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2. Prediction of
Formation Pressure
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Prediction of Formation Pressure
Kind of Pressure
1. Overburden Pressure.
2. Pore Pressure.
3. Hydrostatic Pressure.
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Kind of Pressure
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Formation pressure can be predicted:
1. Before drilling (Planning).
Information from nearby wells
Seismic data→ Formation Velocity.
Analogy to known characteristics of thegeologic basin.
2. While Drilling.
Penetration rate, “d’ exponent Method. Flow line MW.
Flow line Temperature.
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Formation pressure can be predicted:
MWD & LWD.
Resistively Log.
Kicks- D.P Pressure.
- Pit Level (Volume)
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Formation pressure can be predicted:
3. After Drilling
Wireline logging.
Pressure Measurements DST
Pressure Measurement from Production
Testing.
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Kind of Pressure
1.Overburden Pressure - is due to the combined weight of the rock matrix
itself and the fluid (water, oil and gas) which
occupy the pore space.
ρo=weigh( rock matrix & fluid) / area
= D ( 1- Ф ) ρ ma & ρ fl
Where: D = Vertical height in feet or meters
Ф = Porosity of a formation, in fraction
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Kind of Pressure
ρ ma = Density of rock matrix in lb/ft³ or gm/cc
ρ fl = Density of fluid in lb/ft³ or gm/cc
Pressure – Depth relation are commonly spoken interm of Gradient.
Fresh water gradient =0.433 psi/ft
( 62.4 lb/ft³/144 in²/ ft²)
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Kind of Pressure
Earth or overburden gradient =1.0 psi/ftOverburden gradient calculated as follows:
Density of a thick sedimentary sequence is ≈ 2.3
{Average rock sp.gr = 2.3}
Overburden gradient = 2.3 x 0.433 ≈ 1.0 psi/ft
Overburden pressure = overburden gradient ( 1.0
psi/ft ) x VD
VD= Vertical depth
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Kind of Pressure
Typical densities of rocks and fluids
Lithology Matrix density (S.G)• Sandston 2.65
• Limestone 2.71
• Anhydrite 2.98• Gypsum 2.35
• Fresh water 1.0
• Salt water 1.15• oil 0.80
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Kind of Pressure
2. Pore Pressure or Formation Pressure:
Normal pore pressure is equal to thehydrostatic pressure of water column from
that depth to the surface.
If the pore pressure is less than normal
hydrostatic pressure of formation is said to
be subnormal.
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Kind of Pressure
If the pore pressure at that depth exceeds
the expected hydrostatic pressure for thatdepth the zone is termed abnormallypressured.
Example:
Calculate Pore Pressure @ 10200 ft, if thewell drilled to 9100 ft, normal PorePressure Gradient is 0.433 psi/ ft. theoverburden Pressure Gradient is 1.0 psi/ ft
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Abnormal Pressure
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Solution:
At 9100 ft, Pn = 0.433 x 9100 = 3940 psi
At 9100 ft, Po¹ = 1.00 x 9100 = 9100 psi
At 10200 ft, Po² =1.00 x 10200 = 10200
psi Pp = Pn + ( Po² - Po¹)
= 3940 + ( 10200 - 9100 )
= 5040 psiPressure Gradient Pg = 5040/ 10200 = 0.494
psi/ ft
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3. Hydrostatic Pressure:
The hydrostatic pressure is the pressure
executed by the column of fluid or due tothe weight of fluid. (mud weight)
The following pressure profile is an example
of overburden, pore pressure profile and
how casing setting depth is designed
according to overburden, pore andhydrostatic pressures.
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Kind of Pressure
Hydrostatic Pressure varies with the density of
the fluid and the height of the column.
Phy = MD x D
Where MD = Mud Density
D = Depth ( or Vertical Fluid Column )
Mud gradient in psi/ ft = 0.433 psi /ft for pure
water ( Sp.gr. = 1.0 ) = PPG x 0.052
= S.G / 2.31= PCF /144 = PPG /19.23
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Kind of Pressure
Example :
A well is being drilled to a true vertical depthof 10 000 feet.
What will be the hydrostatic pressure on
bottom if the mud density is 10.4 ppg (1.25sp.gr.)
Hsp =Mud density x constant x depth
= 10.4 x 0.052 x 10 000 = 5408 psi or
= 1.25 x 9.8 x 10 000 /3.281 = 37,336 KPa
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Kind of PressureRemarks
1 psi /ft = 22.6 KPa /m = 6.9 KPa/ ftor 0.433 psi /ft = 9.8 KPa / m ( SI unit )
2. Calculate the mud density needed to
produce a Hydrostatic pressure of 4879 psi (33 665 KPa) over a depth of 8580 ft ( 2615
m)
Mud density = HSP/ constant /depth= 4879 / 0.052 / 8580 = 10.94 ppg
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Kind of Pressure
or
= 33665/ 9.8 / 2615 = 1.31 sp. gr.
1.31 x 8.33 = 10.91 ppg
3. Express a mud gradient of 0.647 psi /ft (14.63 KPa/m) as a mud weight?
Mud density = MG / Constant
= 0.647 / 0.052 = 12.44 ppgor = 14.63 / 9.8 = 1.49 s.g
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3. Pressure PredictionAbnormal Pressure Prediction Methods
Source of Data Parameter Time of Recording
Seismic Methods Formation Velocity Prior Drilling
Drilling Parameters Drilling rate While Drilling
“d” Exponent
Drilling Mud Kicks While Drilling
Flow line MWFlow line Temp.
D.P Pressure
Pit level, volume
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Pressure Prediction
Shale Cuttings Shape and Size While Drilling
Bulk Density
Resistivity Log
MWD, LWDWell Logging Resistivity After hole is drilled
Conductivity
Interval Transit Time
Pressure Measur. DST After Drilling
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Pressure Prediction
Abnormal Pressure and Penetration Rate
Bit drill through over pressured rock faster
than normally pressured rock (if everything
alse remains the same)
When drilling in clean shale this fact can
be utilized to detect the presence of
abnormal pressure and even to estimate
the overpressure.
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Pressure Prediction
Differential Pressure between the
Hydrostatic Pressure and Pore Pressure
have a large effect on drilling rate as
shown below.
If mud weight remains constant and pore
pressure increases, penetration rate will
generally increase.
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Effect of Differential Pressure on Drilling Rate
Differentialpressure is the
difference between
wellbore pressure
and pore fluidpressure
Decrease can be due to:
• The chip hold down effect
• The effect of wellbore
pressure on rock strength
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Pressure Prediction
The most important factor to predict the pore
pressure is the Penetration rate.
Penetration rate α ∆P( Hydrostatic pressure – Pore Pressure)
Methods based on Drilling Parameters:1. d-Exponent Method or Normalized Rate
of Penetration
2. Modified d - Exponent MethodThe following equation developed by Jorden
& Shirley
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Pressure Prediction
1.“d ” Exponent Method or Normalized Rate
of PenetrationDepends on:
Penetration Rate.
Weight on Bit. RPM
Hole size
Mud Weight
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Drilling Rate as a Pore
Pressure Predictor
Or, in its most
used form:
inDiameter,Bitd
lbf ,Bit WeightW
exponentdd
rpm N
ft/hr R
10
12
log
60log
b
6
bd
W
N
R
d
d
bd
W N K R
3
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d - Exponent
Where:
d -Exponent is dimensionlessd - Exponent should increase with depth when
drilling in a normal pressure section. A reversal
of this trend is an indication of drilling into
potential overpressure.
2. Modified d - exponent or corrected d -exponent:
d c = d (MWn /MW)
MWn = Mud weight @ normal pressure (ppg)
MW = used Mud weight (ppg)
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Example
While drilling in XXX field, North IraqR = 50 ft/hr
W = 20,000 lbf
N = 100 RPM
ECD = 10.1 ppg
d h = 8 ½ in
Calculate d and dc
And Trend of formation Pressure???
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Solution
34.1d
554.1
079.2
5.8*10
000,20*12log
100*60
50log
d
6
bd
W
N
R
d
6
10
12log
60log
c
nc dd
19.1d
1.10*052.0465.034.1d
c
c
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Under normal compaction, R should
decrease with depth. This would cause ”d”
to increase with depth.
Any deviation from the trend could be
caused by abnormal pressure as shown in
data below.
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TABLE
d-EXPONENT AND MUD
DENSITY DATA
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Differential Pressure between the
Hydrostatic pressure P2 and Pore
Pressure P1 have a large effect on Drilling
rate as shown in the next slide .
If mud weight remains constant and pore
pressure increases, penetration rate will
generally increase.
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Drilling
underbalanced
can further
increase the
drilling rate.
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THE END
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