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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
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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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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
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PRE-DRILL METHODSFOR OVERPRESSURE
ANALYSIS FROMSEISMIC DATA
'++ &':/'-+,
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&&&
." ."
." ."
." 0." 0
&7&7
'=*%/' : '++ '-+,
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*/(/*-+, -'%
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• 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/' +,%(- *-*
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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(','
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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
*/(/*-+, (-%(-
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!
"!!!
!!!
;!!!
D!!!
E!!!
"F "G !
!450
D
6
9 :
Sediment densitycalculated from
seismic data
2'&3(&', )&*+',- */(/*-+,
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) !
.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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%&' )&*+',- */(/*-+,
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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
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0
3
*
1 4
TRANSIT TIME
( t in sec/ft):
Transit time ofsonic waves
through formations
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*(%-+,
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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 '-
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==
99..::99""::++99::
6
9 :
,- +
*/(/*-+, '-
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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 &*-+ '-
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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(','
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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
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?@
!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
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!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)
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Dc Exponent ΣΣΣΣ-logThe two methods used in this case are: and
'- 3*' , &+//+,) %*&*'-'&
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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 */(/*-+,
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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 +,-'&%&'-*-+,
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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
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Due to Mud Weight density (MW), d-Exponent is corrected according
A'=%,',- '-
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to the following correlation: MW dExpdcExp =
6
6
''CC ""''CC
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dc-Exponent
A'=%,',- '- +:- +,-'&%&'-*-+,
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Shifts can be composed in acontinuous curve bytranslating the shiftedportions until they overlay to
Normal Compaction Trend D e p t h
A'=%,',- '- */(/*-+, '-
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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*/',- '%-
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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
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( )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*(
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A
norm A
dc
dcG ×= 03.1
""
..
"""0"3
A'=%,',- '- *3*( +,-'&%&'-*-+,
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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 '-
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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
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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
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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
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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
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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
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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
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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
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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
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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
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E87
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-'& 5#+/' &+//+,)6 '- -'%'&*-(&'
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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+-$
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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 '- /&+'
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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',-
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Pipe connection gasTrip gasBackground gas
HOLE TIGHTENINGHigh torqueOverpull/drag
Reaming/backreamingPresence of cavingsBreakouts
MUD PUMPING PRESSURE
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POST-DRILLINGMETHODS
OVERPRESSUREANALYSIS FROM LOGS
The analysis methods are based on the measurement of clay electrical
*,*/$+ '-
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behavior. In particular the methods are:
• ∆∆∆∆t Shale method, based on transit time measurement, by soniclogs, of an elastic perturbation which propagates along wellborewalls
• Resistivity method, based on the measurement of resistivity metby electric field transmitted through borehole walls and generatedby electric logs
D
∆t (µs/ft)
∆- */' '-
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*
*
*
V e r t i c a l d e
p t h ( m )
The assumption is, again, thatpropagation velocity of elastic
waves increases with depth(for higher rock density).
Consequently, transit time (∆t)decreases regularly and it istherefore possible to draw aNCT.
DD ))
∆t (µs/ft)
∆- */' '- 3*+ %&+,+%/'
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Assuming that density, porosityand relative pressures (effective
and pore ones) areintercorrelated, if by increasingdepth and assuming otherconditions unvaried the transit
time decreases (deviating fromclean shales NCT), theinterested layers areoverpressured
V e
r t i c a l
V e
r t i c a l d e p t h
d e p t h m m
**
**
*
OVERPRESSURESTOP
∆t (µs/ft)
)D
∆- */' '- %&'',' : +:-
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V e
r t i c a l
V e
r t i c a l d e p t h
d e p t h m m
In ∆t-shale method shifts can be
identified, even if they are not sofrequent. These must bedistinguished from NCT slopevariation. The main cause of
shifts can be related togeological issues.
*
*
Availability of an electrical log (resistivity, SP)/geological (GRay)
Availability of acoustic log (ex BHC Sonic Log)
∆- */' '- ,- '-'&+,*-+,
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Availability of acoustic log (ex. BHC Sonic Log)
Availability of Caliper/Image log
Identification of CLEAN shales (and isolate the corresponding ∆tvalues)
Plotting ∆t vs depth (in a semilog plot)
Drawing NCT
INTERPRETING ∆t Shale trend.
IN DEVIATED WELLS, DEPTH SHALL BE VERTICALIZED
/*$ +',-+:+*-+,
S P
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G R
R e s
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"!!!
!
E!!
"! "!!
∆ 1µ74
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,-
"!!!
"E!!
!!!
E!!
;!!!
;E!!
D!!!
2 1 4
• Estimation of bulk density from acoustic log (if density log not
∆- */' '- */(/*-+, 'C(','
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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
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mt t ∆−∆=φ
∆∆∆∆t VS. POROSITY CORRELATIONS
Consolidated soils and rocks
∆- */' '- ',+-$ '-+*-+, 7;
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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
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
The following correlation, developed by Agip, was built by comparing itsresults to density values coming from Formation Density Correlated
∆- */' '- ',+-$ '-+*-+, ;7;
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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+-$ '-
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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+-$ '- */(/*-+, '-
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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
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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
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+
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;
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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
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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;
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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+-$ '- /++-
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• 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
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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
+,-&(-+,
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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
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redistribute around its lateral surface.
σσσσ
θ ′
r σ ′
w p
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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
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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
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( ) 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:
"-&
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0
3
&
Adding fracture
gradient calculationto the previouslymentioned curvesgenerates a plot
similar to the one infigure.
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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.
/'*. :: -'-
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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' %&'(&'
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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
/- %/-
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P r e s s
u r e ( p s i )
Pumped volume (bbl)
*
*
7
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7
7 7> >( -
"- -;; ;- 6- ; - .- -
/- :&*-(&+,) 7
u r e
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7
++++
;;
+
8 - "!
P r e s s u
Time
=/- 1'? /@ -4 /-
'=-',' /'*. :: -'-
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/-
M A
M B @ B B
B9
M @ B B
+ A - ? B 9
- :+- /-
9
:&*-+, +,-')&+-$ -'-
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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
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∆t vs Depth
ρ bulk
OBG
NCT
PPG
FG
Equiv.depth, Eaton, R ratio
(*&$ '' #+/' &+//+,)
INPUT: mudlog ROP, RPM,
WOB, D, MW
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Dc-Exp, ΣALog
NCT
PPG
Equiv.depth, Eaton, abacus
(*&$ '' %-A&+//
INPUT: logs
Caliper GR, Res, SP Sonic, Res Density
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NCT
OBG
PPG FG
Shale Sonic
Filtered Sonic