Viscoelastic surfactants are known to be the best for chemical EOR (cEOR) applications as compared to polymers and surfactants because of its dual capability of reducing the oil/water interfacial tension (IFT) and improving the sweep efficiency. Carbonate reservoirs become most challenging cEOR targets when they are characterized as high temperature and high salinity (HTHS). In this work, a systematic study is carried out to design a formulation using a sequential viscoelastic surfactant (VES) and chelating agent (CA) solutions for recovering heavy oil from carbonate reservoirs.

The objective of this work is to investigate the chemical formulations of viscoelastic surfactant and chelating agent with heavy reservoir oil of 17°API and 234,189 ppm salinity formation brine. Thermogravimetric analysis (TGA), nuclear magnetic resonance (NMR) and fourier transform infrared (FTIR) are used to evaluate the thermal stability of VES. Encouraging results with TGA demonstrate that VES is thermally stable and show high resistance to temperatures up to 250°C. NMR and FTIR results also show good long-term thermal stability when aged for 30 days. Ultra-low oil/water interfacial tension measurements are recorded using spinning drop tensiometer and lie in the range of 10−1 to 10−2 m-Nm−1. Effects of concentration, time and temperature on IFT are evaluated. It is also noted that ultralow IFT is obtained at 50°C and 80°C. Static adsorption has been evaluated by shaking the powdered core samples with VES at two different temperatures for 24 hours. Concentration measurements are performed using total organic carbon (TOC) analysis. Static adsorption results give less adsorption when we have higher temperatures compared to lower temperatures. Coreflooding experiments are designed with VES and VES-CA at 90°C using carbonate core samples to determine the additional oil recovery. Overall oil recovery factor with VES and VES-CA of 47% is calculated when plotted against injected pore volumes. Pressure drop profile, oil rate and water rate are also determined, and their graphs are presented with injected pore volumes.

This study provides an integrated approach of evaluation and application of viscoelastic surfactant and chelating agent solutions for chemical EOR in heavy oil reservoirs. Thermal stability, ultralow IFT's, adsorption and additional oil recovery obtained with viscoelastic surfactant and chelating agent show that they are excellent additives that can be used in cEOR for high salinity and high temperature conditions. They are also very useful when we have heavy crude oil and where IFT reduction is required to mobilize the oil and improve the sweep efficiency of the reservoir.

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