The percentage of effective porosity occupied by the wetting phase fluid that cannot be removed.

• A. Residual
• B. Irreducible or Connate
• C. Wettability
• D. Capillary pressure

Answer: (B)

The key idea is that, in a pore network, capillary forces trap some of the wetting-phase fluid so it cannot be removed even after extensive drainage or displacing with a non-wetting phase. The fraction of the effective porosity that remains occupied by the wetting phase and cannot be drained is known as the irreducible saturation (also called connate water saturation). It represents the minimum amount of wetting phase that stays behind, expressed as a percentage of effective porosity. This is the best choice because it directly describes the immobile, trapped portion of the wetting phase. Wettability relates to which fluid wets the solid surface, capillary pressure is the driving pressure difference between fluids, and while the concept of a residual wetting-phase amount exists, the standard term for the immobile fraction here is irreducible (connate) saturation.