*2.2.4 Hydroxystearic acid*

Heller and co-workers [65] proposed a small organic compound, known as 12-hydroxystearic acid (HAS). This compound (H3C(CH2)5 CHOH(CH2)10COOH) had previously been used to gel hydrocarbon and chlorinated solvents. However, the essential assessment results indicated that the HAS was insoluble in CO2 unless a significant amount of a co-solvent (i.e. ethanol) was added. For instance, the addition of 15 wt% of ethanol co-solvent enabled a solubility of up to 3 wt% of HAS in CO2 resulting in a nearly 100-fold increase in viscosity in the temperature range of 300–307 K. As the temperatures decrease, the solution exhibits a slight viscosity increase. In addition, microfibres in the gel solution form an opaque solution that would probably impede the fluid flow in the reservoir formation.
