基于DLVO理论的低矿化度水驱对岩石润湿性的影响

Effect of low-salinity waterflooding on rock wettability based on DLVO theory

为明确润湿性改变机制的主控因素,基于胶体稳定性(DLVO)理论和扩散双电层理论,建立表征岩石-水膜-原油系统界面间相互作用的分离压力数学模型,从微观受力角度对水驱矿化度与离子价型等分离压力影响因素进行敏感性分析;结合Young-Laplace方程和相互作用势能理论,计算不同水驱矿化度与离子价型下的稳定水膜厚度和岩石润湿角。结果表明:水驱离子价型是影响岩石润湿性的主控因素,少量二价阳离子即可产生极负的分离压力,形成厚度为0.375 nm的较薄水膜,水相润湿角维持在约20°;采用一价阳离子水驱时,随着矿化度的降低,分离压力逐渐增大并产生较高的正峰值,形成厚度为5.334 nm的较厚水膜,岩石润湿性转变为完全水湿(水相润湿角为0°),原油最终脱离岩石表面;在油田注水开发过程中,采用一价阳离子的低矿化度水驱在提高采收率上具有更高的潜能。

To clarify the main controlling factors of wettability alternation mechanism, a mathematical model of disjoining pressure for surface interaction forces was established in terms of the Derjaguin-Landau-Verwey-Overbeek(DLVO) theory and electric double layer theory of colloid stability. And the sensitivity analysis for disjoining pressure was conducted at the microcosmic level to examine the impact of salinity and cation type. Also, the stable water film thickness and contact angle of water on rock were calculated by combining the Young-Laplace equation and interaction potential theory. The results show that cation type is the main factor of controlling the rock wettability. A small amount of divalent cation can trigger large negative disjoining pressure, forming a thinner water film with a thickness of 0.375 nm and the contact angle of water is maintained at about 20°. With the decrease of monovalent cation salinity, the disjoining pressure increases gradually and produces a higher positive peak value, forming a thick water film with a thickness of 5.334 nm. The rock wettability is altered to complete water-wet state and the contact angle of water is maintained at 0°, which causes crude oil to detach from rock surface eventually. It is found that low salinity waterflooding with monovalent cation has a higher potential for improving oil recovery in oil field development.