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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.

A hybrid pore-scale simulation method using Lattice-Boltzmann (LB) coupled with Langevin-Dynamics (LD) is proposed to investigate the transport physics of nanoparticles in microchannel. The controlling factors (i.e., ionic strength, particle diameter and Reynolds number) are investigated in the attachment process of NPs. It is observed that a threshold value of attachment efficiency exists as the ionic strength increases to about 0.01 M. Moreover, the ionic strength of aqueous phase has critical effect on the transport behavior of NPs. For the purpose of quantitatively characterizing the structure of NP suspensions under varying conditions, a general phase diagram including three flow patterns (isolated, transitional and clustered regime) is first proposed for NP suspension with specified ionic strength and Reynolds number. The outcomes of this work provide valuable insight on the critical importance of the particle size, ionic strength and hydrodynamic effects on the attachment and transport process of NPs in porous media

The reservoir heterogeneity and unfavorable oil/gas mobility ratio lead to gas channeling and low CO2 sweep efficiency during CO2 flooding in low-permeability reservoirs. A dispersed particle gel (DPG) could migrate deep into the reservoir and coalesce, which has the potential for CO2 gas channeling control. In this work, a CO2-resistant bulk gel was prepared by a cross-linking reaction of a copolymer with acid-resistant groups and catechol–hexamethylenetetramine, which exhibited excellent CO2 resistance. Subsequently, a CO2-resistant dispersed particle gel (SCDPG) used in supercritical CO2 was successfully prepared from the CO2-resistant bulk gel by a high-speed mechanical shearing method. Meanwhile, the coalescence behavior of the SCDPG particles in supercritical CO2 was systematically investigated from the microstructure, particle size, ζ potential, and mechanical strength. The results showed that the SCDPG particles were dispersed in the liquid phase as a single particle. SCDPG had good dispersion stability during storage and injection. In supercritical CO2, the dispersion stability of SCDPG decreased, and the particles coalesced with each other to form aggregates with a stereostructure instead of degradation. The SCDPG particles maintained high mechanical strength, showing the long-term effectiveness for gas channeling control during CO2 flooding. In addition, the interparticle force of SCDPG particles was measured by an AFM colloid probe based on the reservoir characteristics. The interparticle force of SCDPG particles changed from repulsion to adhesion with increasing salinity. At a salinity of 0.5 mol/L, the adhesion force increased with the increase of temperature and the decrease of the pH value. According to the type of intermolecular force, the adhesion force originated from the hydrogen bond, π–π stacking, and cation−π interaction. This work provides theoretical support for the field application of the dispersed particle gel and promotes the development of utilization of carbon dioxide in oilfields.