Wettability alteration is an important method to increase oil recovery from oil-wet carbonate reservoirs. Chemical agents like surfactants are known as wettability modifiers in carbonate systems. However, the effectiveness of these agents can be increased by the addition of chemicals such as polymers, ionic materials, and nanoparticles. The impacts of nanoparticles on the wettability of carbonate systems have not been reported yet, and it is still in its infancy. In this work, the effect of ZrO 2 -based nanofluids on the wettability alteration of a carbonate reservoir rock was experimentally studied. Several nanofluids were made composed of ZrO 2 nanoparticles and mixtures of nonionic surfactants. The effect of nanofluids on the wettability of carbonate samples were investigated by measuring the contact angles, and it was shown that designed nanofluids could significantly change the wettability of the rock from a strongly oil-wet to a strongly water-wet condition. Scanning electron microscopy (SEM) images and X-ray Diffraction (XRD) data verify adsorption of nanoparticles on the rock and formation of nanotextured surfaces. Moreover, this paper reports the quick imbibitions of ZrO 2 nanofluids into oil-wet core plugs saturated with stock tank oil. The results show that a considerable amount of oil can be quickly recovered by free imbibitions of the nanofluids into the core plugs. A theoretical approach is also presented to explain the wettability alteration by formation of composite nanotextured surfaces.

The effects of surface acidity and basicity of metal oxide nanoparticles on the thermodynamics of asphaltene adsorption were studied. Three different categories of metal oxides/salts with acidic (WO3 and NiO), amphoteric (Fe2O3 and ZrO2), and basic (MgO and CaCO3) surfaces were synthesized, and their textural, structural, and acid-base properties were characterized. Asphaltenes were extracted from a dead oil sample and characterized by X-ray powder diffraction and Fourier transform infrared spectroscopy. The acid and base numbers of the asphaltenes were measured. The nanoparticles were added to the asphaltene-toluene solutions, and the amount of adsorbed asphaltene was obtained through centrifugation followed by UV-vis spectroscopy of the supernatant liquid and temperature-programmed oxidation analysis of the precipitated solid. The concentrations of organic acid and base groups in the asphaltenes are 2.75 and 12.34 mg of KOH/g, respectively, indicating that the asphaltenes are more basic in nature. Isotherms of the asphaltene adsorption onto the six metal oxides/salts fit the Langmuir model closely. The asphaltene adsorption capacity of the nanoparticles is 1.23-3.67 mg/m(2) and decreases in the order of NiO > Fe2O3 > WO3 > MgO > CaCO3 > ZrO2, concomitant with the synergetic effects of acidity and the net charge of the surfaces. High-resolution transmission electron microscopy illustrates that the asphaltenes are spread out over the surfaces with no short-range/long-range order. The adsorption of the asphaltenes onto the six samples is exothermic and spontaneous with the Gibbs energy change of -27.80 to -28.79 kJ/mol at 25 °C. The absolute value of the enthalpy change of the adsorption is calculated to be within the range of 5-20 kJ/mol. Acid-base interaction and electrostatic attraction seem to be the dominant forces contributing to the adsorption of the asphaltenes onto the metal oxide/salt surfaces.

Nanofluids have been recently proposed as new chemical agents for enhanced oil recovery from oil reservoirs. Various nanofluids have been studied in that regard and reported in the literature, verifying the capability of nanostructured materials in enhancing the oil recovery through alteration of rock wettability. In this study, the impacts of different nanofluids of zirconium dioxide (ZrO 2 ), calcium carbonate (CaCO 3 ), titanium dioxide (TiO 2 ), silicon dioxide (SiO 2 ), magnesium oxide (MgO), aluminum oxide (Al 2 O 3 ), cerium oxide (CeO 2 ), and carbon nanotube (CNT) on the wettability of carbonate rocks were investigated. A series of preliminary contact angle evaluations were performed to screen the nanoparticles. The performances of the selected nanofluids were evaluated by spontaneous imbibition and core flooding experiments. Results of spontaneous imbibition tests and coreflooding experiments confirm the active roles of CaCO 3 and SiO 2 nanoparticles for enhancing oil recovery. In addition, the effect of nanofluid injection on rock surface wettability was examined by drainage capillary pressure measurement. It is shown that the irreducible water saturation and the entry capillary pressure were both increased after treatment by CaCO 3 nanaofluid. Moreover, the structural disjoining pressure gradient is proposed to be the responsible mechanism for changing wettability. Both experiments and theoretical calculations prove that disjoining pressure of the nanoparticles layer near the contact point can be high enough to remove oil from the surface.

Asphaltene precipitation causes several problems during crude oil production, transportation, and refinery processes. Therefore, finding an inhibitor to prevent or delay asphaltene precipitation is of paramount importance. In this work, effects of TiO2, ZrO2, and SiO2 fine nanoparticles in organic-based nanofluids have been investigated to study their potential for stabilizing asphaltene particles in oil. To this end, polarized light microscopy has been applied to determine the onset of asphaltene precipitation by titration of dead oil samples from Iranian crude oil reservoirs with n-heptane in the presence of nanofluids. Results show that rutile (TiO2) fine nanoparticles can effectively enhance the asphaltene stability in acidic conditions and act inversely in basic conditions. It was found that the required amount of n-heptane for destabilizing the colloidal asphaltene is considerably higher in presence of TiO2 nanofluids at pH below 4. The FTIR spectroscopy indicates changes in n-heptane insoluble asphaltene when acidic TiO2 nanofluid is used as inhibitor. According to the results obtained by FTIR spectroscopy, TiO2 nanoparticles can enhance the stability of asphaltene nanoaggregates through formation of hydrogen bond at acidic conditions. This is while other materials used in this experiment, as well as the TiO2 nanoparticles in basic conditions, are unable to form any hydrogen bond – hence their incapability to prevent asphaltene precipitation. Dynamic light scattering (DLS) measurements also have been performed to explain the mechanism of asphaltene precipitation in the presence of nanoparticles.

No abstract

The surface phase approach of Butler has been used to derive a model of the surface tension (ST) of surfactant solutions in terms of the ST of the surfactant in the absence of water, an area parameter corresponding approximately to the limiting area per molecule, and the critical micelle concentration (CMC). This isotherm is then used to account for the ST behavior of aqueous solutions of weakly interacting polymer-surfactant (P-S) and strongly interacting polyelectrolyte-surfactant (PE-S) mixtures. For P-S systems, no additional parameters are required other than the critical aggregation concentration (CAC) and the onset of the ST plateau at micellization (T3). The model accounts for experimental isotherms for sodium dodecyl sulfate (SDS) with poly(ethylene oxide) or poly(vinylpyrrolidone). For PE-S systems, the initial CAC has no effect on the ST and is well below the decrease in ST that leads to the first ST plateau at T1. This decrease is modeled approximately using a Langmuir isotherm. The remaining ST behavior is analyzed with the model surfactant isotherm and includes modeling the ST when there is separation into two phases. The behavior in the phase separation region depends on the dissociability of the PE-S complex. Loss of surface activity accompanied by a peak in the ST may occur when there is part formation of a nondissociable complex (neutral with segment/surfactant = 1). The model successfully explains the ST of several experimental systems with and without ST peaks, including poly(dimethyldiallylammonium chloride)-SDS and poly(sodium styrenesulfonate)-alkyltrimethylammonium bromide (C(n)TAB) with n = 12, 14, and 16.

No abstract

The properties and phase diagrams of aqueous mixtures of dodecyltrimethylammonium bromide (C(12)TAB) with the sodium oligoarene sulphonates (POSn), POS2, POS3, POS4, and POS6 have been studied using surface tension and neutron reflectometry to study the surface, and neutron small angle scattering and fluorescence to study the bulk solution. The behavior of POS2 and POS3 is reasonably consistent with mixed micelles of C(12)TAB and POSn-(C(12)TA)(n). These systems exhibit a single critical micelle concentration (CMC) at which the surface tension reaches the usual plateau. This is contrary to a recent report which suggests that the onset of the surface tension plateau does not coincide with the CMC. In the POS3 system, the micelles conform to the core-shell model, are slightly ellipsoidal, and have aggregation numbers in the range 70-100. In addition, the dissociation constant for ionization of the micelles is significantly lower than for free C(12)TAB micelles, indicating binding of the POS3 ion to the micelles. Estimation of the CMCs of the POSn-(C(12)TA)(n) from n = 1-3 assuming ideal mixing of the two component surfactants and the observed values of the mixed CMC gives values that are consistent with the nearest related gemini surfactant. The POS4 and POS6 systems are different. They both phase separate slowly to form a dilute and a concentrated (dense) phase. Fluorescence of POS4 has been used to show that the onset of aggregation of surfactant (critical aggregation concentration, CAC) occurs at the onset of the surface tension plateau and that, at the slightly higher concentration of the phase separation, the concentration of POS4 and C(12)TAB in the dilute phase is at or below its concentration at the CAC, that is, this is a clear case of complex coacervation. The surface layer of the C(12)TA ion in the surface tension plateau region, studied directly by neutron reflectometry, was found to be higher than a simple monolayer (observed for POS2 and POS3) for both the POS4 and POS6 systems. In POS6 this evolved after a few hours to a structure consisting of a monolayer with an attached subsurface bilayer, closely resembling that observed for one class of polyelectrolyte/surfactant mixtures. It is suggested that this structured layer, which must be present on the surface of the dilute phase of the coacervated system, is a thin wetting film of the dense phase. The close resemblance of the properties of the POS6 system to that of one large group of polyelectrolyte/surfactant mixtures shows that the surface behavior of oligoion/surfactant mixtures can quickly become representative of that of true polyelectrolyte/surfactant mixtures. In addition, the more precise characterization possible for the POS6 system identifies an unusual feature of the surface behavior of some polyelectrolyte/surfactant systems and that is that the surface tension can remain low and constant through a precipitation/coacervation region because of the characteristics of two phase wetting. The well-defined fixed charge distribution in POS6 also ...