Sonatrach Casing Design

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September October 2005abalt solutions limited - 2005

INTRODUCTION TO HYDROCARBON EXPLOITATION

Introduction to Hydrocarbon Exploitation

2005 Abalt Solutions Limited. All rights reserved

Casing Design

Section

By Pratap Thimaiah

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Well Planning


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DrillingTechnology-CasingDesign


Well Construction

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Drilling Team


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Operational Pressures

Overbalance

In balance

Underbalance

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Overbalance

Condition where the wellborepressure >pore pressure.

Margin of over balance isoften called safety or tripmargin



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In Balance

Pore pressure of formation=wellbore pressure


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Underbalance

Wellbore

pressure


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Pressure

Force exerted on a cross section area

Pressure gradient is pressure exerted per unitlength

Key objectives of evaluation

Gain an accurate knowledge of formationpressure to ensure well control.

Estimate formation fracture pressure in order toset working pressures.

Reduce risks


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Key types of Operating Pressures

Hydrostatic

BHCP

Pump pressure

Overburden pressure

Formation pressure

Fracture pressure



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Hydrostatic Pressure

Pressure due to unit weight and vertical heightof a static column of fluid.

Expressed in field units as:

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Bottom Hole Circulating Pressure (BHCP)

On circulation, pressure losses occur in the

annulus due to friction.

For circulation to be maintained, these lossesmust be overcome by pumps.

This leads to:

BHCP can be expressed in terms of equivalentcirculating density in pounds per gallon (ppg)


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Pump Pressure

Pump pressure mustovercome all the losses in

the mud path:

Surface equipment

(rotary hose,kelly,swiveletc)

Drill pipe

BHA

Bit

BHA Annulus

Drill Pipe Annulus

All other losses are termed parasitic

losses and calculated from standardpressure equations

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Overburden Pressure

Pressure exerted bytotal weight of solidsand fluids in theformation

Caused by weight ofrock above the areaof interest

z is unique andconstant for aparticular formation

ob can be calculatedby


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Formation Pressure

Pressure exerted by fluids contained in porespaces of rocks.

Depends upon fluid column density & verticaldepth.

For a normal formation, pore pressure is:

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Normal formation Pressure

Saline water is common in porous rocks.

Formation pressure is a function of formationwater density.

Depending on salinity, pressures range from

0.433 psi/ft (fresh water) to 0.465psi/ft

Abnormal formation Pressure

When Pf0.465psi/ft -Geopressured or Overpressured

Formation Pressure


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Fracture Pressure

Critical pressure to break downformation or induce fractures.

Fracture gradient is a plot ofPressure vs. depth necessary forthis to occur.

Knowledge of fracture gradienthelps to:

Determine setting depthsfor intermediate casingstrings

Maximum allowableannular surface pressure tocontrol a Kick.

Maximum allowable Muddensity during drilling.

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Leak off Test

Method for positive determination of maximum mud weight permittedin an open hole section of well.

The crew performs test in first few feet of new hole drilled below anew casing shoe.

The results when converted to equivalent mud weight, determinesmaximum mud weight the section can withstand without losscirculation.

Test consists of well closure at surface and pressure application untilmud just begins to inject into formation.

Procedure1. After Cementing casing, run with bit and drill string

2. Pressure test casing and drill out casing shoe to about 10 ft of newformation.

3. Pull bit to casing shoe.

4. Shut off pumps, wait for flow to cease, then close kelly cock and BOP.

5. Using cementing unit pump mud into hole annulus.

6. Monitor pressure build up and volume.

7. Pressure build up is linear until mud begins to bleed into formation.This is called Leak-off Pressure (PLOT)


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8. as pumping continues, build up curve flattens out until pressure nolonger increases. This is called Injection Pressure.

9. At injectivity point, pump should be shut off and choke closed.

10. Monitor pressure until shut-in pressure falls to equilibrium called Bleed-off Point.

11. Hold bleed-off pressure for several minutes to confirm no break down.

Leak off Test

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Casing Strings

Conductor

Surface

Intermediate

Production

Liner


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Casing Strings-Configurations

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Casing Strings

Reasons for casing off open hole:

Prevent unstable formations from caving in

Protect weak formations from mud weights that causezones to break.

Abnormal pressure isolation in zones.

Seal off lost circulation

To complete and produce zone effectively

Structural support for Bops and Wellheads.


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Casing String Types

Conductor Casing

First casing string to be run from surface toshallow depth and has the largest

diameter.

Surface formations usually have lowfracture strength which could exceed

hydrostatic pressure exerted by drillingfluids.

Conductor provides a conduit for mudreturns and always cemented to surface.

Its functions mainly are

Seal off unconsolidated formationsat shallow depths.

Prevent wash outs.

Protection of shallow gas flows.

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Casing String Types

Surface Casing

The main function is to seal offthe fresh water sands.

Support the BOP Equipment.

The casing setting depth isimportant where abnormalpressures are expected.

Casing is usually set incompetent formations


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Casing String Types

Intermediate Casing

Isolate troublesome formations/Transition zones which wouldcause drilling problems such as lostcirculation, sloughing shale, highpressure zones, salt zones etc.

Number of intermediate stringsmay be required depending uponpore and fracture gradients.

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Casing String Types

Production Casing

Usually the last string ofcasing to be run eitherthrough pay zone or abovepay zone.

Isolation of productioninterval from other zones.

Protection of tubing andother equipment, basis forwell completions.


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Casing String Types

Liner Short string of casing that does not

extend back to surface. Run back inside the previous

casing to provide overlap.

If required tie-back string can beextend to wellhead.

Advantages

Complete wells with lightweight on wellheads andsurface pipe.

Total cost of production stringis reduced & alsorunning/cementing timesallowing optimal size of tubing.

Improved completion flexibility.

Disadvantages

Possible leak in liner hanger. Difficulty in obtaining a good

cementing job due to narrowannulus.

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Properties of Casing

Casings are manufactured from different sizes,

lengths, grades and weights.

API Bulletin 5C2 has specifications for minimumstandards that must be met for each type of

casing.


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Outside Diameter (OD): Casings vary from 4.5 to 36 .They can be found in the

manufactures catalogue or field books.

Casing Grade and Properties:

Grades refer to physical properties of steel in the manufacturing process. API Specification contains various grades designated by a letter and number

referring to minimum yield strength. Ex- N80 casing has a minimum yield strength of 80,000 psi.

Manufactures also produce their own grades but have almost similarproperties to API Casings.

Grade Yield Strength Mini (psi) Max Yield Strength (psi) Tensile Strength (psi)H-40 4000 - 60000

J-55 55000 80000 75000K- 55 55000 80000 95000

C-75 75000 90000 95000L- 80 80000 95000 95000N-80 80000 110000 100000

C-95 95000 110000 105000

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Length of Joint : Casings are

available in three ranges:

Range 1: 16 to 25 ft

Range 2: 25 to 34 ft

Range 3: 34 + ft

Weight of Casing: Wall thickness isindicated by weight per foot.

0.3958.8359.62540

0.4358.7559.62543.5

0.4728.6819.62547

0.5458.5359.62553.5

Wall Thickness (in)ID (inches)OD (inches)Weigh t (lb/ft)


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Connections

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Casing Wear

Adhesive Wear Occurs when contact pressure >250 psi Pressure and movement cause part of

casing to weld themselves to tool joint. Solid phase welding of chip to tool joint

Transport Chip dislodged

Abrasive Machining Wear

Particles of metal shaved off Tong die marks and hard facing Rapid wear of metal

Abrasive Grinding Wear Contact pressure


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Casing Wear

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Casing Wear

Internal Diameter Changes

Changes in casing sizes

Change in casing hanger to top of joint

Wear bushing ID


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Minimize Casing Wear

Minimize rotation hours for High ROP Bits andDHMM.

Change tong dies in good time

Specify correct hard banding on tool joints

Use drill pipe protectors

Weighted drilling mud

Smooth transition of ID at changes of casingsize.

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Basic Casing Design

Casing Design Process

Data Collection

Preliminary Design

Detailed Design

Triaxial analysis

Documentation


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Data Collection

Pore Pressure and Fracture Pressure

Calculations for burst and collapse pressures.

Kick tolerance for casing setting depths.

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Data Collection

Lithology

Formation types from offset wells vs depth.


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Data Collection

Final Hole Size

Minimum size determined by completions.

Exploration wells require logging to determinefinal hole sizes.

Contingency hole size maybe required forexploration wells.

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Data Collection

Temperature gradient

Yield strength of casing decreases with

temperature increase.

Problematic zones Offset well analysis

Identification of problematic zones like saltdiapers, lost circulation zones etc.

Casing designed to withstand mobile saltmovement.

Drilling with salt saturated mud.


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Data Collection

Bit and Casing sizes

Using sizes available in yard or area ofoperations

Life cycle loads

Artificial lift methods

Injection systems

Abandonment plans

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Preliminary Design

Identify

Wellbore fluid densities

Casing diameters Lowest hole section

determined by IPR and

completion tubing size

From lowest hole section bit

size, next casing string upmust have an ID which

allows this bit size to be

passed.

Casing Shoe setting depths

Casing shoes should be

cemented in strong &competent formations.

Isolating troublesome zones.


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Preliminary Design

Casing Setting Depths1. Initial setting depths is based on pore and fracture pressure gradients.

2. The total depth of well and setting depth of production casing or liner

is driven by logging, testing and completion requirements.

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3. Draw mean pore pressure gradient curve along with lithology.Note anyintervals of problematic zones-such as differential sticking, lostcirculation or high pressure gas zones.

4. Draw mud weight curve. It should include 200-400 psi trip margin.specific companies may require different levels of over balance.

5. Draw predicted fracture gradient curve. Draw a fracture gradientdesign curve parallel to predicted with 0.3 to 0.5 ppg reduction, forkicks and ECD during cementing.

6. Plot mud weights and Leak off tests to provide check of pore pressurepredictions.

To estimate Casing setting Depths: Enter mud at point A (Total depth)

Move up to point B which determines initial estimated setting depth forintermediate casing.

Move across to point C which determines mud weight requirement f orthat depth.

Move up to point D which determines preferred setting depth forsurface casing.

Move across to point E to identify mud weight required at that depth.

Preliminary Design


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Other factors for casing setting depth Shallow gas

Lost circulation zones

Formation stability

Directional well profile

Fresh water sands

Salt sections

High pressure zones

Competent formations

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Detailed Design

Burst and Collapse Loads

Installation Loads

Service Loads


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Burst Loads

Burst Pressure=Internal Pressure- ExternalPressure exceeding casing burst strength.

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Burst Loads

Internal Pressures for Burst

Burst pressures occur when formation fluidsenter the casing while drilling or produce nexthole section. (fig 5.3 pg110 rabia)

Assuming a gas kick of pressure Pf from next TDand gas filling the entire well then internalpressure at surface and shoe are:

Internal pressure at surface= Pf G x TD

Internal Pressure at Shoe = Pf G x (TD-CSD)

Where G= gas gradient (generally 0.1 psi/ft)


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Burst Loads

External Pressures for Burst

1. Ext pressure =0.465 psi/ft x csd (ft)

2. External pressure is not based on the cementcolumn.

3. If casing is cemented to surface i.e.- conductor andsurface casing

Ext Pressure = maximum expected pore pressure

4. If casing is uncemented i.e.- Intermediate and

production strings then In open hole use column of mud to balance lowest pore

pressure.

Inside another casing, use mud down to TOC and fromTOC to shoe use column of mud to balance lowest porepressure in open hole.

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Collapse Loads

Collapse Pressure = External pressure

Internal Pressure.


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Collapse Loads

Occurs mainly due to

Cementing

Salt loading

Partial or total casing evacuation

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Tension Load

Due to weight of casing itself.

Uppermost part/joint of string is weakest, as itcarries total weight of string.

Other loads arise due to:

Bending

Drag

Shock

Pressure testing


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Tension Load

Tensile forces are calculated by:

Weight of casing in air using TVD.

Buoyancy force

Bending force in deviated wells

Drag force while POOH.

Shock load due to arresting casing in slips

Pressure testing forces.

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Tension Calculations

Weight of casing in air = casing weight (lb/ft) xTVD (ft)

Bending Force =63Wn x OD x Where Wn = weight of casing

=dogleg severity , degrees/100ft

Buoyancy force for open ended casing= Pe (Ae-Ai) (Fig 5.7,pg 123,Rabia)

Buoyancy force for closed casing= Pe Ae Pi AiWhere Pe=external hydrostatic pressure ,psi

Pi=internal hydrostatic pressure , psi

Ae and Ai are external and internal areas ofcasing.


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Drag Forces- Usually in order of 100,000 lbf. Last for the duration of running a joint of casing. Results usually when POOH due to tight hole

Shock Load results when- Sudden decelerations are applied Casing picked off slips Slips kicked in while pipe is moving Casing jumps off edge downhole

Fshock = 1780 V Aswhere V = pipe velocity (ft/s)

As=C/s area.

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Pressure Testing

Casing should be tested to maximum pressure

which it sees during drilling and productionoperations

Test Pressure force Ft (lb) = (ID)2 x test pressure

4


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Compression Loads

Arise in casings that carry inner casing strings.

Weight of inner strings is transferred to larger

supporting string.

Integrity of surface casings are checked withbuoyant weight of all subsequent strings.

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Service Loads

Loads anticipated while the well is used in service

during production, injection etc through toabandonment.

Pressure

Temperature

Buckling

Point Loads


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Service Loads

Buckling Changes in pressure, temperature

etc increase at bottom end ofcasing

Point Loads Acting on single location on casing

strings Packer setting force

Weight on packer

Pressure test against packer

Temperature Thermal expansion of

uncemented casings & fluids.

Pressure Tubing leaks

Extra pressure exerted intoproduction casing

Gas leak below hanger

Mobile formations

Well stimulation

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Safety Factor

Safety Factor = Failure Load

Actual applied load

Safety factor is always >1

Used in relation to catastrophic failure andlifting equipment


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Design Factors

DF = Rating of PipeMaximum anticipated service load

Uses API minimum Yield. May be equal or >1

Used in downhole tubular.

Typical design factors:

Burst Design

API no recommendations Generally 1.1 to API Minimum internal yield pressure.

Collapse Design 1.0 generally used.

Compression design factors

API no recommendations Generally 1.00

Tension design factors

1.3 (Neal Adams) 1.8(Preston Moore)

API Recommends at least 1.11

Sonatrach Casing Design

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