02_pressure school

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4/21/2013 Formation Pressure EvaluationSchool

1

Day Two

Objectives :

• What is Pore Pressure?• Abnormal Pressure Origins

• Pore Pressure Evaluation• ROP & Drilling Exponents

• MWD and Wireline Logs

• Shale Density and Factor

• Formation Gases

• Temperature

• Borehole Condition

• Pore Pressure Estimation

Formation Pressure




2

WHAT IS POREPRESSURE?

Formation Pressure




3

What is Pore Pressure?

• Pore pressure is the pressure exerted by pore fluids.

• Normal pore pressure = Normal hydrostatic pressure

• Subnormal pore pressure < Normal hydrostatic pressure

• Abnormal pore pressure > Normal hydrostatic pressure

Formation Pressure

Water Type Salinity

Cl-

mg/l

Salinity

NaClmg/l

Water Density

gm/cc

Fresh Water 0 to 1500 0 to 2500 1.00

Sea Water (Example)

18000 30000 1.02

FormationWaters

(Example)

Salt Water

saturated in NaCl

1000036000

4800060000192667

1650060000

80000100000317900

1.011.04

1.051.071.20

• Normal pore pressure

reflects the water density in

the basin of deposition.

• Fluid density is a function

of the concentration of dissolved salts. Varying

salinity causes varying fluid

density.




4

What is Normal Pore Pressure?

Formation Pressure

Pore Pr. = Hyd. Pr.

• Pore pressure will remain

normal if there is good

hydraulic communication

between the sedimentsand the depositional basin.

• Fluids will escape during

compaction and the rock

grains accept all of the

overburden stress.• Pore fluids will maintain a

normal hydrostatic

pressure.




5

What is Abnormal Pore Pressure?

Formation Pressure

Pore Pr. > Hyd. Pr.

• Abnormal formation

pressures develop when

some process limits the

hydraulic communication.• In this case, the trapped

pore fluids accept a

greater share of the

overburden stress.

•

This has the effect of raising the pore pressure

above normal hydrostatic

pressure.




7

How to Measure Pore Pressure?

Formation Pressure

• Production Samples such as DST’s.

• RFT Samples - restricted to potential reservoir areas.

• TesTrak LWD data.

•Mud Chlorides - show gross changes in pore water salinity.

• Resistivity Logs - used to calculate Rw.

• Rw Tables

– Rw is the total resistivity of the water and assumes thepresence of NaCl. It does not differentiate between other saltsand dissolved gases with different densities.

• Offset Data - regional curves or formation water densitytables generally only give an approximation for the area.

• When all else fails ... a bucket and a mud balance.




8

Magnitude of Pore Pressure

• The magnitude of pore pressure will

depend on one or any of ...

• Surface communication

• Concentration of dissolved salts

• Percentage of effective porosity

• Degree of overburden

• Geothermal gradient

• Percentage of gas

• Most important of these is Surface Communication

• Once communication has been halted the other factors will take effect.

Formation Pressure




9

• Offset Well Data

• Rw Catalogues

• Resistivity Logs

• Bucket on a Rope

Formation Pressure

PORE WATER DENSITY

rf

Normal Hydrostatic Pr.Normal Pore Pressure

“P” in psi, bars, atm

ThePlan

(2)Pore

Pressure

(Fluid Properties)

Normal Pore Pr. Gradient(“P” / TVD from water level)

Formation Balance Gradient(“P” / TVD from flowline)

Gas,

Dxc,

Elogs,

Temp.

Flows,

Kicks,

etc.

Estimated PorePressure and

FB Gradient

MINIMUM

STATIC MUD

DENSITY

“S” (from 1)




10

ORIGINS OF ABNORMALPRESSURE

Formation Pressure




11

Abnormal Pressure Environments

Formation Pressure

• There are several geologic

conditions favorable to the

development of abnormal

pressure. – Young sediments




12


Formation Pressure





– Large total thickness

– Presence of clay rocks




13


Formation Pressure






– Presence of clay rocks – Interbedded sandstones

of limited extent




14


Formation Pressure






– Presence of clay rocks – Interbedded sandstones

of limited extent

– Rapid loading and burial




15

Abnormal Pressure Mechanisms

Formation Pressure

• Abnormal pressure develops when de-watering is restricted.

• There are three main mechanisms

• Ineffective pore space (volume) reduction

• Volume expansion

•

Fluid movement Mechanisms within Mechanisms

• Compaction Disequilibrium

• Aquathermal Pressuring

• Clay Diagenesis

• Sulphate Diagenesis

•Salt Diapirism

• Tectonic Activity

• Hydrocarbon Maturation & Placement

• Piezometric Changes

• Osmosis




16

Normal Sediment Compaction

Formation Pressure

• Under normal conditions sediments will de-water withburial.

• Overburden acts as the main cause for fluid expulsion.

• De-watering decreases the porosity and increases the

density of the sediment.• Normal clay compaction will depend on an overall balance

between :

• Clay permeability

• Sedimentation and burial rate

•Drainage efficiency




17

Normal Sediment Compaction

Formation Pressure

Porosity can vary from 80% to less than 10% over a 5,000m interval.

Using data for the Gulf Coast of Mexico, at a depth of 3,000m the total volume of water

expelled is more than 75% of the original volume of the argillaceous sediment.

Interstitial Water

(% initial vol.)

Surface 300m 1000m 3000m

75.9

4.1

20

7366.6

80 13.3 7

202020

Expelled Water

(% initial vol.)

Solid(% initial

vol.)




18

Compaction Disequilibrium

Formation Pressure

• Some sort of seal must be

in place.

• De-watering is stopped or

slowed down.• Overburden pressures are

transferred to the pore

fluids rather than normal

grain to grain contact.

•This has the effect of increasing the formation

pore pressure




19

Differential Compaction

Formation Pressure

• Can occur in interbedded

sand/shale formations.

• Water escapes along the

path of least resistance.• Shales next to the sands

de-water more readily,

become compacted and

less permeable.

•Eventually further de-watering from within the

shale body stops.

• Pore pressure increases.




20

Volume ExpansionDue to Hydrocarbon Generation

Formation Pressure

• With temperature and

pressure, kerogens are

converted to oil and gas.• This conversion is

associated with a volume

expansion.

• This will give rise to an

increase in the porepressure.

}

VolumeIncrease

Type IIKerogen

> Oil

> Wet Gas,Condensate

> Dry Gas




21

Volume ExpansionDue to Aquathermal Pressuring

Formation Pressure

• If temperature is applied to pore water it

will increase in volume.

•The amount of pressure rise will dependupon

– density of the fluid

– amount of temperature increase

– effectiveness of the seal

The resultant pressure increase for a fluid of 1.0 SG (8.34 PPG) with a

rise in temperature from 50C to 75C is 5,600psi.

There is the question whether the seal can actually withstand the

aquathermal pressure. It is more likely that this is an extra drive to

break seals and keep systems dynamic.




22

Clay Diagenesis

Formation Pressure

• Smectite rich clays dehydratelosing their interstitial water ascompaction occurs.

• As the clays compact, space is

created which is filled by thereleased water.

• This water may be able toescape or it may be trapped bythe now low porosity/lowpermeability Illite.

• Abnormal pressure may becaused by the presence of trapped water and the formationof impermeable seals by Illite.




23

Sulphate Diagenesis

Formation Pressure

• Gypsum is the precipitated form of CaSO4

• Gypsum will transform to anhydrite above 40 C.

• In the presence of halite this may be around 25 C.

• Gypsum dehydrates to form anhydrite and free water.

• CaSO4.2H2O << >> CaSO4 + 2H2O

• Gypsum << >> Anhydrite + Water

• Up to 38% of the original water volume is released so

abnormal pressure can develop if this fluid cannot escape.

• Rehydration is accompanied by an increase in volume which

may also generate abnormal pressures.




24

Tectonic Activity

Formation Pressure

• Tectonic activity will cause stress regimes which will

extensional or compressional.

• Extension causes fractures to open and therefore fluid

dissipation or movement to other zones• Compression has two main effects:

• The easy expulsion of fluids, leading to compaction and

therefore the formation of normal fluid pressures.

• The difficult expulsion of fluids, which causes

undercompaction, the formation of abnormal pressures.

• Abnormal pressures can progress to induce hydraulic

fracturing, leading to the expulsion of pore fluids and

ultimately the formation of normal pore pressure.




25

Tectonic Activity

Formation Pressure

Amount of

Shortening Possible Geopressured Zones

Extension Extension

Compression

Compression




26

Salt Diapirism

Formation Pressure

osmosis osmosis

trapped pressure

and confinement

uplifted

paleopressure

isolated rafts with

paleopressure

trapped pressure

under salt sheets Banff Diapir

& Salt Field

• Salt diapirism can cause the formation of abnormal pressure in a variety

of ways, most of which are noted in the cross-section above.

• Note that besides pressure in the isolated rafts, you may also have

dangerous gases such as hydrogen sulphide trapped.




27

Piezometric Changes

Formation Pressure

• A low water table, or an aquifer with an outcrop below the water table, will show a pressure that is subnormal for drilling purposes.This is not as dramatic as abnormal pressure but the resulting lostcirculation will cause a loss of control of the hydrostatic pressurein the well which could result in well control problems.

• A water table above the

height of the rig will have

abnormal pressure on

penetrating the aquifer,

which will cause the fluid to

rise to the piezometric level

to equalise the pressure

imbalance.

When the seal is punctured fluids in the

aquifer will rise to this level to equalise

the pressure.




28

Osmosis

Formation Pressure

• Osmosis is the spontaneous

movement of ions in water

down a concentration gradient

from fresh to saline.

• Movement will continue untilthe salinity’s are equal or

pressure prevents further

movement.

• That pressure may be up to

4000psi where shales act as

semi-permeable membranes. SALTS

Clays

MWD Resistivity




29

PORE PRESSUREEVALUATION

Formation Pressure

i



4/21/2013 Formation Pressure EvaluationSchool 30

Pore Pressure Evaluation

• All methods are a function of mechanism.

– Compaction

– Tectonic

– Thermodynamic • Compaction techniques are best developed.

• The techniques only provide expectations.

• Developed for argillaceous rocks.

• Use direct or indirect porosity

determination.

Of course, operator experience is very important

Formation Pressure

F i P




Pressure Evaluation Tools

• Prior to drilling

– Surface geophysical

– Regional geology

– Seismic – Offset data

• While drilling

– Drilling parameters - drill rate, torque, dxc, pump pressure etc.

– MWD / LWD / PWD - gamma ray, resistivity, sonic, density etc.

– Drilling fluid - gas, temperature, pit volume, salinity etc.

– Geology - shale density, volume, shape, size, shale factor etc

• After drilling

– Wireline logs

– Pressure tests

–Data analysis

Formation Pressure

F i P




Formation Pressure

Pressure Evaluation Tools

F i P




Formation Pressure

x

x

Resistivity

Porosityshould

decreasewith depth

Resistivityshouldincrease

withdecrease

of porosity

Trendreversal

mayindicate

porepressureincrease

Overpressure

Normal Trend

• Of the many techniques

used to detect and quantify

abnormal pore pressure,

all generally have the same

features in common

– Use clay/shale lithologies.

– Give indirect

measurements of porosity.

– Provide estimations based

on expected normalcompaction.

Pressure Evaluation Techniques

F ti P




Formation Pressure

ROP’s and Drilling Exponents

• All factors being equal, the penetration rate will gradually

decrease with increasing depth due to the decreasing

formation porosity.

•

ROP can be used in abnormal pressure detection providingthe following factors are taken into account:

• lithology

• compaction

• differential pressure

•weight on bit

• rotary speed

• torque

• hydraulics

• bit type and wear

•

personnel and equipment

F ti P




Formation Pressure


• Drilling Exponents are used to

“normalise” the ROP.

• They aimed to eliminate the effects

of drilling parameter variations andtry to give a measure of formation

“drillability”.

• Historically the Dxc (Corrected

Drilling Exponent) we use came

from: – Bingham (1964)

– Jorden & Shirley (1966)

– Rehm & McClenden (1971)

Normal trend

Normal

Observed

F ti P




Formation Pressure


• Rehm & McClenden (1971) suggested correcting the “d”exponent to take into account the effects of differential pressurebetween the formation pressure and the regional hydrostaticpressure:

d = log10 R/ 60N x NFBG R = ft/hr ECD = ppg

log10 12W / 106

D ECD N = RPM NFBG = ppgW = pounds

D = inches

d = 1.26 - log10 R / N x NFBG R = m/hr ECD = SG

1.58 - log10 W / D ECD N = RPM NFBG = SG

W = tonnes

D = inches

• This had the effect of removing the “masking effect” when mud

weights were increased and also emphasised the shift in Dxc

values when entering an abnormal pressure zone.

F ti P




Formation Pressure


• Drilling Exponents do not take into

account

– Lithology, unconformities etc.

–

Mud hydraulics – Bit type and wear

– Post 1970’s technology

• highly deviated long reach wells.

• rotary closed loop systems.

• improved mud systems, better hole

cleaning control.

• PDC bits.

• improved cutting efficiency of insert

bits.

Normal trend

Normal

Observed

F ti P




Formation Pressure

Drilling Exponents

Soft Clay

Calc Clyst

Claystone

Calc Clyst

Claystone

N o r m a l P r

e s s u r e

G e o p r e s s u r e

Silty Clyst

Interpretation

• In soft clays the bit jets away theformation and gives a low &scattered dxc curve.

• The trend line has beenestablished in the normallypressured claystones.

• Geopressure can be seen below

the calcitic claystone. Caremust be taken to confirm thatthe trend shift is due to pressureand not just the lithology.

F ti P




Formation Pressure

Drilling Exponents

Interpretation

• Drilling through an alternating

sequence of shales and sands a

trend line can be established for

the shales only in a normallypressured zone.

• When entering the abnormally

pressured zone then shales will

show deviations from its trend

line.

• Sands may or may not show asimilar change in trend. Do Not

try to draw trends through

sandstones.

N

o r m a l P r e s s u r

e

G e o p r e s s u r e

F ti P




Formation Pressure

Drilling Exponents

Interpretation

• When drilling with different

types of bit, trends can be

established for each bit run.

• These trends can be smoothedinto a continuous plot.

• Care must be taken when doing

this for if a geopressured zone

is entered at the start of a new

bit run then the data could be

misinterpreted as being normal.

Smoothed

DataRockBit

Insert

Bit

Rock

Bit

F ti P




Formation Pressure

Drilling Exponents

Interpretation

• When a new hole section is

drilled there will be a shift in the

Dxc values to the right. Again

the data can be smoothed usingtrend lines.

Smoothed Data

12 1/4”

8 1/ 2”

F ti P




Formation Pressure

Drilling Exponents

Interpretation

• Towards the end of a bit run, bit

wear can be seen as an increase

in the Dxc values, this is

because the bit is not drilling asefficiently as previously.

NB #3

NB #4

Bit wear

F ti P




Formation Pressure

Drilling Exponents

Interpretation

• If an abnormally pressured zone

is drilled with a dull bit then the

magnitude of the shift to the left

is reduced when compared todrilling with a fresh bit.

Fresh Bit Dull Bit

Normally

Pressured

Over

Pressured

Formation Pressure




Formation Pressure

Drilling Exponents

Beware the Failings of Dxc

• You have to be judge and jury on the quality of Dxc data

before you present you material to the drilling team.

• You have to look at every argument and set of evidence; both

in favour of, and against your prediction of pressure.

• Do not be afraid to say that you cannot make an estimation at

this time on the evidence so far presented - You can say that

the evidence is inconclusive but only as long as you have

looked at all of the evidence available on the well.

• As we will see later, trendline placement will be a key concern

in the evaluation!

Formation Pressure



4/21/2013 Formation Pressure Evaluation

School

45

• All of the following e-logs

have and can be used to aid

in pressure evaluation.

–

Resistivity Logs – Sonic/Acoustic Logs

– Gamma Ray Logs

– Density Logs

• We will concentrate on

resistivity and sonic logsfor pressure evaluation.

• Gamma will be used to pick

the shale points.

Wireline and MWD Logs

Formation Pressure

Formation Pressure




School

46

Formation Pressure

Resistivity Logs

• With compaction pore water

will be released and the

formation resistivity should

increase.

• In a homogeneousargillaceous formation an

increase in porosity may

indicate an increase in pore

pressure.

• This will be reflected by adecrease in resistivity.

x

x

Overpressure

Normal Trend

Formation Pressure




School

47

Formation Pressure

Resistivity Logs

x

x

Picking Shale Points from Resistivity Logs

• Use the gamma ray (or SP) curve to identify shale beds.

• Shales should be clean and at least 30 feet thick.

• Plot consistent resistivity values from the curve.

•

Don’t use values within 10 feet of the top of a sand.• Values above 3500 ft may be influenced by possible

freshening of formation pore water and low temperatures.

• Age boundaries and unconformities will probably cause a

shift in trends.

• Shales near salts may give values that are too low.

• Shale gas may give values that are too high.

• Again, trendline placement may be a key concern.

Note : It’s better to have a few, good, hand picked data points, than hundreds of

data points from a wireline file. The latter STILL need evaluation for shale points.

Formation Pressure




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48

Formation Pressure

Sonic / Acoustic Logs

x

x

• Sonic logs can be used both for bulk density calculations

as we have seen, and pore pressure evaluation.

• Sonic logs register the Transit Time (Delta T) of a

formation. Delta T is measured in usecs/ft.

• The delta t for a rock is a measure of its porosity• Lower transit times = faster acoustic velocity

= lower porosity = higher density

• On encountering a zone of abnormal pressure, the Delta T

will increase due to increased porosity.

•

As with Dxc and resistivity, only shale points should beused for pressure evaluation.

Formation Pressure




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49

Formation Pressure

The Other Logs

• Gamma Ray : it has been proposed

that greater porosity reduces the

strength of the gamma ray & this

may be used to calculate pore

pressure. The method was notsuccessfully proven, so GR is used

mostly to identify lithology.

• Density Logs : can be used for OBG

and pore pressure evaluation.

Limitations include their infrequent

use during the well and their shallow

depth of investigation and need for

caliper correction.

Formation Pressure




School

50

Formation Pressure

Temperature Analysis

• With uninterrupted drilling, the

flowline temperature should

increase with depth.

• The earth’s core radiates heat

outwards.

• The rate of temperature increase

with depth is the Geothermal

Gradient.

• Fluid in pore spaces cause

abnormalities in the heat transfer.

• An overpressured zone will have

high fluid content and will distort

the temperature gradient.

Formation Pressure




School

51

Formation Pressure


• Evaluations based on geothermal

gradient reflects changes in the

return mud flow.

• Any changes seen in the mud

temperature after correction mustindicate changes in the borehole.

• Therefore the temperature probe

in the possum belly must be kept

clear of cuttings and other debris.

• Temperature data from MWD tools

may also be used when available.

Formation Pressure




School

52

Formation Pressure


• Temperature may be affected by

• Lithology

• Penetration rate

• Mud additions

• Flow rate & pump speed

• Hole size

• Depth

• Mud type

• Length of riser

• Type of bit

• Hole and string geometry• Surface temperature

• Breaks in rotation

Formation Pressure




School

53

Formation Pressure


SEALED GEOPRESSURED ZONE

Good Heat Absorption

Good Insulator but

Poor Heat Conductor

ISOTHERMS

Formation Pressure




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54

Formation Pressure


Transition Zone

GEOPRESSURE

Geothermal GradientAverage = 3 degrees C / 100m

When plotted, you may see a

decrease in the gradient in the

transition zone, followed by a sharp

rise in the pressured zone itself.

Mud Temperature Out

Formation Pressure




School

55

Formation Pressure

Gas Analysis

• For the mudlogger this is probably the

most important evidence for pressure

evaluation available.

• Note that any evaluation is dependant

on lagtime.• Be aware of ...

• Drilling Rate against Gas

• background gas

• Static Mud against Gas

• connection, trip & swab gas

• Mobile Drillstring against Gas

• background gas & swab gas • ECD against Gas

• background gas

Formation Pressure




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56

Formation Pressure

Gas Analysis

• Cuttings Gas - gas released from the

drilled formation.

• Produced Gas - gas issuing from the

borehole walls. May be due to caving,

swelling ordiffusion if differential

pressure is negative.• Recycled Gas - gas that is returned

down the hole from surface if, for

example, degassers are not working.

• Contamination Gas - gas from

petroleum products in the mud or

from thermal breakdown of additives.

Breakdown of organic matter such asshales or thermal effects of the bit

can also give rise to hydrocarbons.

Caving

Swelling

Gas

Diffusion

Gas in Water

Shale Gas

Eruptive Oil

Cuttings

Invasion

Formation Pressure




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57

Formation Pressure

Gas Analysis

%GASLithologyROP ft/hr Depth0 10400 0

3250

3300

3350

3400

3450

3500

Here you see the background

gas rising gradually as the

rates of penetration increase.

Is this pressure related or

simply a function faster drilling

producing more cuttings which

produce more gas?

What other data would help

you decide if necessary?

Formation Pressure




School

58

Formation Pressure

Gas Analysis

Background Gas stable with

sporadic Connection Gas.

This is not characteristic of

formation pressure variation.

This is indicative of swabbing,

lithology variations or gas

from cavings.

Depth

Background

Gas

Connection

Gas

Formation Pressure




School

59

Formation Pressure

Gas Analysis

Background Gas stable with

increasing Connection Gas.

Characteristic of entering a

transition zone.

The stable background

indicates that there is still a

positive differential pressure

with ECD but the fact that the

connection gas is increasing

indicates this is in decline.

Depth

Formation Pressure




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60

Formation Pressure

Gas Analysis

Background and connection

gas are indicating that drilling

is proceeding into a negative

differential pressure condition

at the bit through entry into an

overpressured zone.

Depth

Formation Pressure




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61

Formation Pressure

Gas Analysis

%GASLithologyROP ft/hr Depth0 10400 0

3250

3300

3350

3400

3450

3500

Cxn Gas

with

Kelly System

Cxn Gas

with Top Drive

DANGER

Less Frequent

&Less Visible

Formation Pressure




School

65

Cuttings Character

Formation Pressure

Broad face

Side view

Cross section

Typical cavings from drilling

underbalanced

Note the delicate, spikey shape.

Size: Starting small , 1 cm.

Growth dependant on amount of

underbalance and lithology.

Formation Pressure




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66

Cuttings Character

Formation Pressure

Broad face

Side view

Cross section

Stress relief or borehole failure

cavings.Typical blocky shape

showing fine cracks.

Failure due to rock mechanics

and borehole angle.

Size based on internal stresses

and rock competence.

Formation Pressure




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67

Cuttings & Borehole Condition

Formation Pressure

• Review how cuttings and cavings are behaving and tie this in to

what is happening in the borehole in terms of hole stability.

• Are you seeing torque build up from cuttings beds? The beds

generally form in hole angles from 45 and 55 degrees, is this so?

• Do you have caved material in block form illustrating that the well

direction is in one of the major stress directions ?

• Log the performance of each trip in and out. If the crew do

something that has a positive benefit then tell them. It is more

critical to drilling the well than being able to tell them that their performance got worse. Whatever they were doing right was cost

effective in a problem hole.

Formation Pressure




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68

Cuttings & Borehole Condition

Formation Pressure

Hookload Deviation -100 0 +100 Reamed Stuck Break Circulation Trip In

Trip Out NB #4 NB #5 NB #6 RRB #6 NB #7 NB #8 9 5/8” Csg

DEPTH

metres13

3/8”Csg 2000

-

3000

-

2500

-

3500

-

0 ANGLE 90

0 AZIMUTH 359

T R I P C O N D I T I O N L O G

Formation Pressure




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CALCULATING POREPRESSURE

Formation Pressure

Formation Pressure




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70

• The ratio method works on the principle that the difference between

the observed values for the compaction parameter and the normal

parameter extrapolated to the depth is proportional to the increase

in pressure. This means that the method works on the basic ratio

method.

• The ratio method is :

PPo = Dxcn x PPn / Dxco Where :

PPo = Observed pore pressure

Dxcn = Normal Dxc

PPn = Normal pore pressureDxco = Observed Dxc

Using Dxc - Ratio Method

Formation Pressure

Normal trend

Normal

Observed

Log Dxc

Formation Pressure




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71

• This method assumes that rock properties (porosity) at the depth of

interest will be essentially the same as those at a higher depth with

the same Dxc values. The difference will be higher pressure due to

overburden.

• The equivalent depth method is : PpA = [(OBGA x DA) - DB (OBGB - PpnB)] / DA

Where :

PpA = Observed pore pressure

PpnB = Normal pore pressure

OBGA , OBGB = Overburden pressure

DA , DB = Depth

Using Dxc - Equivalent Depth Method

Formation Pressure

Normal trend

B

A

Log Dxc

DB

DA

OBG

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72

• Showed the importance of using overburden gradient

• Showed the importance of using a variable overburden

• Depends heavily on a trendline placement

• Developed equations for a number of parameters

• Eaton’s method is : Po = S - [(S - Pn) * (Dxco / Dxcn)^1.2]

Where :

Po = Observed pore pressure

Pn = Normal pore pressureS = Overburden pressure

Dxco = Observed Dxc

Dxcn = Normal Dxc

Using Dxc - Eaton’s Method

Formation Pressure

Normal trend

Normal

Observed

OBG

Formation Pressure




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73

• Trendline placement is the biggest source of error

• This is true for any trendline method!

• If a pressure point is known the true

position of the trend can be calculated

Dxcn = Dxco* [(S - Po) * (S - Pn)]^-0.833

Where :

Po = Observed pore pressure

Pn = Normal pore pressureS = Overburden pressure

Dxco = Observed Dxc

Dxcn = Normal Dxc

Using Dxc - Eaton’s Method

Formation Pressure

Normal trend

Normal

Observed

OBG

Formation Pressure




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74

• Eaton developed equations for three MWD or Wireline Logs

• Resistivity

• Conductivity

• Sonic

• Eaton’s equations : Po = S - [(S - Pn) * (Ro / Rn)^1.2]

Po = S - [(S - Pn) * (Cn / Co)^1.2]

Po = S - [(S - Pn) * (Dtn / Dto)^3 ]

Where : Ro, Rn = Observed & Normal Resistivity

Co, Cn = Observed & Normal Conductivity

Dto, Dtn = Observed & Normal Sonic Dt

Using E-Logs - Eaton’s Method

Formation Pressure

Normal trend

Normal

Observed

OBG

Formation Pressure




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• This is a petrophysical-mechanical method

• It is based on derived formation properties

• porosity from Archie’s equation • matrix stresses from Terzaghi

• assumes power law compaction

• uses formation fluid resistivity

• uses formation resistivity from logs

• It does not need a trendline!

Resistivity - Bryant’s Method

Formation Pressure

Shale Resistivity

OBG

Formation Pressure




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• Calculate the overburden pressure - (S)

• Find the normal pore pressure - (Pn)

• Calculate normal effective stress - (ESnorm)

ESnorm = S - Pn

• Calculate the actual effective stress - (ESact)

ESact =

11225 * [1-exp((ln Rw - ln Ro) / 2)]^7.47

•

Calculate the pore pressure - (P)If ESact > ESnorm … P = Pn

If ESact < ESnorm … P = S - ESact

Resistivity - Bryant’s Method

Formation Pressure

Shale Resistivity

OBG

At the depth of interest ...

Note : Rw will be in a range of 0.015 to 0.03 ohmms

Formation Pressure




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• Pore pressure will only ever be an “educated best guess” if

using wireline, MWD, Dxc or other formation evaluation data.

• The engineer should bring together as much

information as possible to present to the client.

Pore Pressure - Conclusion

o o essu e

• Whether using paper,

pencil and ruler,

spreadsheets or computer

programs, the end result

will depend on the

knowledge and experienceof the pressure engineer

evaluating the data.

Formation Pressure




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Formation Pressure Worksheet Eaton Dxc Pore Pressure Calculations

Air Gap 95.1 feet 29.0 metres

Water Depth 728.4 feet 222.0 metres 0.59

Normal PP 8.7 ppg 1.04 sg 0.000089

1.2

TVD (ft) TVD (m) OBG (sg) Dxc Trend (ppg) (sg)

984.3 300.0 1.09 0.80 0.68 8.6 1.03

1148.3 350.0 1.22 0.78 0.69 8.5 1.02

1312.3 400.0 1.32 0.98 0.71 7.6 0.91

1476.4 450.0 1.39 0.98 0.72 7.4 0.89

1640.4 500.0 1.45 0.88 0.74 7.9 0.94

1804.5 550.0 1.51 0.85 0.75 8.1 0.971968.5 600.0 1.55 0.88 0.77 7.9 0.95

2132.5 650.0 1.58 0.90 0.78 7.9 0.94

2296.6 700.0 1.62 0.94 0.79 7.6 0.91

2460.6 750.0 1.64 1.00 0.81 7.2 0.87

2624.7 800.0 1.67 0.98 0.82 7.5 0.90

2788.7 850.0 1.69 1.05 0.84 7.0 0.84

2952.7 900.0 1.71 1.10 0.85 6.7 0.80

3116.8 950.0 1.73 1.15 0.87 6.4 0.77

3280.8 1000.0 1.75 1.10 0.88 6.9 0.83

3444.9 1050.0 1.76 1.20 0.90 6.2 0.74

3608.9 1100.0 1.77 1.15 0.91 6.7 0.81

3772.9 1150.0 1.79 1.28 0.93 5.7 0.69

3937.0 1200.0 1.80 1.25 0.94 6.1 0.73

4101.0 1250.0 1.81 0.95 0.95 8.7 1.05

4265.1 1300.0 1.82 0.85 0.97 9.6 1.16

4429.1 1350.0 1.83 1.00 0.98 8.6 1.03

4593.1 1400.0 1.84 0.90 1.00 9.5 1.14

4757.2 1450.0 1.85 0.85 1.01 10.0 1.20

Well - Bideford - 31/7 : Grossenschmuck : Celtic Petroleum

Pore Pressure

Eaton Slope & Tre nd Paramete rs

Displacement

Slope

Exponent

300.0

400.0

500.0

600.0

700.0

800.0

900.0

1000.0

1100.01200.0

1300.0

1400.0

1500.0

1600.0

1700.0

1800.0

1900.0

2000.0

2100.0

2200.0

2300.0

2400.0

2500.0

0.10 1.00 10.00

dxc/pp

d e p t h m

Formation Pressure




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Formation Pressure Worksheet Bryant Pore Pressure Calculations

Air Gap 95.1 feet 29.0 metres

Water Depth 728.4 feet 222.0 metres 0.028

Normal PP 8.7 ppg 1.04 sg

TVD (ft) TVD (m) OBG (sg) Res ES Norm ES Act (ppg) (sg)

984.3 300.0 1.08 0.80 18 2389 8.7 1.04

1148.3 350.0 1.21 0.78 82 2337 8.7 1.04

1312.3 400.0 1.30 0.98 149 2815 8.7 1.04

1476.4 450.0 1.38 0.98 218 2815 8.7 1.04

1640.4 500.0 1.45 0.88 289 2587 8.7 1.04

1804.5 550.0 1.50 0.85 362 2514 8.7 1.041968.5 600.0 1.55 0.88 437 2587 8.7 1.04

2132.5 650.0 1.60 0.90 513 2634 8.7 1.04

2296.6 700.0 1.64 0.94 592 2726 8.7 1.04

2460.6 750.0 1.67 1.00 672 2858 8.7 1.04

2624.7 800.0 1.71 0.98 754 2815 8.7 1.04

2788.7 850.0 1.74 1.05 838 2963 8.7 1.04

2952.7 900.0 1.76 1.10 923 3064 8.7 1.04

3116.8 950.0 1.79 1.15 1010 3161 8.7 1.04

3280.8 1000.0 1.81 1.10 1099 3064 8.7 1.04

3444.9 1050.0 1.84 1.20 1188 3254 8.7 1.04

3608.9 1100.0 1.86 1.15 1280 3161 8.7 1.04

3772.9 1150.0 1.88 1.28 1372 3396 8.7 1.04

3937.0 1200.0 1.90 1.25 1466 3344 8.7 1.04

4101.0 1250.0 1.92 1.00 1561 2858 8.7 1.04

4265.1 1300.0 1.94 1.00 1657 2858 8.7 1.04

4429.1 1350.0 1.96 1.00 1755 2858 8.7 1.04

4593.1 1400.0 1.97 1.00 1853 2858 8.7 1.04

4757.2 1450.0 1.99 1.23 1953 3308 8.7 1.04

Form ation Water Resistivity

Resistivity

Water resistivity used in

this method can be f rom

0.015 to 0.04 ohmms.

Bryant PPr.

Well - Bideford - 31/7 : Grossenschmuck : Celtic Petroleum

300.0

400.0

500.0

600.0

700.0

800.0

900.0

1000.0

1100.01200.0

1300.0

1400.0

1500.0

1600.0

1700.0

1800.0

1900.0

2000.0

2100.0

2200.02300.0

2400.0

2500.0

0.10 1.00 10.00

res/pp

d e p t h m

Formation Pressure



0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

800.0

900.0

1000.0

1100.0

1200.0

1300.0

1400.0

1500.0

1600.0

1700.0

1800.0

1900.0

2000.0

2100.0

2200.0

2300.0

2400.0

2500.0

2600.0

0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40 1.50 1.60 1.70 1.80 1.90 2.00 2.10 2.20 2.30

pore pressure s.g.

d e p t h m . Dxc

Res

Sonic

Bryant PPr.

02_pressure school

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