Rock surface, altered wettability

Sandstone rock surfaces are normally highly water-wet. These surfaces can be altered by treatment with solutions of chemical surfactants or by asphaltenes. Increasing the pH of the chemical treating solution decreases the water wettability of the sandstone surface and, in some cases, makes the surface medium oil-wet [1644]. Thus the chemical treatment of sandstone cores can increase the oil production when flooded with carbon dioxide. 

Wettability is defined as "the tendency of one fluid to spread on or adhere to a solid surface in the presence of other immiscible fluids" (145). Rock wettability can strongly affect its relative permeability to water and oil (145,172). Wettability can affect the initial distribution of fluids in a formation and their subsequent flow behavior. When rock is water-wet, water occupies most of the small flow channels and is in contact with most of the rock surfaces. The converse is true in oil-wet rock. When the rock surface does not have a strong preference for either water or oil, it is termed to be of intermediate or neutral wettability. Inadvertent alteration of rock wettability can strong alter its behavior in laboratory core floods (172). 

Another interesting result reported by Sarkar and Sharma (55) was that after saturating the core initially with a polar crude oil, the damage ratio was only 1/1.5 and the permeability decline was very slow. Thus, allowing the rock surfaces to come in direct contact with the polar oil did make the rock less susceptible to damage. This effect is apparently related to adsorption of polar components on the rock surfaces and altered wettability of the fines. 

In the alkaline flood process, the surfactant is generated by the in situ chemical reaction between the alkali of the aqueous phase and the organic acids of the oil phase The surface-active reaction products can adsorb onto the rock surface to alter the wettability of the reservoir rock and/or can adsorb onto the oil-water interface to lower the interfacial tension. At these lowered tensions (1-10 dyne/cm), surface or shear-driven forces promote the formation of stable oil-in-water emulsions or unstable water-in-oil emulsions the nature of the emulsion phase depends on the pH, temperature, and electrolyte type and concentration. These different paths of the surface-active reaction products have created different recovery mechanisms of alkaline flooding. The four alkaline recovery mechanisms which have been cited in the recent literature are (i) Emulsification and Entrainment, (ii) Emulsification and Entrapment, (iii) Wettability Reversal from Oil-to Water-Wet, and (iv) Wettability Reversal from Water- to Oil-Wet. These four mechanisms are similar in that alkaline flooding enhances the recovery of acidic oil by two-stage processes. 

The recovery of naturally acidic oils by alkaline flooding fits into the phase alteration category. The recovery mechanisms of these floods are varied since the surface active salts, which are formed by the in situ acid-base reaction, can adsorb onto the oil-water interface to promote emulsification or can absorb onto the rock surface to alter wettability. The exact recovery mechanism, recently reviewed by Johnson (3) depends on the pH and salinity of the aqueous phase, acidity of the organic phase and wettability of the rock surface (4,5). In this study an additional alkaline recovery mechanism is explored. This mechanism. Emulsification and Coalescence, depends on the valency of the electrolyte as well as the pH and salinity of the aqueous phase. The Emulsification and Coalescence mechanism for the recovery of acidic oils is similar to the Spontaneous Emulsification mechanism suggested by Schechter et al. (6) for the recovery of nonacidic oils with petroleum sulfonate solutions. 

The effect of alkaline materials can cause wettability alteration if the rock surface is intermediate or oil-wet. The enhanced cation concentration influences the size and density of the random coils in a known way. Both facts increase the amount adsorbed. 

This imbibition method is dependent on both the chemical properties of the rock surface and the geometry of the pore network, particularly the pore throat size and shape. Since pore stmctnre can affect the measurement results, the reduction of the interfacial tension between pore fluids by surfactants can affect fluid movement through the pore structure. This method, therefore, is not strictly a measnre of surface wettability, but is a measure of the efficiency of fluid displacement that is normally most strongly affected by the wettability state of the pore surfaces. Within this framework of understanding, wettability alteration of the pore surfaces using snrfactants can be stndied with this measurement method. 

Kinetics. Kinetics also play a role in wettability alteration of porous media by surfactants. In reservoirs, surfactants must be transported through the pore networks by an injected fluid phase, usually water or oil. The ability to alter wettability is related to surfactant diffusion rates and adsorption rates. Surfactants must diffuse through the bulk fluid phase to the meniscus interface and the fluid-pore interface. Surfactants must also interact and adsorb on the pore surfaces. If the diffusion rate or the adsorption rate for surfactant is slow relative to the creation of new water-rock interfaces, because of the water displacement rate, the wettability of the pore surfaces may vary with time. These types of non-... 

Since surfactant adsorption can alter rock surface wettability, it is possible that a surfactant could change a water-wet surface to oil-wet and break the foam. Such foam effects on porous media surfaces must be considered in the design of the foam. 

Carbon Dioxide Flood. Smith et al. [SO] studied the impact of wettability on tertiary oil recovery by carbon dioxide flooding after a secondary waterflood. It was reported that oil recovery could be improved by the wettability alteration of reservoir rock surfaces using surfactants. In this study, water-wet sandstone rock surfaces were modified by treatment with solutions of surfactants to neutral and even moderately oil-wet states. The laboratory results indicated that maximum tertiary oil recovery, after waterflood, by carbon dioxide flooding increased as the wettability of the sandstone decreased from highly water-wet to a neutral-wet or a slightly oil -wet surface. 

Amott method, to be preferentially oil-wet, RDI= —0.82. Laboratory work was undertaken to determine the feasibility of injecting alkaline solutions to improve oil recovery. These experiments were designed to produce surfactants in-situ. The surfactants would both lower the interfacial tension and react with the reservoir rock surface to modify the wettability of the porous media. The experimental work considered the injection of seawater and sodium hydroxide mixtures into cores. The experimental results show that the oil recovery was higher than 50% when the alkaline solution was injected. The conclusion was that surfactant produced by alkaline injection altered the rock wettability from oil-wet to intermediate-wet, increasing oU recovery. One precaution with alkaline flooding is that the range of reactions and the change in pH can cause unexpected variation in oil recovery if the reservoir and fluids are not well characterized. 

The reason for this difference, as postulated by Gidley, is the greater difficulty in cleaning up oil wells following acidizing. It is believed that crude oil can adsorb on acid reaction products, such as the various compounds of silica that reprecipitate on rock surfaces. This can alter wettability from water-wet to partially oil-wet. Also, the fine reprecipitated silica compounds can form solid-stabilized emulsions or sludges with the crude oil present, either of which can cause plugging. 

Increasing the water-wet surface area of a petroleum reservoir is one mechanism by which alkaline floods recover incremental oil(19). Under basic pH conditions, organic acids in acidic crudes produce natural surfactants which can alter the wettability of pore surfaces. Recovery of incremental oil by alkaline flooding is dependent on the pH and salinity of the brine (20), the acidity of the crude and the wettability of the porous medium(1,19,21,22). Thus, alkaline flooding is an oil and reservoir specific recovery process which can not be used in all reservoirs. The usefulness of alkaline flooding is also limited by the large volumes of caustic required to satisfy rock reactions(23). 

The wettability of reservoir rocks can be altered by the adsorption of polar compounds or the deposition of organic material such as asphaltenes in the crude oil. Wettability alteration is determined by the interaction of oil constituents, mineral surface, and brine chemistry including ionic composition and pH. Any extraneous substance such as artificial surfactants that changes the mineral surface will change the wettability of the rock and consequently the flow of fluids inside the reservoir. 

The tendency for a solid to prefer one fluid over another is called wettability. Wettabihty is a function of the chemical composition of both the fluids and the rock. Sin-faces can be either oil-wet or water-wet, depending on the chemical composition of the fluids. The degree to which a rock is either oil-wet or water-wet is strongly affected by the adsorption or desorption of constituents in the oil phase. Large, polar compounds in the oil phase can absorb onto the solid surface this leaves an oil film that may alter the wettability of the surface. 

It is well known that flocculation of asphaltenes in petroleum reservoirs, wells and surface separation-upgrading facilities pose technical problems and increase the cost of production and processing of crudes. Field conditions conducive to precipitation of asphaltenes include natural depletion, miscible flooding, caustic flooding, acid stimulation and gas-lift operations. Asphaltene precipitation is particularly important problem in miscible flooding since it can reduce permeability, affect well injectivities and productivities, alter rock wettability characteristics and even cause plugging of producing wells. ... 

Wettability alteration of porous reservoir roek with surfactants is one means to improve the flow and distribution of fluids in a reservoir. However, much remains to be learned regarding how surfactants interact with the rock minerals and organics found both within the pores and on the pore surfaces of reservoir rock. It is the objective of this chapter to provide an initial understanding and review of the following ... 

Like the other imbibition methods, SII is dependent on both surface chemistry and pore network geometry. It works best for rocks that imbibe only one fluid, but could be modified to a form like the Amott-Harvey RDI. It also has the potential, by its formulation, to evaluate wettability alteration by surfactants in reservoirs. 

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