Wettability Alteration Using Silane to Improve Water-Alternating-Gas Injectivity.

Wettability is a main component that determines multiphase flow characteristics in a porous medium. Altering the wettability of a rock has a wide range of applications in the field of geosystems engineering, such as enhanced oil recovery, improving gas well deliverability, and geological CO2 sequestration. Considering how injectivity in many field water-alternating-gas (WAG) processes is lower than expected, wettability alteration is especially suitable to address the reduction in relative permeability encountered during water injection. Several methods for injectivity improvement exist, including the use of surfactants, nanoparticles, salts, and alkalis. Using silanes to modify wettability has been a prominent technique in surface chemistry for decades but has very rarely been applied to porous mineral rocks, especially carbonates. This work explores the use of silanes to render sandstone and limestone surfaces more hydrophobic, thereby reducing gas blockage that causes injectivity loss. Contact angle measurements were taken and showed good wettability alteration away from water wet, exhibiting contact angles well above 90°, regardless of treatment conditions. Centrifuge tests were carried out, and the resulting residual fluid saturations and capillary pressure curves proved that the treatment is also effective on the pore scale. Corefloods conducted in sandstone and limestone cores showed a 45 and 65% increase in water relative permeability after WAG cycles after treatment, respectively. This translates directly to improvements in injectivity based on this treatment method.

Wettability Alteration and Adsorption of Mixed Nonionic and Anionic Surfactants on Carbonates.

Mixed-surfactant systems consisting of secondary alcohol ethoxylates and anionic sulfonates are evaluated as wettability alteration agents for enhanced oil recovery. The cloud points of the nonionic surfactants are raised by the addition of the sulfonates. The oil/water interfacial tension and contact angles of oil on initially oil-wet calcite are reported at different temperatures and surfactant compositions. Adsorption experiments are performed for select mixed systems at high temperatures. The extent of the increase in the cloud point, changes in the contact angle, and adsorption are influenced by co-surfactants, surfactant concentrations, and temperatures. Mixed surfactant systems were identified which modified the oil-wet surface to a water-wet surface with final contact angles as low as 70°. Mixed surfactants exhibit a linear trend in adsorption and wettability alteration with the thermodynamic descriptor of cloud point temperature difference, which has been used previously for single surfactants. These findings enable the design of surfactant formulations for wettability alteration in high temperature, high salinity reservoirs.

Viscoelastic diamine surfactant for stable carbon dioxide/water foams over a wide range in salinity and temperature.

HYPOTHESIS: The viscosity and stability of CO2/water foams at elevated temperature can be increased significantly with highly viscoelastic aqueous lamellae. The slow thinning of these viscoelastic lamellae leads to greater foam stability upon slowing down Ostwald ripening and coalescence. In the aqueous phase, the viscoelasticity may be increased by increasing the surfactant tail length to form more entangled micelles even at high temperatures and salinity. EXPERIMENTS: Systematic measurements of the steady state shear viscosity of aqueous solutions of the diamine surfactant (C16-18N(CH3)C3N(CH3)2) were conducted at varying surfactant concentrations and salinity to determine the parameters for formation of entangled wormlike micelles. The apparent viscosity and stability of CO2/water foams were compared for systems with viscoelastic entangled micellar aqueous phases relative to those with much less viscous spherical micelles. FINDINGS: We demonstrated for the first time stable CO2/water foams at temperatures up to 120 °C and CO2 volumetric fractions up to 0.98 with a single diamine surfactant, C16-18N(CH3)C3N(CH3)2. The foam stability was increased by increasing the packing parameter of the surfactant with a long tail and methyl substitution on the amine to form entangled viscoelastic wormlike micelles in the aqueous phase. The foam was more viscous and stable compared to foams with spherical micelles in the aqueous lamellae as seen with C12-14N(EO)2 and C16-18N(EO)C3N(EO)2.

Phase behavior and interfacial properties of a switchable ethoxylated amine surfactant at high temperature and effects on CO2-in-water foams.

The interfacial properties for surfactants at the supercritical CO2-water (C-W) interface at temperatures above 80°C have very rarely been reported given limitations in surfactant solubility and chemical stability. These limitations, along with the weak solvent strength of CO2, make it challenging to design surfactants that adsorb at the C-W interface, despite the interest in CO2-in-water (C/W) foams (also referred to as macroemulsions). Herein, we examine the thermodynamic, interfacial and rheological properties of the surfactant C12-14N(EO)2 in systems containing brine and/or supercritical CO2 at elevated temperatures and pressures. Because the surfactant is switchable from the nonionic state to the protonated cationic state as the pH is lowered over a wide range in temperature, it is readily soluble in brine in the cationic state below pH 5.5, even up to 120°C, and also in supercritical CO2 in the nonionic state. As a consequence of the affinity for both phases, the surfactant adsorption at the CO2-water interface was high, with an area of 207Å(2)/molecule. Remarkably, the surfactant lowered the interfacial tension (IFT) down to ∼5mN/m at 120°C and 3400 psia (23MPa), despite the low CO2 density of 0.48g/ml, indicating sufficient solvation of the surfactant tails. The phase behavior and interfacial properties of the surfactant in the cationic form were favorable for the formation and stabilization of bulk C/W foam at high temperature and high salinity. Additionally, in a 1.2 Darcy glass bead pack at 120°C, a very high foam apparent viscosity of 146 cP was observed at low interstitial velocities given the low degree of shear thinning. For a calcium carbonate pack, C/W foam was formed upon addition of Ca(2+) and Mg(2+) in the feed brine to keep the pH below 4, by the common ion effect, in order to sufficiently protonate the surfactant. The ability to form C/W foams at high temperatures is of interest for a variety of applications in chemical synthesis, separations, materials science, and subsurface energy production.

Molecular dynamics studies of aqueous silica nanoparticle dispersions: salt effects on the double layer formation.

The ion distribution around hydroxylated silica nanoparticles (NP-H) dispersed in brine was investigated by fully atomistic molecular dynamics. The NP-H dispersions in aqueous electrolyte media are simulated in solutions of varying salinity (NaCl, CaCl2, and MgCl2), salt concentration (0.06 × 10(-3) to 3.00 × 10(-3) mole fraction [Formula: see text]), and temperature (300 and 350 K) at 1 atm. The NP-H models reproduce the observed experimental concentration of silanol and geminal surface sites, which are responsible for local charge variations on the nanoparticles' surface. Interestingly, under certain salt concentration conditions, the formation of an electrical double layer (DL) around the overall neutral NP-H occurs. The resulting DLs are attenuated with increasing temperature for all evaluated salts. With increasing salt concentration, a sign inversion of the effective charge at the first ion layer is observed, which modifies the electrostatic environment around the nanoparticle. The minimum salt concentration that leads to a DL formation at 300 K is 1.05 × 10(-3), 0.37 × 10(-3), and 0.06 × 10(-3) χs for NaCl, CaCl2, and MgCl2, respectively. The width of the DL decreases sequentially in ionic strength from NaCl to CaCl2 to MgCl2, which is similar to that found for highly charged surfaces. These results are in line with our previous experimental data for negative charged NP-H. All together, these observations suggest an interplay mechanism between the formation and narrowing of electric double layers on the stability of NP dispersions in both neutral and negatively charged NP-H.

Effect of silica nanoparticles on clay swelling and aqueous stability of nanoparticle dispersions.

The results of the effects of electrolyte type and concentration, nanoparticle concentration, pH, and temperature on the mobility and aqueous stability of polyethylene glycol (PEG)-coated silica nanoparticles are presented. Nanoparticle mobility was evaluated based on the ability to inhibit montmorillonite swelling in aqueous solutions through visual swelling tests, and the results were quantified in terms of the swelling index. The presence of PEG-coated silica nanoparticles was found to have a positive influence on the inhibition of clay swelling only in the presence of electrolytes. Quantification of nanoparticle stability in the presence of montmorillonite particles was achieved using ultraviolet-visible (UV-vis) spectrophotometry. At the highest concentration of montmorillonite dispersion studied, interaction between the dispersed montmorillonite particles and PEG-coated silica nanoparticles resulted in nanoparticle aggregation as indicated by increased turbidity and absorbance readings. Both nanoparticle concentration and montmorillonite dispersion concentration, in addition to the presence and concentration of NaCl, were found to strongly influence the stability of the mixture.

Adsorption of surface functionalized silica nanoparticles onto mineral surfaces and decane/water interface.

The adsorption of silica nanoparticles onto representative mineral surfaces and at the decane/water interface was studied. The effects of particle size (the mean diameters from 5 to 75 nm), concentration and surface type on the adsorption were studied in detail. Silica nanoparticles with four different surfaces [unmodified, surface modified with anionic (sulfonate), cationic (quaternary ammonium (quat)) or nonionic (polyethylene glycol (PEG)) surfactant] were used. The zeta potential of these silica nanoparticles ranges from -79.8 to 15.3 mV. The shape of silica particles examined by a Hitachi-S5500 scanning transmission electron microscope (STEM) is quite spherical. The adsorption of all the nanoparticles (unmodified or surface modified) on quartz and calcite surfaces was found to be insignificant. We used interfacial tension (IFT) measurements to investigate the adsorption of silica nanoparticles at the decane/water interface. Unmodified nanoparticles or surface modified ones with sulfonate or quat do not significantly affect the IFT of the decane/water interface. It also does not appear that the particle size or concentration influences the IFT. However, the presence of PEG as a surface modifying material significantly reduces the IFT. The PEG surface modifier alone in an aqueous solution, without the nanoparticles, yields the same IFT reduction for an equivalent PEG concentration as that used for modifying the surface of nanoparticles. Contact angle measurements of a decane droplet on quartz or calcite plate immersed in water (or aqueous nanoparticle dispersion) showed a slight change in the contact angle in the presence of the studied nanoparticles. The results of contact angle measurements are in good agreement with experiments of adsorption of nanoparticles on mineral surfaces or decane/water interface. This study brings new insights into the understanding and modeling of the adsorption of surface-modified silica nanoparticles onto mineral surfaces and water/decane interface.

Interface tension of silica hydroxylated nanoparticle with brine: a combined experimental and molecular dynamics study.

We have used molecular dynamics simulations to calculate the interfacial tension of hydroxylated SiO(2) nanoparticles under different temperatures and solutions (helium and brine with monovalent and divalent salts). In order to benchmark the atomistic model, quartz SiO(2) interfacial tension was measured based on inverse gas chromatography under He atmosphere. The experimental interfacial tension values for quartz were found between 0.512 and 0.617 N/m. Our calculated results for the interfacial tension of silica nanoparticles within helium atmosphere was 0.676 N/m, which is higher than the value found for the system containing He∕α-quartz (0.478 N/m), but it is similar to the one found for amorphous silica surface. We have also studied the interfacial tension of the nanoparticles in electrolyte aqueous solution for different types and salts concentrations (NaCl, CaCl(2), and MgCl(2)). Our calculations indicate that adsorption properties and salt solutions greatly influence the interfacial tension in an order of CaCl(2) > MgCl(2) > NaCl. This effect is due to the difference in distribution of ions in solution, which modifies the hydration and electrostatic potential of those ions near the nanoparticle.

Effect of branching on the interfacial properties of nonionic hydrocarbon surfactants at the air-water and carbon dioxide-water interfaces.

The interfacial tensions, surface pressures, and adsorption of nonionic hydrocarbon surfactants at the air-water (A-W) and carbon dioxide-water (C-W) interfaces were investigated systematically as a function of the ethylene oxide (EO) unit length and tail structure. Major differences in the properties are explained in terms of the driving force for surfactant adsorption, tail solvation, area per surfactant molecule, and surfactant packing. As the surfactant architecture is varied, the changes in tail-tail interactions, steric effects, areas occupied by the surfactant at the interface, and tail hydrophobicity are shown to strongly influence the interfacial properties, including the surfactant efficiency (the concentration to produce 20 mN/m interfacial tension reduction). For linear surfactants at the A-W interface, high efficiencies result from dense monolayers produced by the high interfacial tension driving force for adsorption and strong tail-tail interactions. At the C-W interface, where a low interfacial tension leads to a much lower surfactant adsorption, the contact between the phases is much greater. Branching or increasing the number of tail chains increase the hydrophobicity, tail solvation, and adsorption of the surfactant. Furthermore, the area occupied by the surfactant increases with branching, number of tails, and number of EO monomers in the head group, to reduce contact of the phases. These factors produce greater efficiencies for branched and double tail surfactants at the C-W interface, as well as surfactants with longer EO head groups.

Morphology and stability of CO2-in-water foams with nonionic hydrocarbon surfactants.

The morphologies, stabilities, and viscosities of high-pressure carbon dioxide-in-water (C/W) foams (emulsions) formed with branched nonionic hydrocarbon surfactants were investigated by in situ optical microscopy and capillary rheology. Over two dozen hydrocarbon surfactants were shown to stabilize C/W foams with Sauter mean bubble diameters as low as 1 to 2 microm. Coalescence of the C/W foam bubbles was rare for bubbles larger than about 0.5 microm over a 60 h time frame, and Ostwald ripening became very slow. By better blocking of the CO(2) and water phases with branched and double-tailed surfactants, the interfacial tension decreases, the surface pressure increases, and the C/W foams become very stable. For branched surfactants with propylene oxide middle groups, the stabilities were markedly lower for air/water foams and decane-water emulsions. The greater stability of the C/W foams to coalescence may be attributed to a smaller capillary pressure, lower drainage rates, and a sufficient surface pressure and thus limiting surface elasticity, plus small film sizes, to hinder spatial and surface density fluctuations that lead to coalescence. Unexpectedly, the foams were stable even when the surfactant favored the CO(2) phase over the water phase, in violation of Bancroft's rule. This unusual behavior is influenced by the low drainage rate, which makes Marangoni stabilization of less consequence and the strong tendency of emerging holes in the lamella to close as a result of surfactant tail flocculation in CO(2). The high distribution coefficient toward CO(2) versus water is of significant practical interest for mobility control in CO(2) sequestration and enhanced oil recovery by foam formation.

Motion of foam films in diverging-converging channels.

Existing theories of the motion of foam films in capillaries often assimilate the pressure drop over the foam films to the static capillary pressure obtained from the Young-Laplace equation. Hence, they ignore the contribution of dynamic effects associated with the rapid stretching and contraction of the foam films to the overall viscous dissipation. This paper reports an investigation of the motion of foam films in axisymmetric diverging-converging channels, taking into account surface viscosity and elasticity. First, a phenomenological theory for the motion of the foam films is developed using simple physical arguments. We show that the displacement of the film obeys a nonlinear second-order differential equation, which can be solved numerically for the (dimensionless) distance from the inlet and the pressure drop as a function of time. Experiments with foam film motion, conducted using glass diverging-converging channels (minimum radius = 3.00 +/- 0,01 mm, maximum diameter = 7,98 +/- 0,01 mm) and nitrogen foam stabilized with sodium dodecyl sulfate (SDS) in brine, are discussed. For a single film motion in the diverging channel, we find that (a) the static pressure drop is a concave-upward function of distance and decreases from 1.0 to about 0.3, whereas (b) the dynamic pressure drop is concave downward and increases from 1 to a maximum of 1.3 and then decreases to 0.7. In the converging channel both the static and dynamic pressure drops are concave-downward functions, but the dynamic pressure drop values are always higher than the static ones. For two films the motions were found to be rather sensitive to the initial arrangement in the channel. The experiments are found to be in excellent agreement with the theoretical predictions. These observations imply that the large flow resistance obtained during foam flow in granular porous media, where converging-diverging channels are abundant, is largely due to the surface elasticity and viscosity of the films.