**4.5 Effect of surfactant concentration**

Proper selection of foaming surfactant and its concentration under reservoir and fracturing conditions is an essential task. Aronson et al. discussed the disjoining isotherm of foam film at two different surfactant concentrations

*CO2 Foam as an Improved Fracturing Fluid System for Unconventional Reservoir DOI: http://dx.doi.org/10.5772/intechopen.84564* 

 [65]. They found that higher surfactant concentration is able to provide high disjoining pressure to the foam lamella which significantly increases the pressure gradient and the resistance to the foam film during the foam flow process. Apaydin and Kovscek discussed the impact of surfactant concentration and noticed that at low surfactant concentration, weak foam generates which offers low resistance to flow [66]. Gu and Mohantay discussed the apparent viscosity of polymer free foam considering two different surfactant concentrations (0.1 and 0.5 wt%) and it was noticed that the higher concentration is able to generate highly viscous foam under fracturing conditions [8]. The increase in viscosity is accredited to the increment in total interfacial area of the foam structure which induces additional lamella stability [8]. Foam lamella should be elastic in order to withstand any deformation and the force which restores lamella comes from the Gibbs-Marangoni effect [67]. Some authors presented that direct and strong relationship exists between surface elasticity of lamella and foam stability [68–72], whereas others disagree and reported no direct relationship [67, 73]. A maximum foaming performance is usually noticed at intermediate surfactant concentration dictated by the Gibbs-Marangoni effect [69]. When the surfactant concentration is high, the surface elasticity of lamella decreases, which negatively affects the foam endurance due to reduced counteraction towards the deformation forces [69].
