01_pressure school

1Formation Pressure Evaluation School

04/08/23

Baker Hughes INTEQ

Surface Logging Systems

WELCOME!!To The FORMATION PRESSURE

EVALUATION COURSE

5 Fun Filled Days with Exam!

Formation Pressure

04/08/23 Formation Pressure Evaluation School

2

Formation Pressure

Course Objectives ...1. To COMMUNICATE.

2. To collect DATA.

3. To recognise TRENDS.

4. To evaluate PRESSURE.

5. And ...

... develop a plan to be

able to undertake pressure

evaluation anywhere!


3

Day One :

• Introduction• Communication• Basic Terminology• Hydrostatic & ECD• Overburden

Day Two :

• What is Pore Pressure? • Origins of Abnormal Pressure• Pressure Evaluation

Day Three :

• Fracture Pressure Gradient

• Casing Seat Placement• Basic Well Control

Course Outline

Day Four :

• Predict Exercise

Day Five:

• Final Exam

Formation Pressure


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Day OneObjectives :• Introduction

• The Plan• Communication• Become familiar with

• Basic Terminology• Hydrostatic & ECD• Overburden

Formation Pressure


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INTRODUCTION

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The PlanIn the beginning there isa Plan …

… and the Plan is best shown as a flow diagram.

Note: If you learn the flow diagramyou can get through the course very easily and undertake pressure evaluation anywhere! You don’t need a computerjust the ability to undertake a few simple exercises with a calculator!

Formation Pressure


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• Density Logs• Porosity Logs

• Regional Curves• Equations

• Rock Cuttings Density

ROCK BULK DENSITY

b

Overburden Pressure“S” in psi, bars, atm.

Overburden Gradient“S” / TVD from rkb

ThePlan

(1)OverburdenPressure

(Rock Properties)

Formation Pressure


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• Offset Well Data• Rw Catalogues• Resistivity Logs

• Bucket on a Rope

Formation Pressure

PORE WATER DENSITY

f

Normal Hydrostatic Pr.Normal Pore Pressure“P” in psi, bars, atm

ThePlan

(2)PorePressure

(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 andFB Gradient

MINIMUMSTATIC MUD

DENSITY

“S”(from 1)


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Formation Pressure

Estimated Fracture Pressure at Any Depth

Maximum Dynamic Mud Pressure &

Maximum Shut-in Casing Pressure

LEAK OFFTEST

Kick Tolerance

“S”(from 1)

“P”(from 2)

Rock CuttingsPoisson’s

Ratio “”

Tectonicstress

ThePlan

(3)FracturePressure


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The PlanThe aim of this Plan and partof the aim of this Course ...

To give employees the knowledgefor drilling safe wellbores in thewindow between

Minimum Static Mud Density and

Maximum Dynamic Mud Pressure

Formation Pressure


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• Abnormal formation pressures occur in most sedimentary basins worldwide.

Introduction

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• Abnormal formation pressures occur in most sedimentary basins worldwide.– They occur in all geologic

age formations.

– They occur at all depths.

Introduction

Formation Pressure


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• Abnormal formation pressures occur in most sedimentary basins worldwide.– They occur in all geologic

age formations.

– They occur at all depths.

– A third to a half of all wells drilled experience abnormal pressures.

Introduction

Formation Pressure


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• Understanding formation pressures is important for several reasons

– Minimize drilling costs due to lost time and equipment problems through ...

Introduction

Formation Pressure


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– Minimize drilling costs due to lost time and equipment problems through

• Well kicks and blowouts

CHING!CHING!

Introduction

Formation Pressure


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CHING!CHING!CHING!



• Well kicks and blowouts• Stuck pipe

Introduction

Formation Pressure


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• Well kicks and blowouts• Stuck pipe• Lost circulation

CHING!CHING!CHING!CHING!

Introduction

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• Well kicks and blowouts• Stuck pipe• Lost circulation

– Ensure rig personnel safety.– Stop environmental pollution.– Minimize loss of reserves.

Introduction

Formation Pressure


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• The evaluation of formation pressure is critical for well planning and completion.

• When a well is drilled there are several kinds of pressure that must be considered.

Introduction

Formation Pressure


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– Overburden pressure

Introduction

Formation Pressure


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– Formation pore pressure

Introduction

Formation Pressure


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– Fracture pressure

Introduction

Formation Pressure


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Hydrostatic Pressure = Vertical Depth * Mud Weight * 0.0519






– Mud hydrostatic pressure

Introduction

Formation Pressure


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Equivalent Circulating Density (ECD)






– Mud hydrostatic pressure

– Equivalent Circulating Density (ECD)

Introduction

Formation Pressure


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• To successfully drill a well to completion, a comprehensive mud and casing program must be made.

Fracture Gradient

PorePressure Gradient

MudWeight

Introduction

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• There must be a balance between mud hydrostatic pressure and formation pore pressure.

Introduction

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• Problems that can occur – Mud hydrostatic < Formation

• Kicks

Introduction

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• Kicks• Stuck Pipe

Introduction

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• Kicks• Stuck Pipe

– Mud hydrostatic > Formation• Lost Circulation

Introduction

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COMMUNICATION

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Pressure EvaluationWhat Data Will You Be Given To Assist You?

• Hopefully as much as possible!

• When in doubt ask for more!!

• If the Client is serious about having

you perform a high quality pressure

evaluation they should be as helpful

as possible.

You should NOT need a crystal ball ...... yet.

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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

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Communication

Who?

Why?

When?

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Who and Why?

• Everyone associated with the drilling process ...

– Mudlogger / Geologist ?– Mud Engineer ?– MWD Engineer ?– Driller ?– Derrick Hand / Shaker Hand ?– Company Man / Drilling Engineer ?– Your Relief ?– Your Coordinator ?

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When?

• In a timely manner

– There should be constant communication– Communicate when conditions are normal– Communicate when you suspect a problem

And remember to ...

– Never ignore changes in trends– Report everything, but prepare to be wrong– Get second opinions when necessary– Write things down!

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TERMINOLOGY

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• Hydrostatic Pressure– The pressure exerted at any point in a static fluid column

• Overburden Pressure– Pressure of the weight of overlying sediments and fluids

• Pore Pressure– Pressure exerted by the formation fluids

• Formation Balance Gradient– Mud density needed to balance the pore pressure

• Fracture Pressure– The pressure needed for hydraulic fracturing to occur

Formation Pressure Terminology

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• Equivalent Circulating Density – Density of the mud while being pumped

• Pressure Gradients

– pressure divided by TVD & measured in psi/ft or bars/m

• Equivalent Mud Weight or Density

– pressure gradient measured in ppg or gm/cc

– note that gm/cc and specific gravity (sg) are used

interchangeably

psi/ft / 0.052 = ppg bar/m / 0.0981 = sg

Formation Pressure Terminology

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Pressure Gradients

Formation Pressure

0 1000 2000 3000 4000 5000 Pressure

Depth

5000

2500

Gas Gradient 0.07 psi/ftOil Gradient 0.30 psi/tfWater Gradient 0.433 psi/ft“Normal” Formation Gradient 0.465 psi/ft10ppg Mud Gradient 0.519 psi/ft15ppg Mud Gradient 0.779 psi/ft21ppg Mud Gradient 1.090 psi/ft


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HYDROSTATIC PRESSURE AND E.C.D.

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• The pressure exerted by a static column of fluid

Hyd. Pr. (psi) = TVD (ft) * Fluid Density (ppg) * 0.0519

Hyd. Pr. (bar) = TVD (m) * Fluid Density (sg) * 0.0981

• The hydrostatic pressure gradient will be

Gradient (psi/ft) = Hyd. Pr. (psi) / TVD (ft)

Gradient (bar/m) = Hyd. Pr. (bar) / TVD (m)

Hydrostatic Pressure

True vertical depths should be used in all pressure calculations. Mud depths should be referenced from the flowline.

- psi/ft / 0.0519 = ppg- bar/m / 0.0981 = sg

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• The normal hydrostatic pressure of a formation depends solely on the pore water density and the true vertical depth.

• The mud hydrostatic pressure is the pressure exerted at any point by the mud column.

Hydrostatic PressureMud Weight =10.00 lb/gal

Vertical Depth =10,000 ftMeasured Depth=10,000 ft

Hydrostatic Pressure:

MW * TVD * 0.0519

= 5190 psi

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• The normal hydrostatic pressure of a formation depends solely on the pore water density and the true vertical depth.

• The mud hydrostatic pressure is the pressure exerted at any point by the mud column.

Hydrostatic Pressure

Hydrostatic Pressure:

MW * TVD * 0.0519

= 3633 psi

Mud Weight =10.00 lb/gal

Vertical Depth =7,000 ftMeasured Depth=10,000 ft

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Hydrostatic & Pore Pressure • Normal hydrostatic pressure = Normal pore pressure

• Quite simply pore pressure is the pressure exerted by pore fluids which reflect the water density in the basin of deposition

• Non-marine • Brackish • Open marine • Partially restricted marine or saline • Very restricted marine or hypersaline

Normal pore pressure can be anywhere

between 8.34 ppg and 9.00 ppg

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• Balanced Situation : Mud Hydrostatic = Pore Pressure

• see “Minimum Static MW” in the “Plan”

• Underbalanced : Mud Hydrostatic < Pore Pressure

• kicks / sloughing / fast drilling / stuck pipe

• Overbalanced : Mud Hydrostatic > Pore Pressure

• fluid loss / fracturing / slow drilling / stuck pipe

Hydrostatic & Pore Pressure

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• Consider that the formation and the annulus form a u-tube.

• To be balanced, both arms need to have the same pressure.

Formation Balance GradientThe U-Tube & J-Tube Effect

Mud Pore Fluid

Mud Hydrostatic = Pore Fluid Hydrostatic

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• Most rigs can be considered to be j-tubes.

• The Formation Balance Gradient is the mud weight needed to balance pore fluids when considering the j-tube effect.

Pore Fluid

Mud Hydrostatic = Pore Fluid Hydrostatic

Mud

Flowline

Air Gap

Formation Balance GradientThe U-Tube & J-Tube Effect

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Equivalent Circulating Density

• The ECD is the apparent density of the mud while circulating.

• Annular pressure losses due to frictional effects increase the apparent density.

• Annular losses are affected by

– Mud rheology– Hole size & pipe geometry– Flow rate & annular velocity

• The ECD is typically greatest at the bottom of the hole.

Mud Weight =10.00 lb/gal

Circulating mudcreates pressurelosses against the hole wallPressure lossesadd to the mudhydrostatic.ECD is typically greatest at the bottom of the wellbore.

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OVERBURDEN

PRESSURE

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Overburden Pressure

• Overburden pressure is the pressure at any point in the formation exerted by the total weight of the overlying sediments.

• It is a function of the vertical depth and density of the rock column.

• The prior calculation of overburden pressure is critical for the accurate determination of pore and fracture pressures.

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• Overburden calculation needs the following data to be input

– fluid density (offshore)

Measurefluid densityoffshore

Overburden Pressure

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• Overburden calculation needs the following data to be input

– fluid density (offshore)– formation bulk density

Measure bulk densityof formations

Overburden Pressure

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Measure vertical depth to point ofinterest.

• Overburden pressure needs the following data to be input

– fluid density (offshore)– formation bulk density– true vertical depth

Overburden Pressure

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TVD



• Onshore, overburden pressure is calculated from ground level.

Overburden Pressure

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TVD



• Onshore, overburden pressure is calculated from ground level.

• Offshore, overburden pressure is calculated from sea level.

• Overburden gradient is calculated from the drill floor.

Overburden Pressure

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• Overburden is variable.

• Sediment densities vary between 1.6 and 3.2 gm/cc

• Fluid densities vary between 1.0 and 1.08 gm/cc

• Overburden pressure is calculated with :

S(psi) = pb(ppg) * tvd(ft) * 0.0519

S(bar) = pb(sg) * tvd(m) * 0.0981Where :

S = Overburden pressure (psi or bars)pb = bulk density (gm/cc or s.g. or ppg)tvd = interval depth or length (ft or m)

NOTE: The equation in the manual : S(psi) = tvd(ft) x density s.g. x 0.433 is best not used due to inaccuracies in wells shallower than 6000m.

Overburden Pressure

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• At 60ft = 0 psi

• At 360 ft, – (300 * 8.67 * 0.0519) + 0– 135.0 psi– 135.0 / 360 = 0.375 psi/ft– 0.375 / 0.052 = 7.21 ppg

• At 700 ft, – (340 * 15.4 * 0.0519) + 135.0– 407.0 psi– 407.0 / 700 = 0.581 psi/ft– 0.581 / 0.052 = 11.18 ppg

• At 1500 ft, – (800 * 16.9 * 0.0519) + 407.0– 1108.0 psi– 1108.0 / 1500 = 0.739 psi/ft– 0.739 / 0.052 = 14.21 ppg

air

water

clay

shale

sandstone

limestone

0

15.4

20.3

16.9

20.9

8.6760ft

360ft

700ft

1500ft

2200ft

2500ft

OBG Calculation

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• The basic data needed is bulk density.• Sources of bulk density data include :

– Drilled cuttings – Density wireline and MWD logs – Sonic wireline and MWD logs– Regional tables and curves – Equations– Seismic velocity data

Overburden Pressure Data

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Using a Mud Balance

(Pyncometer Method)

• Grab some fresh cuttings• Remove obvious cavings• Clean off mud• Dry slightly on paper towel• Be consistent

• Put cuttings in the balance until it reads 8.34 ppg.• Fill the cup with fresh water and weigh again = (W2)• Calculate bulk density : pb (gm/cc) = 8.34 / (16.68 - W2)

OBG Data - Drilled Cuttings

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• MWD and wireline density logs contain bulk density data that can be read directly from the log.

• Density logs only have a small depth of investigation (2-6 inches)

• Caution has to be taken when reading the logs due to washouts in the hole, mixed lithologies, logging speed etc.

• Logging companies will apply correction factors for the above.

• Allowance needs to be made for the effects that can distort density data such as gas filled pores - density reduction.

• Density log data is superior to density from drilled cuttings data or sonic log data but unfortunately it is not commonly run in all hole sections.

OBG Data - Density Logs

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• Sonic logs register the Transit Time (delta t) of a formation. • Delta t is measured in usecs/ft.• The delta t for a particular rock is a measure of its porosity• Lower transit times = faster acoustic velocity

= lower porosity = higher density

• With depth and normal compaction density should increase.

LOWHIGH

Normal Trendfor Sonic (dt)with normalcompaction.

OBG Data - Sonic Logs

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The procedure

• Using Gamma Ray or SP, pick broad lithology intervals• Ignore beds less than 50ft thick• Eyeball an average delta t over the lithology interval

or ...• Calculate the average delta t using Integrated Transit Time tics

• Each tic = 1000 usecs/ft• Count the tics over the interval -

use 1/2’s, 1/4’s if necessary• Calculate delta t :

delta t = #tics * 1000 / interval


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• AGIP’s Bellotti, DiLorenza and Giacca (1978) developed an equation to calculate bulk density from Sonic logs :

pb = 2.75 - [(2.11 * (Dt - Dt Matrix) / (Dt + Dt Fluid)]

Where :

pb = bulk density

Dt = transit time from log

DtMatrix = transit time of rock

DtFluid = transit time of fluid


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• Gardener’s Sonic Equation (used in Predict) was also developed to calculate bulk density from Sonic logs :

pb = A * ( 10^6 / Dt )^B

Where :

pb = bulk density

Dt = transit time from log

A = Coefficient (typically 0.23)

B = Exponent (typically 0.25)


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• Seismic data can be used to calculate bulk density.

• Seismic velocity can be converted to usec/ft and treated as you would Sonic data:

Transit Time (usec/ft) = (1/vint) * 1,000,000

• Data taken around salt domes will not be reliable due to the effect salt has on sound waves - lowers the values.

• Try to get a "shallow" seismic study to check for existing faults.

• Even from the sparsest of data the pressure engineer should be able to produce a fairly reasonable overburden curve prior to drilling. Other data obtained during the drilling phase should be used to compare and correct this initial curve.

OBG Data - Seismic Data

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• Seismic data may look something like this:

subsea depth two-way time velocity interval

6618 2.2672 82726668 2.2846 56846718 2.3008 62176768 2.3152 69096818 2.3292 71576868 2.3452 6242

Transit Time (usec/ft) = (1/vint) * 1,000,000

139.72 (usec/ft) = (1/7157) * 1,000,000

OBG Data - Seismic Data

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• Many areas have regional curves based on known data.

• Problems arise when data is used outside the specific region.• Even within the area water depth corrections must be made.

• Remember to always question offset data and make sure it is not TOO offset by structure or distance.

• Make corrections for water depth. This can make a vast difference in overburden.

OBG Data - Curves and Tables

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• Bryant (1989) came up with the following equation to calculate the overburden gradient for Tertiary basins :

OBG = OGO+2.64E-5(D)-1.97E-9(D^2)+6.60E-14(D^3)-5.97E-19(D^4)

Where:OGO = Overburden gradient offset -

(Land=0.87; Shallow=0.85; Deep=0.82)D = True vertical depth (ft)

• Another equation comes from Amoco/Chevron

• This equation approximates sediment bulk density (OBG will have to be calculated from these densities) :

pb = 16.3 + [ (TVD - Water Depth - Air Gap) / 3125 ] ^0.6

(Note : 16.3 is the estimated weight of sediments at the mudline)

OBG Data - Equations

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• The generation of an overburden curve cannot be ignored.

• This is the curve that all other pressure work depends on.

• You should not wait until the first casing point.

With the reasonable offset data, an overburden curve can be made even before a foot of formation is drilled.

This can then be corrected as drilling progresses and more data is acquired.

Steps to creating your curve:

1) Use equations or data provided2) Use cuttings data to modify3) Use e-log data at casing points4) Continue to use cuttings, sonic, and density data, etc.5) Check curve with LOT’s

Overburden Pressure - Conclusion

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Formation Pressure

Formation Pressure Worksheet Overburden Pressure Calculations

Air Gap 95.1 feet 29.0 metres OB Offsets: Sand dt 50 The OBG from Amoco uses the bulk

Water Depth 728.4 feet 222.0 metres Land 0.900 Silt dt 48 density equation w hich is used in

Normal PP 8.7 ppg 1.04 sg Shelf 0.825 Shale dt 47 "Predict" . The Bell equation needs

Deep 0.850 Sea dt 190 to use an OB Offset based on w ater

Fresh dt 210 depth. These are estimates only!

TVD (ft) TVD (m) pB Amoco pB Cutting dt Sonic dt Matrix pB Sonic pB Used OBP (psi) OBP (bar) OBG (ppg) OBG (sg) (ppg) (sg) (ppg) (sg)

984.3 300.0 1.97 1.97 466.3 32.1 9.1 1.09 9.1 1.09 9.1 1.081148.3 350.0 1.98 1.98 607.2 41.9 10.2 1.22 10.2 1.22 10.1 1.211312.3 400.0 1.99 1.99 748.8 51.6 11.0 1.32 11.0 1.32 10.9 1.301476.4 450.0 2.00 2.00 890.9 61.4 11.6 1.39 11.6 1.39 11.5 1.381640.4 500.0 2.01 2.01 1033.5 71.3 12.1 1.45 12.1 1.45 12.1 1.451804.5 550.0 2.01 2.01 1176.5 81.1 12.6 1.51 12.6 1.51 12.6 1.501968.5 600.0 2.02 1.85 2.02 1319.9 91.0 12.9 1.55 12.9 1.55 13.0 1.552132.5 650.0 2.02 2.00 2.02 1463.8 100.9 13.2 1.58 13.2 1.58 13.3 1.602296.6 700.0 2.03 1.90 2.03 1608.0 110.9 13.5 1.62 13.5 1.62 13.7 1.642460.6 750.0 2.03 1.90 2.03 1752.5 120.8 13.7 1.64 13.7 1.64 14.0 1.672624.7 800.0 2.04 1.90 2.04 1897.4 130.8 13.9 1.67 13.9 1.67 14.2 1.712788.7 850.0 2.04 1.88 2.04 2042.6 140.8 14.1 1.69 14.1 1.69 14.5 1.742952.7 900.0 2.05 2.05 2188.2 150.9 14.3 1.71 14.3 1.71 14.7 1.763116.8 950.0 2.05 2.05 2334.0 160.9 14.4 1.73 14.4 1.73 14.9 1.793280.8 1000.0 2.06 1.95 2.06 2480.1 171.0 14.6 1.75 14.6 1.75 15.1 1.813444.9 1050.0 2.06 1.90 2.06 2626.6 181.1 14.7 1.76 14.7 1.76 15.3 1.843608.9 1100.0 2.07 2.07 2773.3 191.2 14.8 1.77 14.8 1.77 15.5 1.863772.9 1150.0 2.07 2.07 2920.3 201.3 14.9 1.79 14.9 1.79 15.7 1.883937.0 1200.0 2.07 2.07 3067.6 211.5 15.0 1.80 15.0 1.80 15.9 1.904101.0 1250.0 2.08 2.08 3215.1 221.7 15.1 1.81 15.1 1.81 16.0 1.924265.1 1300.0 2.08 2.08 3362.9 231.9 15.2 1.82 15.2 1.82 16.2 1.944429.1 1350.0 2.08 2.08 3510.9 242.1 15.3 1.83 15.3 1.83 16.3 1.964593.1 1400.0 2.09 2.10 150.0 48 2.12 2.10 3660.1 252.4 15.4 1.84 15.4 1.84 16.5 1.97

OBG Amoco OBG Bell

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

OBG Used For Analysis


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Formation Pressure

0.0100.0200.0300.0400.0500.0600.0700.0800.0900.0

1000.01100.01200.01300.01400.01500.01600.01700.01800.01900.02000.02100.02200.02300.02400.02500.02600.02700.02800.02900.03000.03100.03200.03300.03400.0

0.50 0.70 0.90 1.10 1.30 1.50 1.70 1.90 2.10 2.30 2.50

obg s.g.

dep

th m

.

OBG Used For Analysis

OBG Amoco

OBG Bell


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Formation Pressure

• Use the Bideford 31/7 well logs.

• Select Sonic transit time data points every 100 meters.

• Calculate the bulk density using Gardener’s equation.

• Calculate the overburden gradient in EMW.

• Use 1.03 s.g. as the water density.

• Use the Amoco equation to calculate bulk density values every 100 meters from the seafloor to the top of the first sonic data point.

Day One : Homework : OBG

01_pressure school

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