04.politecnico di torino casing programme 2010
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04. CASING PROGRAMME
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C H A P T E R 4
CASING PROGRAMME
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As said, for an adequate characterization of a formation from a pressure
regime standpoint the following parameters have to be determined:
Pore pressureOverburden pressureFra!ure pressure
These pressures, as seen, are strictly dependent one from the other. In
fact, pore pressures and overburden pressures are related between themby the effective pressure, due to compaction, in accordance with the
effective stress principle and together allow the calculation of fracture
pressures.
The methods in use in the Oil Industry to foresee and calculate these
pressures and gradients have been discussed in hapter ! "Abnormal#ressures$.
4.1. INTRO"#CTION
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4.2. %ASIC CRITERIA IN &E'' P'ANNING
%ased on pressure gradient data, all fundamental information needed to plan drilling
programmes can be obtained because the following parameters can be defined in the most
accurate way:
the dens(!) and r*eo+o,(a+ proper!(es o- !*e dr(++(n, ud/
the op!(a+ dep!*s -or se!!(n, !*e as(n,s&the nuber and d(ae!er o- !*e as(n,sto use&
the ,rade and !*(ness o- s!ee+ -or !*e as(n,sto withstand the stresses induced by the
drilling and production processes&
the dens(!) and r*eo+o,(a+ proper!(es o- een! s+urr(es&
the planning and optimization of the*)drau+( pro,rae'pumps and surface equipment
selection, flow rates, horsepower distribution(& dr(++(n, r(, po!en!(a+, which is related to the geometry 'size, length( and weight of the
casing to run in&
the selection and optimization of dr(++(n, b(!s&
the definition of the e*an(a+ *ara!er(s!(s o- dr(++ s!r(n,sand !ub(n,sfor the )*T
and production tests, acid +obs, hydraulic fracturing&
the choice of thee++*ead and thesa-e!) eu(pen!, such as the %O#s, choe manifold
and surface equipment in general&
the installation of the most suitable equipment and sensors for on!(nuous and (n rea+3!(e
on(!or(n, o- !*e dr(++(n, proess&
the estimation of e++ os!with the relative budget allocation.
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4.$. CASING "ESIGN SE#ENCE 5$ s!eps6
Once the three pressure gradient curves versus depth have been
obtained using the methods previously described, it is possible to tae the
following decisions:-. de-(ne !*e dens(!) !*e ud us! *ave versus dep!*&
. the ne/t step consists in determining the depths at which the various
casing strings shall be run, that is !*e as(n, se!!(n, dep!*s. This also
implies the nuber o- as(n, s!r(n,s reu(red to case the hole from
surface to bottom&
!. nowing the depths of setting and the pressures acting in the well, it is
then possible to a+u+a!e !*e s!resses at which the various casing
strings will be sub+ected during drilling and production and a+u+a!e !*e
e*an(a+ proper!(es the casings should have in order to withstand
these stresses.
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4.$.1. M#" "ENSIT8 "ETERMINATION
The ud e(,*! 5or dens(!)6 s*ou+d be s+(,*!+) *(,*er !*an !*e pore
pressure ,rad(en!'usually -00 g1litre(, when in static conditions 'no mud
circulation(, and be+o !*e -ra!ure ,rad(en! 'plus a certain safetymargin depending on the particular situation(, when in dynamic conditions
'with mud circulation(.
This means that in overpressured zones, the mud density must beincreased with the depth as does the pore pressure gradient.
2or practical reasons, at the rig site the mud density is increased following
step by step patterns, and not continuously, as shown in the ne/t slide.
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-.00 -.30 .00 .300
300
-300
-000
!000
300
000
!300
4000
4300
3000
FRACT#RE
GRA"IENT
PORE PRESS#RE
GRA"IENT
M#" "ENSIT8
5a!ua+6
M#" "ENSIT8
5!*eore!(a+6
4.$.1. M#" "ENSIT8 "ETERMINATION
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4.$.2. CASING SETTING "EPTH "ETERMINATION
The ne/t step consists in the determination of the depths at which the
various casing strings shall be run, taing into account safety margins,
nowledge of the area, previous e/periences, e/pected hole problems.
This sequence of calculations will also define the nuber o- as(n,
s!r(n,s reu(redto case the hole from surface down to bottom.
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SE'ECTION CRITERIA AN" PROCE"#RE
The procedure usually followed to determine the casing points is fairly simple and is
based on a
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SE'ECTION CRITERIA AN" PROCE"#RE
4. A first large diameter casing, the ondu!or p(pe '4$, !0$(, is usually set at
around !0530 m with the purpose to protect the shallower formations from caving or
collapsing or for avoiding any eventual stability problem of the drilling rig. If this
casing is driven, it is called the dr(ve p(pe.
3. A second casing, the sur-ae as(n,'!0$, 6$, 0$(, is also positioned at a depth
between -00 and 300 m, with the scope to mae possible the installation of the
%O#s and e/cluding areas with low facture gradients.
6. The number of casing strings required in a well varies between 4 and 7, depending
on the depth, pressure gradients trend and targets to be reached.
7. An e?ap+e o- a+u+a!(onis shown here below.
4.$.2. CASING SETTING "EPTH "ETERMINATION
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E@AMP'E OF CA'C#'ATION
1. &E'' "ATA
5 Total )epth: 3000 m
5 Overpressures Top: -30 m
5 #ore #ressure 8radient at Total )epth: .00 gf1cm1-0 m
5 2racture 8radient at Total )epth: .-3 gf1cm1-0 m
5 9a/imum 9ud )ensity: .06 g 1 litre at Total )epth
5 #ore #ressure 8radient and 2racture 8radient )evelopment: as per slide --
5 9ud )ensity: as per slide --
4.$.2. CASING SETTING "EPTH "ETERMINATION
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-.00 -.30 .00 .300
300
-300
-000
!000
300
000
!300
4000
4300
3000
M#" "ENSIT8
AN" PRESS#RE
GRA"IENTS
"EE'OPMENT
4.$.2. CASING SETTING "EPTH "ETERMINATION
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OPERATIE E@AMP'E 5CASING "ESIGN6
2. CA'C#'ATIONS
2.1 Produ!(on Cas(n, or '(ner
If the well results produ!(ve, the production casing string will be run at
bottom hole '3000 m(
In case the well results dr), it will be plugged and abandoned.
The running of a liner instead of an entire casing string will be evaluated at due
time. In case a liner will appear to be the better solution, it will overlap the last'say the deepest( intermediate casing string for at least 00 m, design
practice.
4.$.2. CASING SETTING "EPTH "ETERMINATION
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E@AMP'E OF CA'C#'ATION
2.2. In!ered(a!e Cas(n,
59a/imum 9ud )ensity, df: .06 g1litre at 3000 m
5 Trace a s!ra(,*! +(ne para++e+ !o !*e dep!* a?(s un!(+ (! (n!erse!s !*e -ra!ure
,rad(en! urvein correspondence of a value of .06 gf1cm1-0 m.
5This happens at a depth of 700 m, where the pressure e/erted by the mud in hole
equals that of the fracture pressure: 336. gf1cm. In theory, this should be the setting
depth of the intermediate casing.
5%ut for safety reasons, a ar,(nof some tens of gf1cm'-05!0 gf1cm( should be
maintained in favour of the fracture pressure 'to tae into account friction losses during
mud circulation, surge pressures, etc.(. 2or this e/ample, suppose to need a safety
margin # of about 0 gf1cm 'even a bit less is O;(.
5If the casing is set in correspondence of a fracture gradient< .- gf1cm 1-0 m, which
occurs at $270 , an acceptable safety margin of -=.3 gf1cm
is obtained.In fact:$270 52.12 2.096D10 1>.7 ,-D2
5 *o, !*(s -(na+ (n!ered(a!eas(n, (++ be run a! $270 .
4.$.2. CASING SETTING "EPTH "ETERMINATION
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4 $ 2 CASING SETTING "EPTH "ETERMINATION
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E@AMP'E OF CA'C#'ATION
2.$. In!ered(a!e Cas(n,
59a/imum 9ud )ensity at !30 m, df
: -.= g1litre
5Trace a s!ra(,*! +(ne'>ine (, starting from the value of the mud density at !30 m
para++e+ !o !*e dep!* a?(s un!(+ (! (n!erse!s !*e -ra!ure ,rad(en! urve in
correspondence of a value of -.= gf1cm1-0 m.
5This happens at a depth of -330 m, where the pressure e/erted by the mud in hole
equals that of the fracture pressure: =7.6 gf1cm.
5In theory, this should be the setting depth of this other intermediate casing.
5 *upposing to maintain a sa-e!) ar,(n o- abou! 17 3 20 ,-D2 for the same
reasons as before, the casing must be set deeper.
5To obtain a safety margin of -7 gf1cm, the new depth is 2127 , where a fracture
gradient < .00 gf1cm1-0 m can be read:
2127 ? 52.0031.>26D10 1: ,-D2
5 T*(s -ur!*er (n!ered(a!eas(n, (++ be run a! 2127 .
4.$.2. CASING SETTING "EPTH "ETERMINATION
4 $ 2 CASING SETTING "EPTH "ETERMINATION
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-.00 -.30 .00 .300
300
-300
-000
!000
300
000
!300
4000
4300
3000
4.$.2. CASING SETTING "EPTH "ETERMINATION
T*eore!(a+ Se!!(n, "ep!*
1770
E--e!(ve Se!!(n, "ep!*
2127
'AST %#T ONE
INTERME"IATE
CASING
4 $ 2 CASING SETTING "EPTH "ETERMINATION
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E@AMP'E OF CA'C#'ATION
2.4. Sur-ae Cas(n,
59a/imum 9ud )ensity at -30 m, df
: -.!7 g1litre
5Trace a s!ra(,*! +(ne, starting from the value of the mud density at -30 m, para++e+ !o
!*e dep!* a?(s. In this case it can be observed that the fracture gradient curve is not
intersected, neither at the surface.
5This means that in theory no casings will be required in this section of hole.
5 %ut for what said before, a casing is necessary at around 300 m 'this depth isevaluated case by case depending on lithology, e/perience, e/pected hole problems,
etc.( in order to have the possibility to install the sa-e!) eu(pen!'%O# stac(, thus
ensuring a certain safety margin between the pressure e/erted by the mud in hole and
the fracture pressure, and not to leave a too long section of hole uncased.
5If it is supposed to run this casing at 700 , it results that:
d-a! 700 1.$: ,D+(!re 5a?(u ud dens(!) a! 2170 6G-ra! 700 1.;0 ,-D
2D10
Sa-e!) Mar,(n 51.;0 ? 7006D10 51B$: ? 7006D10 21.7 ,-D2
5 T*(s (s !*e sur-ae as(n, se! a! 700 .
4.$.2. CASING SETTING "EPTH "ETERMINATION
4 $ 2 CASING SETTING "EPTH "ETERMINATION
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-.00 -.30 .00 .300
300
-300
-000
!000
300
000
!300
4000
4300
3000
4.$.2. CASING SETTING "EPTH "ETERMINATION
E--e!(ve Se!!(n, "ep!*
700
S#RFACE
CASING
4 $ 2 CASING SETTING "EPTH "ETERMINATION
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E@AMP'E OF CA'C#'ATION
2.7. Condu!or P(pe
5 Above the surface casing, in ons*ore e++s, a first large diameter casing, called the
ondu!or p(pe or dr(ve p(pe '4$, !0$(, is usually set at about 0530 m with the
purpose to protect the shallower wea, unconsolidated formations from caving5in or
collapsing or for avoiding any eventual stability problem to the drilling rig itself.
5 The conductor pipe is usually driven into the ground without being cemented at all
????????????????????????.
5 In o--s*ore e++s, the # setting depth, @i, can be calculated 'm( by the followinge/pression:
where:
< rotary table elevation above sea level or "air gap$, m@ < water depth, m
Bsed< overburden density 'e/pected at # shoe(, g1cm!
Bmud< mud density during the ne/t drilling phase, g1litre
( )
( ) mudsed
mudi
HHEH
+
+=
03.167.003.1
03.1
4.$.2. CASING SETTING "EPTH "ETERMINATION
4.$.2. CASING SETTING "EPTH "ETERMINATION
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-.00 -.30 .00 .300
300
-300
-000
!000
300
000
!300
4000
4300
3000
4.$.2. CASING SETTING "EPTH "ETERMINATION
E--e!(ve Se!!(n, "ep!*
70
CON"#CTOR
PIPE
4 $ 2 CASING SETTING "EPTH A ' I " A T I O N
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"ESIGN E@AMP'E 3 O%TAINE" "ATA
2.9. F(na+ as(n, pro-(+e 5*(* resu+!s !o be a 43s, pro-(+e6
The calculations performed indicate that in the case of this well, with these porepressure and fracture pressure profiles, -(ve as(n, s!r(n,sare required, that is:
1. Condu!or P(pe a! 70
2. Sur-ae Cas(n, a! 700
$. F(rs! In!ered(a!e Cas(n, a! 2127
4. Seond In!ered(a!e Cas(n, a! $270
7a. Produ!(on Cas(n, a! 7000
57b. Ins!ead o- a Cas(n,B a Produ!(on '(ner an be run -ro 7000 up !o $000 B
!*a! (s 270 (ns(de !*e s*oe o- !*e Seond In!ered(a!e Cas(n,6
T*e ne?! s!ep ons(s!s (n !*e ver(-(a!(on o- !*ese as(n, se!!(n, dep!*sB !a(n,(n!o aoun! !*e 7 -a!ors desr(bed (n !*e -o++o(n, s+(des 'C99%CID8
T@AT: the calculations made so far have already considered some of these factors(.
4.$.2 CASING SETTING "EPTH A ' I " A T I O N
4 $ $ CASING SETTING "EPTH A ' I " A T I O N
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After having roughly calculated the depths to which the various
casings must be run, validation must then be carried out to be sure
that these depths are satisfactory.
hecing the accuracy of the casing setting points is based on the
analysis of the following five '3( basic items, including:
1. MA@IM#M PRESS#RE A''O&A%'E AT THE CHOEB P*
2. MA@IM#M "IFFERENTIA' PRESS#REB Pd
$. "RI''ING %A'ANCEB Pdb
4. IC TO'ERANCE
7. E@PECTE"DPOSSI%'E "RI''ING PRO%'EMSREMEM%ER "at the A*ID8$ always means at the TO# of the
ADDE>AC sections
4.$.$ CASING SETTING "EPTH A ' I " A T I O N
4 $ $ 1 MA@IM#M PRESS#RE A''O&A%'E AT THE CHOE P *
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alled also the Ma?(u A++oab+e ANN#'AR Sur-ae Pressure or
MAASP, it represents the ma/imum pressure that can be allowed toaccumulate at the wellhead in case a ic has been controlled, without
causing the fracturing of the formation below the shoe of the last casing
run in hole. It is clear that as mud density increases, #chdecreases.
The minimum acceptable value can be not less than -0 gf1cm for
surface casings and 30 g1litre difference between the fracture gradientbelow the casing shoe and the density of the mud in hole 'or 40530
gf1cm( for the others. In some cases, in particular when deep wells
are drilled, values of #ch very low and sometimes close to zero are
accepted, otherwise the target could not be reached.
At a certain depth, H, the P*
or MAASPis given by the equation:
P* 5G-rBs*oe d-BH6 ? Hs*oeJ D 10
4.$.$.1. MA@IM#M PRESS#RE A''O&A%'E AT THE CHOEB P*
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4.$.$.2. MA@IM#M "IFFERENTIA' PRESS#REB Pd
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It is the difference between the pressure e/erted by the mud at the
ma/imum density foreseen in that given hole section 'generally this is
the value of the mud density at the end of each hole section( and the
pore pressure as a function of depth.
The nowledge of #d is e/tremely important because it can give
indications about the drill string going to get stuc or not, especially in
presence of porous and permeable formations. /perience shows that
#d values can vary from 00 gf1cm and more down to only few
gf1cm depending on local features.
The ma/imum differential pressure Pd as a function of depth is
calculated by means of the formula:
Pd 5d-Ba? GpBH6 ? HJ D 10
4.$.$.2. MA@IM#M "IFFERENTIA' PRESS#REB Pd
4.$.$.2. MA@IM#M "IFFERENTIA' PRESS#REB Pd
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Esually, drilling practices do not fi/ strict reference limits for the values
the differential pressure can assume. This depends on the particularsituation of the area where operations tae place, specifically the
characteristics of formations drilled 'permeability, strength( and
composition and properties of the mud in use.
@owever, it is always necessary to remember that e/cessive
differential pressures can cause:
5 pipe sticing 'vs uncased hole wall(pipe sticing 'vs uncased hole wall(
55 circulation lossescirculation losses
55 low penetration rateslow penetration rates..
B d
4.$.$.2. MA@IM#M "IFFERENTIA' PRESS#REB Pd
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E@AMP'E
The ma/imum differential pressure in the interval -35!30 m
develops as shown in the Table here below.
"EPTHB Gpvs "EPTHB
,-D2D10
MA@ M#" "ENSIT8IN SECTIONB ,D+(!re
PdB ,-D2
2127 1.0$ 1.>2 1;>2700 1.4$ 1.>2 122
2:70 1.77 1.>2 102
$000 1.9; 1.>2 :2
$100 1.:2 1.>2 92
$270 1.:; 1.>2 46
B d
4.$.$.$. "RI''ING %A'ANCEB Pdb
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It is the difference between the pressure due to the drilling mud at its
density and that of the formation, as a function of depth. This measure
indicates how much the pressure e/erted by the mud e/ceeds the pore
pressure. In most case #db increases with depth, if the mud density
and the pore pressure gradient remain constant.
The nowledge of this parameter is very important because it can give
indication about riss of (s ourrene'#dbtoo much close to the
pore pressure, say too low( and about the e--e! on pene!ra!(on ra!e
'too high #db can negatively affect penetration rates(.
The )rilling %alance, Pdb, as a function of depth, H, is calculated with the
equation:
Pdb 5d-Gp6J ? H D 10
db
4.$.$.$. "RI''ING %A'ANCEB Pdb
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$0
E@AMP'E
In the section -35!30 m, the drilling balance varies as shown below.
"EPTHB Gpvs "EPTHB
,-D2D10
M#" "ENSIT8 vs"EPTHB ,D++(!re
Pd%B ,-D2
2127 1.0$ 1.$9 :0
2700 1.4$ 1.90 4$
2:70 1.77 1.:1 44
$000 1.9; 1.;1 $>
$100 1.:2 1.;1 28
$270 1.:; 1.>2 49
4.$.$.4. IC TO'ERANCE
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$1
It represents the vo+ue o- a?(u (n-+u?'ic( that, once entered the
wellbore ' annular space(, can be circulated out by using a "constant
bottomhole pressure$ method without fracturing the formation below theshoe of the previous casing. SEE THE &E'' CONTRO' CHAPTER
This volume (BH is calculated by the following e/pression:
where:
5 Fi,@< ic tolerance, m! 5 Bmud< mud density at @, g1litre
5 a< annular capacity below shoe per unit length, litre1m 5 @ < current depth, m
5 8frac< fracture gradient at the casing shoe, gf1cm1-0 m 5 Bi < density of influ/ fluid, g1litre
5 @shoe< casing shoe depth, m 5 pp< pore pressure at @, gf1cm
It has A>GAH* to be verified at least for the *EC2A and the IDTC9)IAT
casings.
( )[ ]
( )imudp
pmudmudfracshoe
shoefracaHi p
pHGH
HGCV
+
=
1010
4
,
4.$.$.4. IC TO'ERANCE
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IC TO'ERANCE MA@IM#M A''O&A%'E A'#ES
Ho+e S(KeB "(n Ma?(u ( To+eraneo+ue
$ bb+
" L 2$= 40 270
2$= " L 1: 1D2= 24 170
1: 1D2= " L 12 = 19 100
12 1D4= "L ; = ; 70
" ; = 4 27
T*e ( To+erane us! be a+u+a!ed and ver(-(ed -or a++ sur-ae and
(n!ered(a!e as(n,s (n order !o ,uaran!ee 5a! +eas!6 !*ese a?(u va+ues.
4.$.$.7. E@PECTE"DPOSSI%'E "RI''ING PRO%'EMS
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$$
Ghen selecting the casing setting depths, other factors should be taen into
consideration, especially for what regards the shallower casings, such as:
S*a++o Gas
Ho+e A,e(n,'"time dependent$ deformations of the rocs: reep(
#ns!ab+e Fora!(ons
Seepa,es and C(ru+a!(on 'osses
"ev(a!ed or Hor(Kon!a+ "r(++(n,
Produ!(on Reu(reen!s Open Ho+e vs Cased Ho+e
Eono(s
1B0 1B7 2B0 2B70
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CASING POINT
SE'ECTION
Pressure Grad(en!sPressure Grad(en!s
5,-D5,-D22D106D106
Fra!ure
Grad(en!
:=
> 7D;=
Pore PressureGrad(en!
20=
Mud"ens(!)
2000
1700
1000
700
2700
$000
1$ $D;=
4.$.$.9. "ESIGN E@AMP'E
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$7
Pro-.
m
300
-!00
300
Press(one "(--erenK(a+e5 ;g1cmCas(n, Grad(en!( d( Press(one 5 ;g1cm
1-0m Mar,(ne a++a C*oe5 ;g1cm
0
300
-000
-300
000
300
!000
-.0 -.3 .0 .3
#rofonditI
m
0 0 40 60 J0 -00 -0 -40 -60 -J0 00 00 0 40 60 J0
-0
0
-
0
-4
0
-6
0
-J
0
0
0
0
4
0
6
0
20Q
1$ $D;Q
> 7D;Q
4.$.$.9. "ESIGN E@AMP'E
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F(na+ Cas(n, Pro-(+e
After having verified the casing setting depths on the basis of the 3 factors previously
seen, five casing strings are required for the well under e/amination, that is:
-. onductor #ipe at 30 m
. *urface asing at 300 m
!. 2irst Intermediate asing at -3 m4. *econd Intermediate asing at !30 m
3. #roduction asing at 3000 m
T*e ne?! s!ep ons(s!s (n !*e de!er(na!(on o- !*e s(Kes 5d(ae!ers6 o- !*e
*o+es !o be dr(++ed and !*ose o- !*e as(n,s !o be run.
4.$.$.9. "ESIGN E@AMP'E
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asings are +oints --5-! m long, which, once threaded together, can completely case
the well at the planned depths.
4.$.4. F#N"AMENTA' CASING F#NCTIONS
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Allow the installation of theWellheadand BOPs.
Allow to circulatethe drilling fluid up to the surface.
Isolate formations with different pore pressure and fracture
gradients.
Exclude as soon as possible formations which can cause
drilling problems because of their geological characteristics and
(mainly) their hydraulic features.
Protect potentially productive formations.
4.$.7. CASING T8PES
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K CON"#CTOR PIPE
K S#RFACE CASING
K INTERME"IATE CASINGDS
K PRO"#CTION CASING
4.$.7.1. CON"#CTOR PIPE
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The Condu!or P(pe, CP, is the first string to be run in a well and has the main
functions :to protect the shallower formations from any contamination due to the drilling fluids&
to prevent dangerous wash5outs and erosion of the loose, unconsolidated topsoil,which can, sometimes, result in big problems for the stability of the rig itself&it is of large dimensions, generally from -6, -J531J, 0, 4, !0 and !6$.
The setting depth of the onductor #ipe is usually very shallow '!0530 m and at
ma/imum around -00 m( and is chosen in such a way to permit that the drilling fluid
may be circulated to the mud pits while drilling the surface hole. The # seat must be
in an impermeable formation with enough resistance to fracturing to allow the
circulation of the mud to the surface.
The # can be either driven by a pile driver down to a depth where the casing does not
enter anymore into the topsoil or run in a drilled hole. In this last case, a guide shoe is
usually welded on the last +oint of the # and the cement slurry is pumped through a
swedge, which is screwed to the +oint closest to the surface. The cement slurry isnormally pumped up to the surface.
4.$.7.2. S#RFACE CASING
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The second string, which is run immediately after the #, is the sur-ae as(n,B set
usually around -005300 m& its size can range between -!5!1J, -6, -J531J, 0 and 4$.
The main ob+ectives of the surface casing are:to protect the potential fresh5water levels from the contamination of the drilling fluids&
to allow the installation of the %O# stac&
to help supporting the weight of the successive drilling strings and the production
equipment.
Its setting depth should be into an impermeable level below any fresh5water bearing
formation, sometimes as deep as -000 5 -300 m. Of course, if there is the ris to
encounter shallow gas5containing formations or gravel is e/pected, the setting depth of
the surface casing can be shallower. @owever, the setting depth should be deep enough
to allow drilling to the ne/t casing setting point without any ris of fracturing and
providing reasonable assurance that broaching to the surface will not occur in case the
%O# are closed to contain a ic '(n(u *oe ar,(n reu(red 10 ,-D2(.
4.$.7.$. INTERME"IATE CASING5S6
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One or more (n!ered(a!e as(n, s!r(n,s'-5!( are often required to deepen a well
with the purpose:
to seal off troublesome formations, such as wea zones with low fracture gradients,
overpressured intervals, salt beds, sloughing shales, reactive formations, deviated
sections, etc.&
to get closer to the target&
to allow the use of higher density muds, being placed in higher fracture gradient
formations '(n(u *oe ar,(n reu(red 40 ,-D2(&
to install higher performance %O# stac&
to isolate intermediate mineralized formations.
The diameter of these casings normally ranges between -! !1J$ and 7$ and the depth
can reach 3000 m or more. The intermediate casing is often the longest and the
heaviest string run in the well and its role is essential to reach the target.
4.$.7.4. PRO"#CTION CASING
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The produ!(on as(n,or produ!(on +(neris the last and most important string of a
well& its primary functions are:
the isolation and protection of all the zones above and within the production levels&
housing for all the production equipment&
the e/ecution in safe conditions of all those operations which are usually required
during the production life of a well&
the containment of the producing fluids in case of production string 'tubings( failure.
ommon sizes of the production strings vary between = 31J$ to 4 L$ and can be set at
depths from -300 m down to 7300 m and more.
4.$.7.7. 'INERS
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'(nersare sections of a casing
string, that cover the hole frombottom up to as certain depth,
but not up to the surface. The
liner top is usually hanged
inside a previous casing string,
'the overlapping length is about00 M 30 metres(.
'INER
TOP
CASINGSHOE
4.$.7.7. 'INERS
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The a(n advan!a,esof a liner are:costs containment&decrease of the weight of the tubulars to handle 'effects on rig selection(&
better hydraulics&mechanical integrity when production starts&more fle/ibility in completion schemes.
The a(n d(sadvan!a,esof liner installation are:ris of poor pressure integrity in correspondence of the liner top because
of poor cementation or due to wear of the casing on which the liner is
hanged&ris to cement the liner running equipment& difficulty in obtaining good cementing +obs due to the small clearance
between casing and liner or hole and liner&
need to install a bridge plug above the liner top in case of %O# removalin case the completion string has not been run and landed yet..
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4.$.7.7.1. "RI''ING or
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The setting of a "RI''ING +(ner"RI''ING +(ner is often an economically convenient decision
particularly in deep wells as opposed to running a full string. *uch a decision must
be carefully considered, since it requires that the casing 'run before the liner( is
suitable to withstand the pressure conditions as it were set at the depth of the liner.
If drilling is planned to continue below the drilling liner, then its requirements, in
particular for what concerns the burst resistance, must be further increased. This
well profile, obviously, is e/pensive, because of the higher performances the
intermediate string must possess. 2urthermore, the continuous wear, at which the
intermediate string is sub+ected to, should be carefully evaluated and predicted.
A I M
"r(++(n, or
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If a PRO"#CTION +(nerPRO"#CTION +(neris planned, the intermediate drilling liner is usually tied5bac up
to the surface& good drilling practices '8O2#s( suggest that this operation has to be
accomplished before drilling the ne/t hole section, where the production liner will be
placed. T*a! eans !*a! 5*en neessar)6 (! (s usua+ !o des(,n a drilling liner,
which is tied5bac and then followed 'in time( by the drilling activity leading to run a
deeper production liner.
Often, the produ!(on +(ner !oo (s bound dur(n, (!s or(n, +(-e !o be !(ed3baup !o !*e sur-ae. If this operation is planned, the intermediate casing string can be
designed to withstand lower mechanical stresses, resulting in running lower level riss
and1or achieving considerable cost savings.
N O T I C E
ementing production liners is usually difficult, because zonal isolation is essential
during the production life of the well and any subsequent wor over activity.
4.$.7.7.2. PRO"#CTION 'INERS
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'INER TOP
'INER TIE3%AC
4.4. CASING AN" %IT SIE SE'ECTION C H A R T
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To select the casing and bit sizes, the C*ar! s*on (n !*e -o++o(n, s+(descan
help in this tas. Its ordinary "bottom up$ use is as follows:
-. define the size the casing or the linermust have when the bottom of the hole will be
cased '@* is one of the fundamental data contained inside any official agreement
) M O, usually decided by Os(. This will be the production casing or liner size&
. enter the hartwith that size&
!. follow the arrows on the hart& they indicate the hole sizes that may be required to
run the last casing. If, for instance, the production casing has a 3$ diameter, thehole to be drilled could have a 6 L$ or 6 -1J$ size&
4. continue upwards 'in the well( until surface casing has been selected.
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4.4. CASING AN" %IT SIE SE'ECTION
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30" CP
4.4. CASING AN" %IT SIE SE'ECTION
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20" CASIN
! # CASIN
$% CASIN
&3 '% CASIN
STAN"AR" CASING PROFI'ESTAN"AR" CASING PROFI'E
2(% )O*E
&(% O+ &$ ,# )O*E
&2 -# )O*E
. ,# )O*E
4.4. CASING AN" %IT SIE SE'ECTION
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$0Q CP
20=
s,
> 7D;Q s,
*o+e ; 1D2Q
1$ $D;Q s,
S!andardS!andard
Cas(n,Cas(n,Pro-(+ePro-(+e =7D;
; = :=
9= 7=
Ho+e Cas(n,
2;= 24=1D2
22= 1;=7D;
1:=1D2 19=
14=$D4 1$=$D;12=1D4 512=:D;6 11=$D4
10=7D; >=7D;
;=1D2 :=
9= 7=
4.4. CASING AN" %IT SIE SE'ECTION
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"ESIGN E@AMP'E
9aing reference to the calculation e/ample of *lide - and other ones, the
hole vs casing profile results to be:
-. $9=@ole for $0=onductor #ipe at 30 m
. 29=@ole for 20=*urface asing at '400 m or( 300 m
!. 1: 1D2=@ole for 1$ $D;=2irst Intermediate asing at -3 m
4. 12 =@ole for > 7D;=*econd Intermediate asing at !30 m
3. ; 1D2=@ole for :=#roduction asing at 3000 m
T*e ne?! s!ep (n Cas(n, "es(,n (s !*e a+u+a!(on o- !*e e*an(a+
*ara!er(s!(s !*e var(ous as(n,ss*ou+d *ave (n order !o (!*s!and !*es!resses *(* are ,o(n, !o so+((! !*e dur(n, !*e *o+e +(-e o- !*e
e++.
4.7. APPROACH TO CASING "ESIGN
Cas(n, des(,nCas(n, des(,n actually includes s!ress ana+)s(s proedure The tas
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Cas(n, des(,nCas(n, des(,nactually includes s!ress ana+)s(s proedure. The tas
of that procedure is to design a pressure vessel which can withstand a
variety of e/ternal, internal, thermale/ternal, internal, thermal andself weight loading conditionsself weight loading conditions,
while at the same time being sub+ected to wearwearand corrosioncorrosion.It is obvious that loads that can affect casings during the life of the well
can not be e/actly predicted& for this reason they are designed taing
into account the most demanding situations that can realistically occur
according to the policy in force and the e/perience and statistical data
available in each Oil ompany.
It is also necessary to have as much information as possible about:
pressure and temperature gradients&
characteristics of the formations that have to be drilled&
potential hole problems&formation fluid content N characteristics.
It is also necessary to now the mechanical and geometric characteristics of
the tubulars required or available
4.7. APPROACH TO CASING "ESIGN
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the tubulars required or available.
The fundamental mechanical features of tubulars must concern and include:
res(s!ane !o (n!erna+ pressure or burs!&
res(s!ane !o e?!erna+ pressure or o++apse&
res(s!ane !o a?(a+ !ens(on.
The mechanical performance of tubulars depends on several factors, such as:
ou!s(de d(ae!er&
a++ !*(ness&
s!ee+ ,rade&
!)pe o- onne!(on.
International Organizations and 9anufacturers too usually provide these data.
4.7. APPROACH TO CASING "ESIGN
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4.7. APPROACH TO CASING "ESIGN
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STRESS3STRAIN "IAGRAMME
To determine the behaviour of a material, this is tested. Tests of material
performance may be conducted concerning:
tension&torsion&compression&shear.
The tension test is the most common and is usually represented by plotting the
apparen! s!ress 'the total load applied on the material specimen divided by its
cross5sectional area( on the ordinates against the apparen! s!ra(n 'elongation
between two gauge points mared on the specimen divided by the original gauge
length( on the abscissae.
A typical plot is shown in the ne/t slide, presenting a stress M strain diagram..
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2rom the analysis of the curve, it comes out that: th fi t t f th i t i ht li d d t l ti d f ti
4.7. APPROACH TO CASING "ESIGN
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the first part of the curve is a straight line and corresponds to elastic deformations
according to @ooes law and represents the e+as!( re,(on. The slope of the curve 'that
is the ratio between the stress and the strain in the elastic region( is the modulus of
elasticity "$,called also the8oun,Vs odu+us& beyond the elastic limit, peranen! or p+as!( de-ora!(on occurs and the stress5
strain curve assumes a curvilinear trend. If the stress is released in the region between
the elastic limit and the yield strength, the material will contract along a line nearly straight
and parallel to the original elastic line, leaving a permanent set& in steels, a phenomenon, nown as )(e+d(n,, occurs after the elastic limit. It can be
observed, in fact, that once a ma/imum value of stress is reached, a period of
deformation follows, though the stress remains constant or even decreases. The
ma/imum stress reached in this region is called the upper )(e+d po(n!, while the
minimum is called the +oer )(e+d po(n!. In materials that do not e/hibit a mared yield
point, it is customary to identify a quantity called (n(u )(e+d s!ren,!*, which can be
defined as the stress at which the material has a specified permanent set 'a deformation
of 0.P is generally accepted by the Industry(. 2or materials used to produce tubular
goods, the A#I specifies the )(e+d s!ren,!* as !*e !ens(+e s!ress reu(red !o produea !o!a+ e+on,a!(on o- 0.7030.97W 'depending on the grade of the steel( between the
gauge length&
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"ESIGN FACTOR 5"F6
The "F is defined as the ra!(o be!een !*e p(pe res(s!ane to a certain load
4.9. APPROACH TO CASING "ESIGN THE "ESIGN FACTOR "F
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The "F is defined as the ra!(o be!een !*e p(pe res(s!ane to a certain load
'burst, collapse, tension( and theorrespond(n, +oad es!(a!ed inside the well.
)ifferent "es(,n Fa!orsare specified for the three load types and for the varioussteel grades 'high grade steels require higher )2 because they have a smaller
margin between Hield *trength and Tensile *trength(. The following )2s must be
used in casing design calculations:
4.9.1. CASING "ESIGN CRITERIA %#RST
%urs! +oad(n, on the casing is induced when internal pressure
d t l d i th f i b fi t l l ti
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e/ceeds e/ternal pressure and is, therefore, given by first calculating
the probable pressure acting e/ternally to the casing string and the
pressure e/pected internally '*AH inside it(. The burst pressure is,
then, the difference between the e/pected internal pressure and the
e/pected e/ternal pressure
)epending on type of casing, different boundary conditions are
assumed for calculating the stresses which tend to burst the casing&
usually a distinction is made between casings in the following way, in
order to perform the %#RST 'OA" COMP#TATION :
surface casing&
intermediate casings&
production casing&
liners, if any 'drilling or production(.
S#RFACE CASING
In!erna+P
The e++*ead pressure +((! is arbitrary and is generally set equal to that of thei ti f th llh d d %O# i t b t ith i i
4.9.1. CASING "ESIGN CRITERIA %#RST
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Pressure
O(+ Copan)no *o XGOFP a--e!!*eassup!(onsand !*eonseuen!opu!a!(one!*ods
woring pressure rating of the wellhead and %O# equipment, but with a minimumvalue of -40 gf1cm
Ghen an oversize %O# having a capacity greater than that necessary is selectedor in case of a subsea wellhead, the e++*ead (n!erna+ pressure +((! will be 60Pof the calculated pressure obtained as the difference between the fracture pressureat the casing shoe and the hydrostatic pressure of a gas column up to the well head'methane with a density of 0.! g1dm!is normally considered(. In any case it shallnever be taen less than ,000 psi '-40 gf1cm(. The use of methane for thiscalculation is the "worst case$ when the specific gravity of the fluid is unnown.The bo!!o3*o+e (n!erna+ pressure +((! is set equal to the predicted fracturegradient of the formation below the casing shoe.
onnect both the wellhead and bottom5hole internal pressure valueswith astraight line to obtain the ma/imum internal pressure load versus depth.
E?!erna+Pressure
In wells with surface wellheads, the e/ternal pressure value is taen equal to thehydrostatic pressure of the drilling fluid in which the casing has been run.In wells with subsea wellheads:A! !*e e++*eadGater )epth / *eawater )ensity / 0.- 'gf1cm(
A! !*e s*oe'*hoe )epth 5 Air 8ap( / *eawater )ensity / 0.- 'gf1cm
(Ne! Pressure The resu+!(n, burs! +oad, or ne! pressureBwill be obtained by subtracting, at
each depth, the e/ternal from the internal pressure
S#RFACE CASING 400
4.9.1. CASING "ESIGN CRITERIA %#RST
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In!erna+ Pressure
at bottomhole: fracture pressure '82.2 ,-D2
at wellhead: %O# G#, C 140 ,-D2
E?!erna+ Pressureat bottomhole: hydrostatic pressure of mud
'-.-0 g1litre(, " 44 ,-D2
at wellhead:E 0 5re+a!(ve pressure6
5R E " *ere STRAIGHT 'INE6
Resu+!(n, burs! +oad 5Ne! Pressure6Internal #ressure 5 /ternal #ressure
at wellhead, C 140 0 140 ,-D2at bottomhole, F 9>.2 44 27.2
,-D2
INTERME"IATE CASINGS 5see one s+(de -or ea* o- !*e6
4.9.1. CASING "ESIGN CRITERIA %#RST
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In!erna+ Pressure
O(+ Copan)no *o XGOFP a--e! !*eassup!(ons and!*e onseuen!opu!a!(ons
The e++*ead (n!erna+ pressure value is taen as the 60P of the calculatedvalue obtained as the difference between the fracture pressure at the casing
shoe and the pressure of a gas column at the wellhead. If this value is lowerthan -40 gf1cm, the %O# G# of -40 gf1cmis considered at the wellhead.The bo!!o*o+e (n!erna+ pressure +((! is equal to that of the predictedfracture gradient of the formation below the casing shoe.onnect both the wellhead and bottom5hole internal pressure limitswith astraight line to obtain the ma/imum internal pressure load.
E?!erna+ Pressure The e?!erna+ pressure va+ue is taen to be equal to that of the formationpressure.
K Gith a subsea wellhead, at the wellhead, hydrostatic seawater pressureshould be considered.
Ne! Pressure The resu+!(n, burs! +oad, or net pressure, will be obtained by subtracting, ateach depth, the e/ternal from internal pressure. IN THIS CASE THETREN" IS NOT RECTI'INEAR YYYYYYYYYYYYYY
INTERME"IATE CASING 1200
In!erna+ Pressure
t b tt h l f t '- =3 f1 1-0 (
4.9.1. CASING "ESIGN CRITERIA %#RST
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at bottomhole: fracture pressure '-.=3 gf1cm1-0 m(,A 2$4 ,-D2
at wellhead:
560P of bottom hole fracture pressure minus thehydrostatic head of a column of gas '0.! g1dm!(,
% 0.9 52$4 3 $96 11;.; ,-D2
5%O# G#, C 140 ,-D2
E?!erna+ Pressurepore pressure:
5 at J00 m, "1 51.0$ ? ;006 D 10 ;2.4 ,-D2
5at -000 m, "2 51.20 ? 10006 D 10 120.0 ,-D2
5at -00 m, "$ 51.$:7 ? 12006 D 10 197.0 ,-D2
Resu+!(n, burs! +oad 5Ne! Pressure6Internal #ressure 5 /ternal #ressure
5at surface. C 140 3 0 140 ,-D2
5at J00 m, 200 3 ;2.4 11:.9 ,-D2
5at -000, 21; 3 120 >;.0 ,-D2
5at -00, 2$4 3 197 9>.0 ,-D2
INTERME"IATE CASING 2700
In!erna+ Pressure at bottomhole fract re press re '- =! gf1cm1-0
4.9.1. CASING "ESIGN CRITERIA %#RST
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at bottomhole: fracture pressure '-.=! gf1cm1-0
m(, A 4;2.7 ,-D2at wellhead: 60P of bottomhole fracture pressure
minus the hydrostatic head of a column of gas '0.!g1dm!(,
% 0.9 ? 54;2.7 :76 244.7 ,-D2
E?!erna+ Pressurepore pressure:
5 at J00 m, "1 51.0$ ? ;006D10 ;2.4 ,-D2
5 at -J00 m, "2 51.:0 ? 1;006D10 $09.0 ,-D2
5 at 300 m, "n 51.0$ ? 27006D10 27:.7 ,-D2
Resu+!(n, burs! +oad 5Ne! Pressure6Internal #ressure 5 /ternal #ressure5at surface, % 244.7 3 0 244.7 ,-D2
5at J00 m, $$0.0 3 ;2.4 24:.9 ,-D2
5at -J00 m, 410.0 $09.0 104.0 ,-D25at 300 m, 4;2.7 3 27:.7 227.0 ,-D2
PRO"#CTION CASING
4.9.1. CASING "ESIGN CRITERIA %#RST
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The "worst case$concerning burst
load conditions on produ!(on
as(n, occurs when the well isshut5in and there is a lea in
correspondence of the top of the
tubing, or in the tubing hanger,
and the shut5in well head pressure
is applied to the top of the pacer
fluid 'i.e. completion fluid( in thetubing
'EAAGE
PRO"#CTION CASING
4.9.1. CASING "ESIGN CRITERIA %#RST
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In!erna+ Pressure
O(+ Copan)no *o XGOFP a--e! !*eassup!(ons and!*e onseuen!opu!a!(ons
The e++*ead (n!erna+ pressure va+ue is obtained as the difference betweenthe pore pressure of the reservoir fluid and the hydrostatic pressure of theproduced fluid which is inside the tubing. In case of uncertainty on the nature
of produced fluid 'hence of its density(, a column of gas having density < 0.!g1dm!will be considered. Actual gas1oil gradients can be used if informationon these is nown. The bo!!o3*o+e (n!erna+ pressure va+ue is obtained by adding thewellhead internal pressure to the annulus hydrostatic pressure e/erted by thecompletion1pacer fluid. 8enerally the completion fluid density is equal to, orclose to, the mud weight in which the casing has been installed. onnect both the wellhead and the bottomhole internal pressure with astraight line to obtain the ma/imum internal pressure.
E?!erna+ Pressure The e/ternal pressure is taen to be equal to that of the formation pressure. Gith a subsea wellhead, at the wellhead, hydrostatic seawater pressureshould be considered.
Ne! Pressure
The resulting burst load, or net pressure, is obtained by subtracting, at eachdepth, the e/ternal from the internal pressure. A'SO IN THIS CASE THETREN" IS NOT RECTI'INEAR YYYYYYYYYYYY
PRO"#CTION CASING $000
In!erna+ Pressure
4.9.1. CASING "ESIGN CRITERIA %#RST
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at wellhead: reservoir pressure minus the
hydrostatic head of the fluid to be produced 'if the
density of the fluid is unnown, consider gas at 0.!
g1dm!(, A $0> 3 >0 21> ,-D2
at bottomhole: wellhead pressure plus the
hydrostatic head of completion fluid,
% 21> Z 51.1 ? $0006D10 74> ,-D2
E?!erna+ Pressurepore pressure:
5 at J00 m, "1 51.0$ ? ;006 D 10 ;2.4 ,-D2
5at -00 m, "2 51.9> ? 21006 D 10 $74.0 ,-D2
5at !000 m, "n 51.0$ ? $0006 D 10 $0> ,-D2
Resu+!(n, burs! +oad 5Ne! Pressure6Internal #ressure 5 /ternal #ressure
5 at surface, 21> 3 0 21> ,-D2
5at J00 m, 24$ Z ;; ;2.4 24;.9 ,-D2
5at -00, 2;2 Z 2$1 $74 17> ,-D2
5at !000 m, 74> $0> 240 ,-D2
'INERS &ITH INTERME"IATE CASINGS
If d illi li h t b d th i i hi h th li i d d t
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If a drilling liner has to be used, the casing in which the liner is suspended must
withstand the burst pressure that may occur while drilling below the liner.
The design of the intermediate casing string is, therefore, altered slightly:
-( *ince the fracture pressure and mud weight may be greater or lower below
the liner shoe than below the casing shoe, these values must be used to design
both the intermediate casing string and the liner.
( Ghen performing well testing operations or producing through a liner, the
casinginside which the liner is suspended is part of the production stringandmust be designed according to this criterion.
TIE3%AC STRING
In a high pressure well, the intermediate casing string above a liner may be
unable to withstand a tubing lea at surface, according to the production burstcriteria seen before. One solution to this problem is to tie5bac the liner by a
string going from the liner top to surface, thus isolating this intermediate casing.
Co++apse +oad(n, of the casing is induced if the e/ternal pressure e/ceeds the
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Co++apse +oad(n, of the casing is induced if the e/ternal pressure e/ceeds the
internal pressure. It occurs as a result of either or a combination of:
reduction in internal fluid pressure&increase in e/ternal fluid pressure& additional mechanical loading imposed by plastic formations 'i.e. clay, salt(
movement.
The design of a casing string in collapse mode consists in selecting the lowest cost
pipe that has sufficient strength to meet with the desired design criteria and "designfactor$ )2.
N O T I C E
Ghen maing a selection, if a choice e/ists between a lower grade heavy pipe and a
higher grade but lighter pipe, both of them providing adequate strength at similar cost,
the higher grade 'lighter( pipe should be chosen due to the reduction of tension
loading.
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INTERME"IATE CASING
In!erna+Pressure
The worst case for collapse loading occurs when a loss of circulation isencounteredwhile drilling the ne/t hole section. This results in the mud level insideth i d i t ilib i l l h th d h d t ti h d l
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O(+ Copan)no *o X
GOFP a--e!!*eassup!(onsand !*eonseuen!opu!a!(one!*ods
the casing dropping to an equilibrium level where the mud hydrostatic head equalsthe pore pressure of the thief zone. onsequently, it will be assumed that thecasing is empty to the height '@(calculated as follows:
5 '@loss5 @( / dm< @loss / 8p 5 @ < @loss'dm 5 8p(1dm
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F+u(d He(,*! Copu!a!(on
'@ loss5@( / dm< @ loss/ 8p
@ < @ loss'dm5 8p( 1 dm
If 8p< -.0! gf1cm1-0m
then
H H +oss 5d3 1.0$6Dd
or
H H +oss513 GpDd6
@
INTERME"IATE CASING 1200 In!erna+ Pressurein case the depth at which circulation losses are
t d i th l l d i l l t d
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e/pected is nown, the level drop is calculated
with: H H+oss51 GpD dud6
If:dmud < -.J0 g1litre
8p< -.0! gf1cm1-0 m
@loss< 300 m
the well will be empty down to H 109>.4 5a! sur-ae. 05a! 109>.4 0
5a! 1200 512003109>.46?1.;0?0.12$.7 ,-D2
E?!erna+ Pressure5pressure of the mud used to run the casing
% 51.4; ? 12006 D 10 1::.9 ,-D2
Resu+!(n, o++apse +oad 5Ne! Pressure6
5/ternal #ressure 5 Internal #ressure5at surface, A 05at -06=.4 m, F 17;.$ ,-D2
5at -00, E 51::.9 3 2$.76 174.1 ,-D2
INTERME"IATE CASING 1200
In!erna+ PressureIf the depth of e/pected circulation losses
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is unnown, it is assumed the casing half
void and half filled with the mud to be used
in the ne/t hole section at its ma/imumdensity:
5 from 05600 m, C 0
5 at -00 m, d f,ma/ ne/t phase / L @
" 51.;0?9006D10 10; ,-D2
E?!erna+ Pressurepressure of the mud used to run the
casing, % 51.4;?12006D10 1::.9 ,-D2
Resu+!(n, o++apse +oad 5Ne! Pressure6/ternal #ressure 5 Internal #ressure5at surface, A 05at 600m, F 51.4;?9006 D 10 3 0;;.; ,-D2
5at -00 m, E 51::.9 3 10;.06 9>.9 ,-D2
INTERME"IATE CASING 2700
I ! + P
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In!erna+ Pressure5casing half void and for the remaining half
the mud at its ma/imum density3 -ro 031270 0
3 a! 2700 51.1?12706D10 1$:.7 ,-D2
E?!erna+ Pressure5pressure of the mud used to run the
casing, % 51.;?27006 D 10 470 ,-D
2
Resu+!(n, o++apse +oad 5Ne! Pressure6/ternal #ressure 5 Internal #ressure5at surface, A 05at -30 m, F 51.;?12706 D 10 227 ,-D2
5at 300 m, E 470.0 3 1$:.7 $12.7 ,-D2
PRO"#CTION CASING
In!erna+ The worst case occurs when the casing is completely empty. It is in fact probable
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Pressure
O(+ Copan)no *o XGOFP a--e!!*eassup!(onsand !*eonseuen!opu!a!(on
e!*ods
that during the productive life of a well, tubing leas often occur. Also wells may beon artificial lift or have plugged perforations or very low internal pressure values
and, under these circumstances, the production casing string could be partially orcompletely empty. This must be taen into consideration in the design and theideal solution is to design for zero pressure inside the casing which provides fullsafety.In particular situations, the Gell Operations 9anager may consider that thelowest casing internal pressure is the level of a column of the lightest densityproducible formation fluid.
E?!erna+Pressure
Assume the hydrostatic pressure e/erted by the mudin which casing is installed.In case of salt sections, consider uniform e/ternal loading equal to theoverburden pressure at the true vertical depthof the relevant point.
Ne! Pressure In case of the casing being empty the resultant collapse load, or net pressure, isequal to the e/ternal pressureat each depth.
In other cases, it will be obtained by subtracting, at each depth, the internal fromthe e/ternal pressure.
PRO"#CTION CASING $000
In!erna+ Pressure casing completely void
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casing completely void
3 a! sur-ae 0
3 a! bo!!o*o+e 0
E?!erna+ Pressure5pressure of the mud used to run the
casing, % 51.1 ? $0006 D 10 $$0
,-D2
Resu+!(n, o++apse +oad 5Ne!
Pressure65/ternal #ressure 5 Internal #ressure5at surface, A 0 5re+a!(ve pressure65at !000 m, % $$0 ,-D2
'INERS AN" INTERME"IATE CASINGS
-( If a drilling liner has to be used, the casing above, in which the liner is
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suspended, it must withstand the collapse pressure that may occur while drilling
below the liner. Therefore the design of the intermediate casing string is slightly
altered.( Ghen well testing or producing through a liner, the casing above the liner is part
of the production casing5liner string and must be designed according to this
criterion.
TIE3%AC STRING
If the intermediate string above the liner is unable to withstand the collapse
pressure calculated according to the production collapse criteria, it will be
necessary to run and tie5bac a string of casing from the liner top to the surface.
Tens(+e -a(+ureoccurs if the longitudinal force e/erted on a pipe e/ceeds either the tensile strength
of the pipe or of its connections. 8enerally, the connection used in a string of casing is stronger
than the pipe body although this must always be checed out.
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2or situations where a connection coupling has to be of special clearance type 'i.e. smaller
diameter than the normal( or if a flush +oint pipe must be used, the connection will be weaer thanthe pipe body.
Cas(n, !ens(+e +oadsare usua++) (posed b)-( T*e e(,*! o- !*e p(pe (!se+-. The highest tensile stresses will occur at the uppermost portion
of the pipe. The tension is the weight of the pipe in air less buoyancy.
( %up(n, a een! p+u,.
!( %end(n,.4( S*o +oad(n,:
K Ghile lowering casing through unstable formations such as cavings where the casing string may
get temporarily stuc before suddenly slipping through, thereby inducing tensile shoc loads.
K Ghen landing casing in a subsea wellhead from a floater.
3( #pard and donard re(proa!(n, oveen!scarried out where there is a tendency to
become differentially stuc. To free the pipe considerable pull may be necessary.
6( H(,* (n!erna+ pressurewill induce tensional stresses caused by radial e/pansion and, hence,
a/ial contraction.
Esually, mainly the first three parameters are taen into consideration when designing a casing.
%#O8ANC8 FORCE
The effect of buoyancy is generally assumed to be the redu!(on (n e(,*! of the
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casing string when it is suspended in a liquid compared to its weight in air.
The buoyancy or reduction in string weight, as observed on the blocs of the rotarytable, is the resultant of pressure forces acting on all the e/posed horizontal faces of a
body and in calculations is defined as negative as it acts upwards, hence reducing the
pipe weight.
The areas referred to are the tube end areas, the shoulders at point of changing casing
weights and, to a smaller degree, the shoulders on collars. The forces acting on theareas of collar shoulders are for practical purposes negligible in casing design as the
upward and downward facing shoulders countered each other over short distances.
%uoyancy 2orce < Geight in Air / '9ud )ensity1*teel )ensity(
Apparent Geight < Geight in Air / R'*teel )ensity M 9ud )ensity(1*teel )ensityS
Apparent Geight < Geight in Air / %uoyancy 2actor
%uo)an) Fa!or 1 5ud dens(!)6D5s!ee+ dens(!)6
*teel )ensity < 7.J3 g 1 litre *teel *pecific Geight < 7.J3 g force 1 litre
%EN"ING
Ghen calculating tensile loading, the effect of bending must also be considered, if
li bl Th b di f th i dditi l t i th ll f th i
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applicable. The bending of the pipe causes additional stress in the walls of the pipe.
%ending causes tension on the outside of the pipe and compression on the inside of
any bend, assuming the pipe is not already under tension.
%EN"ING
%ending is caused by any deviations in the wellbore from the vertical resulting from
side5tracs, build5ups and drop5off angles. *ince bending load increases the total tensile
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, p p g g
load, it must be deducted from the usable rated tensile strength of the pipe.
2or determination of the effect of bending, the following formula should be used:
T% 17.72 [ \ [ " [ A-
where:
5 < build5up rate or drop5off rate 'degrees 1 !0 m(
5 ) < outside diameter of casing 'in(
5 Af < cross5sectional area of casing 'cm(
5T% < additional tension due to bending 'g force(.
*ince bending load, in effects, increases tensile load at the point applied, it must be
deducted from the usable strength rating of each section of pipe that passes the point of
bending. The section which is ultimately set through a bend must have the bending loaddeducted from its usable strength up to the top of the bend. 2rom that point up to the
top of the section the full usable strength can be used.
E@AMP'E OF %EN"ING CA'C#'ATION
"a!a
K asing: O) -! !1JU, 7 lbf1ft '-07.-4 gf1m(, Af< -!!.= cm, 73, %TC
K )irectional well with casing shoe at ,000 m '9)(
;i ff i t t !00
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K ;ic5off point at !00m
K %uild5up rate: !V1!0m
K 9a/imum angle: !0V reached at 600 mK 9ud weight : -.- gf1dm!
K #ipe body yield strength: -,33J,000 lbf '707 t force(
K )esign factor : -.7 707 1 -.7 < 4-3 t force
Ca+u+a!(on
-( asing weight in air 'Ga(
Ga< -07.-4 / ,000 < -4 t force( asing weight in mud 'Gm(
Gm< -4 / 0.J3= < -J4 t force
!( Additional tension due to the bend(n, e--e!'T%(
T% 17.72 ? $ ? 1$.$:7 ? 1$$.>2 ;$441 , -ore
This stress will be added to the tensile stress already e/isting on the curved section of hole
4( Tension in the casing at !00 m due to its weight< 0.J3= / '-700 / -07.-4( < -36 t force
3( Total tension in the casing at !00 m 'weight in mud W bending( < -36 W J! < != t force6( Tension in the casing at 600 m due to its weight < 0.J3= / '-400 / -07.-4( < -= t force
7( Total tension in the casing at 600 m 'weight in mud W bending( < -= W J! < - t force
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"ESIGN PROCE"#RE
-( alculate the e(,*! o- !*e as(n, s!r(n, (n a(r.
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( alculate the e(,*! o- !*e as(n, s!r(n, (n ud by multiplying theprevious weight by the buoyancy factor '%2( in accordance with the mud
weight in use.
!( Add the additional load due to bup(n, an) een! p+u, to the casing
string weight in mud. Tae into account possible pressurization when
performing operations such as opening1closing )F valves and settingpacers.
No!e T*(s +oad an be a+u+a!ed b) u+!(p+)(n, !*e e?pe!ed bup3p+u,
pressure b) !*e orrespond(n, as(n, (nner se!(on area.
4( In deviated wells, calculate the additional tension due to bend(n,.
3( ompare the obtained weight value to the csg body H* 'including )2 effect(
S#RFACE CASING 400
TENSION
30 -00 t2
%"
4.:.$.$. CASING SE'ECTION E@AMP'E TENSION
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TENSION
asing >ength: 400 mGeight in Air, A%< '400 / -3J.47( < 6!.! t force
Geight in 9ud 'dm< -.-0 g1litre(, ) < 34.4 t force
AC
There is no reliable methods of predicting casing wear and determining the reduction
in casing burst and collapse properties due to the reduction in the thicness of casing
walls. @owever, when needed, some theoretical models can be applied to mae a
4.9.7. CASING "ESIGN CRITERIA CASING &EAR
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, , pp
prediction about possible wear occurrences.
In ver!(a+ e++s, casing wear usually occursin the first few +oints below the wellhead
or in correspondence of intervals with a high dogleg severity 'abrupt variations of
inclination and direction of the wellbore tra+ectory(. In casing design, usually casing
wear in vertical wells is not considered unless specific cases.
In dev(a!ed e++s, wear will occur in correspondence of the build5up and drop5off
sections. The casing covering these sections has to be of a higher grade or heavier
wall thicness, in particular when long drilling times are e/pected.
The ma+or factors affecting casing wear are:
K Cotation of drill pipes
K Tool +oint lateral load and diameter
4.9.7. CASING "ESIGN CRITERIA CASING &EAR
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+
K )rilling rate
K Inclination of the holeK *everity of dog legs
K *teel wear factor.
?? %i5center bit employment
The wear factor depends upon several
variables including:
K 9ud properties
K >ubricants
K )rill solids
K Tool +oint roughness
K Tool +oint hardness.
CORROSION
)uring the drilling phase, if there is any lielihood of a sour corrosive fluid influ/,
consideration should be given to setting a sour service casing string before drilling into
4.9.9. CASING "ESIGN CRITERIA CORROSION
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the reservoir. The %O# stac and wellhead components must also be suitable for sour
service.
O?),en 5O26 O/ygen dissolved in water drastically increases its corrosion action
potential. It can cause severe corrosion at very low concentrations of less than -.0 ppm.
Carbon d(o?(de 5CO26#ressure increases the solubility of Oand lowers the p@ of
drilling fluids, while temperature decreases its solubility raising the p@. orrosion causedby dissolved O is commonly called "sweet$ corrosion. The partial pressure of carbon
dio/ide can be determined by the formula:
Par!(a+ Pressure To!a+ pressure ? Mo+ Fra!(on o- CO2(n !*e ,as.
Esing the partial pressure of O as a reference to predict corrosion, the following
relationships have been found:
K partial pressure Q!0 psi usually indicates high corrosion ris.K partial pressure between 75!0 psi may indicate medium corrosion ris.
K partial pressure X7psi generally is considered non corrosive.
H)dro,en Su+p*(de 5H2S6 @ydrogen sulphide is very soluble in water and when
dissolved behaves as a wea acid and usually causes pitting. Attac due to the presence
of dissolved hydrogen sulphide is referred to as "sour$ corrosion.
4.9.9. CASING "ESIGN CRITERIA CORROSION
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Other serious problems which may result from @* corrosion are hydrogen blistering andsulphide stress cracing.
The S.S.C. 5Su+p*(de S!ress Cra(n,( phenomenon is triggered off when, at
temperatures below J0V and in the presence of tension stress, the @* comes into
contact with @O freeing @W ions. Temperatures above J0V inhibit the *.*..
phenomenon, therefore if the temperature gradient is nown, this may be useful in theselection of the tubular materials most suited to resist @* attac. valuation of the
problem depends on the type of well.
The combination of @* and O is more aggressive than @* alone and is frequently
found in oilfield environments.
It should be pointed out that @* also can be generated by microorganisms '*C%( which
attac sulphur5containing chemicals used in the formulation of drilling fluids
Tepera!ure E--e!s: 2or deep wells, reduction in yield strength must be considered
due to the effect on steel by *(,* !epera!ures. If no information is available on
temperature gradients in an area, a gradient of .35!V1-00m should to be assumed.
4.9.:. CASING "ESIGN CRITERIA OTHER EFFECTS
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Operations at +o !epera!uresrequire tubulars made from steel with high ductility toprevent brittle failures during *EC2A transport and handling.
There is a wide range of casings available from *uppliers, which goes from
plain carbon steel for standard applications to e/otic duple/ steels 'duple/
stainless steels are called "duple/$ because they have a two5phase
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stainless steels are called duple/ because they have a two phase
microstructure consisting of grains of ferritic and austenitic stainless steel(
for e/tremely sour service conditions.
The available casings can be classified under two main specifications:
API SPECIFICATION 5Aer(an Pe!ro+eu Ins!(!u!e6
non3API SPECIFICATION 'materials out of A#I *pecifications have been
developed to meet demand for higher classes of casings able to cope with
e/treme conditions beyond the range of application of A#I casings(
The Aer(an Pe!ro+eu Ins!(!u!e 5API( has an appointed ommittee on
*tandardization of Tubular 8oods which publishes, and continually updates, a
series of *pecifications %ulletins and Cecommended #ractices covering the
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series of *pecifications, %ulletins and Cecommended #ractices covering the
manufacture, performance and handling of oilfield tubular goods. They also
licence manufacturers to use the A#I 9onogram on products which meet with
their published specifications.
The %ulletin regarding tubular goods specifications is the API 7TR C$DISO
10400 "Technical Ceport on quations and alculations for asing, Tubing,
and >ine #ipe Esed as asing or Tubing& and #erformance #roperties Tablesfor asing and Tubing, 2irst dition 'Identical to I*O -0400:007(, dition: 7th
American #etroleum Institute 1 0-5)ec500J 1 !7J pages 'based on the %ull
3 "%ulletin on #erformance #roperties of asing, Tubing and )rill #ipe$,
-st dition, October -===(.
It should not be interpreted that only A#I tubulars and connections may be
used in the field& in fact, there are situations, which can be solved only if
special materials, not yet addressed by A#I specifications, are used.
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special materials, not yet addressed by A#I specifications, are used.
@owever, when using non5A#I pipes and connections, the designer mustchec the methods by which the strengths have been calculated. Esually it will
be found that the manufacturer will have used the published A#I formulae
'reported in the above mentioned A#I 3TC !(, baced up by tests to prove if
the performance of the product conforms or e/ceeds these specifications.
*ometimes, the non5A#I manufacturers claim the performances of theirmaterials be better than those calculated using the A#I formulae& in these
circumstances, claims must be critically e/amined by the designer.
If needed, an end5user can specify more stringent chemical, physical and
testing requirements on orders.
A casing is classified according to:
ou!s(de d(ae!er: generally e/pressed in inches
4.:.1. API CASING SPECIFICATIONS
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no(na+ un(! e(,*!: e/pressed in lb force 1 ft 'g force 1 m(
,rade o- !*e s!ee+: indicated by a letter ';, #, Y( and a number '40, 73, --0,
etc.(, which represents the yield strength of the material divided by -000
a++ !*(nessand (ns(de d(ae!er&
e*an(a+ proper!(esof pipe body and couplings
!)pe o- onne!(on: A#I Cound Thread '*hort or >ong(, %uttress, /treme
>ine
An e/amples of casing designation is as follows: = 31J$ #--0 47 lb force 1 ft
%T Z73J 'shoe depth in feet(
API PRO"#CTS
There are eleven A#I casing grades and seven tubing grades.
4.:.1.2. API CASING GRA"ES
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CASINGS T#%INGS
H340 H340
]377 ]377
377
M397
N3;0 N3;0
'3;0 '3;0
C3>0 C3>0
C3>7
T3>7 T3>7P3110 P3110
3127
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4.:.1.$. API CASING CONNECTIONS
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NON3#PSET CO#P'ING
#PSET CO#P'ING
API THREA"E" CONNECTIONS
The four most common A#I threaded connections are:'(ne P(pe T*reads ; R d S* ! '*TN( d ' '>TN( T* d
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; Round S*or!'*TN( and 'on,'>TN( T*reads
%u!!ress'%T(E?!ree '(ne T*reads
>ong and short threads are identical e/cept for thread length.
These connections are of a type called interference connections, i.e. the
connection seals by the wedging action 'interference( of two tapered surfacescoming into contact. @owever the surfaces are not in full contact over their
entire area. *ealing of the clearance area of both eight round and buttress
threads is accomplished with A#I modified thread lubricant 'and sometimes by
"premium$ thread lubricants.(
'INE PIPE THREA"S
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RO#N" 5'ONG AN" SHORT6 THREA"S4.:.1.$. API CASING CONNECTIONS
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%#TTRESS THREA"S
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E@TREME3'INE THREA"S4.:.1.$. API CASING CONNECTIONS
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4.:.1.4. API CASING TA%'ES
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API Ran,e 'en,!*B
C*e(a+
Copos(!(on
o- API
Tubu+ars
4.:.1.4. API CASING TA%'ES
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04. CASING PROGRAMME
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4.:.2. NON3API CASING SPECIFICATIONS
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CASING PROFI'E
4.:.$. CASING SE'ECTION E@AMP'E
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1. CAON"#CTOR PIPE $0= AT 70
2. S#RFACE CASING 20= AT 400
$. FIRST INTERME"IATE CASING 1$ $D;= AT 1200
4. SECON" INTERME"IATE CASING > 7D;= AT 2700
7. PRO"#CTION CASING := AT $000
20= CASING4.:.$. CASING SE'ECTION E@AMP'E
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20= CASING4.:.$. CASING SE'ECTION E@AMP'E
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S#RFACE CASING 20= AT 400
Resu+!(n, burs! +oad 5Ne! Pressure6Internal #ressure 5 /ternal #ressure at wellhead, C 140 0 140 ,-D2
4.:.$.1. CASING SE'ECTION E@AMP'E %#RST
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at wellhead, C 140 0 140 ,-D
at bottomhole, F 9>.2 44.0 27.2 ,-D2
%#RST RESISTANCE
"es(,n Fa!or 1.07
[533 =4 lbf1ft: --0 psi
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5at surface, A 05at bottomhole, %< 44 3 0 44 ,-D2
CO''APSE RESISTANCE
"es(,n Fa!or 1.10
[533 =4 lbf1ft: 30 psi < !6 gf1cm, ')2
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]377 109.7 +b-D-!'-3J.47 g force 1 m(asing body Hield *trength: -6J3000 lbf < 74= t
force
')2 < -.70( 440.3 t force
asing >ength: 400 m
Geight in Air, A%< 400 / -3J.47 < 6!.! t force
Geight in 9ud 'dm< -.-0 g1litre(, asing GG 0 Z >06 vs 440.7 OAC
]77
10
9.7
+bD-!E
440
!
1$ $D;== CASING
4.:.$. CASING SE'ECTION E@AMP'E
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1$ $D;== CASING
4.:.$. CASING SE'ECTION E@AMP'E
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INTERME"IATE CASING 1$ $D;= a! 1200
Resu+!(n, burs! +oad 5Ne! Pressure6Internal #ressure 5 /ternal #ressure5at surface. C 140 3 0 140 ,-D2
4.9.1. CASING "ESIGN CRITERIA %#RST
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5at J00 m, 200.0 3 ;2.4 11:.9 ,-D25at -000, 21; 3 120 >; ,-D2
5at -00, 2$4 3 197 9> ,-D2
%#RST RESISTANCE
"es(,n Fa!or 1.07
]377 74.70 +b-D-! 2:$0 ps( 1>2 ,-D2B5"F1.0761;$ ,-D2
[533 6- lbf1ft: !0=0 psi
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at -06=.4 m, F 17;.$ ,-D
5at -00, E 1::.9 3 2$.7 174.1 ,-D2
[533 34.30 lbf1ft: --!0 psi
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N3;0 :2 +b-D-! 510:.1$ ,-D6
P(pe %od) 8(e+d S!ren,!* 1.991.000 +b-
:72 ! -pre 5"F 1.:06 442 ! -ore
asing >ength: -00 m
Geight in Air A% < -00 / -07.-! < -J.3 t
force
Geight in 9ud 'dm< -.4J g1litre(, '%2
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OTHER S'I"ES
Ghen needed, the position and magnitude of volumetric wear in a casing string can be
theoretically estimated by calculating the energy imparted by the rotating tool +oints to
the casing at different casing points and dividing this by the amount of energy required
to wear away a unit volume of casing. The percentage of casing wear at each point
along the casing is then calculated once the volumetric wear has been obtained.
4.9.7. CASING "ESIGN CRITERIA CASING &EAR
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The vo+ue!r( ear is proportional to an empirical "wear factor$, which is defined as
the coefficient of friction divided by the volume of casing material removed per unit of
energy input. The Gear Folume, F, equals:
590 ? ^ ? F ? ' ? " ? N ? S6DPwhere:F < Gear Folume #er 2oot 'in!1ft(
2 < Gear 2actor 'in1lbf(> < >ateral >oad on )rill #ipe #er 2oot 'lbf1ft(
) < Tool [oint )iameter 'in(
D < Cotary *peed 'C#9(* < )rilling )istance 'ft(
# < #enetration Cate 'ft1hr(
No!e T*e *e(a+ a!(on o- ,ases su* as H2SB CO2 and O2!ends !o redue !*e
sur-ae *ardness o- s!ee+ andB !*usB on!r(bu!es s(,n(-(an!+) !o !*e ra!e o- ear.
The reoended proedure to determine the wear effect on casing mechanical
performances is the following
-( conduct the casing design as previously seen&
( in correspondence of the wear points, calculate the allowable reduction in wall5
4.9.7. CASING "ESIGN CRITERIA CASING &EAR
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thicness of the casing that maes the burst or collapse resistance of the casing equal tothe calculated burst or collapse loads, including the appropriate )esign 2actors&
!( estimate the wear rate in terms of loss of wall thicness per operating day. A wear
percentage below 7P is considered acceptable&
4( from the loss in wall thicness and the rate of wear, calculate the allowable operatingtime in the casing string:
5 If the allowable operating time is less than the planned operating time, use
heavier casings or increase their grade -00 m above and to 60 m below the
wear point until the allowable operating time e/ceeds the anticipated operating
time.
5 If the allowable operating time is greater than the planned operating time donot include a wear allowance.
"ETECTION OF CASING &EAR
)etecting casing wear can be achieved by two methods:
K use of magnets in the mud flow return in order to catch any piece of metal present in the
drilling fluid.
K recording of caliper logs immediately after the casing has be run into the hole in order to
4.9.7. CASING "ESIGN CRITERIA CASING &EAR
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provide a base log. A wear log can then be run at any time while drilling the ne/t holesection.
CASING &EAR RE"#CTION
If there are fears about casing wear, the following drilling practices can be applied to
minimize this occurrence:K use of down hole motors and turbines&
K use of rubber protectors on drill pipes&
K use of drill pipes without hard5facing&
K minimization of doglegs&
K minimization of sand content inside the mud&
K use of oil5based mud.
API PRO"#CTS
H340
@540 is the lowest strength casing and tubing grade in the specifications, with minimum yield strength
of 40 si, and a minimum tensile strength of 60 si. @540 is a carbon type steel