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Summary In the past few years, nanoparticle flooding has displayed significant potential for enhancing oil recovery. Among different nanoparticles, hydrophilic (functionalized) nanotubes have not been tested yet. Therefore, in this study, functionalized multiwalled carbon nanotubes (MWCNT) were used in three concentrations—0.01, 0.05, and 0.1 wt%—to conduct micromodel, interfacial–tension (IFT), wettability, viscosity, phase–behavior, and static–adsorption tests. In addition, constancy of nanofluids in the presence of sodium chloride (NaCl) was investigated qualitatively. Obtained results illustrated that by increasing the nanoparticle concentration from 0.01 to 0.1 wt%, nanofluid/oil IFT decreased, because MWCNT were placed between the surfaces of the oil and the nanofluid. Also, viscosity of the nanofluid improved slightly when the nanoparticle concentration increased. Moreover, contact angle was measured before and after coating with nanosuspensions, and the achieved data showed a change from oil–wet to water–wet. The resulting emulsion was a water–in–oil type; therefore, as MWCNT concentration increased, the size and number of water droplets in the oil also increased. This augmentation improved the effectiveness of the emulsion viscosity. Stability tests also provided evidence confirming that the nanofluids were stable for 10 days in concentrations of 0.01 and 0.1 wt% and in salinities of 1 and 0.5 wt%, respectively. Besides, the addition of 0.05 wt% sulfonated polymer to the nanofluids ensured their stability in higher salinities. Moreover, the results of the micromodel test revealed that by increasing the concentration of nanoparticles, sweep efficiency increased and the fingering phenomenon was reduced. In addition, the ultimate recovery of oil obtained by nanosuspension flooding in concentrations of 0.01, 0.05, and 0.1 wt% was, respectively, 12.6, 16.4, and 20% higher than that of waterflooding. Static adsorption on sandstone was found to be 0.575, 1.25, and 1.76 mg/m2 at 0.01, 0.05, and 0.1 wt%, respectively, and it reached its fixed amounts within 24 hours. Therefore, functionalized MWCNT might help increase heavy–oil recovery through mechanisms of reducing IFT, changing wettability from oil–wet to water–wet in oil–wet reservoirs, increasing viscosity (slightly), and developing a water–in–oil emulsion.

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In this study, we initially performed interfacial tension (IFT) tests to investigate the potential of using the Persian Gulf seawater (PGSW) as smart water with different concentrations of NaCl, KCl, MgCl2, CaCl2, and Na2SO4. Next, for each salt, at the concentration where IFT was minimum, we conducted contact angle, zeta potential, and micromodel flooding tests. The results showed that IFT is minimized if NaCl or KCl is removed from PGSW; thus, for solutions lacking NaCl and KCl, the IFT values were obtained at 26.29 and 26.56 mN/m, respectively. Conversely, in the case of divalent ions, minimum IFT occurred when the concentration of MgCl2, CaCl2, and Na2SO4 in PGSW increased. Specifically, a threefold rise in the concentration of Na2SO4 further reduced IFT as compared to optimal concentrations of MgCl2 or CaCl2. It should be mentioned that eliminating NaCl from PGSW resulted in the lowest IFT value compared to adding or removing other ions. Whereas the removal of NaCl caused the contact angle to decrease from 91.0° to 67.8° relative to PGSW and changed surface wettability to weakly water-wet, eliminating KCl did not considerably change the contact angle, such that it only led to a nine-degree reduction in this angle relative to PGSW and left wettability in the same neutral-wet condition. At optimal concentrations of MgCl2, CaCl2, and Na2SO4, only an increase in Na2SO4 concentration in PGSW could change wettability from neutral-wet to weakly water-wet. For solutions with optimal concentrations, the removal of NaCl or KCl caused the rock surface to have slightly higher negative charges, and increasing the concentration of divalent ions led to a small reduction in the negative charge of the surface. The results of micromodel flooding indicated that NaCl-free PGSW could raise oil recovery by 10.12% relative to PGSW. Furthermore, when the Na2SO4 concentration in PGSW was tripled, the oil recovery increased by 7.34% compared to PGSW. Accordingly, depending on the conditions, it is possible to use PGSW so as to enhance the efficiency of oil recovery by removing NaCl or by increasing the concentration of Na2SO4 three times.

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