致密储层表面双电层分子动力学模拟

Molecular dynamics simulation of electric double layer on tight reservoir surface

致密储层润湿性受储层表面双电层的微观影响显著,注CO_(2)已成为致密油藏高效开发的关键技术。为探究致密储层提高采收率方法如何影响界面双电层特性及润湿性,运用分子动力学模拟建立了致密储层表面双电层模型,探究温度为313 K、压力为20 MPa时致密储层表面双电层的特征,进而研究了相同条件下超临界CO_(2)在油水体系的传质作用以及在水岩界面的作用机制。分析结果表明:地层水中离子受不同静电作用影响,在岩石表面和体相中呈现不同密度分布,离带电岩石表面越远,净电荷密度越低;超临界CO_(2)扩散进入油相中降低原油黏度,扩散至水岩界面产生黏附作用,CO_(2)溶解使溶液中H+浓度增加,削弱了静电引力,强氢键作用产生的范德华力和静电力的共同作用使水膜更稳定,增强了水润湿性。该研究结果对致密储层微观理论发展具有一定的参考价值。

The wettability of tight reservoirs is greatly influenced by the microscopic effect of the electric double layer on the reservoir surface.CO_(2)injection has become a key technology for the efficient development of tight oil reservoirs.To explore how the methods of improving oil recovery in tight reservoirs affect the characteristics and wettability of interface electric double layers,a surface electric double layer model of tight reservoirs was established by using molecular dynamics simulation.The characteristics of the electric double layer on the surface of tight reservoirs at 313 K and 20 MPa were explored,and then the mass transfer effect of supercritical CO_(2)in the oil-water system under the same conditions and its mechanism of action at the water-rock interface were studied.The analysis results showed that ions in the formation water were affected by different electrostatic interactions and exhibited different density distributions on the rock surface and in the bulk phase.The farther away from the charged rock surface,the lower the net charge density.Supercritical CO_(2)diffuses into the oil phase to reduce the viscosity of crude oil,and diffuses into the water-rock interface to generate adhesion.CO_(2)dissolution increases the concentration of H+in the solution,weakening the electrostatic attraction.The synergistic effect of van der Waals force generated by strong hydrogen bonding and electrostatic force makes the water film more stable and enhances water wettability.The research result has certain reference value for the development of microscopic theory of tight reservoirs.