SKIN EFFECTS

James A. Craig

TABLE OF CONTENTS Concept of Skin Effective Wellbore Radius Skin Factor Flow Efficiency Skin Components

CONCEPT OF SKIN Skin has no physical dimension. It is analogous to the film coefficient in heat

transfer. Skin can be zero (no effect), positive or

negative.

Positive Skin 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

Negative Skin Flow enhancement May result from:

• Matrix stimulation (near-wellbore permeability exceeds the natural value)

• Hydraulic fracturing• Highly inclined wellbore

EFFECTIVE WELLBORE RADIUS

r’w = effective wellbore radius, ftrw = wellbore radius, ftS = skin factor

Positive skin has the effect of reducing wellbore radius.

Negative skin has the effect of increasing wellbore radius.

Sw wr r e

Pressure drop (psia) due to skin is:

qo = oil flow rate, STB/Dμo = oil viscosity, cpBo = oil FVF, bbls/STBk = reservoir permeability, mDh = reservoir thickness, ftS = skin factor

141.2

2o o o

S

q BP S

kh

SKIN FACTOR

Und

am

aged

zoneDamaged

zone

No damage – no skin (ks = k) Ideal drawdown:

Damage (ks < k) Real drawdown:

,

141.2ln

2o o o s

S wf idealw

q B rP P

kh r

,

141.2ln

2o o o s

S wf realS w

q B rP P

k h r

Pressure drop due to skin Therefore:

, ,

141.2

2o o o

S wf real wf ideal

q BP S P P

kh

, ,wf real wf idealP P

141.2 141.2 141.2ln ln

2 2 2o o o o o o s o o o s

S w w

q B q B r q B rS

kh k h r kh r

141.2 141.2 1 1ln

2 2o o o o o o s

S w

q B q B rS

kh h k k r

1 1ln s

S w

rS

k k k r

1 ln s

S w

rkS

k r

141.2 141.2 1 1ln

2 2o o o o o o s

S w

q B q B rS

kh h k k r

FLOW EFFICIENCY

F < 1: Damaged well (skin is positive) F = 1: No change (skin is zero) F > 1: Stimulated well (skin is negative)

Ideal drawdown

Real drawdownF

,

,

S wf ideal s

S wf real

P P kF

kP P

SKIN COMPONENTS

S = total skin effect of a wellSd = skin due to damageSc = skin due to partial penetration completionSθ = skin due to deviationSp = skin due to perforationSpseudo = skin due to rate-dependent effects & phase-

dependent effects

d c p pseudoS S S S S S

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.

Skin Due To Deviation

Sθ = skin due to deviationθ = angle between the well & the verticalkh = horizontal permeabilitykv = vertical permeability

2.06 1.865

log41 56 100

h

w v

khS

r k

1tan tan v

h

k

k

Skin Due To Completion (Partial Penetration)

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.

0.825

1.35 1 ln 7 1.95 ln 0.49 0.1lnh hc wc

p v v

k khS h r h

h k k

exp 0.2126 2.753mwc w

zr r

h

REFERENCES 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.