Adhesion Forces of Shale Oil Droplet on Mica Surface with Different Roughness: An Experimental Investigation Using Atomic Force Microscopy

In order to investigate the effect of rock surface roughness on the occurrence state of shale oil, muscovite mica was firstly characterized by performing atomic force microscopy (AFM). Two-dimensional (2D) images and the three-dimensional (3D) structure of the mica surface were obtained. Wettability of the micas was measured according to the sessile drop method using shale oil, collected from a lacustrine shale oil well drilling through the Yanchang Formation, Ordos Basin. Then, the adhesion forces between shale oil and mica surface with a different roughness were finely measured using AFM mounted with the shale oil modified probe tips. The adhesion force curves at the approaching and retract modes were obtained. The results show that the average roughness value of the mica samples was about 1 nm, while the maximum height was up to 4 nm. The contact angle between shale oil and mica ranged from 128.73 degrees to 145.81 degrees, and increased with increasing surface roughness, which can be described by the Wenzel model. The adhesion force between shale oil and mica also increased with an increasing contact area. Shale oil can fill the deep valleys on the rough surface of rocks and then form microscopic storage for oil droplets. The maximum adhesion force, reached at a distance of about 5-10 nm between shale oil droplets and micas, was between 14 and 30 nN. The adhesion force disappeared when the distance was larger than 40 nm. These indicate that shale oil in pores with a diameter of less than 10 nm was tightly adsorbed, and formed a layered accumulation pattern. Additional energy is needed to decrease the disjoining pressure and then separate shale oil from these tight pores. Shale oil is freely movable at pores with pore diameters of larger than 40 nm. This work provides a new insight about the interaction between shale oil and rock, and helps to understand the occurrence mechanism of shale oil.