Surfactants impact on CO₂ sequestration for enhanced gas recovery and in depleted carbonate reservoirs

Indiana limestone cores were crushed and sieved to particle size of 125-250 microns, half of the sample was kept pure and the remaining has been treated with NaCl brine containing 0.1% (VES) viscoelastic surfactant to study the effect of surfactant on CO₂ and CH₄ adsorption. The carbonate rock was characterized using X-ray diffraction to determine the crystallinity. The adsorption measurement has been conducted at 50, 100 and 150°C and to 45 bars maximum pressure. At 50°C, adsorption uptake of CH₄ and CO₂ for untreated samples was higher than VES treated samples. However, at 100 and 150°C Both CH₄ and CO₂ adsorption capacities of untreated limestone was considerably lower than VES treated limestone. Thermodynamic studies indicate that adsorption on untreated samples was exothermic with ΔH_ads of -16.7 and -13.5 KJ/mol for CH₄ and CO₂ respectively and spontaneous adsorption at 50°C. In contrast, adsorption of CH₄ and CO₂ is endothermic with corresponding ΔH_ads of 60.1 and 36.2 KJ/mol and spontaneous at high temperatures. through all measurements CO₂ adsorption was three times CH₄ adsorption uptake due to high affinity between CO₂ and calcite. Adsorption isotherms fitting shows that Freundlich isotherm is the best model to represent the adsorption behavior for all conducted experiments at all temperatures with lowest sum of squared errors (SSE) and highest R². These results suggest that previously treated carbonate reservoirs that has mid-high temperatures has great adsorption potential for CO₂ and CH₄ making them an excellent candidate for CO₂ sequestration.