Molecular Dynamics Simulations of Oil-Water Wetting Models of Organic Matter and Minerals in Shale at the Nanometer Scale

Wettability is an important physical property of shale. This parameter is related to the shale material composition and the fluid properties in the shale pores and plays an important role in the exploration and development of shale oil. Wettability is affected by the scale and roughness. The contact angle at the nanoscale on a smooth surface can better reflect the wettability of shale than the contact angle at higher scales. Molecular dynamics simulations can be used to measure the contact angle on a smooth surface at the nanoscale. This paper focuses on the effects of organic matter and minerals in shale and different components of shale oil on shale wettability. Wetting models of "organic matter-oil component-water," "quartz-oil component-water" and "kaolinite-oil component-water" at the nanoscale were constructed. Molecular dynamics simulation was used to study the morphological changes of different oil components and water on different surfaces. Studies have shown that organic matter is strongly oleophilic and hydrophobic. Polar components in shale oil can make organic matter slightly hydrophilic. It was recognized by quartz wettability experiments and simulation methods at the nanoscale that the cohesive energy of a liquid has a significant influence on the degree of spreading of the liquid on the surface. The "liquid-liquid-solid" wettability experiment is an effective method for determining mineral oleophilic or hydrophilic properties. The nanoquartz in the shale is strongly hydrophilic. The water wetting angle is related to the crude oil component. Nanokaolinite can have a tetrahedral or an octahedral surface; the tetrahedral surface is oleophilic and hydrophobic, and the octahedral surface exhibits strong hydrophilicity. The wettabilities of both surfaces are related to the crude oil component.