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Pore Pressure in Drilling

Engineering07/10/2014

Kanad Kulkarni

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Pore Pressure

• Pore Pressure is present in various forms

• It is important as it governs the overall pressurethroughout the reservoir.

• elps in developing a safe drilling environment• !an prepare "harts for mud densit# and $eights

development.

• %ll formations penetrated $ill "ontain pressure in

di&erent formats –   'he magnitude of pressure e(erted $ill depend on

• Depth

• )ormation

• Pro(imit# to the stru"tures

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#drostati" * +ver,urden Pressure

#drostati"Pressure

+ver,urdenPressure

)ormationPressure

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#drostati" Pressure

Pressure e(erted ,# a "olumn of -uid.

)un"tion of 

• )luid Densit#

• erti"al eight depth of the -uid "olumn

athemati"all# e(pressed as

HP=g*ρf*D P #drostati" Pressure

ρf   )luid densit#

D 'rue/ erti"al Depth

g gravitational a""eleration

)or Imperial units it is "al"ulated as Ppsi0.032 ρf  ppgDft

)or interpretation all $ell,ore pressures are measured interms of h#drostati" pressure

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Pressure 5radient

)or interpretation all $ell,ore pressures are measured interms of h#drostati" pressure

 – )ormation pressure

 – )ra"ture Pressure

 – )luid Densit#

 – +ver,urden Pressure

• Pressure 5radients are used $hile planning a $ell. – 5P/D psi/ft

• Pressures are generall# "onverted to a relative 6(ed datum mean sea level ground level

• 8hen used to e(press magnitudes of $ell,ore pressure it isusual to re"ord these as E9uivalent ud 8eight E8 inppg.

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E(ample

1. ud $eight :ppg

ome Depth 10100ft D ::00 ft 'D

2. ud 5radient0.4;< psi/ft

ole depth 10100 ft D ::00 ft 'D

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+ver,urden Pressure• Pressure e(erted ,# the total $eight of overl#ing

formations a,ove the point of interest – !om,ined $eight of formation soilds ro"k

matri(

 – )ormation -uids in pore spa"es

• !om,ined densit# is kno$n as ,ulk densit#athemati"all# it "an ,e "al"ulated as

σov=0.052*ρb*D 

σov= Overburden pressure(psi)

ρb= Formaion !u"# densi$ (ppg)D= rue veri%a" dep&(f)

%s gradient E8 in ppg= σov=0.'*ρb0.052 

0.4>> "onverts ,ulk densit# from gm/"" to psi/ft.

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+ver,urden gradient not"onstant

+ver,urden gradient is not "onstant throughout dueto the "ontri,uting fa"tors su"h as

• ?itholog#

•

Pore and -uid densit#• Degree of "ompa"tion

+ver,urden under 6eld "onditions

@ovg 0.4>>A1 B Cma C(fF

@ovg over,urden gradient psi/ftG porosit# e(pressed as a fra"tion

f formation fluid densit# gm/""

ma matri( densit# gm/""

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Home Important densities

Substance Density (gm/cc)Handstone 2.;3

?imestone 2.71

Dolomite 2.

5#psum 2.>3

!la# 2.72.<

)resh$ater 1.0

Hea$ater 1.0>1.0;

+il 0;0.7

5as 0.13

o$ $ould #ou "onvert gradient to psi/ft * ho$ from psi/ft to ppg

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E(ample 2

!al"ulate the over,urden gradient forfollo$ing )ormation

Jo"k Handstone

Porosit# values

7 10 * 13

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+ver,urden gradient graph

8ell Engineering = Ja,iaussain

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E&e"t of $ater Depth on

+ver,urden 5radient

• Hea $ater redu"es overall densit#and inturn over all pressure gradient

• ?oosel# "ompa"ted sedimentar#material further redu"es the densit#for o&shore

• +n shore the near surfa"e densit# is

determined ,# the "ompa"tion ofsedimentar# material

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Deri"k

Hea 8ater

)ormation 1

)ormation2

;3ft

300 ft

1000f t

>000f t

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atri( stress

 'he stress under $hi"h the ro"k material is "on6nedat a parti"ular position in the earths "rust.

 'his a"ts in all dire"tions and

Jelation ,et$een verti"al "omponent and matri(stress

σov=Pf σma  

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Pore Pressure

De6ned as Pressure a"ting on the-uids in the pore spa"es of the ro"k. formation pore Pressure

Lased on agnitude1. Mormal Pore pressure

2. %,normal pore pressure

>. Hu,normal pore pressure

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Mormal Pore pressure

• Pore #drostati" pressure

•  'his pressure is not "onstant

•

agnitude varies $ith –  dissolved salts

 – '#pes of -uid

 – 5ases

 – 'emperature gradient

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%,normal pressure

• Pore Pressure N #drostati" Pressure

• It "an o""ur at an# depth

• agnitude is "ontri,uted to – 5eologi"al

 – 5eo"hemi"al

 – 5eothermal

 – e"hani"al•  'here needs to ,e a distur,an"e or

interruptions

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Hu,normal Pressure

• Pore Pressure O #drostati" pressure

• ?ess fre9uent than %,normal

•

Developed long after deposition• Jeasons for pressure redu"tion

 – Htratigraphi"al

 – 5eologi"al

 – 5eo"hemi"al

 – Jeservoir prdu"tion

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!auses of %,normalPressure

• Depositional E&e"t

• Diageneti" E&e"t

•

 'e"toni" E&e"ts• Htru"tural "auses

•  'hermod#nami"s E&e"ts

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Depositional pro"ess

8ell Engineering = Ja,iaussain

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8ell Engineering = Ja,iaussain

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Diageneti" Pro"ess

• Pro"ess of "hemi"al and ph#si"al"hanges to the sediments

• %lter the "hemi"al "onstituents ofmaterial

• Pro"ess in"ludes formation of ne$minerals

Je"r#stallisation and lithi6"ation.

• Hme"tite to Illite

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 'e"toni" %"ivit#

8ell Engineering = Ja,iaussain

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Htru"tural !auses

8ell Engineering = Ja,iaussain