2.1 casing seat selection

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© COPYRIGHT 2001, NExT. All Rights Reserved

Casing Point Selection

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© COPYRIGHT 2001, . All Rights Reserved

Casing Point SelectionCasing Point Selection• Lecture Contents;

– Lecture Objectives,

– Why Do we Set Casing?

– Exploration Well Casing Setting Depths,

– Development Well Casing Setting Depths,

– Exploration versus Development (Same Target),

– Restrictions on Casing Shoe Depth,

– Special Criteria,

– Example.

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Casing Point SelectionCasing Point Selection• Lecture Objectives;

– At the end of this lecture YOU will be able to:

• State the two Drilling related reasons for running casing,

• State the difference in selecting casing points for exploration and development wells,

• Describe functions of different type of casings

• State restrictions that govern shoe depth selection,

• Calculate the minimum conductor setting depth offshore to enable returns to be taken to the flowline.

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Casing Point SelectionCasing Point Selection• Why do we set casing?

– Casing is set for two Drilling reasons;

• Consolidate the hole already drilled (protect sensitive formations, water sources etc),

• Provide pressure control integrity to drill ahead (be able to safely handle a kick).

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Casing Point SelectionCasing Point Selection• Exploration Well Casing Setting Depths;

– Exploration wells are planned with little offset data,

– Each casing should be set as deep as it is safe This is to:

• Allow maximum contingency in case extra casings are needed lower down,

• Minimize the number of casing strings run to minimize well cost,

– Each casing string is run as deep as possible based on kick tolerance, unless other reasons dictate it be run higher.

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Casing Point SelectionCasing Point Selection• Development Well Casing Setting Depths;

– Casings are run as minimum as possible. This to:

• Reduce the cost of the well (less time running casings; less casing)

• Run smaller casings to reach the reservoir at a particular hole size which will also lead to large cost savings (smaller casings cheaper than larger; same for drillbits etc.)

– Each casing string is only set deep enough to enable the next casing point to be reached,

– Contingency is less critical.

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Casing Point SelectionCasing Point Selection• Exploration vs. Development (same target);

Exploration well 5 casings Development well 4 casings

30”

20”

13-3/8” Intermediate


7” Production casing

Reservoir

Conducr

Surface casing

20”

13-3/8”


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Casing Point SelectionCasing Point Selection• Restrictions on Casing Shoe Depth;

– Kick tolerance:

• The hole section should not be drilled deeper than is possible to safely circulate out a kick, based on formation pressures, fracture gradients and kick tolerance policies.

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– Wellbore Stability:

• We may be restricted by deterioration of the wellbore with time,

• If a formation is stabilized by our mud system for up to 10 days, then drilling ahead for 12 days might prevent us logging the hole or running casing.

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– Mud Requirements:

• We may have formations which have mutually exclusive requirements for the mud,

• For instance if we have very reactive shale requiring ideally OBM at a higher density to drill, but the mud will cause emulsion blocking in the reservoir, then we need to separate the two with casing.

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– Directional Requirements:

• We may anticipate problems related to torque and drag, or differential sticking due to stationary string while using downhole motors,

• The build up section may need protection before drilling ahead.

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– Fracture gradient at the shoe:

• We need to set our casing in a competent formation for drilling ahead,

• We could drill to the limit for kick tolerance on a hole section; but if the place where we set the casing is fractured or underpressured then the shoe will be too weak to drill ahead.

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Casing Point SelectionCasing Point Selection

• Conductor Pipe;• Purpose:

– Prevents washing out under the rig,

– Provides elevation for flow and bell nipple.

• Common Sizes and Depths:

– 30” - 20” Welded,

– 20” - 16” Threaded,

– 30’ - 200’ (< 100’ common).

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Casing Point SelectionCasing Point Selection• Special Criteria;

– Conductor Pipe (offshore):

• The Conductor needs to be set deep enough in a formation which allows returns to the flowline,

• The minimum depth for the conductor shoe to allow returns to the flowline can be calculated:

ρReturnsρFormation

ρSeayρReturnsyxnPenetratio

Penetration = depth of shoe from seabed, feetx = Height of flowline above sea, feet y = Water depth, feet Sea = Sea water density, psi/ft Returns = Returns density, psi/ft Formation = Formation density, psi/ft.

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Casing Point SelectionCasing Point Selection• Example;

– Calculate the minimum setting depth BRT, on a development offshore Well:

• Height of flowline = 80' MSL,

• Height of drill floor = 90' MSL,

• Water depth = 100'

• Formation density gradient = 0.85 psi/ft,

• Seawater in the area = 0.45 psi/ft ,

• Expected returns gradient = 0.5 psi/ft.

'129

5.085.0

45.01005.0180nPenetratio

Therefore;The casing shoe will be at 129 + 100 + 90 = 319’ below the rotary table.

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Casing Point SelectionCasing Point Selection

• Surface Casing:• The Surface Casing is usually set in the first competent

formation which is strong enough to close in on a kick,

• The setting depth is selected to allow a BOP to be nippled up for drilling ahead.

• Other considerations:

– Support subsequent casing strings,

– Isolate of surface fresh water formation,

– Case-off unconsolidated or lost circulation areas,

– On a multi well platform, well may be kicked off in surface hole and surface casing will also protect the build up section Common sizes and depth: 20” - 9 5/8” threaded,

» 100’ - 3000’ (or more).

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Casing Point SelectionCasing Point Selection• Intermediate Casings:

• The Intermediate Casing is set as deep as possible to allow sufficient shoe strength for drilling ahead,

• It is often planned to be set in a pressure transition zone, where pore pressures and fracture gradients are increasing. Care must be taken to not drill into transition zone-well kicks,

• Other considerations:

– Directional (reduce torque and drags in an extended reach hole) or wellbore stability reasons - Case off troublesome zones (Lost Circulation, Salt Section, Over-pressured Zones, Heaving Shales) prior to drilling ahead. Separates hole into workable sections.

• Common sizes and depths:

– 13 3/8”, 10 3/4”, 9 5/8” - 3000’ to 10,000’

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Casing Point SelectionCasing Point Selection• Production Casing;

• The Production Casing is set through or just above the reservoir, depending on the type of completion to be used,

• The completion type will dictate the production casing setting depth:

– If a production liner is run, then generally the production casing will be set somewhere above the reservoir,

– If the completion calls for the production casing or liner to be perforated, it will be set through the reservoir,

– In a non-cemented completion (such as a prepacked screen in a horizontal well) then the casing may be set just inside the reservoir.

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Casing Point SelectionCasing Point Selection• Production;

• Purpose:

– Isolates the pay zone from other formations and the fluids in them,

– Protective housing for production equipment,

» Subsurface artificial lift,

» Multiple zone completion,

» Screens for sand control,

– Covers worn or damaged intermediate string.

• Common sizes:

– 4 1/2”, 5”, 7”, & 9 5/8”.

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Casing Point SelectionCasing Point Selection• Liners;

– Prime reason:• Save money,

• Cover Corroded/Damaged Casing,

• Cover:

– Lost circulation zones,

– Shales or plastic formations,

– Salt zones.

• Deep Wells:

– Rig unable to lift long string of casing.

Drill pipeWiper Plug

LinerHanger

Liner WiperPlug

Running Tool

Shear Pin

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Casing Point SelectionCasing Point Selection• Liners;

• Tie-Back (Liner Complement);

– The integration of the liner with casing from top of existing liner to surface,

– To further up casing to cover corroded or damaged zone,

– This is often done if production is commercially viable or there is damage to casing above the liner.

TIE BACK

STINGER WITHSEALS

LINER

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• Casing String Types;

– To Conclude;

• Drive Pipe,

• Conductor Pipe,

• Surface Casing,

• Intermediate Casing,

• Production Casing:

– Liner,

– Liner & Tieback,

– Full String.

Casing and Casing DesignCasing and Casing Design

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Casing Point SelectionCasing Point Selection• Now You should be able to:

– State the two Drilling related reasons for running casing,

– State the difference in selecting casing points for exploration and development wells,

– Describe functions of different type of casings

– State restrictions that govern shoe depth selection,

– Calculate the minimum conductor setting depth offshore to enable returns to be taken to the flowline.

2.1 casing seat selection

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