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APPROPRIATE SKIN FACTOR FOROPTIMAL WELL PRODUCTIVITY
By
WILLIAMS ACHESE
DE.2008/1223
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INTRODUCTION
The formation can be damaged during drilling, casing and cementing,
completion, well servicing, well stimulation and production operations.
The skin due to damage can be defined as the additional pressure drop inthe near wellbore area that results from the drilling, completion and
production practices used (Van Everdingen, 1953).
Formation Damage can be defined as any near wellbore alteration thataffects permeability due to well operations (Byrne et al, 2007).
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Damage Caused by
Drilling Fluid
Mud filtrateinvasion Damage Caused by
Production
p < pbp > pb
Damage CausInjection
dirtywater
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Reservoir Model of Skin Effect
Bulk
formation
h
rw
ka
ra
k
Altered
zone
R i P P fil
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Reservoir Pressure Profile
500
1000
1500
2000
1 10 100 1000 10000Distance from center of wellbore, ft
Pressur
e,psi
ps
skh
qB2.141ps
Skin and Pressure Drop
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Skin and Pressure Drop
spqB
hk00708.0s D
Skin Factor and
Propertiesof the Altered Zone
w
a
a r
rln1
k
ks
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Effective Wellbore Radius
w
wa
r
rlns
s
wwa err
Productivity Index
wfpp
qJ
Flow Efficiency
w
ideal
actualf
p
pp
J
JE
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ANALYSIS OF A BUILD-UP CURVE
In a buildup test, a well which is already flowing (ideally at
constant rate) is shut in and the downhole pressure measured
as the pressure builds up. Ever since van Everdingen and Hurst
(1953) introduced the concept of a skin factor, the primary
focus of research has been on evaluation and minimization of
formation impairment.
The skin effect and its influence on the productive capacitive of
a well is illustrated more efficiently in well before and after
perforation as illustrated in the table 1
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Before After
Perforated interval 6258-70ft 6258-70ft
h, producing interval 548cm 548cm
Cumulative production 3956 B/D 5500 B/D
Production rate 96 B/D 60 B/D
Production time 41.21 days 91.67 days
t, production time 3561000 sec 7920000 sec
Shrinkage factor 0.795 0.795
q at reservoir conditions 222 cc/sec 139 cc/sec
Pressure increase per cycle 6 psi 4 psipr , flowing pressure 2060 psi 2502 psi
, viscosity 0.65 cp 0.65 cp
, porosity 0.219 0.219
c, fluid compressibility 0.00017/atm 0.00017/atm
rw,well raduis 6.3 cm 6.3 cm
Reperforating Job
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PRESSURE BUILD-UP TEST BEFORE REPERFORATION PRESSURE BUILD-UP TEST BEFORE R
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Before Reperforation After Reperforati
Reservoir pressure, PR 2554 psi 2554 psi
Pressure drop, PRPf 494 psi 52 psi
Pressure drop due to skin, ps 441 psi 16 psi
Skin, s 84.8 4.6
Permeability, k 118 md 111md
Effective well bore radius, rwa 9.35 x 10-37cm 6.33 x 10-2 cm
Production Index 0.194B/D-psi 1.154B/D-psiFlow Efficiency 0.107 0.692
TABLE 2RESERVOIR PRESSURE GOTTEN FROM THE BUILDUP
CURVE INTERPRETATION
The result show that the first perforation job was not efficient a
that the reperforation essentially removed a large resistance wh
existed near the well bore
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CONCLUSION
It is always possible to reduce the damage skin in the vicinity of a we
stimulation techniques such as fracturing, so both damage and
improvement can occur. The appropriate skin factor for optimal wel
productivity can be achieved and control during well testing. In the
reperforation of the well as describe in this paper. After reperforatio
resulting skin factor is lesser when compared after reperforation.
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