1 - drilling technology - gradients

8/18/2019 1 - Drilling Technology - Gradients

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PRESSURE GRADIENTS


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WELL PLANNING

PURPOSE OF THE WELL PLANNING

• The primary purpose of the well plan is to provide guidelines for the safe and efficient

drilling and completion of the well.

• A secondary, but important purpose, is to provide a reasonably accurate time and cost.

• The third purpose of the well plan is to drill a hole that is usable once drilling is finished.

This will be the automatic result after a well-thought-out plan is created and followed.

Important topics:

• Casing Design

• Mud Density•

• Drilling Rig Selection


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Where does the well plan come from?The well lan is a roduct of man different eo le in the oil com an .

Team Members

Geoscience Department

Geophysicist

Engineering Department

Drilling

Reservoir

Operations Department Support Department

Drilling manager

Drilling superintendent

Loss prevention – safety

Environmental

r ng superv sor

Drilling coordinator

urc as ng


4/142

Contents of a well Plan

• Well summary

1.Drilling and geological prognosis• Drilling procedure

-2.Drawings

a.Well schematic

b.BOPs and manifold

.

2.Conductor hole

3.Surface hole4.Intermediate hole.d.Location

e.Structural map

3.Pore pressure analysis

5.Production hole

6.Completion

7.Standard procedures

4.Type log5.Drilling time curve

6.Drilling cost curve and estimate

8.Abandonment

7.Support

a.Vendors list

b.Transportc.Communications

8.Directional plan


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Contents of a well Plan

• Drilling parameters

.

2.Drilling mechanics

3.Bits.

b.BHA / drillstring

c.Hydraulic program

4.Casin ro ram

5.Cement program6.Well control program

7.Wellhead equipment

8.Rig specs

9.Logging, coring, and testing

10.Emergency proceduresa.Hurricane procedures

b.Fire drills and rig evacuation

c.Blowout control procedures


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DRILLING TIME CURVES

0Phase 16"

""

500Phase 12 1/4"

Depth vs. Time

1.000

csg 9 5/8"

1.500Phase 8 1/2"

2.000Well Testingcsg 7"

2.5000 5 10 15 20 25giorni


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T es of casin s

Conductor pipe

Intermediate Production Liner


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Most common diameters

The normal dimensions of the casing or liner and in which open-hole they are

run-in are shown below; the dimensions are given in inches:

(inches)

”

-

(inches)

”

18 5/8”13 3/8”

9 5/8”

24”17.5”

12.25”

7”

5”

8.5”

6.5”


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Setting depth is usually shallow, from 24 to 50 m. (80 to 150 ft) and

while drilling the surface hole.

The casing seat must be in an impermeable formation with

su c ent ractur ng res stance to a ow u to c rcu ate to t e

surface.

Large sizes (usually 16 to 30 in.) are required as necessary to

accommodate subsequent required strings.


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SURFACE CASING

Setting depth should be in an impermeable section below fresh-water

formations.

In some instances, near-surface gravel or shallow gas may need to be

cased off.

sufficient to allow drilling to the next casing setting point and to providereasonable assurance that broaching to the surface does not occur in

.

In hard-rock areas the string may be relatively shallow, from 90 to 240

m. (300 to 800 ft), but in soft-rock areas deeper strings are necessary.

regulatory bodies to protect fresh-water sands.


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INTERMEDIATE CASING

A protective string may be necessary to case off lost circulation, salt

beds, or sloughing shales. ,

to allow reduction of mud density.

The most predominant use is to protect normally pressured formations

.

It is sometimes necessary to alter the setting depth of the intermediatecasing during drilling if:

•hole problems prohibit continued drilling

•pore pressure changes occur substantially shallower or deeper thanoriginally calculated or estimated


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Production casing is used to isolate production zones and contain

orma on pressures n e even o a u ng ea .

It is set into the reservoir and may also be a liner .

A good primary cement job is very critical for this column.

Liner

Liner is a casing string that does not extend back to the wellhead, but is

hung from another casing string.Liners are used instead of full casing strings to:

• Reduce cost

• Improve hydraulic performance when drilling deeper

• Allow the use of larger tubing above the liner top

•

Liners can be either an intermediate or a production string. Liners are

typically cemented over their entire length.


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PRESSURES AND PRESSURE GRADIENTS

Importance of knowing formation pressure gradients

While Drilling:

•

> to avoid kicks o blow-outs

> To avoid mud absorption and/or mud loss circulation

> to avoid sticking of drilling string due to caving hole

> to reduce drilling times

, , .

• To reduce the drilling problems and reach the planned well depth.

• To cut drilling costs.


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PRESSURES AND PRESSURE GRADIENTS

• “ ” .

• Pressure and “OVERBURDEN Gradient”.

• “Pressure of COMPACTION”.

“ ” .


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HYDROSTATIC PRESSURE

by the weight of the fluid column with a given density.

H f

where

10

P = hydrostatic pressure expressed in kg/cm2

H = examined depth expressed in meters

= 3 , .

kg/dm3


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Hydrostatic Pressure Gradient

Pressure Gradient is defined as a ratio of pressure value and depth:

G H

hyd 10

where:

Ghyd = hydrostatic gradient expressed in kg/cm2/10m

P = pressure expressed in kg/cm2

H = examined depth in m


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OVERBURDEN PRESSURE

SEDIMENT PRESSURE or GEOSTATIC PRESSURE or OVERBURDEN

PRESSURE is the pressure exerted on bottom of a vertical column by the weight of

sediments of a certain density, that extends from the surface to the considered depth.

It’s ex ressed in K /cm2 b use of the followin formula:

H Sed OV

10

where:

POV = overburden pressure expressed in kg/cm2

H = examined depth expressed in msed = average sediment density expressed in kg/dm

3


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SEDIMENTARY ROCK DENSITY

The sedimentary rock density (bulk density) is given by of the density of the matrix

so par mu p e y p us e ens y o e u con a ne n s pores y e roc

porosity:

sed = f + (1 - ) m

sed = sediment density (bulk density) in kg/dm3 = rock porosity expressed as a ratio

= matrix density expressed in kg/dm3

f = fluid density contained inside pores expressed in kg/dm3


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OVERBURDEN GRADIENT - GOV

• The OVERBURDEN GRADIENT is the value of the pressure variation as afunction of de th.

• It’s generally expressed in kg/cm2 /10 m and is obtained by dividing pressure by

depth.

The Overburden Gradient will therefore be e ual to:

P

GOVGOV = x 10H

where:

POVERBURDEN = Overburden pressure in kg/cm2

at H metersH = Examined depth in m


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COMPACTION PRESSURE

COMPACTIONCOMPACTION Pressure is the pressure exerted by the weight of the rock matrix

that, in normal compaction condition, is totally supported by the rock matrix by

COMPACTIONCOMPACTION Pressure is the pressure exerted by the weight of the rock matrix

that, in normal compaction condition, is totally supported by the rock matrix by

means of intergrain contacts. It’s expressed by the formula:means of intergrain contacts. It’s expressed by the formula:

= 2

= – m w ere

Φ = rock porosity expressed as a ratiom = rock matrix density expressed in kg/dm

3

“SEDIMENT PRESSURE“ (or Overburden Pressure) in kg/cm2 , can be expressed

by the formula: PSED = CP + FPwhere: CP = Compaction Pressure in kg/cm2

FP = Fluid Pressure in kg/cm2


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( ) m f sed φ φ δ −+= 1

= +


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FORMATION PRESSURE (PPORE)

ABNORMAL Pore Pressure

The Formation Pressure can be :

• OVERPRESSURE. Its value is > than the hydrostatic Pressure

• UNDERPRESSURE. Its value is < than the hydrostatic Pressure


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Formation Gradient

FORMATION GRADIENT

NORMALPore Gradient is considered normal when its value is

between 1.03 and 1.07 kg/cm2/10m.

Pore Gradient is considered

different from the ones mentioned above.

Hence there might be:

ABNORMAL

Gradient > 1.03-1.07 kg/cm2

/10m• OVERPRESSURED:

• UNDERPRESSURED: Gradient < 1.03-1.07 kg/cm2/10m


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ABNORMAL PRESSURES ABNORMAL PRESSURES

ABNORMALPRESSURES

OVERPRESSURESOVERPRESSURES

Sedimentation Speed

UNDERPRESSURESUNDERPRESSURES

TectonicsTectonics

Reservoir GeometryReservoir Geometry

Depleted ReservoirsDepleted Reservoirs

Artesian Pressure Artesian Pressure

DiapirismDiapirism

rop o a er a erop o a er a e

Dilatation due toDilatation due to

OsmosisOsmosis

Clay DiagenesisClay Diagenesis

Sulfate DiagenesisSulfate Diagenesis

Volcanic Ash DiagenesisVolcanic Ash Diagenesis


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GpGp >>

k /cmk /cm22/10/10 mm

Overpressure IndexOverpressure Index


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TECTONICSTECTONICS -- FAULT CREATIONFAULT CREATION

Normal

Side

Compressed

Side Fault Plane

1) Overturned Fold

2) Compressed Fold

3) Fault


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TECTONIC UPLIFT

BB

A C

A - C = Normal PressureBB = Overpressure


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C - D = Normal pressureA - B = Overpressure


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POSSIBLE EFFECTS OF A FAULT

BC A

D

E

GG

A - B - C - D = Normal PressureFF - GG - HH - II = Overpressure


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OVERPRESSURES DUE TO COMPRESSIVE

1

A A

BB

CC

A A

BB 2

CC


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RESERVOIR GEOMETRY

1800

Overpressure0.1

2100

Water Water

2500


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RESERVOIR GEOMETRY

OverpressureOverpressure1000

OilOil

d = 0.7d = 0.7 1500Water d = 1.03Water d = 1.03

2000 m PPORE = (2000 * 1.03)/10 = 206 kg/cm2 ; GPORE = (206/2000) * 10 = 1.03

2000

g cm m

1500 m PPORE = 206 - (1.03 * 500/10) = 154.5 kg/cm2; GPORE = (154.5/1500) * 10 =1.03

kg/cm2

/10m

1000m PPORE=154.5 kg/cm2 -(0.7 * 500/10) = 119.5 kg/cm2 - GPORE = (119.5/1000) * 10 =

= 1.195 kg/cm2/10 m

1.195 > 1.031.195 > 1.03

RESERVOIR GEOMETRY


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RESERVOIR GEOMETRY

OverpressureOverpressure1000

1500

Gasd. = 0.1

Water d = 1.03Water d = 1.03

2000 m PPORE = (2000 * 1.03)/10 = 206 kg/cm2 ; GPORE = (206/2000) * 10 = 1.03

2000

g cm m

1500 m PPORE = 206 - (1.03 * 500/10) = 154.5 kg/cm2; GPORE = (154.5/1500) * 10 =1.03

kg/cm2

/10m

1000m PPORE=154.5 kg/cm2 -(0.1 * 500/10) = 149.5 kg/cm2 - GPORE = (149.5/1000) * 10 =

= 1.495 kg/cm2/10 m 1.495 > 1.031.495 > 1.03


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PRESSURE GRADIENT Vs DEPTH IN THE CARBONATEROCKS OF THE PO VALLEY (ITALY)

l e v e l

f r o m s

e a

e p t h ( m )

Pressure Gradient - K /cm2/10 m


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PIEZOMETRIC LEVEL

+ 300 m

RKB 0 m

- 250 m


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DIAPIRITIC STRUCTURES

21


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DIAPIRISM

Overpressure

Salt


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Montmorilloniteontmorillonite is a very plastic clay whose original water content is reduced to

about 30% during the depositional phase. This clay, which is found at low depths,reaches the hydrostatic value rather rapidly, and its pore pressure has a normal

gradient.

When, by effect of subsidence, this clay is found at a lower depth and under the

action of pressure and temperature it undergoes a metamorphosis, losing some

features while acquiring a MONTMORILLONITIC - ILLITIC composition and has a

overpressure gradient.


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CLAY DIAGENESIS

--MONTMORILLONITE

before diagenesis

Free Waterinside Pores20002000 -- 3000 m3000 m

diagenesis

and compaction

Volume Loss

--


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UNDERPRESSURESUNDERPRESSURES


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kg/cmkg/cm22/ 10/ 10mm

Underpressure indexUnderpressure indexn ep e e we s, or ns ancen ep e e we s, or ns ance


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45/142


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OVERPRESSUREANALYSIS METHODS

+,-&(-+,


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• Sediment compaction increases in function of depth (at higher depthsa higher sediment compaction is expected).

• Overpressure analysis is carried out, where possible, taking into

consideration pure clay levels.

• Shales are overpressured when they did not have the possibility tothrow out interstitial water, thus resulting more porous and under-

compacted.

ALL THE ANALYSIS METHODS FOR OVERPRESSUREDETERMINATION ARE BASED ON THE FOLLOWING ASSUMPTIONS:

)','&*/ ,'%-


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6

6

!!

6

6

!!

NORMAL compactionNORMAL compaction

)','&*/ ,'%-


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, ,

Clay undercompaction =Clay undercompaction = OVERPRESSUREOVERPRESSURE

6

6

! 7! 7

6

6

! 7! 7

*,*/$+ '-


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• All the overpressure analysis methods are based on normal-compactionconcept

• The available methods are different in fuction of their utilization time:before, during or after drilling

• Their effectiveness increases if they are used successively: beforedrilling to build the model, during and after drilling to update and refinethe model

• The use of different methods within one phase increases predictioncapability

*,*/$+ '-


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8+ $ $+ $

'++ %&' &+//+,) :

&+//+,) %*&*'-'& #+/' &+//+,) &% #3 # %*'

+*'-'& &%

?

ΣA

(--+,) *,*/$+ #+/' &+//+,) (--+,)

-'%'&*-(&' #+/' &+//+,) ( -'%'&*-(&'

+7

#'// +),*/ #+/' &+//+,)- &) -

B ?

%

# /))+,) #+/' &+//+,) &

&

#+&' /+,' /) %- &+//+,) & &

)(v f


52/142

PRE-DRILL METHODSFOR OVERPRESSURE

ANALYSIS FROMSEISMIC DATA

'++ &':/'-+,


53/142

&&&

." ."

." ."

." 0." 0

&7&7

'=*%/' : '++ '-+,


54/142

*/(/*-+, -'%


55/142

• INTERVAL VELOCITY (24

• TRANSIT TIME (∆-4 of sonic waves between two reflections (µsec/ft)

• DEPTH (attention to reference “datum” from seismic!)

• SEDIMENT DENSITY

• SEDIMENT PRESSURE

• “R” RATIO

%+3/' +,%(- *-*


56/142

1. Seismic section with interpretation (it shows the curve on which twoway time and average velocities can be read).

2. Table with the following couple of values for each reflection:- two way time- average velocities of sound waves through formations

3. The following couple of values:- depth- interval velocity between two reflections

STARTING FROM TWT AND VELOCITY FUNCTION

*/(/*-+, 'C(','


57/142

STARTING FROM TWT AND VELOCITY FUNCTION

1) Interval velocity calculation

12

1

2

12

2

2

t t

t vt vv mmi −

−= V m average velocity

t TWT

2) Transit time calculationiv

t 304800=∆

3) Calculation of the distancebetween two reflectors i

vt t h

−=∆

2

12 ∆t in µs/ft

4) Calculation of averagedensity between tworeflectors

+

−−=

min

i

max

i

maxsed

vv

vv

1

111.2δ δ

δ max = 2.75 g/cm 3

v max = 7000 m/s

v min = 1500 m/s

v i interval velocities

!450

*/(/*-+, (-%(-


58/142

!

"!!!

!!!

;!!!

D!!!

E!!!

"F "G !

!450

D

6

9 :

Sediment densitycalculated from

seismic data

2'&3(&', )&*+',- */(/*-+,


59/142

) !

.7;

*

*

*

0

0*

3

3*

*

**

%) %)* %1 %1* %%* %

Overburden gradient is calculated by integrating sediment density afterhaving added to the latter curve the missing portion of data from groundsurface to the first seismic datum (extrapolation the first available data tothe surface)

SEISMIC DATUMSEISMIC DATUM 200 m200 m


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%&' )&*+',- */(/*-+,


61/142

In absence of offset wells, interstitial pressure gradient trend forecast isdone by elaborating seismic data coming from one or more shot points in

the nearby of well location.

Pore gradient estimation is drawn by applying two different methodologies:

• Transit time method ( µ sec/ft)

• “R” ratio method

*/(/*-+, /)+ 3*' *(%-+,


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The calculation is based on the assumption that transit time of sonicwaves is a linear function that decreases in semi-logaritmic scalewith depth (sediment burial by meands of other sediments increasestheir density and, consequently, sonic waves propagation velocityincreases)

+

6

( )nn vh ,

( )11,vh

( )00,vh

( )22,vh

- A µ7


63/142

0

3

*

1 4

TRANSIT TIME

( t in sec/ft):

Transit time ofsonic waves

through formations


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65/142

*(%-+,


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1. Overburden pressure acting at depth “z” is the sum of effective and pore pressure

2. If, at depth ”z1”, the rock has had time to dissipate the pore pressure that generatesduring burying process, pore pressure will be hydrostatic

3. If, instead, at depth ”z2” the rock has had no time to dissipate the pore pressure thatgenerates during burying process, pore pressure will be higher than hydrostatic

4. If at the two depth transit time is equal (obviously, in case of equal lithology) the twopoints have the same effective pressure

5. Finally, having calculated the two overburden pressures and the two gradients, the

difference between overburden and effective pressures will be:

• hydrostatic pressure “p1“at depth ”z1”• overpressure “p2“at depth “z2”

p p p peff ovbd

+=


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+6 9: & # &

*/(/*-+, '=*%/'


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0

1000

2000

3000

4000

5000

6000

10 100 1000Dt (ms/ft)

D e p t h ( m )

=

=

( )

10

23003.1275.2

10121

×−=

×−==

zGG p p povbd eff eff 8 GF;E

10

3500335.2

10

22 ×=×

= zG

p ovbd ovbd 8 G"H;E

35.28625.817222 −=−= eff ovbd p p p p 8 E;!I!

3500

109.53010

2

22 ×=×=

z

pG

p

p %*

" ;!! HE "!;

;E!! ;;E 000

6 9: &,#9>5":5

&9>5":5

∆∆∆∆ 9µµµµ"5:

'*-,J '-


69/142

==

99..::99""::++99::

6

9 :

,- +

*/(/*-+, '-


70/142

Eaton’s correlation is based on the relation, at analyzed depth,between normal ∆t, on Normal Compaction Trend, and the valuemeasured through seismic prospection.

( )

∆

∆×−−=

n

meas

NCT sed sed p

t

t GGG 03.1

The exponent n depends on available input data. A value equal to 3 isused in case of Sonic Log, while 1.5 is used for Resistivity Log.

It’s an empirical graphic method developed by eni (formerly Agip) based

%&' )&*+',- */(/*-+, #+- 5&6 &*-+ '-


71/142

p g p p y ( y g p)

on calculating and plotting R ratio

vi and va, expressed in µs/ft, are, respectively interval velocity andreference velocity in clean clay, considered at normal pressure.

In function of the value of R ratio, the interpretation will be:

R = 1 Formations with Normal Pressure Gradient

R > 1 Overconsolidated or carbonatic Formations

R < 1 Porous or overpressured Formations

a

i

v

v R =

With Two Way Times and average velocities (vm) of the nearest shot

*/(/*-+, 'C(','


72/142

point to well location, interval velocity (vi), depth, pore pressure (pp),overburden pressure (povb) and effective pressure (peff) can becalculated.

Velocity in shales assumed at normal pressure (va) is calculated

according to the correlation:

R ratio is calculated in function of depth according to the correlation:

min

eff

eff max

a v B p A

pvv +

+×

×=

a

i

v

v R =

Coefficients A and B vary in function of the analyzed area. Forexample, in Pianura Padana their value is, respectively, 0.85 e 650

!A


73/142

?@

!E!F !G A " "D "F "G !

E!!

"!!!

"E!!

!!!

E!!

;!!!

;E!!

D!!!

DE!!

E!!!

EE!!

F!!!

FE!!

H!!!

Very porous oroverpressured

formations

6

Example of R ratio

trend in function ofdepth in PianuraPadana

!

E!!

A


74/142

!E!F !G A " "D "F "G !

E!!

"!!!

"E!!

!!!

E!!

;!!!

;E!!

D!!!

DE!!

E!!!

EE!!

F!!!

FE!!

H!!!

Overcompacted

Formations

6

?@

Very porous oroverpressured

formations

Example of R ratio

trend where in theupper part R>1 valuescan be seen(undercompacted

Formations orcarbonates)


75/142

Dc Exponent ΣΣΣΣ-logThe two methods used in this case are: and

'- 3*' , &+//+,) %*&*'-'&


76/142

They are semi-empirical methods based on the following assumptions:

1. The index obtained by combining drilling parameters is an indication

of rock DRILLABILITY, intended as rock capability to be drilled by thebit

2. This drillability index, assumed everything else fixed, is inversely

proportional to depth, therefore it decreases while depth increases3. Being this index linked to rock density (higher rock matrix content,

lower pore volume in a bulk volume), where an overpressure can belocated (less rock matrix, more voids) the rock becomes more

drillable

Dc Exponent ΣΣΣΣ logThe two methods used in this case are: and

.. - .(! # +;7;B.. - .(! # +;7;B.. - .(! # +;7;B

Conceived by Jorden & Shirley in 1966, it represents rock drillability as

A'=%,',- '- A */(/*-+,


77/142

normalization of ROP (Rate Of Penetration) according to the followingcorrelation:

D

WOB

RPM

ROP

dExp

*10

*12log

*60log

2=

where ROP, RPM, WOB and D are expressed, respectively, in ft/h, rpm, lb

and in

Using m/h, rpm, t and in, the correlation becomes:

D

WOB RPM ROP

dExp

*0264.0log

*60*281.3log

=

dd--ExponentExponent

A'=%,',- '- A +,-'&%&'-*-+,


78/142

pp

D e p t h

D e p t h

In the example here beside,

the well is characterized byformations with hydrostaticpore pressure (normalgradient). d-Exponent

increases with depth andfollows a NCT


79/142

Due to Mud Weight density (MW), d-Exponent is corrected according

A'=%,',- '-


80/142

to the following correlation: MW dExpdcExp =

6

6

''CC ""''CC


81/142

dc-Exponent

A'=%,',- '- +:- +,-'&%&'-*-+,


82/142

Shifts can be composed in acontinuous curve bytranslating the shiftedportions until they overlay to

Normal Compaction Trend D e p t h

A'=%,',- '- */(/*-+, '-


83/142

As well as overpressures calculation procedures from seismic data, it ispossible to perform a similar estimation while drilling, by using thefollowing methods:

• Equivalent depth

• Eaton’s

A further estimation method, formerly used, consists in using abacuses

opportunely built.

2 zdc-Exponent

A'=%,',- '- 'C(+2*/',- '%-


84/142

122 z

eff

z

ovbd

z

p p p p −=

1022 zG p ovbd zovbd ×=

10

1

11

12

zGG

p p

z z

zeff zeff

povbd ×−

==

102

2

2

×= z

p

G

z

p z

p

V e r t i c a l d e p t

h

=

=

A'=%,',- '- '*-,

dc-Exponent


85/142

( )2.1

03.1

×−−=

norm

measovbd ovbd p

dc

dcGGG

V e r t i c a l d e p t

h

"=

"

A'=%,',- '- *3*(


86/142

A

norm A

dc

dcG ×= 03.1

""

..

"""0"3

A'=%,',- '- *3*( +,-'&%&'-*-+,


87/142

This system was developed in eni (ex AGIP) in the ’70s in occasion of Pianura

P d ll d illi Th d f i t t ti it i t d t d

ΣA '-


88/142

The method foresees the calculation of

Padana wells drilling. The need of a new interpretation criterion came out due to dc-Exponent inability to “see” overpressures in carbonatic layers.The method takes directly into consideration Mud Weight influence and is based ondrillability concept. Drillability is drawn from ROP normalization. The used drillingparameters for this calculation are (m/h), RPM (rpm), WOB (t) and Bit Size (in).

t σ '

t σ and

The final value on which the analysis is performed is obtained by the followingcorrelation:

'

0 t F σ σ =

corrected by factor, which accounts as pressure difference between mud

pressure and formation pressure and

This depends on value

F p∆n

't σ

*/(/*-'

ΣA '- */(/*-+, %&'(&' "7

25.05.0

RPMWOB


89/142

pn

pnF

∆

∆+−+=

22

* 111 ( )10

zGG p pmud ×−=∆

*/(/*-'

1' ≥t σ

−=

'

75.04

640

1

t

nσ

1' ≤t σ '64025.3

t

nσ

=

−+= 3

'

107028.0 zt t σ σ

'*

0 t F σ σ =

-',

*, :+,*//$

25.0 ROPd RPM WOB

bit

t ××=σ

Function is plotted, and for it a NCT is defined

NCT i li d fi d b h i hi h hb

0σ

ΣA '- */(/*-+, %&'(&' 7


90/142

NCT is a line defined by the equation which crosses the

abscissa axis at point = 0.088

b zar +=1000

σ

PORE GRADIENT IS CALCULATED BY THE CORRELATION:

z

pG mud p

10×∆−= ρ

And by calculating again differential pressure between mud andformation with the following correlation

( ) 12' )1(1

12 −×−−−=∆∧= n

F

F pF

t

r

σ

σ '0 t σ σ ∧@B

σσσσ0


91/142

V e

r t i c a l d e p t h m

NCT INTERPRETED

ON FUNCTION

0σ

σσσσNormal compaction trend

σσσσNormal compaction trend

As well as dc Exp also Σ log

σσσσ0


92/142

As well as dc-Exp, also Σ-logcan show some translations(shifts) caused by:

LithologyTransgressions/regressionsDifferent hole diameterBit type

Bit wearEtc…

In this case NCT will appear

shifted, but angular coefficientwill remain constant.

V e

r t i c a l d e p t h m

ΣA +,-'&%&'-*-+, "7D

σσσσ0


93/142

In presence of shifts in

overpressured Formations, thecurve is characterized by avisible variation of angularcoefficient

OVERPRESSURES TOP

V e r t i c

a l d e p t h m

ΣA +,-'&%&'-*-+, 7D

σσσσ0


94/142

Calculation of coefficientCalculation of coefficient ““bb””

in Formations with normalin Formations with normal

gradientgradient

V e r t i c a

l d e p t h m

ΣA +,-'&%&'-*-+, ;7D

σσσσ0


95/142

1r σ

2

r σ 2

0σ 10σ

Shifts can be calculatedby means of an analyticalmethod (method I)

1

0

2

0

1

2

σ σ σ σ ×= r r

V e r t i c a

l d e p t h m

A A3 AD#

0σ #

ΣA +,-'&%&'-*-+, D7D


96/142

1

0

2

012

σ σ ×= bb

V e r t i c a

l d e p t h m

2

0σ 1

0σ

Shifts can be calculatedby means of an analyticalmethod (method II)

'=*%/' : +,-'&%&'-' ΣA


97/142

E87


98/142

-'& 5#+/' &+//+,)6 '- -'%'&*-(&'


99/142

Overpressured shales aremore porous and, for thisreason, they represent a sortof thermal barrier whichprevents heat coming frombelow to pass uniformlytowards the upper layers.

Where overpressures can bespotted, the GeothermicalGradient (usually 3°/100m)shows a sharp increase.

-'& 5#+/' &+//+,)6 '- &'+-+2+-$


100/142

D e p t h m

Resistivity

OVERPRESSURESTOP

MUD RESISTIVITY – Mud contamination by means of formation waterdue to overpressure not sufficiently balanced causes a decrease ofresistivity value, since formation fluid is assumed with higher salinity thandrilling mud.

-'& 5#+/' &+//+,)6 '- /&+'


101/142

D e

p t h m

Chlorides

OVERPRESSURESTOP

MUD CHLORIDES CONTENT – The chemical analysis of chlorides in

drilling mud as it comes out of the well can highlight an overpressuresince the contamination could have been caused by formation fluidinflux. Formation fluid is assumed with higher salinity than drilling mud.

GAS INFLUXES

-'& 5#+/' &+//+,)6 '- +),*/7'2',-


102/142

Pipe connection gasTrip gasBackground gas

HOLE TIGHTENINGHigh torqueOverpull/drag

Reaming/backreamingPresence of cavingsBreakouts

MUD PUMPING PRESSURE


103/142


104/142

POST-DRILLINGMETHODS

OVERPRESSUREANALYSIS FROM LOGS

The analysis methods are based on the measurement of clay electrical

*,*/$+ '-


105/142

behavior. In particular the methods are:

• ∆∆∆∆t Shale method, based on transit time measurement, by soniclogs, of an elastic perturbation which propagates along wellborewalls

• Resistivity method, based on the measurement of resistivity metby electric field transmitted through borehole walls and generatedby electric logs

D

∆t (µs/ft)

∆- */' '-


106/142

*

*

*

V e r t i c a l d e

p t h ( m )

The assumption is, again, thatpropagation velocity of elastic

waves increases with depth(for higher rock density).

Consequently, transit time (∆t)decreases regularly and it istherefore possible to draw aNCT.

DD ))

∆t (µs/ft)

∆- */' '- 3*+ %&+,+%/'


107/142

Assuming that density, porosityand relative pressures (effective

and pore ones) areintercorrelated, if by increasingdepth and assuming otherconditions unvaried the transit

time decreases (deviating fromclean shales NCT), theinterested layers areoverpressured

V e

r t i c a l

V e

r t i c a l d e p t h

d e p t h m m

**

**

*

OVERPRESSURESTOP

∆t (µs/ft)

)D

∆- */' '- %&'',' : +:-


108/142

V e

r t i c a l

V e

r t i c a l d e p t h

d e p t h m m

In ∆t-shale method shifts can be

identified, even if they are not sofrequent. These must bedistinguished from NCT slopevariation. The main cause of

shifts can be related togeological issues.

*

*

Availability of an electrical log (resistivity, SP)/geological (GRay)

Availability of acoustic log (ex BHC Sonic Log)

∆- */' '- ,- '-'&+,*-+,


109/142

Availability of acoustic log (ex. BHC Sonic Log)

Availability of Caliper/Image log

Identification of CLEAN shales (and isolate the corresponding ∆tvalues)

Plotting ∆t vs depth (in a semilog plot)

Drawing NCT

INTERPRETING ∆t Shale trend.

IN DEVIATED WELLS, DEPTH SHALL BE VERTICALIZED

/*$ +',-+:+*-+,

S P


110/142

G R

R e s


111/142

"!!!

!

E!!

"! "!!

∆ 1µ74


112/142

,-

"!!!

"E!!

!!!

E!!

;!!!

;E!!

D!!!

2 1 4

• Estimation of bulk density from acoustic log (if density log not

∆- */' '- */(/*-+, 'C(','


113/142

available or incomplete);

• Calculation of overburden gradient, by integrating density curve;

• Acoustic (sonic) log analysis and NCT determination;

• Pore pressure gradient calculation by means of equivalent depthor Eaton’s method


114/142

mt t ∆−∆=φ

∆∆∆∆t VS. POROSITY CORRELATIONS

Consolidated soils and rocks

∆- */' '- ',+-$ '-+*-+, 7;


115/142

153

568.1 mt t ∆−∆

×=φ

153=φ

89

28.3 t

sed

∆+=δ

20011.275.2

+∆

∆−∆×−=

t

t t msed δ

200228.1 +∆

∆−∆×= t

t t mφ


Slightly or not consolidated terrigenous

Consolidated soils and rocks (alternative)

∆∆∆∆t VS. BULK DENSITY CORRELATIONS


Slightly or not consolidated soils

The following correlation, developed by Agip, was built by comparing itsresults to density values coming from Formation Density Correlated

∆- */' '- ',+-$ '-+*-+, ;7;


116/142

2004711.275.2

+∆−∆×−=

t

t sed δ

esu ts to de s ty a ues co g o o at o e s ty Co e atedLogs. The results of this comparison revealed the wide validity of this

correlation, which can be used with good reliability for every

formation type.

Resistivity depends on rock porosity (fluid in rock pores). Rockscharacterized by low porosity have high resistivity (ex. compact

&'+-+2+-$ '-


117/142

limestone, volcanic rocks..).

Having other conditions fixed, rock resistivity depends on:

• salt concentration• rock composition

• temperature

Shales density increases with increasing depth, thus increasingcompaction and decreasing porosity. For this reason, resistivity

increases.

The methods based on shales resistivity for pore pressure estimationb i ll t

&'+-+2+-$ '- */(/*-+, '-


118/142

are basically two:

I°method – from an electric log, shales resistivity is obtained andthen it is plotted vs depth in a semi-logarithmic scale. Loginterpretation is performed directly on this curve, without furthercalculation.

II°method – F-shale factor (clay formation factor) is identified fromresistivity curve and is used for the interpretation by plotting it vsdepth in a semi-logarithmic scale.

Clay resistivity

Resistivity of clean shales isplotted in semi-logarithmic scalein function of vertical depth The

&'+-+2+-$ '- '- + "7


119/142

D e p t h

in function of vertical depth. Thecorrelation between resistivity

and porosity (fluid content, sincesaturation = 1 is assumed) isinversely proportional andgenerates an increasing NormalCompaction Trend.In case of Formations withnormal pore gradient, resistivityvalues allign around a line withincreasing trend in function of

depth.

Clay resistivity

&'+-+2+-$ '- '- + 7


120/142

+

D e p t h

In case of overpressuredlevels, the trend of measured

resistivity values depart fromNormal Compaction Trend.The deviation is high or lowin function of absolute

pressure value.

%%)%D %

In this cases the analyzed trend is not resistivity one, but shales formation factor

F-Shale. It is calculated from the ratio between measured shales resistivity and

formation fluid one:

&'+-+2+-$ '- '- ++ "7;


121/142

V e r t i c

a l d e p t h m

*

*

*

“F shale”Normal gradient

Formations

wshalew

shaleshale

RC R

RF

×== 1


122/142

The operational sequence to be followed for F-Shale analysis is

illustrated here below:

1 Calculate or measure formation water resistivity R throughout

&'+-+2+-$ '- '- ++ ;7;


123/142

1. Calculate, or measure, formation water resistivity Rw throughoutthe well.

2. Plot Rw values on a semi-logarithmic scale.

3. Read resistivity value from log data for clean shales throughoutthe wellbore profile.

4. Calculate F-Shale value for the analyzed clay points.

5. Plot F-Shale values on a semi-logaritmic scale.

6. Draw F-Shale Normal Compaction Trend.

7. Evaluate the presence of overpressures and interpret their trend.

The main limits of resistivity log analysis can be resumed as follows:

• It can not be applied in carbonatic layers

&'+-+2+-$ '- /++-


124/142

• It can not be applied in carbonatic layers

• It can be applied only in wells with frequent shale-sandinterbedding

• Spontaneous Potential (SP) value shall be easily distinguishedbetween sands and shales

• Shales shall be clean

• Hydrocarbons in shales (oil or gas) can modify conductivity values

• Wellbore must be in gauge


125/142

FRACTURE GRADIENTESTIMATION AND

VERIFICATION

Once having calculated Overburden and Pore curves, in order tocomplete the pressure model Fracture Gradient shall be estimated.This value is an indication of borehole wall propension to break

+,-&(-+,


126/142

p p(fracture opening) due to excessive Mud Weight.

Knowing fracture ggradient curve throughout the whole well length,together with pore gradient one, is of the utmost importance for themain planning and drilling phases of a well:

• During planning phase, it allows establishing the optimal casingshoe depth in function of choke margin and differential pressure

• During drilling phase, it allows safe operations in case ofkick/blowout

The correlations used for fracture gradient calculation are based on theassumption that, in case of homogeneous, elastic and isotropic mean,in situ stress state is modified by the presence of the well and stresses

:&* */(/*-+, "7


127/142

redistribute around its lateral surface.

σσσσ

θ ′

r σ ′

w p


128/142

From the solution of elastic equations and in function of formation type, inparticular concerning Poisson’s Ratio coefficient, fracture pressure isobtained from the following correlations:

&&'/*-+, :& :&* */(/*-+, "7


129/142

ELASTIC FORMATIONS with low permeability and minimum filtrateinvasion:

( ) povbd p frac p p p p −−

+=ν ν

1

2

INCONSOLIDATE OR SLIGHTLY CEMENTED FORMATIONS with highpermeability and sensible filtrate invasion:

povbd p frac p p p p −+= ν 2

PLASTIC FORMATIONS:

ovbd frac p p =

+ B 9

9 - %J & ν8!E B 9

&&'/*-+, :& :&* */(/*-+,7


130/142

( ) p psed frac GGGG +−=32

The 2/3 coefficient shall be modified as follows:

• in slightly consolidated sands = 1/2;

• in shales or silty marl = 3/4.

( ) povbd p frac GGGG −−+=

ν ν

1

A A* A A*

,#-&

945"5:

"-&


131/142

0

3

&

Adding fracture

gradient calculationto the previouslymentioned curvesgenerates a plot

similar to the one infigure.


132/142

The LOT is performed in a well during drilling phases. It is carried out inopen hole and consists in pressurizing the well until pressure causes areaction to the well.

/'*. :: -'-


133/142

reaction to the well.

The LOT can be performed for two main reasons:

• Verification, after casing setting, of the real value of fracturegradient below the last casing shoe;

• Verify, after crossing a level characterized by high porosity andpermeability, a more realistic value of fracture pressure andgradient.

1. Drill cement and casing shoe and then drill 10m of virgin formation.

2. Circulate for mud density conditioning in the whole well.

3 Close BOP

/- %'&*-+2' %&'(&'


134/142

3. Close BOP.

4. Pump at low flow rate (¼ - ½ bbl/h) and plot flow rate and pressure valueson a diagram.

5. Carry on pumping until no more than two values depart from linear pumpingtrend.

6. Wait for pressure stabilization and read final value.

7. Add to the read value the hydrostatic pressure applied at bottom depth bymud column. This will be the value of fracture pressure.

8. Calculate fracture gradient.

*

0

7

/- %/-


135/142

P r e s s

u r e ( p s i )

Pumped volume (bbl)

*

*

7


136/142

7

7 7> >( -

"- -;; ;- 6- ; - .- -

/- :&*-(&+,) 7

u r e


137/142

7

++++

;;

+

8 - "!

P r e s s u

Time

=/- 1'? /@ -4 /-

'=-',' /'*. :: -'-


138/142

/-

M A

M B @ B B

B9

M @ B B

+ A - ? B 9

- :+- /-

9

:&*-+, +,-')&+-$ -'-


139/142

B 9 A

B @ @

M "!

M B B

M 3%

M % B B 1N A O 9974 B

M A 1 B4

(*&$ '' %&'A&+//

INPUT: seismic vm e TWT

vi vs Depth


140/142

∆t vs Depth

ρ bulk

OBG

NCT

PPG

FG

Equiv.depth, Eaton, R ratio

(*&$ '' #+/' &+//+,)

INPUT: mudlog ROP, RPM,

WOB, D, MW


141/142

Dc-Exp, ΣALog

NCT

PPG

Equiv.depth, Eaton, abacus

(*&$ '' %-A&+//

INPUT: logs

Caliper GR, Res, SP Sonic, Res Density


142/142

NCT

OBG

PPG FG

Shale Sonic

Filtered Sonic

1 - drilling technology - gradients

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