The percentage of non-wetting phase (usually oil or gas) left in the ground after production.

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

Answer: (A)

This item tests understanding of residual saturation: the portion of the non-wetting phase that remains trapped in the pore space after displacement. Capillary forces and pore-scale geometry prevent all of the non-wetting fluid (oil or gas) from being removed during production, leaving disconnected pockets that cannot be mobilized by usual displacement. This trapped fraction is expressed as a residual saturation, often referred to as residual oil saturation or residual gas saturation, and it’s given as a percentage of the pore volume. The remaining fraction is what stays in place after production, influencing ultimate recovery and strategies to improve it. The other terms describe different ideas: irreducible or connate saturation is the minimum amount of the wetting phase that remains due to pore structure in a particular wettability context; capillary pressure is the pressure difference driving phase distribution but is not itself a saturation; and wetting index relates to wettability behavior rather than the amount of trapped non-wetting fluid.