Estimation of skin factor by using pressure transient

Prepared by :
Muhammad .S. Amin
Estimation of Skin Factor by Using
Pressure Transient Testing Results
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Objectives
 Introducing skin factor and formation damage.
 Introducing well testing as a tool to describe the productivity
and future of a reservoir.
 Different methods of well testing such as Drawdown and
Build Up tests has been applied to determine Skin factor
 We will integrate the pressure analysis approach with the
pressure integral analysis approach and develop a consistent
and applicable method for the analysis of well test data to
evaluate skin factor.
 In this project, PanSystem program will also be applied to
evaluate the effect and value of skin factor


 Formation damage is an undesirable operational and
economic problem that
can occur during the
various phases of oil
and gas recovery.
introduction
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
 . Formation damage indicators include permeability
impairment, skin
damage, and
decrease of well
performance
Introduction
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
 Formation damage can occur at any time during a
well’s history from the initial drilling and completion
of a wellbore through depletion of a reservoir by
production. Operations
such as drilling,
completion,
workovers and
stimulations.
Formation Damage in Oil Wells:
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
 1. Shallow damage is the most common and makes
the biggest impact on production.
 2. It may take significant damage to create large
drops in production
 3. The problem, however, is that the highest
permeability zones are the easiest to damage, and
that can have a major impact on productivity
Observations on Damage:
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
Causes of damage
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
 Skin factor denoted by the symbol S .
 What ever is the reason of the pressure loss or losses
in production (flow efficiency) there is a measure of
severity of for this reduction which is called skin
factor (S).
 Skin has no physical dimension.
 Skin can be zero (no effect), positive or negative.
Concept of Skin factor
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
 A restriction to flow.
 A distortion of the flow lines from the perfectly
normal to the well direction.
 May result from:
Positive Skin
St = Sfm + Sc +Spp
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
Flow enhancement
May result from:
 Matrix stimulation (near-wellbore permeability
exceeds the natural value)
 Hydraulic fracturing
 Highly inclined wellbore
Negative Skin
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
Effective wellbore radius
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
Positive skin has the effect of reducing
wellbore radius.
Negative skin has the effect of
increasing wellbore radius
S
w wr r e
 
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
Radius distribution
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
ΔΡskin = (ΔΡ2- ΔΡ1 ) = ( p3 – p2 ) - ( p3 – p1 )
ΔΡskin = ( p1 – p2 ) ………… (A)
Skin factor equations
141.2
2
o o o
S
q B
P S
kh


 
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No damage – no skin (ks = k)
Ideal drawdown:
Damage (ks < k)
Real drawdown:
,
141.2
ln
2
o o o s
S wf ideal
w
q B r
P P
kh r


 
   
 
,
141.2
ln
2
o o o s
S wf real
S w
q B r
P P
k h r


 
   
 
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
Flow efficiency
Ideal drawdown
Real drawdown
F 
 
 
,
,
S wf ideal s
S wf real
P P k
F
kP P

 

F < 1: Damaged well
(skin is positive)
F = 1: No change (skin
is zero)
F > 1: Stimulated well
(skin is negative)4/21/2015 16


To determine the following parameters
 Initial pressure (p)
 Average pressure within the drainage boundary (p)
 Permeability thickness product(kh) and
permeability(k)
 Mechanical skin factor (S)
 Area drained (A)
Why test a well
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
• Pressure Drawdown
• Pressure Build up
• Multirate
• Injection/fall-off
• Interference
• Pulse
Types of Well Tests
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
shut in the well till pressure reaches static level & then
flowing the
well at a constant
rate ,q &
measuring Pwf.
Pressure Drawdown Tests
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Semilog plot of pressure drawdown data4/21/2015 20


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
•Advantages:
–Suitable in new wells
–No need to lose production
– K, S, reservoir size can be determined
•Disadvantages:
–Difficult to maintain constant production rate.
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
•Most preferred well testing technique
•The well is first produced at a constant rate till
pressure is stabilized
and then the well is
shut in
•Pressure is recorded
as a function of time
Pressure Build-up Tests
Shut in
flowing
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•
Horner relationship on semi-log plot
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
•Advantages:
–Precise control of rate
–P* can be determined
•Disadvantages:
–Loss of production due to shut in.
Pressure Build-up Tests
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 Formation damage can be occurred in different stages of oil
well operations.
 Skin factors can be positive or negative .
 Well testing can help reservoir engineer as a formation
evaluation tool to describe the future performance of oil wells
in terms of pressure drop and flow efficiency.
 Different methods of well testing such as Build-up,
drawdown, injection, fall of, can be applied at different case
to determine skin factor of oil wells.
 Petroleum engineer need to monitor oil well timely, therefore
different action need to be don,
 Sometime formation damage can be due to partial penetration
. of the pay zone .
Conclusion
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
Execution remained testes
PanSystem
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Future catalogue


 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.
Refrences
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Estimation of skin factor by using pressure transient

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