Universal scaling of adsorption of nonionic surfactants on carbonates using cloud point temperatures

Adsorption of Nonionic Surfactants (Nonylphenols) on Sandstone Rock via Alcoholic Micellar Solution

The adsorption of surfactants on rock surfaces can modify their hydrophobicity, surface charge, and other important properties that govern advanced oil recovery processes, such as decreasing the interfacial tension between water and oil and increasing permeability. Generally, the need to control and/or reduce surfactant adsorption on reservoir rock surfaces has been a challenging task in enhanced oil recovery (EOR) methods, as it directly impacts the project's economics. This requires a comprehensive study and understanding of the adsorption mechanism on rocks. This work investigates the adsorption process of nonionic surfactants from the family of ethoxylated nonylphenols in alcoholic micellar solutions on sandstone rock surfaces. The systems used in the experiments consisted of NP 9.5EO, NP 11EO, and NP 15EO, butanol as an amphiphilic solvent, and a saline solution (2% KCl) as the aqueous phase. The experiments were conducted according to the Scheffé network and showed an adsorption efficiency of 66.89% for NP-15EO, 67.15% for NP-11EO, and 70.60% for NP-9.5EO, thus proving that the higher the degree of ethoxylation of nonylphenols, the lower the adsorption capacity. Point F was chosen as the optimum point since this point remained constant during the experiments, besides being a water-rich region with low butanol content. The sandstone exhibited oil-favorable wettability, which after treatment resulted in wettability inversion, with a decrease in the contact angle with water, a factor that can increase oil recovery. Adsorption isotherm modeling was also performed to investigate the adsorption mechanism. All adsorption tests followed and best fit the Redlich-Peterson isotherm, showing that the adsorption process occurs in monolayers and multilayers. The experimental methodology also involves analyses of mineralogy, morphology, thermal stability, and surface charge of the sandstone rock.

A review of wettability alteration using surfactants in carbonate reservoirs

The wettability of carbonate rocks is often oil-wet or mixed wet. A large fraction of oil is still remained in carbonate reservoirs, it is therefore of particular significance to implement effective methods to improve oil recovery from carbonate reservoirs. Altering wettability from oil-wet to more favorable water-wet has been proven successful to achieve this goal. Surfactants are widely investigated and served as wettability modifiers in this process. Yet a comprehensive review of altering wettability of carbonate reservoirs with surfactants is ignored in literature. This study begins with illustration of wettability evolution process in carbonate reservoirs. Techniques to evaluate wettability alteration extent or to reveal behind mechanisms are also presented. Several surfactant systems are analyzed in terms of their wettability alteration mechanisms, influential factors of performance, applicable conditions, and limitations. Mixture of different types of surfactants could obtain synergic effects, where applicable conditions are extended, and final performance is improved. Gemini surfactants have many desirable properties, which warrants further investigation for understanding their wettability alteration mechanisms and performance. At the end, this review discusses strategies related with surfactant cost, surfactant adsorption, and challenges at high temperature, high salinity reservoirs conditions. Also, some unclear issues are discussed.

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.

Molecular Dynamics Simulations of Aqueous Nonionic Surfactants on a Carbonate Surface

The interactions and structure of secondary alcohol ethoxylates with 15 and 40 ethoxylate units in water near a calcite surface are studied. It is found that water binds preferentially to the calcite surface. Prediction of the free-energy landscape for surfactant molecules shows that single-surfactant molecules do not adsorb because they cannot get close enough to the surface because of the water layer for attractive ethoxylate-calcite or dispersion interactions to be significant. Micelles can adsorb onto the surface even with the intervening water layer because of the integrative effect of the attractive interactions of all the surfactant molecules. Adsorption is found to increase because of the closer proximity of the micelles to the surface due to a weakened water layer at higher temperatures. The free-energy well and barrier values are used to estimate surface to bulk partition coefficients for different surfactants and temperatures, and qualitative agreement is found with experimental observations. The combined effect of surfactant-water and surfactant-solid interactions is found to be responsible for an increased adsorption for nonionic surfactants as the system approaches the cloud point.