan Jian , Chen Shaowei 
Published: April 2022

Surfactant-based chemical flooding systems, e.g. alkali-surfactant-polymer (ASP) flooding, surfactant-polymer (SP) flooding and surfactant (S) active water flooding, is an efficient enhanced oil recovery (EOR) methods for sandstone reservoirs. The previous mechanism research of surfactant-based chemical flooding focused mainly on reducing the interfacial tensions (IFT) and phase behavior characteristics. There are few researches on removing oil film on the rock in three phases (crude oil-water-rock) interface. This mechanism is very important under the condition that oil film is mainly involved in the residual oil of low/ultra-low permeability reservoir and unconventional resources. Our research will focus on removing the oil film from the hydrophobic rock surface using surfactant-based flooding systems (S, SP and ASP). The factors on performance of stripping oil film were systematically studied.

Experiments related performances of crude oil drop contact angle and detachment time from oil-wet rock model surface with different surfactant-based flooding systems were conducted. The contact angle and detachment time of oil drops in three phases interface were measured by OCA20 contact angle instrument. IFT between oil and surfactant solutions were tested using Texas-500C spinning drop interfacial tensiometer. The mechanism of different surfactant-based chemical systems on removing oil film at low surfactant concentration was discussed.

The main conclusions are as follows: (1) The contact angle of oil drops on oil-wet rock surface could reach a stable equilibrium in brine and low concentration of petroleum sulfonate solution. But the contact angle of oil drop in nonionic surfactant orampholytic surfactant solution was changing with time and the oil film could be removed from the rock surface ultimately. (2) The oil detachment times were changing with different surfactant systems. Oil drop detachment behavior was also related with its lowest instantaneous IFT. When the lowest instantaneous IFT was below around 5×10−2 mN/m, the contact angle changed with time and oil drop could be detached from the rock surface by necking and snap-off phenomena. (3) The addition of alkali to surfactant solution contributes to oil film stripping and enhances its ability to reduce oil/water IFT. Compared with S system, the oil film detached times by SP system was longer. Compared with S or SP systems, the oil film detachment time by ASP system was much shorter. Different alkali types significantly affected the oil film detached times. (4) The kinetics equation of stripping oil film from rock was established.

The results were important for deeply understanding of oil displacement mechanism of surfactant-based chemical flooding in porous medium. Through combined wettability alteration and reducing IFT, the residual oil on oil-wet rock could be recovered more efficient. For the first time, the effects of stripping oil film by surfactant-based chemical flooding systems were studied both considered wettability and IFT performance. This fundamental research will set a foundation for screening and optimizing more efficient chemical flooding systems for IOR/EOR application.