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skin effect

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1. n. [Formation Evaluation]

The loss in amplitude and change in phase of an electromagnetic field as it penetrates into a conductive medium. In an induction log, the skin effect causes a reduction of the R-signal (in-phase) and an increase in the X-signal (out-of-phase) at the receiver. It has a significant effect on the 6FF40 array, particularly below 1 ohm-m. Since the magnitude of the reduction depends on the conductivity, the skin effect can be corrected for by using a fixed function of the measured conductivity. A much improved method is to estimate the correction from the X-signal measured in balanced arrays.

See: balanced array, induction

2. n. [Well Testing]

An increase or decrease in the pressure drop predicted with Darcy's law using the value of permeability thickness, kh, determined from a buildup or drawdown test. The difference is assumed to be caused by the "skin." Skin effect can be either positive or negative. The skin effect is termed positive if there is an increase in pressure drop, and negative when there is a decrease, as compared with the predicted Darcy pressure drop. A positive skin effect indicates extra flow resistance near the wellbore, and a negative skin effect indicates flow enhancement near the wellbore. The terms skin effect and skin factor are often used interchangeably. In this glossary, the term skin effect refers to the numerical value of the skin factor.

See: buildup test, drawdown test, permeability thickness, skin factor