(1) In well logging. Sometimes called propagation effect. As applied to induction logging, a reduction in the conductivity reading (increase in the resistivity reading) observed in very conductive media. A thick-bed correction for the effect is usually provided in the instrument. Residual corrections, when needed in thin beds, may be made by means of charts.
Simply stated, skin effect results from the interactions between adjacent ground loops when, because of their high conductivity. they carry induced currents of appreciable magnitudes. The magnetic fields of the ground-loop currents induce additional eddy currents in adjacent ground loops which are superimposed on those induced by the transmitter coil field. The net result is to nullify somewhat the magnetic field of the transmitter coil, and the resultant field may be shifted in phase. The conductivity signal seen at the receiver coils and the depth of investigation are thereby decreased.
The term “skin effect” originated from the tendency in metallic conductors for high-frequency alternating-current flow to be concentrated near the surface or “in the skin” of the conductor.
(2) In pressure transient testing and analysis. Skin effect is the additional resistance to fluid flow which causes an additional pressure drop at the well bore and/or in the formation near the well bore. The additional pressure drop is the result of some form of damage occurring during the drilling, completion. and/or production operations. Some factors which can cause this alteration are: invasion by mud filtrate or whole mud; cement; partially plugged perforations; movable fines; introduced contaminants such as rust, pipe dope, etc. The zone where permeability has been reduced by these factors creates an additional pressure drop during production or injection and is referred to as skin. The resulting effect is skin effect. The skin factor reflects the effects of the additional pressure drop. A positive value of the skin factor denotes formation damage, or damage at the well bore. A negative value of skin effect can result from man-made fractures or any form of stimulation which increases the ease with which fluid can flow in the region immediately surrounding the well bore.