Csg_seat_1

CASING SEAT

SELECTION

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Selection criterion:

• Hole to be drilled successfully and safely at minimum cost.

• Casing shoe normally set in competent formation which should be able to withstand the forces imposed upon it during well activity.

• High-compressive-strength formation is best.

• Casing can be set in a competent shale.

• Compared to sandstones at same depths, shales have higher strength margin (in order of 1,000psi).

• Unconsolidated formations not suitable

• Limestone ????

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METHODOLOGY OF CASING SEAT SELECTION

• But shales also slough or swell.• Can be mitigated by selecting correct mud type• Oil Base Mud (OBM) and SOBM systems are effective in

preventing shale destabilization• Stronger the casing seat, higher the pr. it can withstand

during drilling in terms of ECD's, and well control operations without breaking down

• Care to be taken in following cases:– Fresh water sands – High pressure zones– Depleted zones– Mud loss zones– Faults and stressed zones– Heaving formations

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CONDUCTOR CASING

• Shoe depth selected for conductor casing should be– Strong enough to withstand fracture during drilling the

next hole interval which is assumed to have no hydrocarbon bearing intervals.

• To estimate anticipated fracture pr, following conditions must be considered:– Drilling rate with loading effect in annulus– Equivalent circulating density– Mud weight to be used

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SURFACE CASING

• Surface casing is treated as conductor casing if no hydrocarbons are expected in the next hole interval or alternatively as intermediate casing in the event that hydrocarbons are expected in the next phase of drilling.

• In many countries it is statutory requirement to cover all the fresh water sands with the surface casing.

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INTERMEDIATE CASING• Shoe selected for intermediate casing should be strong

enough to withstand fracture during drilling the next hole section and should be able to take a kick of pre defined size.

• Other major considerations for selection of intermediate casing seat are:– Differential pr consideration for safe lowering of the

casing– Isolation of troublesome or unstable formations which

may include heaving shales, loss circulation zones– Pressure regression– length of open hole

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METHODOLOGY OF CASING SEAT SELECTION

1. Well objective is clearly defined.2. Potential problems encountered in nearby wells

to be short listed.3. Pore and fracture pressure for well is estimated.4. Pore and fracture pr profile is overlaid against

the lithological column, potential troublesome zones and the hydrocarbon bearing zones.

5. A basic casing program is prepared.

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Methodology Of Casing Seat Selection7. Production casing shoe depth: Requirements

are studied and suitable formation and depth are selected so as to meet these requirements as an absolute min.

8. Intermediate casing shoe depth: To satisfy designed kick tolerance & differential pr consideration & a suitable casing point is selected to meet these requirements as an absolute min.

9. Surface & conductor casing shoe depth: Requirements to be studied & accordingly suitable formation & depth are selected.

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PORE & FRACTURE PR DATADepth (m) Pore pr grad (MWE) Fracture pr gradient (MWE)

400 1.07 1.46

800 1.07 1.52

1200 1.07 1.57

1600 1.07 1.61

2000 1.07 1.65

2400 1.07 1.69

2800 1.07 1.77

3100 1.07 1.81

3200 1.40 1.82

3320 1.50 1.839

Depth (m) Pore pr grad (MWE) Fracture pr grad (MWE)

3400 1.57 1.83

3600 1.64 1.86

3800 1.65 1.80

3920 1.66 1.90

4000 1.67 1.91

4100 1.68 1.92

4320 1.70 1.94

4520 1.72 1.95

4680 1.75 1.97

4800 1.78 1.99

5000 1.80 2.0010

DESIGN CONSIDERATIONS

SN Design factor Design limits

1 Swab consideration 0.04 gm/cc

2 Surge consideration 0.04 gm/cc

3 Safety factor 0.02 gm/cc

4 Differential pr (For normal pr zones) 170 kg/cm2

5 Differential pr (For abnormal pr zones) 215 kg/cm2

6 Design kick size 0.06 gm/ccDifferential pr limit:• These values can vary for different field conditions. • It also depends on the general attention given to mud

properties and drill string configuration. 11

Fig-1:Formation Pr and Fracture Pr Vs Well Depth

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First casing (9-5/8”) seat selection

Function MWE (gm/cc)

Max anticipated formation pr at 5,000 m

1.80

Swab pr + 0.04

Min acceptable MW = 1.84

Surge pr + 0.04

Min fracture gradient = 1.88

Safety factor + 0.02

Design fracture gradient = 1.90

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Fig-2:1st Intermediate casing seat selection (Initial shoe depth)

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First casing (9-5/8”) seat selection

Function

Projected casing shoe depth

3,960 m

Formation pr at 3,960 m 1.66 gm/cc MWE

Swab pr + 0.04 gm/cc

Min MW at 3,960 m during drilling

= 1.70 gm/cc

Formation pr at 3,100 m 1.07 gm/cc MWE

Diff pr at 3,100 m when drilling at 3,960 m

{(1.70-1.07) * 3100}/10

Differential pr at 3,100 m 195.3 kg/cm2

Is it acceptable No

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First casing (9-5/8”) seat selection: Revised depth

Function

Max acceptable diff pr for normal formations

170 kg/cm2

So MW at which this diff pr will be achieved

170 = {(MW-1.07) * 3100}/10

So MW obtained is {170*10/3100}+1.07= 1.61 gm/cc

Swab pr Subtract 0.04 gm/cc

So acceptable formation pr = 1.57 gm/cc MWE

So revised shoe depth 3,400 m

Fracture grad at 3,400 m 1.83 gm/cc MWE

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Fig-3:1st Intermediate casing seat selection (Modified shoe depth)

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Liner Seat Selection

MWE (gm/cc)

Fracture gradient at 3,400 m 1.83

Swab+ Surge+ SF Subtract 0.10

Formation pr 1.73

Projected liners setting depth at formation pr of 1.73 gm/cc

4,560 m

Is it final No

Check differential pr criterion

Check kick criterion

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Fig-4:Liner seat selection

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Liner seat selection: Diff Pr Criterion

Formation pr at 4,560 m 1.73 gm/cc

Swab pr Add 0.04 gm/cc

Min MW at 4,560 m Gives 1.77 gm/cc

Formation pr at last casing shoe (3,400 m)

1.57 gm/cc

So differential pr at 3,400 m {(1.77-1.57)*3400}/10 = 68 kg/cm2

Is it acceptable 170 kg/cm2 for normal 215 kg/cm2 for abnormal pr zones

Yes

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Liner seat selection: Kick Criterion

Kick size 0.06 gm/cc


Min MW at 4,560 m 1.77 gm/cc

EMW at 3,400 m 1.77 + {(4560/3400)*0.06} = 1.85 gm/cc

Fracture pr at 3,400 m 1.83 gm/cc

So liner shoe at 4,560 is suitable

No

What to do Try again to set at some higher depth by trial

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Revised Liner seat selection: Kick Criterion

What depth we try 4,200 m


So MW at 4,200 m 1.73 gm/cc

So EMW at 3,400 m 1.73 + {(4200/3400)*0.06} = 1.80 gm/cc

Fracture gradient at 3,400 m 1.83 gm/cc

Is it satisfactory Yes

Lower liner here No

What to do Try again to set liner as deep as possible. Try again

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Second casing (Liner) seat selection: Kick Criterion

What depth we try 4,320 m



So EMW at 3,400 m 1.74 + {(4320/3400)*0.06} = 1.83 gm/cc

Fracture gradient at 3,400 m 1.83 gm/cc

Is it satisfactory Too close

Lower liner here Set somewhere between 4,200 and 4,320 m

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Third casing (13-3/8”) seat selection

MWE (gm/cc)

Max anticipated formation pr at 3,400 m

1.57

Swab + Surge + SF Add 0.10

Design fracture gradient Gives 1.67

Depth selected to lower 13-3/8” casing with this frac gradient of 1.67

2,200 m

What to do Find out 20” casing seat by

trial

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Fig-5:2nd

Intermediate casing seat selection (Shoe depth)

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Fourth casing (20”) seat selection

What depth we try 320 m



So EMW at 320 m with 0.06 gm/cc kick from 2,200 m

1.11 + {(2200/320)*0.06} = 1.52 gm/cc

Fracture gradient at 320 m 1.45 gm/cc

Is it satisfactory No way

Lower 20” casing here Keep shoe somewhere deeper

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Fourth casing (20”) seat selection

What depth we try 400 m



So EMW at 400 m with 0.06 gm/cc kick from 2,200 m

1.11 + {(2200/400)*0.06} = 1.44 gm/cc

Fracture gradient at 400 m 1.46 gm/cc

Is it satisfactory YES

Lower 20” casing here YES

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Summary

Csg size

Shoe (mtr)

20” 400

13-3/8”

2,200

9-5/8” 3,400

7” liner

4,320

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Casing seat selection in weak Formations in Well

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Pressure Regression in Well

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Pore Pr & Fracture Pr Gradient of ONGC Well

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Pore Pr & Fracture Pr Gradient of ONGC Well

• 13-3/8” shoe• Proposed at 300 m to seal fresh water sands

• 9-5/8” shoe:• Proposed at 1400/ 1450 m with objective of

covering upper pay sands with pore pr of 0.98 MWE

• 5-1/2” shoe:• Proposed at 2000 m covering lower pay

zones from 1800-2000 m and deeper zones below 2000 m

• Higher MW of 1.50 required to drill charged Cambay sand in 8-1/2” hole

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