低盐水改变砂岩表面润湿性的pH升高机理

The pH increase mechanism of wettability alteration on sandstone surface by low saline water

低矿化度水驱能改变砂岩表面润湿性,注入低矿化度水使体系pH升高。当岩心初始饱和水为酸性时,利用岩心自渗吸和接触角实验,在宏观上研究自渗吸盐水pH升高对砂岩表面润湿性的影响,结合Zeta电位测试解释pH升高改变砂岩表面润湿性的机理;利用极性模拟油组分砂岩表面吸附实验,结合分子模拟在微观上分析自渗吸盐水pH升高改变砂岩表面润湿性的机理。实验及分子模拟结果表明:当初始饱和水为酸性时,原油中的石油酸电离被抑制,以羧酸分子形式存在,碱组分电离而带正电,羧酸分子、正电性碱组分分别通过氢键和库仑力(静电引力)吸附在砂岩表面的负电性硅氧基团上。由于吸附的酸碱组分另一端为烃基,砂岩表面亲水性减弱,当体系pH升高时,吸附在砂岩表面上的羧酸分子电离而成负电性羧酸根,碱组分电离被抑制而成电中性碱,在负电性羧酸根与砂岩表面负电性硅氧基团之间存在库仑力,电中性碱组分与硅氧基团之间的范德华力不足以自行吸附在砂岩表面的基团上,导致酸碱组分从砂岩表面脱落,砂岩亲水性增强。

Low salinity waterflooding can change the wettability on sandstone surface but also increase pH of crude oil/brine/rock system.The spontaneous imbibition macroscopically studied the effect of pH increase of imbibing brine on wettability of sandstone surface when the saturated brine is acidic,and zeta potential measurements were used to interpret the mechanism of wettability alteration on sandstone surface by pH increase.The adsorption of simulated polar oil on sandstone and molecular simulation were used to microscopically analyze the mechanism of wettability alteration on sandstone surface by pH increase.The experiments and simulation results show that when saturated brine was acidic,the ionization of petroleum acid was inhibited and they existed as the form of acid molecule and the base were ionized to be positive,then the petroleum acid and positive base can adsorb onto negative oxygen of sandstone surface by hydrogen bond and coulomb force respectively.The hydrocarbon chain of adsorbed polar components can change the sandstone surface to less water-wet.However,as pH increased,the adsorbed carboxylic acid were ionized to be negative and the base were ionized to be neutral,then the coulomb force between negative acid and oxygen group on sandstone surface and the weak van der waals between neutral base and oxygen group resulted in desorption of acid and base from sandstone surface and alteration of sandstone surface to be more water-wet.