Skin Factor Formula |
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| \( S \;=\; \left( \dfrac{ k }{ k_s } -1 \right) \; ln \left( \dfrac{ r_s }{ r_w } \right) \) | ||
| Symbol | English | Metric |
| \( S \) = Skin Factor | \(dimensionless\) | - |
| \( k \) = Undamaged Formation Permeability | \(mD\) | - |
| \( k_s \) = Damaged Zone Permeability | \(mD\) | - |
| \( r_s \) = Damaged Zone Radius | \(ft\) | - |
| \( r_w \) = Wellbore Radius | \(ft\) | - |
Skin factor, also called skin, a dimensionless number, is used to quantify the impact of wellbore conditions on the flow of fluids into or out of a well. Specifically, it represents the additional pressure drop near the wellbore compared to an ideal, undamaged well with the same reservoir properties.
Skin Factor Interpretation
- Negative Skin Factor (S < 0) - This indicates enhanced permeability and improved flow conditions near the wellbore. Creating fractures in the rock to provide higher conductivity flow paths. Dissolving formation damage or creating larger flow channels with acid treatments.
- Zero Skin Factor (S = 0) - This represents an ideal well with no damage or stimulation around the wellbore. The flow is as predicted by the reservoir's inherent properties.
- Positive Skin Factor (S > 0) - This indicates damage or restrictions in the near-wellbore region that impede flow. This damage can be caused by various factors during drilling and completion, Drilling fluids can invade the formation, depositing solids and reducing permeability. Clay swelling, fines migration, or scale buildup can plug pore spaces. The wellbore or perforations may not extend through the entire productive zone. The holes connecting the wellbore to the reservoir may be blocked or inadequate.
A positive skin factor is often an initial concern due to the potential for formation damage caused by the drilling process itself. Engineers then might employ completion techniques and stimulation methods to minimize or reverse this damage, aiming for a skin factor close to zero or even negative to maximize well productivity.
