Pressure Prediction

7/18/2019 Pressure Prediction

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2. Prediction of

Formation Pressure

Prediction of Formation Pressure

Kind of Pressure

1. Overburden Pressure.

2. Pore Pressure.

3. Hydrostatic Pressure.

Kind of Pressure

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.

MWD & LWD.

Resistively Log.

Kicks- D.P Pressure.

- Pit Level (Volume)

3. After Drilling

Wireline logging.

Pressure Measurements DST

Pressure Measurement from Production

Testing.

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

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)

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

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

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.

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

Abnormal Pressure

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

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.

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

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

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

Mud density = HSP/ constant /depth= 4879 / 0.052 / 8580 = 10.94 ppg

Kind of Pressure

= 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

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

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

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.

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.

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

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

Pressure Prediction

1.“d ” Exponent Method or Normalized Rate

of PenetrationDepends on:

Penetration Rate.

Weight on Bit. RPM

Hole size

Mud Weight

Drilling Rate as a Pore

Pressure Predictor

Or, in its most

used form:

inDiameter,Bitd

lbf ,Bit WeightW

exponentdd

ft/hr R

W N K R

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)

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???

Solution

5.8*10

000,20*12log

100*60

1.10*052.0465.034.1d

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.

d-EXPONENT AND MUD

DENSITY DATA

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.

Drilling

underbalanced

can further

increase the

drilling rate.

THE END

QUESTIONS ???

Pressure Prediction

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