Which of the following is a mechanical cause of positive skin?

• A. Partial completion
• B. Phase changes reducing the main fluid relative permeability
• C. Turbulence
• D. Gravity effects on oil density

Answer: (A)

Positive skin is an extra pressure drop caused by a restriction in the near-wellbore region that makes it harder for fluids to enter the reservoir. A partial completion is a mechanical change to the well that can leave portions of the wellbore or perforated zones isolated or less connected, effectively reducing the flow area into the reservoir. That added restriction increases friction and resistance right at the well, which is captured as a positive skin factor.

The other options describe effects that aren’t mechanical restrictions near the well. Phase changes altering relative permeability change how fluids flow through the rock, not a localized near-well obstruction. Turbulence refers to dynamic flow behavior and energy losses, not a persistent near-well skin effect. Gravity effects on oil density influence vertical segregation and mobility, but don’t create the kind of near-wellbore damage or restriction that’s counted as positive skin.