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A Note on the Skin Effect
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1956
Year
EngineeringFluid MechanicsDermatologyVisual ArtsHydraulicsWell Performance EvaluationDrillingGeotechnical EngineeringCompressible FlowAltered Permeability KaAesthetic SurgerySkin PharmacologyHydraulic PropertyCutaneous BiologyWorkover FluidsMultiphase FlowSkin EffectExternal Permeability KeCivil EngineeringGeomechanicsFormation EvaluationMedicineDermal StructureSkin Pressure Drop
Pressure drop in a wellbore produced at constant rate is governed by skin effects arising from reduced permeability near the wellbore, as demonstrated by Horner and van Everdingen and confirmed by incompressible flow analysis. The study introduces the skin effect factor S to quantify the additional pressure drop caused by this reduced permeability. The authors model the wellbore as having an altered permeability zone (ka) extending to radius ra, beyond which the external permeability (ke) applies, and derive the skin pressure drop from this configuration, noting its sign depends on whether ka is smaller or larger than ke. Equation 2 defines the.
Horner and van Everdingen have shown that the pressure drop within the wellbore, as a result of having produced the well at a constant rate q for time t, where t is sufficiently large, is: (Equation 1) van Everdingen observed that better agreement between theory and well performance can be obtained if, instead of assuming the permeability is ke everywhere about the well, it is assumed the permeability near the wellbore is substantially reduced as a result of drilling, completion and/or production practices. In order to account for the additional pressure drop he introduced the dimensionless quantity S, the skin effect factor, so that Eq. 1 becomes: (Equation 2) Eq. 2 might have also been obtained as follows. Assume a zone of altered permeability ka exists about the well out to a radius ra, and beyond that the unaltered, external permeability ke. The additional pressure drop required to overcome this skin of reduced permeability may be calculated with sufficient accuracy using the incompressible flow equation; for Brownscombe and Collins have shown almost no difference between compressible and incompressible steady-state flow, in the vicinity of the wellbore, and the small volume of fluid in the vicinity of the wellbore makes unsteady-state mechanics unnecessary. Then, (Equation 3) The sign of this skin pressure drop will be positive or negative depending upon whether the altered permeability ka is smaller or larger, respectively, than the external permeability ke.