2 1 casing seat selection

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Casing Point Selection

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Casing 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 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 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 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 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 Selection
• Exploration vs. Development (same target);
Exploration well 5 casings

Development well 4 casings

Conducr

30”

20”

Surface casing

20”

13-3/8”
13-3/8”
Intermediate
9-5/8”
Intermediate
9-5/8”
Intermediate

Reservoir
7

7” Production casing

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Casing 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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Casing Point Selection
• Restrictions on Casing Shoe Depth;

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Casing Point Selection
• Restrictions on Casing Shoe Depth;
– 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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Casing Point Selection
• Restrictions on Casing Shoe Depth;
– 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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Casing Point Selection
• Restrictions on Casing Shoe Depth;
– 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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Casing Point Selection
• Restrictions on Casing Shoe Depth;
– 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 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 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:
Penetration 

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  x y  ρReturns    y  ρSea 
ρFormation  ρReturns

Penetration = depth of shoe from seabed, feet
x = 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 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.

Penetratio n 

 180 0.5    100 0.45 
0.85  0.5

 129'

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

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Casing 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,
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» 100’ - 3000’ (or more).
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Casing 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 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 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 Selection
• Liners;
Drill pipe
Wiper Plug

– Prime reason:
• Save money,
• Cover Corroded/Damaged Casing,
• Cover:
– Lost circulation zones,
– Shales or plastic formations,
– Salt zones.

Liner
Hanger
Running
Tool
Shear

Pin

Liner
Wiper
Plug

• Deep Wells:
– Rig unable to lift long string of
casing.

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Casing Point Selection
• Liners;
TIE BACK

• Tie-Back (Liner Complement);

STINGER WITH
SEALS

– The integration of the liner
with casing from top of

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

LINER

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

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Casing and Casing Design
• Casing String Types;
– To Conclude;
• Drive Pipe,
• Conductor Pipe,
• Surface Casing,
• Intermediate Casing,
• Production Casing:
– Liner,
– Liner & Tieback,

– Full String.

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Casing 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.
24

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Network of Excellence in Training

Casing Point Selection

End of Lecture

© COPYRIGHT 2001, NExT.

All Rights Reserved