Mechanistic Study of Wettability Alteration Using Surfactants with Applications in Naturally Fractured Reservoirs

Abstract

In naturally fractured reservoirs, oil recovery from waterflooding relies on the spontaneous imbibition of water to expel oil from the matrix into the fracture system. The spontaneous imbibition process is most efficient in strongly water-wet rock where the capillary driving force is strong. In oil- or mixed-wet fractured carbonate reservoirs, however, the capillary driving force for the spontaneous imbibition process is weak, and therefore the waterflooding oil recoveries are low. The recovery efficiency can be improved by dissolving low concentrations of surfactants in the injected water to alter the wettability of the reservoir rock to a more water-wet state. This wettability alteration accelerates the spontaneous imbibition of water into matrix blocks, thereby increasing the oil recovery during waterflooding. Several mechanisms have been proposed to explain the wettability alteration by surfactants, but none have been verified experimentally. Understanding of the mechanisms behind wettability alteration could help to improve the performance of the process and aid in identification of alternative surfactants for use in field applications. Results from this study revealed that ion-pair formation and adsorption of surfactant molecules through interactions with the adsorbed crude oil components on the rock surface are the two main mechanisms responsible for the wettability alteration. Previous researchers observed that, for a given rock type, the effectiveness of wettability alteration is highly dependent upon the ionic nature of the surfactant involved. Our experimental results demonstrated that ion-pair formation between the charged head groups of surfactant molecules and the adsorbed crude oil components on rock surface was more effective in changing the rock wettability toward a more water-wet state than the adsorption of surfactant molecules as a monolayer on the rock surface through hydrophobic interaction with the adsorbed crude oil components. By comparing two anionic surfactants with different charge densities, we propose that wettability alteration processes might be improved through the use of dimeric surfactants, which have two charged head groups and two hydrophobic tails. Gemini surfactants where the molecules are joined at the head end are likely to be effective when ion-pair formation is the wettability alteration mechanism, and bolaform surfactants, in which molecules are joined by the hydrophobic tails, should be more effective in the case of surfactant monolayer adsorption.

Review of surfactant-assisted chemical enhanced oil recovery for carbonate reservoirs: challenges and future perspectives

“…In a study, Zhu et al 2009, reported the use of a mixture of Triton X 100 (nonionic) and cetyl trimethyl ammonium bromide (CTAB) (cationic) microemulsion in lowering IFT between crude oil and the aqueous phase (brine) for additional oil recovery. Recent investigations show that cationic surfactants, for example CTAB, perform better than anionic surfactants in wettability alteration of carbonate rocks to more water wet (Saleh et al 2008).…”

“…The dead oil viscosity (0.6-33.7 cP), saturated oil viscosity (0.05-20.89 cP) and undersaturated oil viscosity (0.2-5.7 cP) having API gravity of 19.9-48, temperatures of 38-150°C, bubble point pressure of 690-25,500 kPa and pressure above bubble point (8875-69,000 kPa) were obtained from the Middle East crude oils. For such challenging reservoirs, an approach using tetraalkylammonium salt (TAS) type cationic surfactant was proposed for enhancing ORF from heavy oil-impregnated calcite cores (Saleh et al 2008). It is commonly observed that spontaneous water imbibition is not possible in oil-wet rock surfaces due to the presence of very small or negative capillary pressure.…”

A critical review on fundamental mechanisms of spontaneous imbibition and the impact of boundary condition, fluid viscosity and wettability

“…The hydrophilic head groups of adsorbed surfactant may form a small layer covering the primarily oil-wet rock surfaces, which could alter the wettability of rocks from oil-wet to water-wet state. In order to verify the ion-pair mechanism of wettability alteration, Salehi et al (2008) conducted the imbibition experiments with cationic and anionic surfactants by use of sandstones. Buckley et al (1998) reported that the pore surface of carbonate rock is usually positively charged and adsorbs acidic components in the crude oil.…”

A Realistic Look at Nanostructured Material as an Innovative Approach for Enhanced Oil Recovery Process Upgrading

“…The increase in nanoparticle concentration (0-5 wt.%) increased the mobility reduction factor from 4000 to 8700 and increased foam stability. It has also been experimentally proven that the presence of nanoparticle creates better foam stability than surfactant or VES fluids and even polymer, and improves mobility control; and efficient for enhancing oil recovery owing to high sweep efficiency and effective viscosity [137][138][139][140][141][142] (Table 2). • Particle size influences porous media retention and transport behavior; SiO 2 with small particle size promote efficient penetration through the porous media…”