8-Casing Setting Depth Design
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Transcript of 8-Casing Setting Depth Design
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CASING SETTING DEPTHDESIGN
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Casings are set in a drilled hole for one
or more of the following reasons: to protect contamination of underground
water formations,
to prevent drilling problems such as holecollapse, differential sticking of pipes,
to prevent fracturing of subsurface formations,
to produce oil or gas from producing pay zoneor zones.
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Three types of casings are used for these
purposes:
Surface casings,
Intermediate casings (and liners),
Production casings (and liners).
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Surface casing setting depth:
As a drilling engineer you will not be in a position
to select a setting depth for surface casing.
This depth is provided by the authorities of local
government by considering depth of water table.
Surface casings should be set deep enough so thatdrilling fluid can not contaminate underground
fresh water sources by mud filtration.
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Production casing setting depth:
As a drilling engineer youwill not select setting depthfor a production casingstring.
Production divisions of oilcompanies provide thisdata for you.
They consider productionaspects of well such as payzone depth, well completiontype, etc.
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You will bedesigning setting depthsfor intermediate casings considering
safety of drilling only,
Properly designed intermediate
casings are the ones which help
drilling engineer reach to specified
targetsafely
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Containing 20 bbl of gas kick means
satisfing condition in the left figure
Pinside
< Pfrac
@ shoe Pinside
> Pfrac
@ shoe
Formations
should never be
fractured by
internal
pressures
Safe
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Avoiding differential sticking
I nternal pressure
should never exceed
1000 psi formationpressure across
permeable zones in
order to avoid
dif ferential sticking
(Pmud
-Pformation
)1,000 psi
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So, how do I select a setting depth
for the next casing set ?
You will be given
a depth at which
surface casing shoe
will reside C.
You need to determine
maximum depth to
which you can drill while
satisfying previous two
criteria D.
This depth
will be the setting depth
for your next set of
casings.
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Procedure When you take 20 bbl CH4
kick in the hole at D,
pressure inside the hole
at c must be less than
formation fracture
strength at C
Pinside @ C
= Pformation @ D
- 0.052kiL
ki
- 0.052 MW (D-C-Lki
)
Pformation @ D
= 0.052 (ePf @ D
) D
ePf @ Dis pressure gradient (ppg) at D
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Satisfy the first design criteria byselecting a formation, which will
satisfy following condition:Pfr acture @ C
Pinside @ C
Note that, the formations getstronger as you drill deeper.
Formation fracture gradients(ePff) can be determinedempirically as a function ofdepth (see the example).
Pfr acture @ C
= 0.052 (ePf f) C
ePff @ C is fracture gradient (ppg)at C
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Pfr acture @ C Pinside @ C
0.052(ePff @ C
) C 0.052 (ePf @ D
) D-0.052kiL
ki-0.052 MW (D-C-L
ki)
( ePf @ D
- MW ) [C (ePff @ C
- MW) - Lki
(MW -ki
)] /D
ePf @ D
-MW is known as kick tolerance(KT)
For a proper casing setting depth, the value of KT is recommended to be
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0.052 ePff C 0.052 ePf D 0.052 L 0.052 MW D C L( )
ePff C ePf D L MW D C L( )
ePff C ePf D L MW D MW C MW L
eP
ff
C MW C MW L L eP
f
D MW D
C ePff MW L MW L L D ePf MW
C
DePff MW L
DMW L L ePf MW
KT C
D
ePff MW L
D
MW L L
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Example
The pore pressure for a projected well are determined as in the
following table (see Dec 2000-newsletter). The surface casing is
set at 5,100 ft. what is the maximum depth to which the second
string may be set and maintain a kick tolerance of
Kick Volume = 20 bbl
Bit Size = 12 inch
ODdp= 5 inch
CH4kick
DCs = 8 x 3, 360
MW = pore pressure + 150 psi
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Table of pore pressures
I assume, you already know how to estimate formation pore
pressures prior to drilling (see previous newsletter if you dont).
Depth, ft Pore
Pressure,
psi0 0
5,100 2 519.4
7,800 3 853.2
8,200 5 628.5
12,000 8 236.8
12,100 5 977.4
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Calculate ePf = Pore Pessure/(0.052 x Depth)
Depth, ft Pore
Pressure, psi
ePf
ppg
0 0 8.6
5,100 2 519.4 9.5
7,800 3 853.2 9.5
8,200 5 628.5 13.2
12,000 8 236.8 13.2
12,100 5 977.4 9.5
for depth=0, ePf= 8.6 ppg
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Calculate over-burden stress (s) based on Ben Eatons data:
Depth, ft Pore Pr.
psi
ePf
ppg
s
0 0 8.6
5,100 2 519.4 9.5 0.9097
7,800 3 853.2 9.5 0.9332
8,200 5 628.5 13.2 0.9363
12,000 8 236.8 13.2 0.9611
12,100 5 977.4 9.5 0.9617
s = 0.84753 + 0.01494 D/1,000 - 0.0006 (D/1,000)2 + 1.199 x 10-5
(D/1,000)3
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Poissons ratio
is the negative ratio of transverse to axialstrain. When a materialis compressed in
one direction, it usually tends to expand in
the other two directions perpendicular tothe direction of compression. This
phenomenon is called the Poisson effect.
Poisson's ratio (nu) is a measure of this
effect. The Poisson ratio is the fraction (or
percent) of expansion divided by the
fraction (or percent) of compression, for
small values of these changes.
http://en.wikipedia.org/wiki/Strain_(materials_science)http://en.wikipedia.org/wiki/Materialshttp://en.wikipedia.org/wiki/Nu_(letter)http://en.wikipedia.org/wiki/Nu_(letter)http://en.wikipedia.org/wiki/Materialshttp://en.wikipedia.org/wiki/Strain_(materials_science) -
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Poissons Ratio (v) based on Ben Eatons data:
Depth, ft Pore Pr. psi ePf
ppg
s v
0 0 8.6
5,100 2 519.4 9.5 0.9097 0.4088
7,800 3 853.2 9.5 0.9332 0.4360
8,200 5 628.5 13.2 0.9363 0.4384
12,000 8 236.8 13.2 0.9611 0.4546
12,100 5 977.4 9.5 0.9617 0.4545
v = 0.23743 + 0.05945 D/1,000 - 0.00668 (D/1,000)2 + 0.00035
(D/1,000)3
- 0.671 x 10-5 (D/1,000)4
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Finally, the formation fracture gradient, ePffcan be predicted
empirically
eP
vv
s eP eP
ff
f f
10 052 0 052
0 052
. .
.
Depth,
ft
Pore Pr. psi ePf
ppg
s v ePff
ppg0 0 8.6
5,100 2 519.4 9.5 0.9097 0.4088 15.10
7,800 3 853.2 9.5 0.9332 0.4360 16.03
8,200 5 628.5 13.2 0.9363 0.4384 16.95
12,000 8 236.8 13.2 0.9611 0.4546 17.60
12,100 5 977.4 9.5 0.9617 0.4545 17.01
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Remember, we are trying to find a value for D at which KT =
KT [C(ePff @ C- MW) - Lki(MW- ki)]/D
C = 5,100 ft and ePff @ C = 15.10 ppg
Lki= 20 bbl/CapacityDC-H
CapacityDC-H= 0.0836 bbl/ft
Lki= 20 bbl / (0.0836 bbl/ft) = 239 ft
since 239 ft < ldc(360 ft), Lki= 239 ft
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Trial & error solution, assume D = 10,000. From 5,100 to 10,000 you
will be drilling open hole. So you will design a MW such that the mud
pressure must not fall below the maximum anticipated formation
pressure in the open hole section.
Determining the required MW
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So, Mud Weight isselected as MW = 13.55ppg
finaly, assuming methanekick, ki @ D= ?
real gas law
PV = z n RT
PV = z (m/MW) RT
P MW = z (m/V) RT
P MW = z () RT
ki@D= (P@DMW)/( z RT@D)
molecular weight of CH4 (MW) = 16
P@D= 0.052 (ePf @ D) D
= 0.052 (13.2) 10,000
= 6,864 psi
T@D= 65 + (3of/1,000 ft) D
= 65 + (3of/1,000 ft) 10,000
= 95 of or (460+95 = 555 oR)
R = 80.186
z = a + (1-a) / eb+ c pprd
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a = 1.39(Tpr- 0.92)0.5- 0.36 T pr- 0.101
b = (0.62-0.23 Tpr) Ppr+ { [0.066/(Tpr- 0.86)] - 0.037 }Ppr
2+ (0.32 Ppr6) / [10 9 (Tpr
- 1)]
c = (0.132 - 0.32 log Tpr
)
d = antilog (0.3106-0.49 Tpr+ 0.1824 Tpr2)
Tpr= T/343,
Ppr= P/667.8
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T= 555
P= 6,864
Tpr= 1.62
Ppr= 10.28
A= 0.48
B= 7.84C= 0.07
D= 0.99
Z= 1.13
ki@10,000= (6,864x16)/(1.13x80.186x555 )
ki@10,000= 2.184 ppg
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substituting,
C=5,100 ft Lki= 239 ft
ePff @ C
=15.1ppg ki@10,000= 2.184 ppg
MW = 13.55 ppg D = 10,000 ft
into [C (ePff @ C
- MW) - Lki(MW -
ki)] /D
[5,100(15.1-13.55)-239(13.55-2.184)]/10,000 = 0.519
0.519 > for your next iteration choose a greater depth (i .e. 10,500 ft).
F inal ly, iterate unti l KT = .