| Recently, the role of organic acids as exogenous or endogenous oil displacement agents to enhance oil
recovery of sandstone reservoirs has gradually attracted attention, but the specific mechanism and effect are still
controversial. Regarding this, through experiment such as core displacement and contact angle, interfacial tension /
interfacial expansion rheological properties and Zeta potential measurement, the mechanism and effect of formic acid in
enhancing oil recovery are explored. Core displacement experiment shows that formic acid can significantly enhance oil
recovery of low porosity and low permeability sandstone reservoirs. As the concentration of formic acid increases to 15
mmol/L, the oil recovery efficiency increases by 6.9%. Contact angle measurement shows that formic acid significantly
enhance the hydrophilicity of sandstone surface. The contact angle of oil drops is 110.3° in formation water, and is 123.0°
in formic acid aqueous solution of 15 mmol / L. Interfacial tension and interfacial expansion rheological properties
measurement shows that formic acid reduces the interfacial tension, and significantly enhances the interface
viscoelasticity. The interfacial tension between crude oil and formation water is 27.7 mN/m, which is greater than the 20.0
mN/m between crude oil and formic acid solution (15 mmol/L). The elastic and viscous moduli of the crude oil-formation
water interface are 11.1 mN/m and 4.0 mN/m respectively, which are much smaller than the 20.3 mN/m and 7.9 mN/m of
crude oil-formic acid solution (15 mmol/L) interface. The Zeta potential measurement show that the adsorption of formic
acid and crude oil on the quartz surface can reduce the negative potential of quartz surface, and formic acid has a more
obvious effect, so formic acid can displace the adsorbed crude oil. Meanwhile, the adsorption of formic acid on the crude
oil surface increases its negative potential and significantly enhances the stability of oil droplets. To sum up, the formic
acid can promote the wettability alteration and enhance the interface viscoelasticity, which works synergically to enhance
oil recovery. |
