What effect does a negative skin have on the effective well bore radius?

What effect does a negative skin have on the effective well bore radius?

• A. Decreases
• B. Increases
• C. No change
• D. Causes instability

Answer: (B)

Negative skin increases the effective wellbore radius. In radial flow, the pressure drop between the outer boundary and the wellbore is proportional to q times [ln(r_e / r_w) + s], where s is the skin factor. A negative s lowers this bracket, reducing pressure drop for a given flow. You can think of it as if the wellbore were effectively larger: define an effective radius r_w_eff so that Δp ∝ q ln(r_e / r_w_eff). Then ln(r_e / r_w_eff) = ln(r_e / r_w) + s, giving r_w_eff = r_w e^{-s}. With s negative, -s > 0, so e^{-s} > 1 and r_w_eff > r_w. Therefore, negative skin makes the well appear larger, increasing the effective wellbore radius.

Negative skin increases the effective wellbore radius. In radial flow, the pressure drop between the outer boundary and the wellbore is proportional to q times [ln(r_e / r_w) + s], where s is the skin factor. A negative s lowers this bracket, reducing pressure drop for a given flow. You can think of it as if the wellbore were effectively larger: define an effective radius r_w_eff so that Δp ∝ q ln(r_e / r_w_eff). Then ln(r_e / r_w_eff) = ln(r_e / r_w) + s, giving r_w_eff = r_w e^{-s}. With s negative, -s > 0, so e^{-s} > 1 and r_w_eff > r_w. Therefore, negative skin makes the well appear larger, increasing the effective wellbore radius.