1. n. [Enhanced Oil Recovery]

Oil in pore spaces that cannot be moved because of capillary forces. Typical trapped or residual oil saturation is in the range of 10% to 50% of the pore space, and it is higher in tighter formations where the pore spaces are small. The amount of trapped oil is a function of the displacement method and conditions, making this oil a target for enhanced oil recovery (EOR) processes. EOR methods introduce fluids that reduce viscosity, interfacial tension or mobility ratio, and thus improve flow and sweep efficiency to release the residual oil.