Skin Effects

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Skin Effects in Petroleum Production Engineering

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<ul><li> 1. James A. Craig </li> <li> 2. Concept of Skin Effective Wellbore Radius Skin Factor Flow Efficiency Skin Components </li> <li> 3. Skin has no physical dimension. It is analogous to the film coefficient in heat transfer. Skin can be zero (no effect), positive or negative. </li> <li> 4. A restriction to flow. A distortion of the flow lines from the perfectly normal to the well direction. May result from: Partial completion (perforation height less than formation thickness) Inadequate number of perforations Phase changes Turbulence (high-velocity flow) Damage to the natural reservoir permeability </li> <li> 5. Flow enhancement May result from: Matrix stimulation (near-wellbore permeability exceeds the natural value) Hydraulic fracturing Highly inclined wellbore </li> <li> 6. S w w r r e rw = effective wellbore radius, ft rw = wellbore radius, ft S = skin factor Positive skin has the effect of reducing wellbore radius. Negative skin has the effect of increasing wellbore radius. </li> <li> 7. Pressure drop (psia) due to skin is: 141.2 P S 2 q B o o o S kh qo = oil flow rate, STB/D o = oil viscosity, cp Bo = oil FVF, bbls/STB k = reservoir permeability, mD h = reservoir thickness, ft S = skin factor </li> <li> 8. Undamaged zone Damaged zone </li> <li> 9. No damage no skin (ks = k) Ideal drawdown: , Damage (ks &lt; k) Real drawdown: 141.2 q B r ln 2 o o o s S wf ideal w P P kh r , 141.2 q B r ln 2 o o o s S wf real S w P P k h r </li> <li> 10. Pressure drop due to skin Therefore: P S P P , , 141.2 2 q B o o o S wf real wf ideal kh wf ,real wf ,ideal P P q B q B r q B r 141.2 141.2 141.2 o o o o o o s o o o s ln ln 2 2 2 S w w S kh k h r kh r q B q B r 141.2 141.2 1 1 o o o o o o s ln 2 2 S w S kh h k k r </li> <li> 11. q B q B r 141.2 141.2 1 1 o o o o o o s S 1 1 r k k k r ln s S w k r 1 ln s S w S k r ln 2 2 S w S kh h k k r </li> <li> 12. Ideal drawdown Real drawdown F P P k S wf , ideal s P P k S wf , real F F &lt; 1: Damaged well (skin is positive) F = 1: No change (skin is zero) F &gt; 1: Stimulated well (skin is negative) </li> <li> 13. d c p pseudo S S S S S S S = total skin effect of a well Sd = skin due to damage Sc = skin due to partial penetration completion S = skin due to deviation Sp = skin due to perforation Spseudo = skin due to rate-dependent effects &amp; phase-dependent effects </li> <li> 14. Rate-dependent skin can be obtained from a well test. Phase-dependent skin effects are associated with phase changes because of the near-wellbore pressure gradient. If Pwf &lt; Pb: a reduction in the effective permeability to oil in the case of oil wells. If Pwf &lt; Pd: a reduction in the effective permeability to gas in the case of gas wells. </li> <li> 15. Skin Due To Deviation </li> <li> 16. 2.06 1.865 log h k 41 56 100 S = skin due to deviation = angle between the well &amp; the vertical kh = horizontal permeability kv = vertical permeability h w v S r k 1 tan tan v h k k </li> <li> 17. Skin Due To Completion (Partial Penetration) </li> <li> 18. 0.825 h k k 1.35 1 ln 7 1.95 ln 0.49 0.1ln h h S h r h c wc h k k p v v z exp 0.2126 2.753 m r r wc w h zm = distance between the top sand &amp; the middle of the open interval. rwc = rw for an interval either starting at the top of the reservoir of finishing at the base. </li> <li> 19. Clegg, J. D.: Production Operations Engineering, Petroleum Engineering Handbook, Vol. IV, SPE, 2007. Economides, M. J., Hill, A. D., and Ehlig- Economides, C.: Petroleum Production Systems, Prentice Hall, PTR, 1994. Bellarby, J.: Well Completion Design, 1st Ed., Elsevier B.V., 2009. </li> </ul>