03.Politecnico Di Torino Abnormal Pressures 2010

8/9/2019 03.Politecnico Di Torino Abnormal Pressures 2010

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CHAPTER 3

ABNORMAL PRESSURES:

THEIR ORIGINS, DETECTION ANDPREDICTION

03. ABNORMAL PRESSURES

1


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2

As said, for an adequate characterization of a formation from a pressure regimestandpoint, the following parameters have to be determined:

Overburden Preure, POverburden Preure, P!v!vP!re Preure, PP!re Preure, P""#r$%&ure Preure, P#r$%&ure Preure, P'r'r

These pressures are strictly dependent one from the other. In fact, pore pressuresand overburden pressures are related between them by the compaction process inaccordance with the effective stress principle and together allow the calculation offracture pressures.

The overburden and pore pressures are lined together through the so callede''e%&(ve "reure. It represents how the forces, due to the weight of thesediments and acting on a certain area laying at a defined depth, are distributed

between the solid and the liquid components of the considered roc. The effectivepressure, )orP%, is, therefore, given by:

))*!r*!r PP%%++ P P!v!v- P- P""


1. BASIC CONCEPTS1. BASIC CONCEPTS


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!

O/ERBURDEN !r GEOSTATIC !r LITHOSTATIC PRESSUREO/ERBURDEN !r GEOSTATIC !r LITHOSTATIC PRESSURE ( &e

"reure eer&ed !n $ 2(ven '!r$&(!n b4 &e 5e(2& !' ed(en&$v(n2 $n $ver$2e den(&4 e6u$7 &! 8b, &$& e&end 'r! &e ur'$%e &!

&e %!n(dered de"&, H:

PP!v!v * *88bb9 H+ 109 H+ 10

where:

"ov# overburden pressure, g$%cm2

& # depth, m'b# average sediment density, g%cm

!


3

1.1. Overburden Preure1.1. Overburden Preure


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(

P!v 2989 9 ;239 > N;2. ;239 > N

> P$

If:

'b# 2 g%cc # 2))) g%m!

h # *) m

"ov# +.)-- 2))) *) #*+-.*!2 "a # 2 g%cm2

* g%cm2# +.)-- "a

"ov# /' &0%*) # /2 *)0%*) # 2 g%cm2

"ov# /g%dm!0 m # /g%*))) cm!0 *)) cm # *%*) /g%cm20



?


5/2131

Ther!%; den(&4r!%; den(&4orbu7; den(&4bu7; den(&4is given by the density of the matri /solid part0

multiplied by the fraction of the volume occupied by the matri plus the density ofthe fluid contained in its pores multiplied by the fraction of volume occupied by the

fluid /porosity0:

8b @ 9 8'7 *1 @+ 9 8$

where:'b # roc density /bul density0, g%cm!

# roc porosity, fraction'ma # roc matri density,g%cm!

'fl # density of the fluid contained in the pores, g%cm!




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THE BUL DENSIT CONCEPTTHE BUL DENSIT CONCEPT

# 3


8b @ 9 8'7 *1 @+ 9 8$


7/2134


T4"(%$7 Den(&(e !' R!%; $nd #7u(d


F


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The overburden pressure consists of two components: the $&r( !re''e%&(ve "reure, ) *!r P%+, and the"!re "reure, P", and is epressedby the Terzaghi relationship:

PP!v!v P P"" ) )

The total overburden pressure, *P!v+, at any depth can be calculated from theoverlying roc bul densities and cumulative pressures. 5ince the pore fluidpressure may be nown or closely estimated in a normally pressuredsequence, the matri or effective pressure, ), can be found by subtraction:

) P) P!v!v- P- P""



=


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+

$or !verburden 2r$d(en&, G!v

, it is meant the rate with which theoverburden pressure changes with depth and is given by the followingrelationship:

GG!v!v *P *P!v!v9 10+H9 10+H

where:6 7ov# overburden gradient, g%cm2%*)m6 "ov# overburden pressure, g%cm2

6 & # depth, m



<


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*)

89"T&

b2.!*

89"T&

b2.!*

The bul density of rocs normally increases with increasing depth. hen other sourcesare not available an average value equal to 2.!* g%cm!/* psi%ft0 is often taen, though itis very approimate. ;ffshore the overburden gradient, if referred to rotary table height,strongly depends on


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**

$or4dr!&$&(% "reure4dr!&$&(% "reure,, PP,, is meant the pressure eerted by a column of

water at rest having a certain density and is given by the equation:

PP *8 *8'7'7 9 H+ 109 H+ 10

where:6 "h# formation or pore pressure, g%cm2

6 'fl# density of water, g%cm!

6 & # height of the water column, m

1.. N!r$7 H4dr!&$&(% Preure1.. N!r$7 H4dr!&$&(% Preure


11


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*2

ATER TPE SALINIT ASC7-

SALINITAS N$C7

DENSIT

mg%> mg%> g%cm!

$resh ater )6*,1)) )62,1)) *.))

5ea ater /9ample0 *,))) !),))) *.)2

$ormation aters6?orth 5ea6Italy ;ffshore67ulf @oast, 5A6Barents 5ea

*,)))24,)))-!,2))4),1))

!),)))(1,)))

*)1,)))**4,)))

*.)2*.)!*.)4*.)

5alt 5aturated ater *+2,--4 !*4,+)) *.2)

S$7(n(&4 $nd Den(&4 !' S!e T4"e !' $&erS$7(n(&4 $nd Den(&4 !' S!e T4"e !' $&er

1.. N!r$7 H4dr!&$&(% Preure1.. N!r$7 H4dr!&$&(% Preure

The pressure gradient for water in 4dr!&$&(%conditions is tied to 5$&er den(&4/and usually is taen equal to *.)! g%cm2%*) m0. It will depend on formation watertemperature, pressure and salinity.


1


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*!

P!re PreureP!re Preure,, also called #!r$&(!n Preure#!r$&(!n Preure, ", "pp,, is the pressure of the fluid

contained in the pore spaces of the rocs.

In a sedimentary basin three categories of "ore "ressure can be encountered:

Ne2$&(ve "reure $n!$74Ne2$&(ve "reure $n!$74/subnormal pressure or underpressure0:PP"" P P

N!r$7 "reureN!r$7 "reure:: PP"" P P

P!(&(ve "reureP!(&(ve "reure$n!$74$n!$74/overpressure0:PP"" P P

The timely and reliable detection and quantification of overpressures is fundamentalfor safe and cost6efficient drilling operations. A great deal of efforts have to be made

to properly predict abnormally pressured formations.

1.3. P!re Preure1.3. P!re Preure


13


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*(



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*1

P!re Preure prediction uses the Terzaghi stress relationship between

total stress, effective stress and the pore pressure, in the simplifiedequation, as already seen:

PP"" P P!v!v ) )

where:6 "p# pore pressure, g%cm2

6 "ov# overburden pressure, g%cm2

6 C /or "c0 # effective stress or effective compaction pressure, g%cm2



1


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*-

The"!re "reure 2r$d(en&"!re "reure 2r$d(en&indicates the rate with which the pore pressure

changes with depth and is given by:

GG"" *P *P""9 10+ H9 10+ H

where:6 7p# pore pressure gradient, g%cm2%*) m6 "p# pore pressure, g%cm2

6 & # depth, m.



1>


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*4

#r$%&ure Preure#r$%&ure Preure is the pressure necessary to fracture a formation. This

pressure is a function of the overburden pressure, the pore pressure and themechanical properties of the roc matri /epressed by the D coefficient0.

The general and empirical formula for fracture gradient is:

PP'r'r P P"" *P *P!v!v P P""++

where:6 "fr# fracture pressure, g%cm2

6 "p# pore pressure, g%cm2

6 "ov# overburden pressure, g%cm2

6 D # matri stress coefficient

1.?. #r$%&ure Preure1.?. #r$%&ure Preure


1F


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*

The matri stress coefficient, , reflects the mechanical behaviour of the rocmatri and is a function of theP!(!nJ R$&(!P!(!nJ R$&(!,,

::

*1 -*1 - ++

hen a material is stretched or compressed, its cross6sectional area changes aswell as its length. "oissonEs Fatio is the constant relating these changes indimensions and is obtained by dividing the change in width per unit width by thechange in length per unit length as given by the equation:

* *KKCC+*CC+*KKLL+ - *LL+ - *&r$n&r$n

$($7$($7

+ - *+ - *

44++



1=
http://upload.wikimedia.org/wikipedia/commons/6/67/Poisson_ratio_compression_example.svg


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*+

7enerally a roc can be uncemented /i.e. sand, silt0, elastic /i.e. carbonates,shales, sandstones0 or plastic /evaporites, clays0.

The values of for different types of rocs are the following:

0.0 for loose and unconsolidated formations lie sands or superficialintervals of offshore wellsG

0.>Ffor elastic formations /shales, sandstones, carbonates0G

0.F for elastic deep formations /carbonates, sandstones0G

1.00 for plastic formations /clays, evaporites0G

default value fordefault value for ( 0.>F ( 0.>F.



1<


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2)

The #r$%&ure Gr$d(enr$%&ure Gr$d(en& represents the rate of change of the fracture

pressure versus depth:

GG'r'r *P *P'r'r9 10+ H9 10+ H

where:6 7fr# fracture gradients, g%cm2

6 "fr# fracture pressure, g%cm2

6 & # depth, m



0


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2*

In 9ni 9H" operations, the following epressions are used to determine thefracture gradient:

a0 for a roc having an e7$&(% be$v(!ur, :

G'r G" * + *G!v G"+ 1-b0 If the drilling fluid is 5$&eror invades in depth the formation:

G'r G" * + *G!v G"+

c0 If the roc has a "7$&(% be$v(!ur:

G'r G!v



1

1 P G d( & C * d & +


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22

0

1000

000

3000

?000

1,0 1, ,0 ,

OverburdenGr$d(en&

*;2%10+

P!re PreureGr$d(en&

#r$%&ureGr$d(en&

1.. Preure Gr$d(en& Curve *v de"&, +1.. Preure Gr$d(en& Curve *v de"&, +


1 > M U (


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2!

Preure :Preure : ;2%;2%*e&r(%+*e&r(%+"( *API+"( *API+

P$ !r NP$ !r N*SI+*SI+

Preure Gr$d(en& :Preure Gr$d(en& : ;2%;2%10 e6u(v$7en& &! ;2L !r 2%% *2+ *e&r(%+10 e6u(v$7en& &! ;2L !r 2%% *2+ *e&r(%+"('& *API+"('& *API+7b'2$7 *API+7b'2$7 *API+""2 e6u(v$7en& &! 7b'2$7 *API+""2 e6u(v$7en& &! 7b'2$7 *API+NN33!r P$ *SI+!r P$ *SI+

#7u(d Den(&4 :#7u(d Den(&4 : ;2L !r 2L !r 2%% *e&r(%+;2L !r 2L !r 2%% *e&r(%+7b2$7 ""2 *API+7b2$7 ""2 *API+;2;233*SI+*SI+

1.>. Me$ureen& Un(&1.>. Me$ureen& Un(&


3

1 > M U (1 > M & U (&


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2(

1.>. Me$ureen& Un(&1.>. Me$ureen& Un(&

PRESSURE UNITS

"$%$7*P$+

b$r*b$r+

&e%n(%$7 $&!"ere

*$&+

$&!"ere*$&+

&!rr*T!rr+

"!und-'!r%e "er6u$re (n%

*"(+1 P$ 1 N 10 1.01103 1?.0?10>

1 b$r 100,000 10>d4n% 1.01. 0.> 1 ;2'% 0.F=? F3.> 1?.3

1$& 101,3 1.013 1.033 1 $& F>0 1?.>

1&!rr 133.3 1.33310

3 1.3


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21

. ORIGINSO#

ABNORMAL PRESSURES


1 Ab 7 P G &( M ( 1 Ab 7 P G &( M (


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Abnormally pressured formations can be encountered everywhere in the world from"leistocene to @ambrian.

7eological environments can be seen as:

!"en 4&e!"en 4&e: formed by permeable formations, which allow hydrostaticconditions to be reachedG

%7!ed 4&e%7!ed 4&e: constituted by formations which prevent or greatly reduce fluid

transfer and thus causing abnormal pressures to occur.

?ormally pressured rocs and abnormally pressured rocs only coeist if separatedby

1 Ab 7 P G &( M ( 1 Abn!r$7 Preure Gener$&(!n Me%$n(


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The most common types of


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2

The mechanisms, that have been proposed during the years to eplain theThe mechanisms, that have been proposed during the years to eplain thegeneration of overpressures in a specific field situation, can be grouped intogeneration of overpressures in a specific field situation, can be grouped intofour main categories, as suggested by 5warbric and ;sborne /*++-0:four main categories, as suggested by 5warbric and ;sborne /*++-0:

&re-re7$&ed e%$n(&re-re7$&ed e%$n(, which cause the compression of the rocs with, which cause the compression of the rocs withpore volume reduction, such aspore volume reduction, such as %!"$%&(!n%!"$%&(!n d(e6u(7(br(ud(e6u(7(br(u/vertical loading/vertical loading

stress0 orstress0 or

&e%&!n(%&e%&!n(%

/lateral compressive stress0G/lateral compressive stress0G

'7u(d v!7ue (n%re$e e%$n('7u(d v!7ue (n%re$e e%$n(, which determine an increase in, which determine an increase involume of the fluids within the pores of a roc, transformed, then into pressurevolume of the fluids within the pores of a roc, transformed, then into pressureincrease in case the volume increase is restricted. 9amples are:increase in case the volume increase is restricted. 9amples are: &e"er$&ure&e"er$&ure

(n%re$e(n%re$e, water release due to mineralogical transformations of rocs, water release due to mineralogical transformations of rocs//d($2ene(d($2ene(0,0, 4dr!%$rb!n 2ener$&(!n4dr!%$rb!n 2ener$&(!n,, b(&uenb(&uen andand !(7 %r$%;(n2 &! 2$!(7 %r$%;(n2 &! 2$GG


=

.1. Abn!r$7 Preure Gener$&(!n Me%$n(.1. Abn!r$7 Preure Gener$&(!n Me%$n(

1 Abn!r$7 Preure Gener$&(!n Me%$n( 1 Abn!r$7 Preure Gener$&(!n Me%$n(


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2+

'7u(d !veen& $nd bu!4$n%4 e%$n('7u(d !veen& $nd bu!4$n%4 e%$n(, which cause the movement, which cause the movementof fluids from a formation to another with increase in pressure, if these etraof fluids from a formation to another with increase in pressure, if these etravolumes of fluids are not accommodated with an increase in volume of thevolumes of fluids are not accommodated with an increase in volume of thereceiving formations. 9amples of these mechanisms are:receiving formations. 9amples of these mechanisms are: !!(!!(,, 4dr$u7(%4dr$u7(%!r $r&e($n "reure!r $r&e($n "reure,, bu!4$n%4 !' 4dr!%$rb!nbu!4$n%4 !' 4dr!%$rb!nabove water due to densityabove water due to densitycontrastsGcontrastsG

red(&r(bu&(!n !' !ver"reured '7u(dred(&r(bu&(!n !' !ver"reured '7u(d, originated by one of the mechanism, originated by one of the mechanismcategories mentioned above, from one formation to another. This occurrence,categories mentioned above, from one formation to another. This occurrence,referred to asreferred to as &r$n'eren%e&r$n'eren%e, though not a real mechanism in itself, may all the, though not a real mechanism in itself, may all thesame eert a strong influence on many of the pore pressure profiles seen in thesame eert a strong influence on many of the pore pressure profiles seen in thesubsurface and may, sometimes, mas the recognition of the true mechanismsubsurface and may, sometimes, mas the recognition of the true mechanismwhich has originated the pressure anomaly.which has originated the pressure anomaly.


.1...1. D($2ene(!' C7$4!' C7$4


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S%e$&(% Re"reen&$&(!n !'M!n&!r(77!n(&e S&ru%&ure

/from 5750

L(N$RbCM2C$SrB$H


?F

.1...1. D($2ene(!' C7$4!' C7$4


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D4n$(%S&ru%&ur(n2 !'

$&er (nM!n&!r(77!n(&e


?=

.1...1. D($2ene(!' C7$4!' C7$4


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A. SMECTITE DEHDRATIONA. SMECTITE DEHDRATION

5everal authors have proposed that e%&(&e de4dr$&(!n ( &$2ed (n &5!!r &ree "u7eand that these pulses of released water were instrumental indriving hydrocarbon from source rocs to traps.

The overall volume change accompanying the comple smectite6illite reactionis not well6nown, but it is thought to be only around (K, not enough to createsignificant abnormal pressures.

The smectite dehydration reaction is therefore considered to be a secondaryrather than a maor cause of overpressures, but may be additive tooverpressures generated by the compaction disequilibrium mechanism.


?<

.1...1. D($2ene(!' C7$4!' C7$4


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Te!r(e !'

M!n&!r(77!n(&eDe4dr$&(!nMe%$n(


0

.1...1. D($2ene(!' C7$4!' C7$4


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B. SMECTITE-ILLITE TRANS#ORMATION

@lay, such as 5mectites, can adsorb huge amounts of water due to an

imbalance in their ionic charge. 8uring burial, 5mectites alter chemically byaddition of Al and D ions and the release of ?a, @a, Jg, $e and 5i ions pluswater, to produce Illite, which does not show the same capacity to adsorbwater. The reaction is inetically controlled and is dependent on the combinedeffects of time and temperature as well as the influence of mineral fabric andpermeability. The transition from 5mectite to Illite occurs at temperatures

between 4)6*1)o

@ and appears to be independent of sediment age and burialdepth. 5ome authors thin that the transformation 5mectite6Illite can cause avolume increase of about 21K, which, accompanied by a concomitant changein the physical characteristics of the roc, determines a release of boundwater with an increase in roc compressibility. If the roc compressibilityincreases, the overburden induces additional compaction requiring epulsion

of the newly released water to achieve equilibrium. If this is impeded, due tothe low permeability of the roc, then overpressure will result, i.e. compactiondisequilibrium due to mineral dehydration.


1

.1...1. D($2ene(!' C7$4!' C7$4


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SMECTITE-ILLITE TRANS#ORMATION

Another consequence of the transformation of 5mectite into Illite is the releaseof 5ilica. This 5ilica can further reduce, according to some authors, thepermeability of the shales to produce a hydraulic seal and hence potentially atransition zone into overpressures.

;thers thin that the reduction in permeability of Illite is due to therearrangement of the 5mectite platelets into Illite ordered pacetsG this lowerpermeability would help to retain fluids and contribute to the preservation ofoverpressure, but would not be responsible for its initiation.


.1...1. D($2ene(!' C7$4!' C7$4


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3

.1...1. D($2ene(!' C7$4!' C7$4


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M!n&!r(77!n(&eTr$n'!r$&(!n (n&! I77(&e


?

.1...1. D($2ene(!' C7$4!' C7$4


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M!n&!r(77!n(&eTr$n'!r$&(!n

(n&! I77(&e


.1...1. D($2ene(!' C7$4!' C7$4


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S&ru%&ure $nd Pr!"er&(e !' &e M!& C!!n C7$4 M(ner$7

Pr!"er&4 $!7(n(&e M(%$ M!n&!r(77!n(&e A&&$"u72(&e C7!r(&e

L$4er T4"e 1:1 :1 :1 :1 :1:1

Cr4&$7 S&ru%&ure ee& ee& ee& ee& ee&

P$r&(%7e S$"e e$2!n$7"7$&e

e&en(ve"7$&e

'7$;e need7e "7$&e

P$r&(%7e S(e, -0. 7$r2e ee&&! 0.

-0.1 1-0.1 -0.1

Sur'$%e Are$BET*N+, 2

BET *HO+, 2

1---- 0-110--- 30-=000-=00 00--- 1?0---

CEC, e6100 2 3-1 10-?0 =0-10 1- 10-?0

/(%!(&4 (n $&er 7!5 7!5 (2 (2 7!5

E''e%& !' S$7& '7!%%u7$&e '7!%%u7$&e '7!%%u7$&e 7(&&7e !r n!ne '7!%%u7$&e


>

.1.... Su7"$&e D($2ene(.1.... Su7"$&e D($2ene(


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14

The dehydration of G4"u, which is @alcium 5ulphate 8i6hydrate/@a5;(L2 &2;0, into An4dr(&e/@a5;(0 is a reaction taing place during

the post6sedimentary diagenetic processes at temperatures around ()6-)o@ and ambient pressures. 5uch reaction involves the release of boundwater and a ()K volume reduction of 7ypsum. If this water can not flowfreely due to the presence of permeability barriers, overpressures develop.

The "en!en!n ( rever(b7e: Anhydrite may become hydrated with aconsequent *1621K volume increase if Anhydrite is only partially hydrated/@a5;(L*%2 &2;0 up to ()K volume increase if Anhydrite is completelytransformed into 7ypsum /@a5;(L2 &2;0.


F

.1...3. Se%!nd$r4 Pre%("(&$&(!n !' Ceen&$&(!n M$&er($7.1...3. Se%!nd$r4 Pre%("(&$&(!n !' Ceen&$&(!n M$&er($7


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1

The precipitation of materials dissolved in formation waters may developoverpressures in a 4dr$u7(%$774 %7!ed 4&e, that was initially under

hydrostatic conditions. The precipitation of %$rb!n$&e%$rb!n$&e,, u7"$&eu7"$&e,,%7!r(de%7!r(de,, (7(%$(7(%$ etc. can cause a reduction of the pore space withconsequent pressurization of the fluids contained in it. The process startswhen there is a variation of the factors that maintain the salts in solution,such as: temperature, pressure, composition of the solution, concentration ofits components.

;verpressure conditions can be also created in a 4dr$u7(%$774 !"en4&e, if the precipitation of salts decreases the permeability of the roc tosuch an etent that the flow of the fluids present in the pores before burial is

somehow impeded.


=

.1...?. De%!"!(&(!n !' Or2$n(% M$&&er.1...?. De%!"!(&(!n !' Or2$n(% M$&&er


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1+

At shallow depths, organic matter contained in the sediments is broen down byb$%&er($7 $%&(!nb$%&er($7 $%&(!n, generating biogenic methane. In a %7!ed env(r!nen& theresulting epansion can lead to overpressure /trapped gas pocet at very shallowdepth0.

ith increase of depth, bacterial activity decreases, giving way to &er!-&er!-%e(%$7 %r$%;(n2%e(%$7 %r$%;(n2 /transformation of heavy product to a lighter one under theinfluence of high temperature0. The cracing process creates hydrocarbons fromorganic matter and also produces light hydrocarbons from heavy one. The

transformation increases the total volume of molecules and therefore the volumethey occupy. If this phenomenon occurs in a %7!ed env(r!nen&, it can causepressure to rise and consequently ;verpressure.

Derogen maturation to oil and%or gas taes place at 4)6*2)o@ and oil cracing togas at +)6*1)o@.


<

.1.3. #7u(d M!veen& $nd Bu!4$n%4 Me%$n(.1.3. #7u(d M!veen& $nd Bu!4$n%4 Me%$n(


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-)

The mechanisms related to fluids movement and buoyancy are:The mechanisms related to fluids movement and buoyancy are:

OSMOSISOSMOSIS

ARTESIAN PRESSURESARTESIAN PRESSURES

HDROCARBONS BUOANCHDROCARBONS BUOANC


>0

.1.3.1. O!(.1.3.1. O!(


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-*

O!(O!(is defined asthe spontaneous flow of a solvent (usually water)through a semi-permeable membrane separating two solutions of

different solute concentration (or between a pure solvent and a solution)until the concentration of each solution (that is their chemical potential)

becomes equal or until the development of osmotic pressure prevents

further movement of water molecules from the solution at lower

concentration to that at higher concentration=.

A @lay layer between two permeable reservoirs containing formationwater of differing salinity may become a semi6permeable membranecausing an osmotic phenomenon.

In a closed environment, ;smosis will create an increase of fluid

pressure in the reservoir containing high salinity water.


>1

8uring the years, it has been demonstrated that the flow of water through a clay bed isdependent on many factors as the following:

.1.3.1. O!(.1.3.1. O!(


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-2

dependent on many factors, as the following:differential pressure across the clay bedGdifferential concentration of the two solutions separated by the clay bedGtheir differential electric potentialGtemperatureGthicness of the clay /a thic bed does not act as a semi6permeable membrane0Gsize of the micropores of the clay bedGdegree of fissuring of the clay bed.

It has been also observed that the efficiency of a clay as semi6permeable membrane

increases with its cation echange capacity, @9@cation echange capacity, @9@, which is as to say with its purity. @9@ isthe capacity of a soil for echange of cations between the soil and the its solution. Thequantity of cations that a clay mineral or similar material can accommodate on its negativelycharged surface is epressed as milli6ion equivalent per *)) g, or more commonly asmilliequivalent per *)) g /meq%*)) g0. @lays are aluminosilicates in which some of the Al and5i ions have been replaced by elements with different valence. $or eample, Al!3 may bereplaced by $e23or Jg23, leading to a net negative charge, which attracts cations when the

clay is immersed in an electrolyte such as salty water and causes an electrical double layer.


>

.1.3.1. O!(.1.3.1. O!(


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-!03. ABNORMAL PRESSURES

>3

Th & &( & ( d 7( d lti f l ti f th

.1.3.. Ar&e($n Preure.1.3.. Ar&e($n Preure


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-(

The "!&en&(!e&r(% !r 4dr$u7(% e$d resulting from elevation of thewater table in highland regions /charge areas0 eerts a pressure in thesubsurface if the reservoir%aquifer is overlain by a seal.

ells drilled in the over6pressured aquifers are nown as $r&e($n 5e77and will produce water flow at the surface due to the ecess pressure.


>?

300 P *H +10 *0 1 00+10 ;

.1.3.. Ar&e($n Preure.1.3.. Ar&e($n Preure


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11 6565

Sand

Sand

Sand

Sand

S$7e

PTR 0

300

-0

S$7e

H1 H

PH1 *H198'7+10 *091,00+10 ;2%

PH *H98'7+10 *091,00+10 ;2%

KP H1-H 30 ;2%

KH

Eni Corporate University 03. ABNORMAL PRESSURES>

All gases and most oils have a density lower than that of water and hence have a

.1.3.3. H4dr!%$rb!n Bu!4$n%4.1.3.3. H4dr!%$rb!n Bu!4$n%4


66/213


>>

lower pressure gradient. 5ince overpressure is the ecess pressure abovehydrostatic for a given depth, there is always a certain amount of overpressurewhere a column of gas or oil is present. This mechanisms is restricted to structural

and stratigraphic traps of hydrocarbons and can not cause regional overpressures.The amount of overpressure within the hydrocarbon accumulation is a function ofthe pressure gradients of oil, gas and water and the height of the hydrocarboncolumnG it increases from the water contact upwards.

The pressure at the top of the reservoir /hydrocarbon column0,", is :", is :

P P 8855 99H10 10H10 10 99**8855 88%%+ *P (n ;2%+ *P (n ;2%++

where:&: height of the top of reservoir, mh: height of hydrocarbon column /m0'w: density of water, g%cc'hc: density of the hydrocarbon /gas: ).21 g%ccG oil: 3%6 ). g%cc0


>>

.1.3.3. H4dr!%$rb!n Bu!4$n%4.1.3.3. H4dr!%$rb!n Bu!4$n%4


67/213


>F

000 PP *000 9 1.03+10 0> ;2%Q GP *0>000+ 9 10 1.03 ;2%10

100 PP 0> - *1.03 9 0010+ 1?. ;2%Q GP*1?.100+ 9 10 1.03 ;2%10

1000 PP 1?. - *0.F 9 0010+ 11


68/213


>=

Tr$n'eren%e ( &e red(&r(bu&(!n !' e%e "!re "reure (n &eubur'$%e. Although not a primary mechanism to create overpressure

within a sedimentary basin, transference can be the principal control onthe distribution of overpressure found there. $luid movement is drivenby differences in ecess pressure and controlled by the permeability ofthe roc.

?ormally pressured reservoirs, especially at a minor depth, may

sometimes be pressurized, which means that they may be brought tohigher than normal pressures, as a result of 4dr$u7(% %!un(%$&(!n&$& %$n be e&$b7(ed 5(& dee" '!r$&(!n $& (2er "reure.5uch communication may tae place along non6sealing faults orbecause of a poor cementing of intermediate and production casingstringsand for various other reasons.


>=

.1.3.. O&er C$ue !' Over"reure.1.3.. O&er C$ue !' Over"reure


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>

@auses of pressures below the hydrostatic trend could be:

DI##ERENTIAL DISCHARGE

DI##ERENTIAL GAS #LO

ROC DILATANC

OSMOSIS

THERMAL E##ECTS

PROLONGED RESER/OIR PRODUCTION

In dry or semi6dry regions such as Arabia, Iran, etc., ground water isfound sometimes at a depth of hundreds or even thousands of metresG

..1. D(''eren&($7 D(%$r2e..1. D(''eren&($7 D(%$r2e


77/213


FF

found sometimes at a depth of hundreds or even thousands of metresGunder these conditions the pore pressure gradient is always lower thannormal. The same is often the case at high altitudes such as the Oagrosmountains in Iran, due to the fact that the ground water table is at asignificant depth, as a consequence of local hydro6geologicalconditions.

ATER TABLE

... D(''eren&($7 G$ #7!5... D(''eren&($7 G$ #7!5


78/213


F=

8uring uplift, 2$ in overpressured, saturated reservoirs %!e !u& !'!7u&(!nas the temperature and confining pressure are reduced. This

esolved gas migrates out of the lower permeability reservoirs at agreater rate than continued gas generation in the source roc.

The (b$7$n%e be&5een 2$ (2r$&(!n $nd 2ener$&(!n 7e$d &! $redu%&(!n (n &e !ver"reur(n2and, depending on the magnitude of

temperature reduction and gas loss from the system, pressures belowthe hydrostatic may result

..3. R!%; D(7$&$n%4..3. R!%; D(7$&$n%4

8uring er!(!nof shallowly buried, clay rich lithologies, d(7$&(!n !' &e"!re can occur which may facilitate the dissipation of overpressure


79/213


F

above mentioned equation the effective compaction pressure can be easily calculated:

Pe'',H1 P!v,H1 P",H1 *$& de"& H1+

-. Joving to the depth &2, the overburden pressure is easily calculated, being the overburdengradient already availableG by subtracting from the overburden pressure the effective

compaction pressure, as obtained at the previous point, the pore pressure is determined by thedifference:

P",H P!v,H Pe'',H1H*$& de"& H+

1.5elect the depth at which you want to compute the "ressure 7radient /i.e. point &2at !1)) m0.

2. Intersect vertically the


107/213


10F

This means that shales at !1)) m are as compacted as those at 2!)) m.

!. $rom the Integrated ;verburden 7radient curve, obtain the ;verburden 7radient at !1)) m and 2!)) m:

GO/ $& 300 *H+ .? ;2%10 GO/$& 300 *H1+ .3? ;2%10

(. @alculate the @ompaction "ressure at 8epth &*:

*GO/H1 - G",n+ H1 *.3? - 1.03+ 300

P% 301,3 ;2% *) *)

1. @alculate the ;verburden "ressure at 8epth &2: GO/H H .? 300 PO/ =?F,0 ;2%

10 10

-. @alculate the "ore "ressure at !1)) m: "" # ";M 6 "c # (4,) 6 !)*,! # 1(1,4 g%cm2

4. @alculate the "ore "ressure 7radient at !1)) m:GP *PP 10+H *?,F 10+300 1.> ;2%10

010 100 1000

00

Tr$n(& &(e - e%'&

VTHE E^UI/ALENTDEPTH METHOD

3....1. In&erv$7 Tr$n(& T(e veru De"&


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10=

De"&0

3

1000

100

000

00

3000

300

?000H?

H3

H1

H

1. .0 . 3.0

Overburden Gr$d(en&, ;2%10

3....1. In&erv$7 Tr$n(& T(e veru De"&


109/213


10

1000

000

3000

?000

Gr$d(en&

P!re PreureGr$d(en&

#r$%&ure

Gr$d(en&


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**4

3.3. PRESSURES PREDICTION AND3.3. PRESSURES PREDICTION ANDE/ALUATIONE/ALUATION

#ROM DRILLING DATA ANALSIS#ROM DRILLING DATA ANALSIS


11F

The processing and interpretation of drilling parameters represents a veryimportant group of techniques, which have the advantage to be availablemore or less in real time and are referred to the well in progress

3.3.1. INTRODUCTION


118/213

**

more or less in real time and are referred to the well in progress.

These methods can be:

6u$7(&$&(ve, but which. if analyzed in their completeness, can providesignificant information on the actual status of the well and alert the drillingengineer in case dangerous and abnormal conditions are evolvingG

6u$n&(&$&(ve, which ensure the quantification of the pressures acting in awell and below it.


11=

Among the 6u$7(&$&(ve &e%n(6ue, based on drilling and geological parametersrecording, processing and interpretation, we can recall the following:

r$&e !' "ene&r$&(!n

3.3.. ^UALITATI/E INDICATORS


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**+

r$&e !' "ene&r$&(!n&!r6ue!ver"u77%$v(n2 $nd !7e &(2&en(n2'7!5 r$&e $nd "u"(n2 "reureud 7eve7 (n &e "(&%u&&(n2 (n%re$e $& $7e $;er

!7e '(77 u"ud re(&(v(&4 $nd %7!r(de %!n%en&r$&(!n"H$7e re(&(v(&4 $nd $7e den(&42$ !5ud &e"er$&ure

!n&!r(77!n(&e %!n%en&r$&(!n (n &e ud *MBT &e&+


11<

DRILLING RATE OR PENETRATION RATE

If everything else remains constant, dr(77(n2 r$&edr(77(n2 r$&e/called also "ene&r$&(!nr$&e or r$&e !' "ene&r$&(!n ROP0 gradually declines as depth increases

3.3..1. ^UALITATI/E DRILLING INDICATORS


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*2)

r$&eor r$&e !' "ene&r$&(!n, ROP0 gradually declines as depth increasesdue to the decreasing porosity caused by the weight of the overlyingsediments and the increase in differential pressure between the hydrostatichead of drilling mud in hole and the pressure of formation fluids.

But raw penetration rate values are affected by so many influencing factors

/characteristics of formations, bit types and wear, ;B, F"J, mud typeand weight, etc0, that it is impossible to use it as a reliable and efficientdetecting method. $urthermore, "ene&r$&(!n r$&e "r!v(de !n74 $6u$7(&$&(ve (nd(%$&(!n about formations porosity and do not allow anyquantitative evaluation of abnormal pressures.


10

TOR^UE

The amount of &!r6ue&!r6ue required during drilling can be an indication of thepresence of an abnormal pressure formation



121/213

*2*

presence of an abnormal pressure formation.

It must be remembered that the amount of required torque depends on theresistance met by the bit, which is a function of the weight on the bit, thecoefficient of friction of the formation and the amount of restoring torque,the latter being dependent on the amount of frictional force developed

against the wellbore walls. Any change in the torque value, therefore, canbe due to a change in weight on the bit, a change in the type of formation orballing of the bit and not only to hole tightening, which could be indicative ofwellbore instability and presence of abnormal pressure.


11

O/ERPULLS

hen tripping out a string, the amount of hoo load is approimatelyproportional to the depth reached $or various causes not always



122/213

*22

proportional to the depth reached. $or various causes, not alwaysimputable to the presence of an abnormal pressure, the predicted valuemay be eceeded, thus causing an !ver"u77!ver"u77condition. The main causes ofhoo load increase can be due to the following:

bit balling,stuc drill stringG

unusual swabbing effectsGdog6legs.

If these above stated causes can be ruled out, the overpull may beattributed to underbalance conditions with hole tightening and then to thepossible presence of overpressures.


1


123/213

MUD SALINIT, RESISTI/IT, "H

If an increase in the $7(n(&4$7(n(&4 /mainlychlorides0 of the mud is observed, thismeans that water, present in the rocpores has entered the hole because of



124/213

*2(

pores, has entered the hole, because ofinsufficient pressure eerted by the mudcolumn. Being the formation watermore salty than the water used forpreparing the mud, an increase insalinity is therefore eperienced.

The inflow of salty formation waters into

the hole determines, not only anincrease in salinity, but at the sametime provoes a decrease in ududre(&(v(&4re(&(v(&4 and in its "H"H. If thesequantities are continuously monitoredand plotted versus depth, theoccurrence of abnormally pressured

formations can be detected.


1?

MUD DENSITGAS RELATIONSHIP

The v!7ue !' 2$v!7ue !' 2$ released from a drilled formation will be dependent upon theporosity, permeability, gas saturation and differential pressure of this formation.

3.3... ^UALITATI/E GEOLOGICAL INDICATORS


125/213

*21

The progressive and continuous increase of gas while drilling /b$%;2r!und 2$0 andwhile connecting pipes to the drill string /"("e %!nne%&(!n 2$0 could be regarded asanother way to monitor the encountering of abnormal pressure conditions, provided thatother causes such as swab or drilling through gas bearing roc could be ecluded.

The influ of gas into the wellbore determines a decrease in mud density, which can be

calculated with the relation:

with:

6 *# gas6cut mud density, lb%gal6 2# uncut mud density, lb%gal


1

CUTTINGS CHARACTERISTICShen underbalance conditionsare approached, an increase inpenetration rate occursG in thiscase the amount of %u&&(n2%u&&(n2



126/213

*2-

produced and lifted to the surfaceincreases too. The quantity ofcuttings produced depends by: length of the interval drilled inunderbalanceGpressure differential between the

mud and the formationGpenetration rate.

hen drilling in underbalancedconditions, also %$v(n2%$v(n2 areproduced. @avings are largepieces of formation detached from

the wellbore walls and notproduced by the bit /cuttings0.


1>

SHALE DENSIT

This has been a very popular technique in the past, when sensors and computers werenot so common at rig sites.



127/213

*24

The method is based on the measurements of $7e den(&4$7e den(&4, assuming thatoverpressured shales be undercompacted, therefore more porous and with a lowerdensity than normally compacted shales. By plotting the density, as obtained on cuttingscollected at the shale shaers, versus depth, a graph is built, which shows an increasein shale density if normal conditions eist and a decrease when overpressures areentered.

The magnitude of the bul density change will vary with the type and magnitude of thegeopressure. Bul density may also decrease, but it may remain constant /due tolithology0 or continue to increase at a lower rate than the previously established trenddue to the geopressure mechanism.


1F

03

03

SHALE DENSITNORMAL

COMPACTIONUNDERCOMPACTION



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*2

O/ERPRESSURED $7e $re UNDERCOMPACTED, $ve $ LOER DENSIT&$n 5$& &e4 !u7d $ve $& &e de"& 5ere &e4 7(e.

O/ERPRESSURED $7e $re UNDERCOMPACTED, $ve $ LOER DENSIT&$n 5$& &e4 !u7d $ve $& &e de"& 5ere &e4 7(e.

/er&(%$7de"&0

/er&(%$7de"&0

DENSIT 2%3

2.2 2.3 2.4 2.5

DENSIT 2%3

2.2 2.3 2.4 2.5

O/ERPRESSURETOP


1=

SHALE DENSIT

Ideal @lay8ensity



129/213

*2+

Fesponses in7eopressuredOones @ausedby 8ifferentJechanisms


1<

TEMPERATURE

The 2e!&er$7 2r$d(en&2e!&er$7 2r$d(en&, or the rate at which subsurface temperatures increase withdepth, can be calculated from:

GT 100 Y*T# T#1+*D D1+Z E6..F



130/213

*!)

where:6 7T# geothermal gradient /U@%!) m06 T$*# temperature /U@ at depth 8*, m06T$2# temperature /U@ at depth 82, m0

hile the average temperature gradient across normally pressured formations may beconstant, pressured formations ehibit abnormally high geothermal gradients, due totheir higher porosity and higher fluid content, which mae them very poor heatconductors. Therefore, overpressured shales will heat more the mud than othernormally pressured rocs.

Jonitoring and recording ud '7!57(ne &e"er$&ureud '7!57(ne &e"er$&ureis a practical method to determine

temperature gradient, provided variable factors such as pump rate, lag time, ambienttemperature, lithology and temperature changes at the surface /due to mud miing andchemical treatments0 can be accounted for.


130

TEMPERATURE

"rior to reaching ageopressured zone, a&e"er$&ure



131/213

*!*

&r$n(&(!n !newill beencountered in which,due to distortion of theisothermal lines, therewill be a reduction in

geothermal gradient,followed by anetremely largeincrease as thegeopressured zone ispenetrated.


131

TEMPERATURE

At the rig6site '7!57(ne&e"er$&ure ("7!&&ed $2$(n&



132/213

*!2

de"&. The transitionzone is characterizedby a decrease ingeothermal gradient,while the entrance inthe overpressures

shows an increase ingeothermal gradient.

The mud temperatureis affected by manyvariables and can give

only qualitativeresponses.


13

NORMALIWED PENETRATION RATE

As seen before, the rate at which a formation can be drilled is determined by a numberof factors, such as:weight on bit, ;BG

3.3.3. ^UANTITATI/E E/ALUATION O# GEOPRESSURES


133/213

*!!

rotary speed, F"JGbit tooth efficiencyGdifferential pressureGhydraulicsGroc matri strengthGformation compaction.

To render penetration rates more useful, several attempts have been made in the pastto correct them for some of the most important parameters which affect drilling, inparticular ;B, F"J, hole size, differential pressure and mud density and,consequently, a number of


134/213

*!(

where:6 FV)# penetration rate with a new bit and P"#), ft%h6 # weight on bit per hole size unit, lbs%in

6 8h# hole diameter, in6 ? # rotary table revolutions per minute, rpm6 W # flow rate, gal%min6 8n# bit nozzle diameter, *%!2ths of an inch6 aw# eponent of weight on bit6 an # eponent of rotary table speed6a

q# eponent of hydraulics

6 P" # differential pressure, psi6 T # bit teeth wear inde


13?

$rom this basic


135/213

*!1

dr(77(n2 e"!nen&dr(77(n2 e"!nen&,, dd%-%-e"e", methodG

the S(2$7!2S(2$7!2method, developed by other ;il @ompanies and also used at 9ni 9H".

*. Both methods are e(-e"(r(%$7 and are based on a relationship betweenpenetration rates and the drilling parameters used.

2. If all other conditions remain the same, &ee dr(77$b(7(&4 (nd(%e $re "r!"!r&(!n$7 &!&e de"&. In other words, in normal compaction conditions, they (n%re$eas the depthof the well increases, because the roc becomes harder and the differential pressurebetween the mud in hole and the formation pressure increases too, while in presence ofoverpressures the drillability inde de%re$e, because the porosity of the roc

increases and the differential pressure decreases..


13

J.7. Bingham /*+-10 proposed the following relationship between the drilling rate,weight on bit, rotary speed and bit diameter.

3.3.3.1. THE DRILLING EXPONENT ANDCORRECTED DRILLING EXPONENT METHOD


136/213

*!-

where:6 F # drilling /penetration0 rate, ft%h6 ? # rotary speed, rpm6 8 # bit diameter, ft6 # eight on bit, lbs6 a # matri constant, dimensionless6 d # drillability eponent, dimensionless


13>

This mathematical relationship was revised and adapted to field requirements by Xordenand 5hirley /*+--0, solving for


137/213

*!4

that vary within an acceptable range. It assumed the following form which is nown as


138/213

*!

where:6 d6ep # drilling eponent. dimensionless6 F # drilling or penetration rate, m%h6 # weight on bit, t6 8 # bit diameter, in6 ? # rotary speed, rpm


13=

5ince differential pressure depends upon the mud density and formation pore pressure,whenever there is any change in the mud density this will promote an unwanted changein the d6epG for this reason, the %!rre%&ed dr(77(n2 e"!nen&or


139/213

*!+

/Fehm and Jc@lendon, *+4*0, whose epression, in metric units, is the following:

6 7pn# normal pore pressure /equal to *.)!0, g%cm2%*) m6 9@8 # 9quivalent @irculating 8ensity /mud density plus friction losses0, g%>

or more simply: d%-e" d-e"M

with:

6 d6ep # uncorrected drilling eponent6 J #mud weight in use, g%>

7!2


13<

The calculation sequence generally adopted in the field is the following:

the dr(77(n2 "$r$e&er/F;", ;B, F"J, 80 are recorded every * meter drilled andprocessed by using the equations previously seenG

h l i d d d 7 l d d h il i h i

3.3.3.1. THE DRILLING EXPONENT AND

CORRECTED DRILLING EXPONENT METHOD


140/213

*()

the resulting d-e"and d%-e" v$7ueare plotted versus depthon a semilogarithmicpaperGin normal compaction conditions, the d6ep, and alsothe dc6ep, will increasewith thedepth and all points, taen in correspondence of shales, will lay on what is called then!r$7 %!"$%&(!n &rend 7(neG as soon as the d6ep and dc6ep values, always taen in correspondence of shale

levels, start to decrease, this means that abnormal formations are entered and themore these values depart from the reference trend line the higher is their overpressure.

;f course, the true overpressure values have to be calculated on the d c6ep curve,because it has been corrected for the mud weight present in the well. The net slideshows that the d6ep curve, with respect to the dc6ep curve, is mased as the mud

weight is increased thus indicating apparently a lower pressure regime.


1?0

d6ep and dc6eptrends versus depth




141/213

*(*03. ABNORMAL PRESSURES

1?1

The 6u$n&(&$&(ve %$7%u7$&(!n !' "!re "reure $nd 2r$d(en&can be performed withone of the three following methods, that is:

A.the


142/213

*(2

in seismicsG

B.the so6called E$&!n e&!dG

@.the Vr r$&(! e&!d/abacus construction0 which is based on the epression:

P! P

N9 Y*d

%-e"+

N*d

%-e"+

"Z

where:6 "o# the actual pore pressure at the depth, &, of interest, g%cm2

6"? # normal pore pressure, obtained from /& *,)!0%*), g%cm2

6 /dc6ep0?# actual dc6ep value at the depth of interestG6 /dc6ep0p# value at the depth of interest as read on the normal compaction trend line


1?




143/213

*(!03. ABNORMAL PRESSURES

1?3




144/213

*((03. ABNORMAL PRESSURES

1??




145/213

*(103. ABNORMAL PRESSURES

1?

131

111

S$nd

L(ne

SHALES

L(neThe interpretation of thedc6ep curve is facilitated

by taing into




146/213

*(-

11

131

131

B(&runde"&03

G" L(ne

1.F 1. 1.3 1.1

b(&

e$r

B(& e$r

T!" !' Over"reure

P!r!(&4 e''e%&

by taing intoconsideration:

lithologyG

bit runs,

casing setting pointsG

hole difficultiesG

nowledge of the area.


1?>

The S(2$7!2S(2$7!2method has been developed during the *+4)s by En( E`Ptoovercome the limits of the dc6ep technique eperienced while drilling deepwells in the "o Malley Basin, in particular its deficiency in identifyingoverpressures within carbonatic reservoirs.

3.3.3.. THE SIGMALOG METHOD


147/213

*(4

The 5igmalog taes into account the mud density effect on penetration ratesand is based on the drillability concept, as all other methods which processdrilling data. Again the parameters involved are F;" /m%h0, F"J /rpm0, ;B/t0 and bit or hole size /in0.

The 5igmalog does not allow only the %$7%u7$&(!n !' &e "!re "reure2r$d(en&, but also those of the !verburden $nd 'r$%&ure 2r$d(en&throughsteps very long and tediousG for this reason, the overburden gradients can bemore easily determined from seismic interpretation or from 5onic >oganalysis.


1?F

The basic equation of 5igmalog is as follows:

[)& OB0, RPM0,

D ROP0,



148/213

*(

where:6;B : weight on bit, t6F"J: revolutions per minute of the rotary table68h: hole size, in6F;": rate of penetration, m%h

To compensate for values ecursion at shallow depth, a correction factor,depending from depth, has been introduced:

[)&J [)& 0,0= *F H1000+where:

6 &: depth, m


1?=

In order to tae into account the effect of the differential pressure, P", on thepenetration rate, the following relationship is introduced:

KP *M G"+ H10

where:



149/213

*(+

where:6J: mud weight, g%>67p: pore pressure gradient at the depth &, g%cm2%*)m /taen equal to *,)!0

Because F;" does not change linearly with P", a correction factor taes intoaccount this occurrence:

# 1 1 - [1 nKP

n KPwhere:6n # !,2%/-() YCtV0 if YCtV Z *

6n # * /( [ ),41 0 if YCtV N * -() YCtV


1?<

The value of 5igmalog, which is plotted versus depth, is given by the followingequation:

[)! # 9 [)&J

The interpretation criterion is the same as per the d e ponent method that is



150/213

*1)

The interpretation criterion is the same as per the dc6eponent method., that is:6if the YCovalues increase with depth in homogeneous formations /shales0, it means

that normal conditions eist and the pore pressure is normalG6if the YCovalues tend to decrease with depth, always in homogeneous formations, it means that abnormal conditions are encountered and that overpressures are

present.

A re'eren%e &rend 7(ne [)r, indicating normal compaction, can be drawn throughthe YCovalues in the section of the hole where they constantly increase with depth.

The departure of the YCovalues from the YCrtrend line is proportional to the

amount of overpressure.


10


151/213

True S(2$7!2/eru De"&P7!& E$"7e



152/213

*1203. ABNORMAL PRESSURES

1

S(2$7!2:De&er(n$&(!n !'&e Re'eren%e

Trend L(ne



153/213

*1!

Trend L(ne


13

C!rre%&(!n !' &eRe'eren%e TrendL(ne b4 S('&

(n &e N!r$774C!"$%&ed



154/213

*1(

(n &e N!r$774C!"$%&edW!ne


1?

C!rre%&(!n !' &eRe'eren%e TrendL(ne b4 S('&

(n &eAbn!r$774



155/213

*11

Abn!r$774C!"$%&edW!ne


1

. THE SIGMALOG METHODCALCULATION O# PORE PRESSURES AND GRADIENTS

;nce the term


156/213

*1-

g pcalculate the values assumed by the reference trend line as seen aboveGcalculate the term $ from the equation:

define the differential pressure P" as:P" # \2 /* [ $0]%\* [ /* [ $02] L /*%n0

calculate the pore pressure gradient as:7p# df6 \/P" L *)0%&]

where:6 df# mud density, g%>03. ABNORMAL PRESSURES

1>

E$"7e !' P!rePreureGr$d(en&Deve7!"en&'r!

$n In&er"re&edS( 7 C



157/213

*14

S(2$7!2 Curve


1F

3 ? PRESSURES PREDICTION3 ? PRESSURES PREDICTION


158/213

*1

3.?. PRESSURES PREDICTION3.?. PRESSURES PREDICTIONAND E/ALUATIONAND E/ALUATION

#ROM IRELINE LOGS#ROM IRELINE LOGS


1=

3.?.1. INTRODUCTION

The analysis of wireline logs allows the calculation of:

6;verburden 7radients

6"ore "ressure 7radients

6$racture 7radients


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*1+

The wireline logs generally used for pressure prediction and evaluation are:

5onic >og /5>0

Induction >og /I950

$ormation @ompensated 8ensity >og /$8@0


1<

The 5onic >og is interpreted in the same way as the seismic dataG in factboth are based on the use of travel times, Pt, plotted versus depth.

The 5onic >og allows the calculation of:

3.?.. SONIC LOG


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*-)

!verburden "reure $nd "reure 2r$d(en&

"!re "reure $nd "reure 2r$d(en&

'r$%&ure "reure $nd "reure 2r$d(en&


1>0

As already seen, the following relationship between Pt and can be written:

K& K&$*1 @+ K&'79 @

where:6 Pt # transit time of sound through the formation /Qsec%ft0

3.?..1. SONIC LOG: O/ERBURDEN GRADIENT CALCULATION


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*-*

g /Q 06 Ptma# transit time of sound through the solid frame /Qsec%ft06Ptfl# transit time of sound in the pore fluid /Qsec%ft0

The previous equation can be also epressed in terms of porosity:

@ *K& - K&$+* K&n- - K&$+

where:6 Ptfl # assumed equal to 2)) Qsec%ft /a conservative value for pore pressuregradient calculations0

6 Ptma# values depend on lithology


1>1

SOLID Den(&4 K&

MATTER 2%3

(%r!e%!nd'&D 7 (& =F ?3

SOLID Den(&4 K&MATTER 2%3 (%r!e%!nd'&

D!7!(&e =F ?3



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*-2

D!7!(&e .=F ?3.

L(e&!ne .F1 ?3. - ?F.

An4dr(&e . 0

S$7e .F0 ?F

D!7!(&e .=F ?3.

L(e&!ne .F1 ?3. - ?F.

An4dr(&e . 0

S$7e .F0 ?F

Tr$n(& T(e $nd Den(&4 '!r S!e R!%; M$&r(%e


1>

L$b!r$&!r4 &e& "r!ved &e '!77!5(n2 re7$&(!n %!rre%&.

Relation between transit time and porosity

^for consolidated and cemented rocks

f lid t d d

= (1)t- t

13

ma

(2)= 1 =t- tma



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*-!

^for unconsolidated sands

^for shales

Relation between density and transit time

^'!r %een&ed $nd %!"$%&ed '!r$&(!n

^'!r n!n-%een&ed '!r$&(!n

t +(2)= 1.=

00

(3)= 1.>= t- t

t +

ma 00

= 3.= -t

=<

(4)

= .F- .11 tt+

- tma

00(5)


1>3

@omparisons with density values detected by the FDlog /formationdensity compensated log0 attested that the following relation is valid forall formations:

?Ft



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*-(

b=

F 1100

. . t

where:

(4 # Pt ma /transit time through the rocy matri assumed to be equal to (4 _sec%ft

2)) # Pt in water, _sec%ft


1>?

To determine the eact transit time in an interval of P& thicness, the following procedurecan be applied:the number of milliseconds elapsed for the sound wave to cross the formation intervalhaving thicness P& is read /this is done by counting the ITT pips on the 5> to the left0 onthe logG this value is multiplied by *,))) to change milliseconds into microseconds, and isdivided by the value of P&G in order to obtain the values of Pt, measured in Qsec%ft, P&must be changed from meters to ft by dividing by ! 2G



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*-1

must be changed from meters to ft by dividing by !.2Gthe final epression represents the average transit time in the P& interval and taes theform:

K& * 9 1000+*3.= 9 KH+

where:6 D # milliseconds required by the sound wave to pass through a section of height P&:6P& # depth interval, m.

The Pt values can be also more rapidly read on the log by taing into account the

intervals characterized by more or less the same values.


1>

SONIC LOG INTERPRETATION

ITT INTEGRATED TRANSIT TIME



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11 166166

KH 1.

=.

K& 1>1 e%'&


1>>

;nce determined the bul density versus depth with the equations seenabove, the procedure for the calculation of the overburden pressure andrelevant overburden gradient using the transit times read on a 5onic >og is thesame as discussed when dealing with seismic data interpretation, to which thereader has to refer. Therefore, the basic equations to consider are, as alreadynown, the following:



167/213


1>F

E6.>.10

E6.>.11

BUL DENSITBUL DENSIT

0

1000

1,> 1.= .02d3

. .?

0

00

1000

1.

"&03

.0 . 3.0

Overburden Gr$d(en&,;2%10



168/213


1>=

000

3000

?000

000

100

000

00

3000

300

?000

?00

/er&(%$7de"

0 >0 100 00

00

1000

The plot of Transit Times Pt, asobtained from 5onic >og readings,versus depth is the most commonmethod for overpressure detectionand calculation, being a quic andreliable tool. As in the case ofseismics the transit times in shales

& *'&+

3.?... SONIC LOG: PORE PRESSURE GRADIENT CALCULATION


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1>0>0 100100=0=0 0000

@onsidering always valid the assumptionthat density, porosity and relevantpressures /effective and pore pressure0are correlated, it derives very clearly thatthe Pt will decrease regularly with the

depth when normal conditions eist andthat an increase in its values, on thef

&*'&+

3.?... SONIC LOG: PORE PRESSURE GRADIENT CALCULATION


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1F0

/er&(%$7De"&0

/er&(%$7De"&

10001000

100100

000000

00

contrary, will determine its departure fromthe reference trend line and will beindicative of abnormal conditions/undercompaction and overpressures0. O/ERPRESSURES TOP

O/ERPRESSURED $7e $reUNDERCOMPACTED $nd $ve $HIGHER PORE ATER CONTENT.

Therefore they have a HIGHER K&,compared to the depth at which they

lie.

0

00

100=0>0 00& *'&+

In the


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1F1

/er&(%$7de"&031000

100

000

leftwards, are not related tooverpressures but to particulargeological conditions /overcompactedformations, for instance in the upper

part of the well0 and, therefore, haveto be corrected during interpretation.

TRUE O/ERPRESSURES TOP

ERRONEOUS O/ERPRESSURES TOP

;nce the curve has been interpreted and adequately corrected, the porepressure gradients are calculated with the usual


172/213


1F

(%r!e%!nd'&

*3+

0

00

1.000

1.00

000

0

00

1000

100

000

1.

er&(%$7

de"&03

.0 . 3.02%10

POINT B:- P!v


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1F3

/er&(%$7de"&0*

10 0 30 0 100 00 300 00 1.000

.000

.00

3.000

3.00

?.000

?.00

.000

00

3000

300

?000

?00

/e

POINT A:

- P!v 11 ;2%

- P% 33 ;2%

- P" 11 - 33 F


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1F?

G'r G" * + *G!v G"+ 1-

b0 If the drilling fluid is water or invades in depth the formation, the relationshipsbecomes:

G'r G" * + *G!v G"+

c0 If the roc has a plastic behaviour, the fracture gradients is given by theformula:

G'r G!v

0

1000

1,0 1, ,0 ,

OverburdenGr$d(en&

*;2%10+

#r$%&ureGr$d(en&

3.?..?. SONIC LOG: GEOPRESSURE CUR/ES *v DEPTH, +


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1F

000

3000

?000

P!reGr$d(en&

The resistivity of a roc depends on its porosity /amount of fluids present in thepores0G therefore low porosity rocs are also high resistivity formations /forinstance, compacted limestone, volcanic rocs, etc0.

If all other conditions are the same, the resistivity of a roc depends on: saline concentration of the fluids within the poresG

3.?.3. SHALE RESISTI/IT: PORE PRESSURE GRADIENTS CALCULATION


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*4-

saline concentration of the fluids within the poresG

roc compositionG

temperature.

As depth increases, shales are more and more compacted and less porous,

therefore their resistivity tends to progressively increase.


1F>

Tere $re &5! e&!d !' !ver"reure $n$74( b$ed !n

VS$7e Re(&(v(&4:

Me&!d 1 - #(nd $7e re(&(v(&4 !n &e re%!rded e7e%&r(% 7!2,

$nd "7!& (& d(re%&74 !n $ e(-7!2$r(&(% %$7e,

3.?.3. SHALE RESISTI/IT: PORE PRESSURE GRADIENTS CALCULATION


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*44

5(&!u& $n4 e7$b!r$&(!n.

Me&!d - An$74e &e V# $7e '$%&!r *$7e '!r$&(!n

'$%&!r+.


1FF

RESISTI/IT LOG

3.?.3.1. SHALE RESISTI/IT PLOT


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*403. ABNORMAL PRESSURES

1F=

METHOD 1The rThe resistivity of the cleanest shales is plotted Ms depth, on a semi6logarithmic scale. Theinversely proportional relationship eisting between resistivity and porosity /meant as fluid content0will produce a diagram, whose values will increase with the depth in case of normal compactionconditions. S$7e re(&(v(&4

Therefore, in case of normallycompacted and normally pressuredformations, the resistivity values

ill i i h h d h d ill



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*4+

De"&0

will increase with the depth and willlie on the normal compaction trendline, as shown in $igure aside.

#!r$&(!n 5(& $ NORMAL"reure 2r$d(en&.

As depth increases, so doescompaction, while porosity

decreases


1F<

METHOD 1In case of overpressured formations, the resistivity values, as obtained from the logreadings, will tend to depart from the normal compaction trend line, assuming lower valuesthan epected for that depth of burial.

S$7e re(&(v(&4

This depends on the fact that more

fluid with more salt is present in thepores of the roc and this maes theresistivity to decrease. The more the



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*)

Over"reure T!"De"&0

points depart from the referencetrend line, the higher the porepressure gradient will be.

O/ERPRESSURED '!r$&(!n

Under%!"$%&ed $7e 5(&

(2 "!r!(&4, %!"$red &!

&e de"& 5ere &e4 7(e.


1=0

F shale- N!r$7-N!r$7-Gr$d(en& #!r$&(!n

METHOD In this case, it is not made reference only to the resistivity values as read on the log, but it isnecessary to determine the


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**

/er&(%$7de"&0

300

1000

100

000

00

where:

6 Fshale# shale resistivity

6 @shale# shale conductivity

6 Fw# formation water resistivity


1=1

00.1000.0=00.0>0 0.00

00

F shale-

Over"reuredOver"reured

#!r$&(!n#!r$&(!n

e"&0

3

METHOD Also in this case, if overpressuredformations are encountered, a decrease in$sh values can be observedG again, higheris the departure of the points from thereference trend line, higher will be the pore

pressure gradient values.

3.?.3.. SHALE RESISTI/IT: SHALE #ORMATION #ACTOR, #SH


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*2

,

1000

100

000

00

/er&(%$7de

O/ERPRESSUREO/ERPRESSURE

In "reen%e !' under%!"$%&ed$7e, hence(n O/ERPRESSURE,V#$7e v$7ue de%re$e 5en%!"$red &! &e Vn!r$7%!"$%&(!n &rend.

;M9F"F955F95T;"


1=

CALCULATION SE^UENCE

Te !"er$&(!n$7 e6uen%e re6u(red &! %!"u&e "!re "reure 2r$d(en& b4 e$n !' &eV#$7e &e%n(6ue ( $ '!77!5:

*. C$7%u7$&e R, that is the formation water F95I5TIMIT` along the whole well profile.

2. Plot Rvalues on a semi6logarithmic scale.



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*!

!. Read the Conductivityvalue in the log in correspondence of clean shales, along the wholewell profile.

(. Calculate the F shalevalue.

1. Plot the F shale on semi6logarithmic paper.

-. Dr$5 &e V# $7e


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*(

on the log heading

(. 9nter the mud and temperatureR'values read on the log heading in the5@&>JB9F79F


185/213

*1

. ;btain Ffor the entire well. 9ntering the /F0evalues and correspondingtemperatures in the 5chlumberger


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*-

000

3000

?000

000

/er&(%$7de"&

MEAN R/ALUES*RA/ENNA SEA WONE+ OHM

9ach well or area ischaracterized by aspecific Fwcurve.


1=>

CALCULATION SE^UENCE *%!n&(nued+CALCULATION SE^UENCE *%!n&(nued+

+. In the Fesistivity%@onductivity log read the @onductivity value in correspondence of

clean shales .

*). @ompute the


187/213

*4

**. "lot the


188/213

*

/

er&(%$7de"&0

000

3000

?000

000

from the normalcompactiontrend line.

Over"reure T!"

Over"reured W!ne


1==

Te $(n 7((& !' &e e&!d b$ed !n Re(&(v(&4 Me$ureen&$re $ '!77!5:

They can not be applied to @AFB;?AT95.

They can only be applied in presence of frequent interbeddings of shales and sands.



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*+

The


190/213

*+)

*/ERI#ICATION STEPS+*/ERI#ICATION STEPS+


1


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*+*

conducting daily operations, such as the maimum mud density, the maimumflow rate and the maimum friction losses allowed.

These techniques are:

LEA O## TEST *LOT+ $nd EXTENDED LEA O## TEST *X-LOT+

#ORMATION INTEGRIT TEST *#IT+


1ea ;ff Test />;T0 will be performed in eploratory wells at each casingshoe after the surface casing has been set and it is suggested in both appraisaland development wells.

>ea ;ff Test and $ormation Integrity Test /$IT0, also called >imit Test, are thenames generally given to formation strength pressure tests made ust below the

casing seat prior to drill ahead.

These tests are carried out to:

?.1. INTRODUCTION


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*+2

investigate the cement seal around the casing shoe which should be at least as high as the predicted fracture pressure for the areaG

investigate the wellbore capability to withstand pressures below the casing shoe in order to allow proper well planning with regard to the setting depth of the net casing, mud weights and alternatives during well control operationsG

collect regional information on formation strengths and stress magnitude for different applications including optimization of future well planning, hole stability

analysis and modelling, reservoir applications.03. ABNORMAL PRESSURES

1


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*+!

6$.I.T. does not allow to obtain information on stress magnitudeG

6>.;.T. is designed, and should be performed, to determine in the better way the

desired data without breaing down the formation.


1ea ;ff or $ormation Integrity Tests can be carried out in any open hole sectionand at any time while drilling the hole, even if it is customary to have performedsomee&er *>-1 e&er0 below the casing.

$or instance, the casing seat can be in a shale and the first sand formation maybe encountered several tens of meters deeper. This formation will certainly bemore permeable and weaer than the shale, and a test can be performed to

?.1. INTRODUCTION


194/213

*+(

p , pascertain the maimum pressure this sand can hold. If it is lower, as usually is,than the shale ust below the casing seat, this sand becomes the limiting factor.


1


195/213

*+1

g p g g g and the estimated />;T0 or predetermined /$IT0 pressure. A pressure recorder with chart should be used during the test. Fig pumps are unsuitable for maing these tests.

10 $ill and test the lines with mud.-0 Brea circulation with cementing unit to mae sure that bit nozzles are clear.40 5top pumping when circulation is established.0 @lose top pipe rams or annular.+0 ;pen annulus of previous casings.*)0 "ump slowly until pressure builds up.


1%min0 up to amaimum of * bbl%min /*-) >%min0G however, values of ).21 bbl /*2 *%(= andsmaller holes0 or ).1) bbl /*4 *%2= hole0 are commonly used. ait for twominutes or the time required for the pressure to stabilise.

*20 ?ote the cumulative mud volume pumped, the final pumping and final static

pressure.*!0 Fepeat items /*)06/*20 and plot pressure versus cumulative mud volume for

each increment of pumped volume.

?.. LEA-O## TEST, LOT


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*+-

*(0 @ontinue this procedure until: two 6 three points on the plot are reached where the pressure deviates and falls below

the approimate straight line /or if the pressure does not increase with the inectedvolume0. The point on the plot where the curve begins to bend away from the straightline is called Le$; O'' P!(n&,

the "rede&er(ned &e& "reureis reached.*10 5top pumping, shut in the well and record and plot pressure versus time until

stabilization /usually it taes *162) minutes0G in the early stage /26! min0 onevalue every *16!) seconds should be collected, while for the remaining avalue of pressure every !)6-) seconds may be sufficient. The use of "A@F or

equivalent device, if available, should be preferred.03. ABNORMAL PRESSURES

1

*-0 Bleed off the pressure and record the quantity of fluid returned into thecementing unitG compare it to the volume used for the test to obtain the amountof fluid lost in the formation.

*40 ;pen B;" and calculate the result of the formation strength test in terms of9quivalent Jud eight using the lowest between lea off point pressure andstabilized pressure.

*0 @ollect the data recorded during the test in a data sheet together with thefollowing information: borehole diameter, depth of test, depth and type of thelast casing, mud density, plastic viscosity, filtrate and gels /see the eample inthe net page0



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*+4

the net page0.

N!&e:6 The pumping rate should be ept in the limits reported in /**0: if the rate is too low,

filtration losses will mas any leaage loss, otherwise, if the rate is too high the formationmay suddenly brea and the lea off pressure will not be determined. Besides, the longerthe open hole section, the higher should be the inection rateG if the initial pumping rate isnot sufficient, the well should be depressurized and the test restarted with a higher rate.

6 If a float valve is used in the drilling string, the test can not be carried out by pumpingdown drill pipe. In this case rig up cementing unit to choe or ill line, fill and test the linesagainst fail6safe and establish circulation through the riser. @lose B;" and perform

formation strength test by pumping down the annulus.


1

>ast @sg. 5hoe F0

7rade a-CC `" C >iners >.C

eight >1 Lb'& 7els = $low Fate 0.C b"

Ja. Burst. "ress. 3103 "( .>. 10,C

E"e%&ed EM 1,>= ;2%10

T(e

*(n+

/!7ue

*bb7+

Preure

*P(+

T(e

*(n+

/!7ue

*bb7+

Preure

*P(+

T(e

*(n+

/!7ue

*bb7+

Preure

*P(+

* ),21 1) + 2 (+) *- 2 (!1

2 ),1 *)) +,1 2 ()

! ),41 21) *) 2 (4)

( * !) *),1 2 (-!

1 *,21 (1) ** 2 (11

- *,1 () **,1 2 (1)

4 *,41 12) *2 2 ((1

2 11) *! 2 (()

2 12) *( 2 (!4

,1 2 1)1 *1 2 (!1

N!&e

8IFAdFA";



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*+

N!&e

"umped with a costant flow Fate of ),21 bpm

Molume pumped: 2 bbl

Molume returned: * bbl

RESULT: /"ress.mud3"ress.>.;.0%8epth # //*.!4+4%*)03(!)).)40*)%4+4 # 1,>=

Note: L.O.T. n.1 Company Man

)

*))

2))

!))

())

1))

-))

) ),1 * *,1 2 2,1 ! !,1 ( (,1

/!7ue *bb7+

Preure*"(+

) * 2 ! ( 1 - 4 + *) ** *2 *! *( *1 *- *4 *

T(e *(n.+

8 minutes

Shut in curve

Stop pumps


1


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*++

Preure

00

1000

100


1;T, attention should be paid n!& &! %!n(der the valueobtained from the test as the


200/213

2))

The true fracture pressure is reached only when to the formation underevaluation a pressure, equal to the minimum horizontal stress plus an etravalue depending on the formation tensile strength, is applied /ee D($2r$ (n

&e ne& 7(de0.

Therefore, the >;T gives a value of a pseudo fracture pressure /at the ea6;ff "oint=, >;"0, which is an acceptable value to refer to in sound drillingpractices, being in most cases CONSER/ATI/E.


00

THE VTRUE #RACTURATION

LOP

PBD

P#P

PC

LOP Le$; O'' P!(n&PBD Bre$;d!5n PreureP#P #r$%&ure Pr!"$2$&(!n PreurePISIP In&$n&$ne!u Su& In "reurePC C7!ure Preure



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2)*

TEST START

PISIP

STOPPUMP Preure '$77-!''


01

The E&ended Le$;-O'' Te& !r X-LOT differs from the standard >;T for thefollow

03.Politecnico Di Torino Abnormal Pressures 2010

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Transcript of 03.Politecnico Di Torino Abnormal Pressures 2010