The document discusses the concept of skin factor in wellbore flow, which is a dimensionless quantity that describes flow efficiency. A positive skin factor indicates damage that restricts flow, while a negative skin indicates flow enhancement. Skin can result from various factors like partial completion, damage near the wellbore, hydraulic fracturing, or deviation of the well from vertical. Equations are provided to calculate the pressure drop and flow efficiency based on the skin factor. The total skin is the sum of individual skin components from different sources like damage, completion, deviation etc.

1. James A. Craig

2. Concept of Skin
Effective Wellbore Radius
Skin Factor
Flow Efficiency
Skin Components

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.

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

5. Flow enhancement
May result from:
• Matrix stimulation (near-wellbore permeability
exceeds the natural value)
• Hydraulic fracturing
• Highly inclined wellbore

7. S
w w r r e  
r’w = 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.

9. 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
μo = oil viscosity, cp
Bo = oil FVF, bbls/STB
k = reservoir permeability, mD
h = reservoir thickness, ft
S = skin factor

10. Undamaged zone
Damaged zone

11. No damage – no skin (ks = k)
 Ideal drawdown:
,
Damage (ks < 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

 
   
 

12. 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
 
   

13. 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
 
   

14. Ideal drawdown
Real drawdown
F 
 
 
P 
P k
S wf ,
ideal s
 
P 
P k
S wf ,
real
F
 F < 1: Damaged well (skin is positive)
 F = 1: No change (skin is zero)
 F > 1: Stimulated well (skin is negative)

15. 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
Sθ = skin due to deviation
Sp = skin due to perforation
Spseudo = skin due to rate-dependent effects & phase-dependent
effects

16. 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 < Pb: a reduction in the effective
permeability to oil in the case of oil wells.
 If Pwf < Pd: a reduction in the effective
permeability to gas in the case of gas wells.

17. Skin Due To Deviation

18. 2.06 1.865
        
       log
          
h k
41 56 100
    
Sθ = skin due to deviation
θ = angle between the well & the vertical
kh = horizontal permeability
kv = vertical permeability
h
w v
S
r k 
1 tan tan v
h
k
k
      
 

19. Skin Due To Completion (Partial Penetration)

20. 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 & the middle of the
open interval.
rwc = rw for an interval either starting at the top of the
reservoir of finishing at the base.

22. 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.