Previous measurements show mixed‐wet behavior of tight siltstone core plugs, initially saturated with air. However, the same plugs show water‐wet behavior when saturated with oil or water. This study aims at explaining these observations by investigating the effects of mineral heterogeneity and intermolecular forces acting on solid/liquid and liquid/liquid interfaces. First, we conduct pore‐scale visualizations to characterize the minerals surrounding the pores of the tight rock samples. Thin‐section and scanning electron microscopy/energy dispersive X‐ray spectroscopy analyses show the existence of multimineral pores, which can be responsible for the mixed‐wet behavior. Next, we apply the DLVO theory to investigate the intermolecular forces acting on solid/liquid and liquid/liquid interfaces. We use disjoining pressure‐distance profiles and contact angles calculated for different minerals to evaluate the stability of the thin film covering the rock grains and to explain the wettability of the core plugs during spontaneous imbibition tests. The disjoining pressure values calculated for the dry core plugs suggest similar wetting affinities toward oil and water. However, the contact angles calculated for the water‐saturated core plugs suggest the water‐wet behavior, which agrees with the countercurrent imbibition results. Finally, we measure the adhesion forces between the tip of a cantilever and (1) pure quartz slide, (2) pure calcite crystal, and (3) the rock thin section using an atomic force microscope. The values of adhesion forces measured for the thin section composed of multimineral pores are between those measured for pure quartz slide and pure calcite crystal.

Fine migration is a noticeable problem in petroleum production engineering. Plugging of throats in porous media occurs due to detachment of fine particles from sand surfaces. Hence the study of interactions between fines and pore surfaces and investigating the governing forces are important issues to describe the mechanism of fines release process. The main types of these forces are electric double layer repulsion and London-van der Waals attraction. It is possible to change these forces by the use of nanoparticles as surface coatings. Nanoparticles increase the effect of attraction forces in comparison with repulsion forces.In this paper, we present new experiments and modeling to observe these properties of nanoparticles. For this purpose, the surface of pores was coated with different types of nanoparticles (MgO, Sio2, and Al2O3). Zeta potential test was used to examine changes in potential of the pore surfaces. Total interaction energy is then mathematically calculated to compare different states. Total interaction energy is a felicitous criterion which gives proper information about the effect of different nanoparticles on surface properties. Accordingly, the total interaction plots are found to be the suitable tool to select the best coating material.10-μm-diameter silica spheres and glass beads packed cwere utilized as fines and porous medium, respectively. Measurements were performed in 1:1 electrolyte aqueous solution of 10−3 mM, and probe-plate separation distances varied from less than 1 nm to 50 nm. Magnitude of change in zeta potential for MgO nanoparticle is 45 mV. Our model demonstrated that the magnitude of the electric double layer repulsion in comparison with the London-van der Waals attraction between probe and plate surface was considerably diminished when MgO nanoparticle was used to coat the surface of plate which is in complete agreement with our experimental observation.

Formation damage of the oil reservoirs because of fines migration is a major reason for productivity decline. Formation fines are defined as unconfined solid particles present in the pore spaces of the formations. Their migration because of fluids flow in the reservoir would cause pore plugging or permeability reduction. In the last three decades, many studies have been done for the characterization of fines and their migration effect on permeability reduction. There are many techniques in the industry to remediate the damage, especially near well bore regions. Nanofluids that contain nanoparticles show specific properties such as high tendency for adsorption and good candidate for the injection into the near wellbore regions because of their very small sizes. In this paper, a packed column is utilized to study the use of different types of nanoparticles to reduce fines migration in synthetic porous materials. Three types of nanoparticles; MgO, SiO2 and Al2O3 are used here to investigate their effects on the fines movement. The results indicate that the fines could stick into the grains hindering their migration when the porous materials are already soaked with nanofluids. Besides, to check the mechanisms of this remediation technique, the effect of nanoparticle concentrations and fluid flow rates in the medium on the fines detachment was studied. A theoretical model was employed to calculate the total energy of interaction for the surfaces to check the experimental results, which was also validated by using SEM pictures for samples from the synthetic cores. The results showed that the addition of 0.1 wt% of MgO and SiO2 nanoparticles would reduce the fines migration by 15% compared to the reference state. MgO nanoparticles were found to be more effective even at high fluid rates when it is used at higher concentration as it is noticed in the macroscopic and microscopic results.

Fines migration is the major reason for productivity decline known as formation damage in oil reservoirs. Sandstone formations are sensitive to brine salinity alteration which disturbs equilibrium condition in porous media. Because of nonequilibrium condition fines migration occurs during various operations. Nanoparticles seem to be good candidates to strengthen the attractive forces between fines and pore wall due to very small size, high specific surface area and electrical surface charge. In this experimental study, several tests were performed using Berea sandstone (8 wt% clays) cores (3 in. length and 1.5 in. diameter). MgO nanoparticles were stabilized in the water uniformly using surface active agent and ultrasonication. Total dimensionless energy of interaction between nano particles in the suspensions was calculated based on the DLVO theory. Various core flooding tests were conducted to determine the effect of MgO nanofluid injection as clay stabilizer at different brine salinities on the cores with the permeability from 600-100 md. The pressure drop across the core was measured. The results indicated that the MgO nanofluid could fix fines effectively where brine salinity became lower than CSC. Besides, measured zeta potential and total energy of interaction calculation confirmed that repulsive force became dominant at the specific concentration of the complex nanofluid which ensures its stability for long time during core flooding tests. Thus, MgO nanofluid significantly prevented water shock problem. Also, no significant reduction in permeability was noticed in any of core flood tests.