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Design and evaluation of surfactant formulations for wettability alteration

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Only about 35% of oil is recovered from carbonate reservoirs through primary and secondary flooding because of oil wet surfaces and unfavorable capillary pressures. Surfactants, with their dual hydrophobic and hydrophilic nature have been known to improve oil recovery significantly by lowering oil-water interfacial tension and by altering wettability of surfaces. However, the process of selecting an efficient surfactant for wettability alteration is dependent on several factors, including mineral type, porosity, temperature, salinity, nature of adsorbed oil, molecular structure and surfactant adsorption. Core-flood experiments usually used for evaluating surfactants tend to be time-consuming and provide very little information on the actual mechanism of surfactant action. To this end, our research focuses on macro and molecular scale analysis of surfactants to understand relevant structure-property relationships and design effective formulations for enhanced oil recovery.

Our initial work on nonionic surfactants with different hydrophobic groups and different lengths of hydrophilic ethylene oxide oligomers revealed that they can alter the wettability of the rock primarily by acting on the water−rock and oil−rock interfaces. It was found that wettability alteration was enhanced by surfactants with shorter hydrophilic units and at increased temperatures. We were also able to summarize the performance metrics of the surfactants using a simple energy model. Furthermore, qualitative experiments were performed to propose a simple mechanism involving a combination of “coating” and “sweeping“ based dewetting of oil films

Aggregative adsorption of nonionic surfactants

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In our following work, we investigated the role of surfactant adsorption in wettability alteration. We found that the adsorption increased with temperature and for surfactants with fewer hydrophilic groups. Along with experiments, we also performed molecular dynamics simulations to confirm the mechanism of aggregative adsorption of these nonionic surfactants. A universal thermodynamic model explaining the correlation between surfactant adsorption, wettability alteration and surfactant cloud point, was put forth as an easy and effective tool for rational selection of surfactants for wettability alteration.

Wettability alteration and improved oil recovery using mixed nonionic and anionic surfactants

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We subsequently investigated mixed-surfactant formulations consisting of nonionic surfactants and anionic cosurfactants to address the issue of surfactant stability and performance at high-temperature and high salinity reservoirs. Our work revealed that features including formulation stability, wettability alteration and adsorption are influenced by the choice of cosurfactant, relative concentrations in the 

mixtures and temperatures. We identified mixed surfactant systems which modified the oil-wet surface to a water-wet surface with final contact angles as low as 70°. These formulations also exhibited a linear trend in adsorption and wettability alteration with the thermodynamic descriptor of cloud point temperature difference, which had been used previously for single surfactants.

Using these formulations, we performed imbibition experiments in porous media and were able to generate additional oil recoveries in the range of 20% - 50% depending on the formulation, initial water saturations and brine compositions. The ultimate oil recoveries when scaled by the system capillary driving force was found to generate a universal oil recovery curve versus the initial water saturation and cloud point temperature difference (CPTD). CPTD was found to correlate with surfactant performance for both single and mixed surfactant systems. Thus, with the information on driving force these curves can be used to predict oil recoveries for a vast number of surfactant systems.

References
 

  • Das, S.; Katiyar, A.; Rohilla, N.; Bonnecaze, R. T.; Nguyen, Q. A Methodology for Chemical Formulation for Wettability Alteration Induced Water Imbibition in Carbonate Reservoirs. J. Pet. Sci. Eng. 2021, 198, 108136. https://doi.org/10.1016/j.petrol.2020.108136.

  • Das, S.; Katiyar, A.; Rohilla, N.; Nguyen, Q. P.; Bonnecaze, R. T. Wettability Alteration and Adsorption of Mixed Nonionic and Anionic Surfactants on Carbonates. Langmuir 2020, 36 (50), 15410–15422. https://doi.org/10.1021/acs.langmuir.0c03022.

  • Das, S.; Khabaz, F.; Nguyen, Q.; Bonnecaze, R. T. Molecular Dynamics Simulations of Aqueous Nonionic Surfactants on a Carbonate Surface. J. Phys. Chem. B 2020, 124 (37), 8158–8166. https://doi.org/10.1021/acs.jpcb.0c03997.

  • Das, S.; Katiyar, A.; Rohilla, N.; Nguyen, Q.; Bonnecaze, R. T. Universal Scaling of Adsorption of Nonionic Surfactants on Carbonates Using Cloud Point Temperatures. J. Colloid Interface Sci. 2020, 577, 431–440. https://doi.org/10.1016/j.jcis.2020.05.063.

  • Das, S.; Nguyen, Q.; Patil, P. D.; Yu, W.; Bonnecaze, R. T. Wettability Alteration of Calcite by Nonionic Surfactants. Langmuir 2018, 34 (36), 10650–10658. https://doi.org/10.1021/acs.langmuir.8b02098.

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