Microscale Reservoir Wettability Evaluation Based on Oil–Rock Nanomechanics

Abstract

Low oil production and rapid production decline epitomize the challenges of unconventional reservoir development. Rocks in the formation with micro-/nanoscale pore-throat units possess an immense surface area. Consequently, the wettability of rocks plays a crucial role in the relative permeability of oil/water, the injection pressure, the distribution and morphology of residual oil, and hence the ultimate oil recovery. Contact angle, as a frequently used method to evaluate wettability, is susceptible to oil-water saturation, imbibition, and surface roughness of rock slices. In addition, the measurement scale (droplet of several millimeters) does not match the flow scale (pore units in micro-or nanoscale). Therefore, this paper established a new wettability evaluation method based on oil−rock interaction. Molecular dynamics simulation results demonstrated that the interaction energy between the oil molecule and walls with different wettabilities shows a distinct discrepancy. Accordingly, nanomechanics by AFM was utilized to measure the oil molecule−pore wall interaction force. It was found that the interaction force increases with the augmented hydrophobicity of the pore wall due to hydrophobic force. In addition, the reservoir rock wettability index was defined using the oil−rock interaction force in the nanoscale. The wettability index has an excellent linear correlation with the intrinsic contact angle. The wettability evaluation method based on the oil−rock interaction force is more direct, facile, and rapid and overcomes the limitation of the traditional contact angle method.

Quantitative Characterization of Adhesion Work on Shale Surfaces and Discussion on the Influence of Roughness Based on Atomic Force Microscopy (AFM)

“…According to the Wenzel and Cassie models, , when rocks are relatively hydrophobic, increasing surface roughness can significantly increase the contact angle of water droplets. Although surface roughness can be controlled through grinding and polishing to some extent, it only addresses the large scratches produced during cutting processes, failing to completely resolve the heterogeneous nature of rocks caused by brittle fractures and depressions between small-sized mineral grains. For traditional contact angle methods, air entering rough areas hinders solid–liquid contact, resulting in measurement inaccuracies that do not fully reflect the adhesion work of samples. , Additionally, traditional methods struggle to effectively characterize the contribution of local adhesion and to evaluate and distinguish the main development positions of oil–solid/water–solid adhesion work, leading to an unclear understanding of the main oil-wet/water-wet spaces of the samples.…”

Mechanism and Quantitative Characterization of Wettability on Shale Surfaces: An Experimental Study Based on Atomic Force Microscopy (AFM)

“…This underscores the direct influence of microscale wettability distribution on contact angle measurements and the limited representativeness of this method. Nevertheless, other methods, such as the Amott wettability index, USBM method, and spontaneous imbibition, face challenges in accurately characterizing neutral wettability, being influenced by pore-throat structures, surface roughness, and the fact that they are complex and time-consuming [20,26,[32][33][34][35][36]. Nevertheless, nuclear magnetic resonance only allows for the analysis of wettability variations between pore-throat channels of different diameters [20,[36][37][38].…”