1 - Drilling Technology - Gradients

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 estimate

estimate.

• 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 Point Selection • Casing Point Selection • Casing Design

• Mud Density

Fracturing Gradient • Fracturing Gradient • Drilling Rig Selection

Where does the well plan come from?

The well plan is a product of many different people in the oil company. p p y p p p y

Team Members Geoscience Department  Geophysicist  Geologist Engineering Department  Drilling  Production  Geologist  Production  Reservoir

Operations Department Support Department

 Drilling manager

 Drilling superintendent D illi i

 Loss prevention – safety  Environmental

P h i  Drilling supervisor

 Drilling coordinator

Contents of a well Plan

• Well summary

1.Drilling and geological prognosisg g g p g • Drilling procedure_{1 Location / pre-spud} 2.Drawings

a.Well schematic b.BOPs and manifold c Wellhead

1.Location / pre spud 2.Conductor hole 3.Surface hole 4.Intermediate hole c.Wellhead d.Location e.Structural map 3.Pore pressure analysis

4.Intermediate hole 5.Production hole 6.Completion

7.Standard procedures

4.Type log

5.Drilling time curve

6.Drilling cost curve and estimate or AFE p 8.Abandonment or AFE 7.Support a.Vendors list b.Transport c.Communications 8.Directional plan

Contents of a well Plan • Drilling parameters 1 Mud program 1.Mud program 2.Drilling mechanics 3.Bits a.Weight and RPM a.Weight and RPM b.BHA / drillstring c.Hydraulic program 4.Casing programg p g 5.Cement program 6.Well control program 7.Wellhead equipment 8.Rig specs

9.Logging, coring, and testing 10.Emergency procedures

a.Hurricane procedures

b.Fire drills and rig evacuation c.Blowout control procedures

DRILLING TIME CURVES

0 _{Phase 16"} csg csg 13 3/8"13 3/8" 500 Phase 12 1/4" csg csg 13 3/813 3/8 Depth vs. Time 1.000 csg 9 5/8" 1.500 Phase 8 1/2" 2.000 Well Testing csg 7" 2.500 0 5 10 _giorni 15 20 25

Types of casings

yp

g

 Conductor pipe  Surfaces  Surfaces  Intermediate  Production  Liner

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:g

casing/ liner dimension open-hole dimension casing/ liner dimension

(inches) 20” open-hole dimension (inches) 26” 20 18 5/8” 13 3/8” 9 5/8” 26 24” 17.5” 12.25” 9 5/8 7” 5” 5 8.5” 6.5”

CONDUCTOR PIPE CONDUCTOR PIPE

Setting depth is usually shallow, from 24 to 50 m. (80 to 150 ft) and selected so that drilling fluid may be circulated to the mud pits

selected so that drilling fluid may be circulated to the mud pits while drilling the surface hole.

The casing seat must be in an impermeable formation with

ffi i t f t i i t t ll fl id t i l t t th

sufficient fracturing resistance to allow fluid to circulate to the surface.

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

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.

The depth should be great enough to provide a fracture gradient The depth should be great enough to provide a fracture gradient

sufficient to allow drilling to the next casing setting point and to provide reasonable assurance that broaching to the surface does not occur in event of closure on a kick

event of closure on a kick.

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. Surface casing setting depths are often specified by government

Surface casing setting depths are often specified by government regulatory bodies to protect fresh-water sands.

INTERMEDIATE CASING

A protective string may be necessary to case off lost circulation, salt beds, or sloughing shales.

In cases of pressure reversals with depth protective casing may be set In cases of pressure reversals with depth, protective casing may be set to allow reduction of mud density.

The most predominant use is to protect normally pressured formations from the effects of increased mud density needed in deeper drilling

from the effects of increased mud density needed in deeper drilling. It is sometimes necessary to alter the setting depth of the intermediate casing during drilling if:

•hole problems prohibit continued drilling

•pore pressure changes occur substantially shallower or deeper than originally calculated or estimated

PRODUCTION CASING PRODUCTION CASING

Production casing is used to isolate production zones and contain

f ti i th t f t bi l k

formation pressures in the event of a tubing leak. 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 g g hung from another casing string.

Liners are used instead of full casing strings to: • Reduce costReduce cost

• Improve hydraulic performance when drilling deeper • Allow the use of larger tubing above the liner top • Not represent a tension limitation for a rig

• Not represent a tension limitation for a rig

Liners can be either an intermediate or a production string. Liners are typically cemented over their entire length.

PRESSURES AND PRESSURE GRADIENTS

Importance of knowing formation pressure gradients

While Drilling:

• To use adequate mud density:To use adequate mud density: > to avoid kicks o blow-outs

> To avoid mud absorption and/or mud loss circulation > to avoid sticking of drilling string for differential pressure > to avoid sticking of drilling string for differential pressure > to avoid sticking of drilling string due to caving hole

> to reduce drilling times

• To change in case of need the casing point depth while drilling • To change, in case of need, the casing point depth while drilling. • To reduce the drilling problems and reach the planned well depth. • To cut drilling costs.

PRESSURES AND PRESSURE GRADIENTS

• Pressure and “HYDROSTATIC Gradient”Pressure and HYDROSTATIC Gradient . • Pressure and “OVERBURDEN Gradient”. • “Pressure of COMPACTION”.

• Pressure and “FORMATION Gradient” • Pressure and FORMATION Gradient .

HYDROSTATIC PRESSURE

Hydrostatic pressure at a certain depth is defined as the pressure exerted Hydrostatic pressure at a certain depth is defined as the pressure exerted by the weight of the fluid column with a given density.

P





f



H

where

P

10

P = hydrostatic pressure expressed in kg/cm2 H = examined depth expressed in meters

 _f = fluid density expressed in kg/dm3 _{usually for water assumed to be equal to 1 03}  _f fluid density expressed in kg/dm , usually for water assumed to be equal to 1.03 kg/dm3

Hydrostatic Pressure Gradient

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

P

G P

H

hyd   10

where:

G_hyd= hydrostatic gradient expressed in kg/cm2_/10m P = pressure expressed in kg/cm2

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 expressed in Kg/cm_{p g} 2 _{by use of the following formula:y g}

P





Sed



H

P

_OV



10

where:

P_OV = overburden pressure expressed in kg/cm2 H = examined depth expressed in m

SEDIMENTARY ROCK DENSITY

The sedimentary rock density (bulk density) is given by of the density of the matrix ( lid t) lti li d b l th d it f th fl id t i d i it b th k (solid part) multiplied by plus the density of the fluid contained in its pores by the rock porosity:

_sed =  _f + (1 - ) _m

where: where:

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

_m = matrix density expressed in kg/dm3

m y p g

OVERBURDEN GRADIENT - GOV

• The OVERBURDEN GRADIENT is the value of the pressure variation as a function of depth.p

• It’s generally expressed in kg/cm2 _{/10 m and is obtained by dividing pressure by} depth.

The Overburden Gradient will therefore be equal to: The Overburden Gradient will therefore be equal to:

P_OVERBURDEN P_OVERBURDEN GOV GOV = x 10 H where:

P_OVERBURDEN = Overburden pressure in kg/cm2 _{at H meters}

H E i d d h i

COMPACTION PRESSURE

COMPACTION

COMPACTION 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 COMPACTION

COMPACTION 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 p y pp y y means of intergrain contacts. It’s expressed by the formula:

p y pp y y

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

CP

= (1 Φ x



) x H

h

CP = compaction pressure in kg/cm2

CP

= (1 – Φ x



_m

) x H

where

CP compaction pressure in kg/cm Φ = rock porosity expressed as a ratio

_m = rock matrix density expressed in kg/dm3

“SEDIMENT PRESSURE“ (or Overburden Pressure) in kg/cm2 _{, can be expressed} by the formula:

P

_SED

= CP + FP

where: CP = Compaction Pressure in kg/cm2

(

)

m f sed

φδ

φ

δ

δ

=

+

1

−

=

+

6(',0(17RU³%8/.´ '(16,7<

FORMATION PRESSURE (P

_PORE

)

ABNORMAL Pore Pressure

The Formation Pressure can be :

•

OVERPRESSURE. Its value is > than the hydrostatic Pressure • UNDERPRESSURE. Its value is < than the hydrostatic Pressure

Formation Gradient

FORMATION GRADIENT

NORMAL Pore Gradient is considered normal when its value is _{between 1.03 and 1.07 kg/cm}2_/10m.

Pore Gradient is considered abnormal when its value is abnormal when its value is

different from the ones mentioned above. Hence there might be:

ABNORMAL

Gradient > 1.03-1.07 kg/cm2_/10m • OVERPRESSURED:

ABNORMAL PRESSURES ABNORMAL PRESSURES ABNORMAL PRESSURES OVERPRESSURES OVERPRESSURES Sedimentation Speed UNDERPRESSURES UNDERPRESSURES Tectonics Tectonics Reservoir Geometry Reservoir Geometry D f W t T bl D f W t T bl Depleted Reservoirs Depleted Reservoirs Artesian Pressure Artesian Pressure Diapirism Diapirism Reservoir Repressurized Reservoir Repressurized

Drop of Water Table Drop of Water Table

Dilatation due to Tectonic PhenomenaDilatation due to Tectonic Phenomena Osmosis Osmosis Clay Diagenesis Clay Diagenesis Reservoir Repressurized

Reservoir Repressurized Tectonic PhenomenaTectonic Phenomena

Sulfate Diagenesis Sulfate Diagenesis Volcanic Ash Diagenesis Volcanic Ash Diagenesis

Gp

Gp

>

>

kg/cm

kg/cm

22

_/10

_{/10 m}

_m

kg/cm

kg/cm /10

/10 m

m

Overpressure Index

Overpressure Index

ORIGIN OF OVERPRESSURES

•Sedimentation Velocity T t i • Tectonics • Reservoir Geometry • Artesian Pressures • Diapirism • Diagenesis • OsmosisOsmosis

TECTONICS

TECTONICS -- FAULT CREATION

FAULT CREATION

Normal

Side Compressed_Side

Fault Plane

1) Overturned Fold

2) Compressed Fold) p

TECTONIC UPLIFT

B

B

A

C

A - C = Normal Pressure

B

B

=

Overpressure

C - D = Normal pressure A - B = Overpressure

&

$

$

'

'

%

029(0(17$/21*$)$8/73/$1(

POSSIBLE EFFECTS OF A FAULT

B _C A D E F F F F G G II HH II HH A - B - C - D = Normal Pressure F F - GG - HH - II = Overpressure

OVERPRESSURES DUE TO COMPRESSIVE

TECTONIC PHENOMENA

TECTONIC PHENOMENA

1 A A B B C C A A B B ₂ C C

RESERVOIR GEOMETRY

Hydrocarbons Hydrocarbons 1800

Overpressure

Hydrocarbons Hydrocarbons 0.1 2100

Water

Water

2500

RESERVOIR GEOMETRY

Overpressure

Overpressure

1000

Oil

Oil

d = 0.7

d = 0.7

₁₅₀₀

Water d = 1.03

Water d = 1.03

2000 m P_PORE = (2000 * 1.03)/10 = 206 kg/cm2 _{; G} PORE = (206/2000) * 10 = 1.03 k / 2_/10 2000 kg/cm2_{/10 m} 1500 m P_PORE = 206 - (1.03 * 500/10) = 154.5 kg/cm2_{; G} PORE = (154.5/1500) * 10 =1.03 kg/cm2_/10m 1000m P_PORE=154.5 kg/cm2 _{-(0.7 * 500/10) = 119.5 kg/cm}2 _{- G} PORE= (119.5/1000) * 10 = = 1.195 kg/cm2_{/10 m}

1.195 > 1.03

1.195 > 1.03

RESERVOIR GEOMETRY

Overpressure

Overpressure

1000 1500

Gas

d. = 0.1

Water d = 1.03

Water d = 1.03

1500 2000 m P_PORE = (2000 * 1.03)/10 = 206 kg/cm2 _{; G} PORE = (206/2000) * 10 = 1.03 k / 2_/10 2000 kg/cm2_{/10 m} 1500 m P_PORE = 206 - (1.03 * 500/10) = 154.5 kg/cm2_{; G} PORE = (154.5/1500) * 10 =1.03 kg/cm2_/10m 1000m P_PORE=154.5 kg/cm2 _{-(0.1 * 500/10) = 149.5 kg/cm}2 _{- G} PORE= (149.5/1000) * 10 = = 1.495 kg/cm2_{/10 m}

1.495 > 1.03

1.495 > 1.03

PRESSURE GRADIENT Vs DEPTH IN THE CARBONATE

ROCKS OF THE PO VALLEY (ITALY)( )

level from sea D epth (m) D

PIEZOMETRIC LEVEL

+ 300 m

RKB 0 m

DIAPIRITIC STRUCTURES

CREATION OF A SALINE DOME

CREATION OF A SALINE DOME

2

1

DIAPIRISM

Overpressure

Salt

CLAY DIAGENESIS

CLAY DIAGENESIS

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.

CLAY DIAGENESIS

1000 1000 -- 2000 m2000 m MONTMORILLONITE before diagenesis 1000 1000 2000 m2000 m

After the ILLITE

Free Water inside Pores

2000

2000 -- 3000 m3000 m

After the ILLITE diagenesis After diagenesis 3000 3000 4000 m4000 m After diagenesis and compaction Volume Loss 3000 3000 -- 4000 m4000 m

UNDERPRESSURES

UNDERPRESSURES

OSMOSIS

If two saline solutions with different concentrations and (initially) equal pressure are separated by a membrane, an “OSMOTIC” flow takes place as ions pass from a solution to the other until saline concentrations are balanced, but final pressures are different.

The solution that initially had lower concentration loses pressure in favor of the solution that initially had higher concentration.

(In nature this phenomenon can take place when two porous formations, with different salinity, are separated by a clayey septum.)

kg/cm

kg/cm

22

_{/ 10}

_{/ 10}

Gp <

Gp <

m

m

Gp <

Gp <

Underpressure index

Underpressure index

(i d l t d

ll f

i t

)

(i d l t d

ll f

i t

)

(in depleted wells, for instance )

(in depleted wells, for instance )

UNDERPRESSURE DUE TO EXTENDED PRODUCTION

UNDERPRESSURE DUE TO WATER TABLE LOWERING

derpressure

Water

OVERPRESSURE

• Sediment compaction increases in function of depth (at higher depths a 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 to throw out interstitial water, thus resulting more porous and under-compacted.

ALL THE ANALYSIS METHODS FOR OVERPRESSURE

DETERMINATION ARE BASED ON THE FOLLOWING ASSUMPTIONS:

6 6 ! ! 6 6 ! ! NORMAL compaction NORMAL compaction

)','&*/

,

'%-, ,

Clay undercompaction =

Clay undercompaction = OVERPRESSUREOVERPRESSURE

)','&*/

,

'%-6 6 ! 7 ! 7 6 6 ! 7 ! 7

• All the overpressure analysis methods are based on normal-compaction concept

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

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

• The use of different methods within one phase increases prediction capability

'-8 + $ $ + $

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

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

? ΣA

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

-' %'&*-(&' # +/' &+//+,) ( -' %'&*-(&' +7

#'// +),*/ # +/' &+//+,) - & ) -B ? % # / ))+,) # +/' &+//+,) & &

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

& )

(v f

'-PRE-DRILL METHODS

FOR OVERPRESSURE

ANALYSIS FROM

SEISMIC DATA

& & & ." ." ." ." ." 0 ." 0 &7 &7

'+ + &':/' -+ ,

• INTERVAL VELOCITY (2 4

• TRANSIT TIME (∆-4 of sonic waves between two reflections (µsec/ft) • DEPTH (attention to reference “datum” from seismic!)

• SEDIMENT DENSITY • SEDIMENT PRESSURE • “R” RATIO

1. Seismic section with interpretation (it shows the curve on which two way 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

1) Interval velocity calculation

1 2 1 2 1 2 2 2 t t t v t v v m m i ₋ − = Vm average velocity t TWT

2) Transit time calculation

i

v

t

=

304800

∆

3) Calculation of the distance

between two reflectors

v

i

t

t

h

=

−

∆

2

1 2 _{∆t in µs/ft} 4) Calculation of average density between two

reflectors

+

−

−

=

min i max i max sed

v

v

v

v

1

1

11

.

2

δ

δ

δmax = 2.75 g/cm 3 vmax = 7000 m/s vmin = 1500 m/s v_i interval velocities

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

! "!!! !!! ;!!! D!!! E!!! " F " G ! !4 5 0 D 6 9 :

*/ (/*-+ ,

(-%(-Sediment density calculated from seismic data

) ! . 7 ; * * * 0 0* 3 3* * ** %) %)* %1 %1* % % * %

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

SEISMIC DATUM

SEISMIC DATUM 200 m200 m

74 ;+< 94 5 0_: & ; + 94 5" 5 : ! E % % ! "!!! !!! ;!!! D!!! E!!! F!!! " E 6 BULK DENSITY INTEGRATED SEDIMENT DENSITY

In absence of offset wells, interstitial pressure gradient trend forecast is done 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

The calculation is based on the assumption that transit time of sonic waves is a linear function that decreases in semi-logaritmic scale with depth (sediment burial by meands of other sediments increases their density and, consequently, sonic waves propagation velocity increases) + 6 ( )h ,n vn ( )h1,v1 (h0,v0) (h2,v2)

*/ (/*-+ , / )+

3* '*

( %-+ ,

0 3 * 1 4 - A µ 7 TRANSIT TIME ( t in sec/ft): Transit time of sonic waves through formations

Being transit time input data available, to calculate pore gradient transit time method can be applied. Its application is done mainly through the use of two different methodologies:

• EQUIVALENT DEPTH method

• EATON’S method

∆∆∆∆ 9µµµµ "5 : = = = = 9 .: 9 .: 99 "" ::++99 :: 6 9 : , - +

'C(+

2*/',-

'%-

'-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 generates during burying process, pore pressure will be hydrostatic

3. If, instead, at depth ”z2” the rock has had no time to dissipate the pore pressure that generates 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 two points 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

_ovbd = _eff + _p

1. Define normal compaction trend

2. Choose the depth at which pore gradient (assumed overpressured) will be calculated

3. Draw a vertical line from the chosen depth (point 2) until Normal

Compaction Trend is reached (point 1). This depth and point 2 one have the same effective pressure

4. From overburden gradient curve, calculate overburden pressure of the two chosen points

5. Calculate effective pressure of point 1, given overburden and pore (hydrostatic) pressures

6. Calculate pore pressure at point 2 from the difference between overburden and effective pressure calculated at step 5

7. Calculate pore gradient

0 1000 2000 3000 4000 5000 6000 10 100 1000 Dt (ms/ft) D ep th (m ) = = +

(

)

(

)

10 2300 03 . 1 275 . 2 10 1 2 1 _{=p =} G −G ×z ₌ − ×

p_{eff eff} ovbd p 8 GF;E

10 3500 335 . 2 10 2 2 ₌ G ×z ₌ × p ovbd ovbd 8 G"H;E 35 . 286 25 . 817 2 2 2 _{= − = −} eff ovbd p p p p 8 E;! I! 3500 10 9 . 530 10 2 2 2 ₌ × ₌ × z p G_p p %* " ;!! HE " !; ;E!! ;;E 000 69 : _{& ,#} 9> 5" :5 & 9> 5" :5

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

∆∆∆∆ 9µµµµ "5 : = = 9 9 ..:: 99 "" ::++99 :: 6 9 : , - +

'*- ,J

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

(

)

∆

∆

×

−

−

=

n meas NCT sed sed p

t

t

G

G

G

1

.

03

The exponent n depends on available input data. A value equal to 3 is used 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 on calculating and plotting R ratio

vi and va, expressed in µs/ft, are, respectively interval velocity and

reference 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 (v_m) of the nearest shot point to well location, interval velocity (v_i), depth, pore pressure (p_p), overburden pressure (p_ovb) and effective pressure (p_eff) can be calculated.

Velocity in shales assumed at normal pressure (v_a) 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 p v v + + × × = a i v v R =

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

? @ ! ! E! F ! G _A " " D " F " G ! E!! " !!! " E!! !!! E!! ; !!! ; E!! D!!! DE!! E!!! EE!! F!!! FE!! H!!! Very porous or overpressured formations A 6 Example of R ratio trend in function of depth in Pianura Padana

! ! E! F ! G _A " " D " F " G ! E!! " !!! " E!! !!! E!! ; !!! ; E!! D!!! DE!! E!!! EE!! F!!! FE!! H!!! Overcompacted Formations 6 ? @ Very porous or overpressured formations A Example of R ratio trend where in the

upper part R>1 values can be seen

(undercompacted Formations or carbonates)

OVERPRESSURES

?

WHILE DRILING

@

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 the bit

2. This drillability index, assumed everything else fixed, is inversely proportional to depth, therefore it decreases while depth increases

3. Being this index linked to rock density (higher rock matrix content, lower pore volume in a bulk volume), where an overpressure can be located (less rock matrix, more voids) the rock becomes more drillable

Dc Exponent ΣΣΣΣ-log

The 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 normalization of ROP (Rate Of Penetration) according to the following correlation: D WOBRPM ROP dExp * 10 * 12 log * 60 log 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 . 0 log * 60 * 281 . 3 log =

A'=% ,',-

'-

A

*/ (/*-+ ,

d d--ExponentExponent D ep th D ep th

In the example here beside, the well is characterized by formations with hydrostatic pore pressure (normal

gradient). d-Exponent increases with depth and follows a NCT

d-EXPONENT METHOD - LIMIT

The main d-exponent limit consist s in the fact that mud

density effect is not considered

density effect is not considered.

Since ROP is influenced by this parameter, d-exponent

values must be corrected according to it

Due to Mud Weight density (MW), d-Exponent is corrected according to the following correlation:

MW dExp dcExp = 6 6 ' ' CC ""'' CC

A'=% ,',-

'-dc-Exponent

D

ep

th

NCT line continuity, when it is

NCT line continuity, when it is

drawn on d

drawn on d--exponent, can be exponent, can be interrupted due to effects not

interrupted due to effects not

depending from overpressures:

depending from overpressures:

•

• lithology, lithology, •

• transgressions/regressions,transgressions/regressions, •

• different hole diameter,different hole diameter, •

• bit type,bit type, •

• bit wear,bit wear, •

• etc.etc.

In this case the curve appears

In this case the curve appears

shifted, but its slope remains

shifted, but its slope remains

constant.

constant.

:-Shifts can be composed in a continuous curve by

translating the shifted

portions until they overlay to Normal Compaction Trend

dc-Exponent

D

ep

th

As well as overpressures calculation procedures from seismic data, it is possible to perform a similar estimation while drilling, by using the

following methods: • Equivalent depth • Eaton’s

A further estimation method, formerly used, consists in using abacuses opportunely built.

'-1 2 2 z eff z ovbd z p

p

p

p

=

−

10

2 2 2

G

z

p

z ovbd z ovbd

×

=

(

)

10

1 1 1 1 2

p

G

G

z

p

z z z eff z eff p ovbd

−

×

=

=

10

2 2 2

=

×

z

p

G

z p z p dc-Exponent V er tic al d ep th = =

A'=% ,',-

'-

'C(+

2*/',-

'%-(

−

1

.

03

)

×

1.2

−

=

norm meas ovbd ovbd p

dc

dc

G

G

G

A'=% ,',-

'-

'*- ,

dc-Exponent V er tic al d ep th " = "

A norm A

dc

dc

G

= 03

1

.

×

" " _.. " " "₀ "3

A'=% ,',-

'-

*3* (

The method foresees the calculation of

This system was developed in eni (ex AGIP) in the ’70s in occasion of Pianura 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 on drillability concept. Drillability is drawn from ROP normalization. The used drilling parameters 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 following correlation:

' 0

F

σ

t

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

'-p n p n F ∆ ∆ + − + = 2 2 * ₁ 1 1

(

)

10

z

G

G

p

=

_mud

−

_p

×

∆

*/ (/*-'

1

'

_≥

t

σ

= − ' 75 . 0 4 640 1 t n σ

1

'

_≤

t

σ

₆₄₀ ' 25 . 3 t n σ = − + = ₃ ' 10 7 028 . 0 z t t σ σ ' * 0

F

σ

t

σ

=

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

ΣA

'-

*/ (/*-+ , %&

' (&'"7

25 . 0 25 . 0 5 . 0

ROP

d

RPM

WOB

bit t

_×

×

=

σ

Function is plotted, and for it a NCT is defined

NCT is a line defined by the equation which crosses the abscissa axis at point = 0.088

b

z

a

r

=

+

1000

σ

0

σ

PORE GRADIENT IS CALCULATED BY THE CORRELATION:

z

p

G

_p

=

ρ

_mud

−

∆

×

10

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

(

)

1 2 '

1

(

1

)

1

2

_×

−

−

−

−

=

∆

∧

=

n

F

F

p

F

t r

σ

σ

' 0

σ

t

σ

∧

@ B

V er tic al d ep th m NCT INTERPRETED ON FUNCTION 0

σ

σσσσ₀ σσσσ

Normal compaction trend

σσσσ

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

Lithology

Transgressions/regressions Different hole diameter

Bit type Bit wear Etc…

In this case NCT will appear shifted, but angular coefficient will remain constant.

σσσσ₀ V er tic al d ep th m

In presence of shifts in

overpressured Formations, the curve is characterized by a

visible variation of angular coefficient OVERPRESSURES TOP

ΣA

+

,-'&%&'-*-+ , "7

D

V er tic al d ep th m σσσσ₀

Calculation of coefficient

Calculation of coefficient ““bb”” in Formations with normal

in Formations with normal

gradient gradient

ΣA

+

,-'&%&'-*-+ , 7

D

σσσσ₀ V er tic al d ep th m

1 r

σ

σ

r2 2 0

σ

1 0

σ

Shifts can be calculated by means of an analytical method (method I) 1 0 2 0 1 2

σ

σ

σ

σ

=

r

×

r

ΣA

+

,-'&%&'-*-+ , ;7

D

σσσσ₀ V er tic al d ep th m

1 0 2 0 1 2

σ

σ

×

= b

b

V er tic al d ep th m A # A3 AD 2 0

σ

0

σ

1 0

σ

#

ΣA

+

,-'&%&'-*-+ , D7

D

Shifts can be calculated by means of an analytical method (method II)

E 8 7

V er tic al d ep th m Density Kg/dm Density Kg/dm33 2,2 2,3 2,4 2,5 V er tic al d ep th m Density Kg/dm Density Kg/dm33 2,2 2,3 2,4 2,5

If overpressured shales are UNDERCOMPACTED (porosity is higher than what expected at the depth where they are located), their density is lower then theoretic one. Its measurement can be performed on cuttings.

OVERPRESSURES TOP

Overpressured shales are more porous and, for this

reason, they represent a sort of thermal barrier which

prevents heat coming from below to pass uniformly towards the upper layers.

Where overpressures can be spotted, the Geothermical

Gradient (usually 3°/100m) shows a sharp increase.

D ep th m Resistivity OVERPRESSURES TOP

MUD RESISTIVITY – Mud contamination by means of formation water due to overpressure not sufficiently balanced causes a decrease of resistivity value, since formation fluid is assumed with higher salinity than drilling mud.

D ep th m Chlorides OVERPRESSURES TOP

MUD CHLORIDES CONTENT – The chemical analysis of chlorides in drilling mud as it comes out of the well can highlight an overpressure since the contamination could have been caused by formation fluid influx. Formation fluid is assumed with higher salinity than drilling mud.

GAS INFLUXES

Pipe connection gas Trip gas Background gas HOLE TIGHTENING High torque Overpull/drag Reaming/backreaming Presence of cavings Breakouts

MUD PUMPING PRESSURE

'2',-MWD systems (Measurement While Drilling) can perform real time downhole measurement of some drilling parameters that can be used as indicators for overpressures interpretation:

• Well inclination and orientation • Resistivity log

• Neutron log • Temperature • Torque

• Weight on bit

LWD tools (Log While Drilling) can measure and transmit in real time some useful data for petrophysical characterization. The same data, with a better

resolution, are memorized in the tool and unloaded when it is pulled out of hole: • Gamma ray log

• Sonic log

• Caliper log (ultrasonic !!)

POST-DRILLING

METHODS

OVERPRESSURE

The analysis methods are based on the measurement of clay electrical behavior. In particular the methods are:

• ∆∆∆∆t Shale method, based on transit time measurement, by sonic

logs, of an elastic perturbation which propagates along wellbore walls

• Resistivity method, based on the measurement of resistivity met

by electric field transmitted through borehole walls and generated by electric logs

'-D * * * V er tic al d ep th (m )

The assumption is, again, that propagation velocity of elastic waves increases with depth (for higher rock density).

Consequently, transit time (∆t) decreases regularly and it is therefore possible to draw a NCT.

∆t (µs/ft)

'-Assuming that density, porosity and relative pressures (effective and pore ones) are

intercorrelated, if by increasing depth and assuming other

conditions unvaried the transit time decreases (deviating from clean shales NCT), the

interested layers are overpressured V er tic al V er tic alde pt h de pt hm m * * * * * D D )) ∆t (µs/ft) OVERPRESSURES TOP

∆-

*/' '-

3* + %&+

, +

%/'

V er tic al V er tic alde pt h de pt hm m ∆t (µs/ft)

In ∆t-shale method shifts can be identified, even if they are not so frequent. These must be

distinguished from NCT slope variation. The main cause of shifts can be related to

geological issues. * * ) D

∆-

*/' '-

%&' ', ' :

+

:-Availability of an electrical log (resistivity, SP)/geological (GRay) Availability of acoustic log (ex. BHC Sonic Log)

Availability of Caliper/Image log

Identification of CLEAN shales (and isolate the corresponding ∆t values)

Plotting ∆t vs depth (in a semilog plot) Drawing NCT

INTERPRETING ∆t Shale trend.

IN DEVIATED WELLS, DEPTH SHALL BE VERTICALIZED

/*$+ ',-+

:+ *-+ ,

G R R es S P

It cannot be applied in carbonatic layers Shales must be clean

Fluids contained in shales (gas or oil) can modify ∆t value The wellbore wall shall be in gauge

Geological age changes increase the risk of wrong interpretation

-"!!! , -! E!! "!!! "E!! !!! E!! ;!!! ;E!! D!!! "! "!! 2 1 4 ∆ 1µ 7 4

• Estimation of bulk density from acoustic log (if density log not 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 depth or Eaton’s method

ROCK Densit! ∆∆∆∆t MATRIX g/cc µµµµsec/ft Dolomite 2.87 43.5 Limestone 2.71 43.5 - 47.5 Anhydrite 2.96 50 Clay 2.70 47

(

)

m

f

sed

φ

φ

δ

δ

=

δ

+

1

−

∆-

*/' '-

', +

-$' -+ *-+ , "7

;

153

568

.

1

×

∆

t

−

∆

t

m

=

φ

153

m

t

t

−

∆

∆

=

φ

89

28

.

3

t

sed

∆

+

=

δ

200

11

.

2

75

.

2

+

∆

∆

−

∆

×

−

=

t

t

t

_m sed

δ

200

228

.

1

+

∆

∆

−

∆

×

=

t

t

t

_m

φ

∆∆∆∆t VS. POROSITY CORRELATIONS

Consolidated soils and rocks

Slightly or not consolidated terrigenous Consolidated soils and rocks (alternative)

∆∆∆∆t VS. BULK DENSITY CORRELATIONS

Consolidated soils and rocks Slightly or not consolidated soils

200

47

11

.

2

75

.

2

+

∆

−

∆

×

−

=

t

t

sed

δ

The following correlation, developed by Agip, was built by comparing its results to density values coming from Formation Density Correlated Logs. 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). Rocks characterized by low porosity have high resistivity (ex. compact limestone, volcanic rocks..).

Having other conditions fixed, rock resistivity depends on: • salt concentration

• rock composition • temperature

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

'-The methods based on shales resistivity for pore pressure estimation are basically two:

I° method – from an electric log, shales resistivity is obtained and then it is plotted vs depth in a semi-logarithmic scale. Log interpretation is performed directly on this curve, without further calculation.

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

'-D

ep

th

Clay resistivity

Resistivity of clean shales is

plotted in semi-logarithmic scale in function of vertical depth. The correlation between resistivity and porosity (fluid content, since saturation = 1 is assumed) is

inversely proportional and

generates an increasing Normal Compaction Trend.

In case of Formations with

normal pore gradient, resistivity values allign around a line with increasing trend in function of depth.

Clay resistivity + D ep th In case of overpressured

levels, the trend of measured resistivity values depart from Normal Compaction Trend. The deviation is high or low in function of absolute

pressure value.

V er tic al d ep th m % % ) % D % * * * “F shale” Normal gradient Formations

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:

w shale w shale shale

R

C

R

R

F

×

=

=

1

&' + -+

2+

-$ '-

'-

+

+"7

;

V er tic al d ep th m % % ) % D % * * * “F shale” OVERPRESSURED Formations

Also with clay formation factor, in presence of overpressured layers curve trend departs from Normal

Compaction Trend line.

The operational sequence to be followed for F-Shale analysis is illustrated here below:

1. Calculate, or measure, formation water resistivity R_w throughout the well.

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

3. Read resistivity value from log data for clean shales throughout the 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

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

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

• Shales shall be clean

• Hydrocarbons in shales (oil or gas) can modify conductivity values • Wellbore must be in gauge

-FRACTURE GRADIENT

ESTIMATION AND

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

(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 the main planning and drilling phases of a well:

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

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

The correlations used for fracture gradient calculation are based on the assumption that, in case of homogeneous, elastic and isotropic mean, in situ stress state is modified by the presence of the well and stresses redistribute around its lateral surface.

σσσσ

θ

σ

′

r

σ

′

w

p

:&*

*/ (/*-+ , "7

The value of tangential stress is maximum in case of empty hole and decreases in function of mud weight increase, since mud weight pressure applied at wellbore replaces the action of the previously removed rock.

An excessive mud density increase could cause wellbore wall fracturing.

σσσσ

↓ ′ θ σ ↑ ′ r σ =↑ w p

:&*

*/ (/*-+ , 7

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

ELASTIC FORMATIONS with low permeability and minimum filtrate invasion:

(

ovbd p

)

p frac

p

p

p

p

−

−

+

=

ν

ν

1

2

INCONSOLIDATE OR SLIGHTLY CEMENTED FORMATIONS with high permeability and sensible filtrate invasion:

(

ovbd p

)

p frac

p

p

p

p

=

+

2

ν

−

PLASTIC FORMATIONS: ovbd frac

p

p

=

(

sed p

)

p frac

G

G

G

G

=

−

+

3

2

+ B 9 9 - % J & ν8! E B 9

The 2/3 coefficient shall be modified as follows: • in slightly consolidated sands = 1/2;

• in shales or silty marl = 3/4.

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

*/ (/*-+ , 7

(

ovbd p

)

p frac

G

G

G

G

−

−

+

=

ν

ν

1

0 3 A A* A A* , #-& 94 5" 5 : & "-& Adding fracture gradient calculation to the previously mentioned curves generates a plot similar to the one in figure.

Seen the importance of fracture gradient for operative practice, it is necessary to verify the estimation done in planning phase by means of direct measurements.

The direct measurements can be performed during drilling phase and provide a good estimation of fracture gradient limits, even though they can not measure its real value. This introduces automatically a safety margin.

The two main sources of calibration values in drilling phase are:

Leak Off Test (LOT)

Formation Integrity Test (FIT)

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

The LOT can be performed for two main reasons:

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

• Verify, after crossing a level characterized by high porosity and permeability, a more realistic value of fracture pressure and gradient.

-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.

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

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

6. Wait for pressure stabilization and read final value.

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

8. Calculate fracture gradient.

P re ss ur e (p si ) Pumped volume (bbl) * * * 0 7

/ -

%/

-ATTENTION: the fracture gradient value calculated with the previously described procedure is NOT the real fracture gradient, though for operative purposes it can be considered a good approximation.

The LOP (Leak Off Point), at best, is coincident to the beginning of mud leak phase but the real fracture limit is not reached.

The real fracture gradient can be obtained only by applying to the formation a pressure equal or higher than minimum horizontal stress plus traction resistance of the analyzed rock.

According to our internal procedures, LOT is a good but VERY CONSERVATIVE control test.

7 + + + + ; ; + 8 7 7 > > ( -"- -; -; ; - 6- ; -. - -- " !

/ -

:&* -(&+

,) 7

P re ss ur e Time

=/ - 1'? / @ - 4 / -M A M B @ B B B 9 M @ B B

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

-+ A - ? B 9 - :+- / -B 9 A B @ @ M "! M B B M 3 % M % B B 1N A O 9974 B M A 1 B 4

: & *-+ , +

,-')&+

$'

-(

*&$

' '

%&'A &+

//

INPUT: seismic v_m e TWT v_i vs Depth ∆t vs Depth ρ bulk OBG NCT PPG FG

(

*&$

' '

# +

/' &+

//+

,)

INPUT: mudlog ROP, RPM, WOB, D, MW

Dc-Exp, ΣALog

NCT

PPG

(

*&$

' '

%

-A &+

//

INPUT: logs Caliper GR, Res, SP NCT OBG PPG FG

Sonic, Res Density Shale Sonic