3. wellheads and casing

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TAMU - PemexOffshore Drilling

Lesson 3Wellheads and Casing

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Wellheads and Casing

Drilling with a Riser Temporary and Permanent Guide Bases Fracture Gradients Subsea Cementing Casing Seals Drilling Procedures - An Example

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Conventional Riser Drilling

SEAFLOOR

SEA WATER HYDROSTATIC

PRESSURE

DEP

TH

MUD HYDROSTATIC

BOP

FLOATER

DRILLING RISER

CHOKE LINE

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Conventional Riser Drilling - Install 30-in Conductor -

FLOATER

DRILLPIPE

Jet 30-in Conductor to ~ 200 ft below mudlineNo riser - “Mud” returns to seafloorNo annulus - no cementing (in GOM)

~200 30”

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Conventional Riser Drilling- Install 20-in Conductor

FLOATER

DRILLING RISER

CHOKE LINE

Drill 26-in hole to 1,050 ft below mudlineRiser optional - Mud returns to surface or seafloorRun 20-in Conductor to ~ 1,000 ft below mudlineCement to mudline

D

~1,05030”

20”

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Conventional Riser Drilling- Install 13 3/8-in Surface Csg.

FLOATER

DRILLING RISER

CHOKE LINE

Run Riser and BOP StackDrill 17 1/2-in hole to 4,050 ft BMLDrill with Mud returns to surfaceRun 13 3/8-in Surface Casing to ~ 4,000 ft below mudlineCement to mudline

D

BOP

Now, finally, we can close the BOP if necessary13 3/8”

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Wellheads and Casing

A subsea wellhead, like a land wellhead:

Must support the BOP’s while drilling Must support the suspended casing while cementing, and Must seal off between casing strings during drilling and production operations.

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Wellheads and Casing, cont.

In floating drilling, the casing hangers, casing seals and cementing heads

differ

from land and platform operations in the following manner:

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Wellheads and Casing

1. The first and second casing strings are cemented with returns to the

seabed.2. Casing is run with the last joint made-

up on a casing hanger and permanently suspended prior to cementing. Mud returns flow through fluting in the hanger.

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Wellheads and Casing

3. Usually, cementing plugs are located at the wellhead and released remotely. The cementing string from the vessel to the wellhead is drill pipe.

4. Casing seals are run and set remotely.

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Wellheads and Casing

5. Special test tools are required for remotely testing the casing seals.

6. Wear bushings are essential for protecting the wellhead.

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Fig.4-10. Typical

sealing arrangement for subsea

wells.

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14

Depth BML

240 ft

1,100 ft

4,100 ft

8,600 ft

10,100 ft

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16

Permanent Guide

Structure.

Temporary Guide Base

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Hole Opener

TemporaryGuide Base

UtilityGuide Frame

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Procedure for Starting a Well

1. To get the well started, place a heavy steel template on the seafloor.

Run on drillpipe.

2. Four guidelines guide bit, casing, etc to the right location on the seafloor.

3. Run 36” hole opener (with guide frame) and drill 36” hole to ~240 ft BMLwith returns to the seafloor.

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Procedure for Starting a Well

4. Run 30” casing and cement with returns to the seafloor. With the 30” casing also run the permanent guide structure and the wellhead housing.

(3 & 4 alt. Sometimes the 30” casing is jetted or driven in. - instead of drilling).

5. Drill 26” hole to 1,050 ft below mudline.

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Procedure for Starting a Well

6. Run 20” conductor casing.With the 20” casing, run the high pressure wellhead. Cement the casing.

NOTE: The 26” hole may be drilled with returns to the seafloor, or with returns to the surface using the marine riser.

Note the guide posts on the permanent guide structure. These are for the BOP stack

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Fig. 4-5. Estimated Fracture gradients at 100 ft below seabed (Santa Barbara Channel).

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Fracture gradient at

100 ft. below seabed (Santa Barbara

Channel).

Why drill with returns to the seafloor?

With this low fracture gradient it is difficult to drill with returns to the surface.

No shallow gas would be expected at this depth below the mudline.

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Fig. 4-5. Estimated Fracture gradients at 1000 ft below seabed (Santa Barbara Channel).

Drill with Diverter to the Surface Casing Point

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Shallow Gas

Blowout

Gas reduces

buoyancy!

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Typical specific gravity variations in a blowout boil have increasing effect nearer the water’s surface.

Fortunately for a semi-submersible, the rig’s primary flotation members are situated below the zones where specific gravity has been reduced the most.

Gas in the Water Column

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If there is sufficient length to the mooring system cables/ chains, the rig will be pushed off location and out of harm’s way.

However, the plume can also cause the rig to list, which reduces its freeboard and makes it more susceptible to capsizing.

Gas in the Water Column

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Increasing the water depth reduces the total overburden gradient and consequently the formation fracture gradient. This can be expressed as:

ppobf gF)gg(g

Fracture Gradients in Deep Water

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Where:

ratio stress vertical/horizontalF

psi/ft gradient, pressure overburdeng

psi/ft gradient, pressure formationg

psi/ft gradient, fractureg

ob

p

f

ppobf gF)gg(g

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For offshore drilling:

)ddd(p4335.0d44.0d

1g FKBfKB

ob

3f

F

KB

g/cm density,bulk formation

ft water, theabove flowline ofheight dft depth,water d

ft bushing,kelly thefrom measured depthd

Where:

f

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0.44 d is the overburden due to water, or simply the hydrostatic pressure at the seafloor.

(dKB - d - dF) is merely the penetration into the seafloor.

)ddd(p4335.0d44.0d

1g FKBfKB

ob f

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Formation bulk density vs. horizontal to vertical stress ratios for the Santa Barbara Channel.

Get f from density log.

Get F from this plot.

Calculate gf

Get gp

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Fig. 4-7. An example of onshore and offshore fracture gradients.

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Cumulative average (BML)formation bulk density

= 5.3 * (TVDBML)0.1356

e.g. = 5.3 * (3,000)0.1356 = 15.70 lb/gal

J. W. Barker and T. D. Woods“Estimating Shallow Below Mudline Deepwater Gulf of Mexico Fracture Gradients”Presented at the 1997 Houston AADE Chapter Annual Technical Forum, April 2-3, 1997.

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At 1,000 ft below mudline, avg. OB. Density,

= 5.3 * (TVDBML)0.1356

gob = 5.3 * (1,000)0.1356 = 13.52 lb/gal

gf = 0.9 * ob= 12.17 lb/gal= 0.663 psi/ft

gp = 0.8 * ob = 10.82 lb/gal= 0.563 psi/ft

NOTE: These are gradients relative tothe mudline!

J. W. Barker and T. D. Woods cont’d

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At 1,000 ft below mudline, in 1,500 ft water:

Total overburden = 0.44 * 1,500 + 0.052 * 13.52 * 1,000 psi

gob = 1,363/2,500 psi/ft = 10.48 lb/gal !!

pf = 0.44 *1,500+ 0.052 * 12.17 * 1,000 psigf = 1,293/2,500 psi/ft = 9.94 lb/gal

gp = 0.44 * 1,500+0.052 * 10.82 * 1,000 psi = 1,223/2,500 psi/ft = 9.40 lb/gal

NOTE: These are gradients relative to SURFACE!

J. W. Barker and T. D. Woods cont’d

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Fracture gradient equation: =

Poisson’sRatio

from Text

Ben A. Eaton and Travis L. Eaton“Fracture Gradient Prediction for the new generation”

World Oil, October 1997, pp. 93-100.

ppobf gF)gg(g

Dp

1Dp

DS

DF

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Fig. 4-8. Plot of a leak-off test.

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Mud Weight 9.5 PPGCasing 13 inchesSet to 3,340 ft-KBFrac. Grad. = ?

Fracture Gradient Calculation

Fracture Pressure = 0.052 * 9.5 * 3,340 + 650 = 2,300 psig

Frac. Grad. = 2,300/3,340 = 0.6886 psi/ft = 0.6886/0.052 = 13.24 ppg

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BOPs

Casing

Drillpipe

Leak-Off Test

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Fig. 4-9. Sub-sea

cementing system.

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Fig.4-10. Typical

sealing arrangement for subsea

wells.

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Metal-to-Metal Casing Annulus Seal

Assures maximum seal over extended periods, even in high-pressure holes

Eliminates dependence on seal materials that deteriorate or “cold flow”.

Available on systems up to 15,000 psi pressure integrity.

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Upper Metal Seal Lips

Resilient Compression

Element

Lower Metal Seal Lips

1. Actuating force is transferred to the2. Resilient compression element which expands, forcing the 3. Metal seal lips into contact with the surface of the4. Wellhead housing and the5. Casing hanger

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Casing Hanger and Pack-off Assembly

Single trip installationThe pack-off seal assembly is run simultaneously with the casing hanger body. All operations - installing the casing hanger, cementing the casing string and actuating and testing the pack-off seal are performed in a single trip of the running string.

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Large Flow-By Areas

Large flow-by areas can handle most drilling fluid applications with a minimal drop in pressure.

Deep 2" wide flow-by slots in the casing hanger body, and ample porting through the pack-off nut assembly, provide clear passage for cuttings and mudcake without plugging.

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Liquid Compressibility

The volume required to compress a liquid is defined by the equation:

Where: Vi = volume of system, bbl Cp = compressibility = 3 * 10-6 per psi for water

= 6 * 10-6 per psi for mud P = test pressure, psi

V = Vi * Cp * P

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Seal Test - Example

Water depth = 500 ft (all depths are KB)Casing string = 13 3/8” ODVolume of system above the seal = 11 bblTest pressure = 3,000 psiTest fluid = waterPrevious casing string = 20”, J-55, 94.0 lb/ftPrevious casing seat = 1,500 ft KBCement top = 996 ft

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Seal Test - Example

V = 11 bbl

500 ‘ KB Mud Line 996’ 20” 1,500’ 13 3/8”

4,000 ft

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With no leak, the system will require

V = 3 * 10-6 * 11 * 3,000 = 0.1 bbl water

to reach test pressure.

If the seal leaks, the volume will be more, but how much more?

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Obviously, 0.1 bbl would be difficult to measure. The annular volume between the seal and the cement is

(996 - 500) ft * 0.1815 bbl/ft = 90 bbl of mud

Now,

V = 6*10-6 * 90 P + 3*10-6 * 11 P bbl = ( 5.4 * 10-4 + 3.3*10-5 ) P bbl = ( 5.73 * 10-4 ) P bbl What should the maximum pressure be?

?

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Pressure in the annulus must always be less than the collapse pressure of the inner casing, and less than the internal yield of the outer casing.

This will depend on both volume and pressure. Table 4-2 shows the relationship for four grades of casing.

Also, the internal yield of the 20-inch casing is reached at 2,110 psi when V = 1.24 bbl.

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55

Plug fortesting

casing seal to full

working pressure.

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Test Procedure

1. Set seal2. Land test plug in wellhead,

sealing off below the seal3. Displace mud with water for test4. Close pipe rams5. Pump slowly down the choke line,

preferably in stages, to protect the casing in case of leaks

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Test Evaluation

During the test, if the wellhead system being tested will not sustain test pressure, several possible causes should be considered:

1. Leak in the surface manifold2. Leak in the test plug (detected

by returns through the drillpipe)

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Test Evaluation, cont.

3. Leak in the casing seal

4. Leak in the BOPs

5. Leak in the hydraulic wellhead

connector

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When the well does not sustain pressure, it is obvious that there is a problem.

There is also a problem if the well takes too much fluid to reach test pressure, just as we have discussed.

Test Evaluation, cont.

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Drilling Procedures from a floater

Install 30” Structural Csg.

Install 20” Conductor

Install 13 3/8” Surface Casing

etc.

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Drilling Procedures Tentative Hole and Casing Sizes

8 1/2” Pilot Hole to 180’ BML 26”x36” Hole Opener to 180’ BML

Install 30” Structural Csg. 8 1/2” Pilot Hole to 1040’ BML 17 1/2” Pilot Hole to 1040’ BML 17 1/2”x26” Under reamer to 1040’

Install 20” Conductor

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Drilling Procedures Tentative Hole and Casing Sizes

12 1/4” Pilot Hole to 3,830’ BML 12 1/4”x17 1/2” Hole Opener to 3,830’ BML

Install 13 3/8” Surface Csg. 12 1/4” Hole to TD (8,530’ BML)

Install 9 5/8” Production Csg. 8 1/2” Hole if Required 7” Contingency

Liner

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General Rules

1. Do not change the tension on the anchor lines until the 30” casing has been run and cemented.

2. Have all the 30” casing and all of the wellhead equipment on board prior to spudding.

3. There will be an SLM prior to any logging or coring run.

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General Rules

4. All casing strings will be strapped and drifted prior to running.

5. Casing will not be run until the hole is in the best possible condition and a trouble free wiper trip can be made.

6. Cement densities will be monitored with a mud balance.

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General Rules

7. The rig will be moved 50’ off location whenever the riser is being run or pulled.

8. No smoking or open flames are permitted on deck whenever the riser is connected to the well.

9. Welding permits (authorized by the drilling supervisor and tool pusher)

will be required at all times.

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General Rules

10. Coring will be at the the discretion of the well site geologist, but only after approval from the task force Manager and the Exploration Coordinator.

11. All information concerning the well will be kept strictly confidential. Any

discussions will be held in a secure area in the quarters or on the rig.

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General Rules

11. Confidentiality - cont’d. Only contractors with “a need to know” will be allowed access to well information.12. All personnel on board and all visitors

will be instructed with the necessary environmental and safety films and instructions.

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General Rules

13. No one will be allowed on the helicopters, work boats, or drilling vessel without the proper authorization or identification.

14. The rotary table must be positioned within a 200 foot radius of the proposed location.

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General RulesAnchoring

1. Place anchors on sea floor 5800’ from the desired final location.

2. Anchor lines should be equally deployed around the rig with an angular spacing of 45 degrees between adjacent lines.

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General RulesAnchoring

3. Pull in opposing lines to set anchors. An indicated line tension of 125 kips is necessary for the anchor to receive any load.

4. A tension level of 440-460 kips should be reached before 600’ of line is taken in with the rig remaining stationary.

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General RulesAnchoring

5. If a line tension of 440-460 kips has not been reached before 800’-1000’ of line has been retrieved, then it may be necessary to use piggy-back anchors.

6. The following Western KDC plan outlines the mooring procedure.

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Shallow Gas Plan

After the rig is properly anchored the following steps will be followed as there is a potential for shallow gas in this area:

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Shallow Gas Plan

1. Leave mooring line pawls or stoppels unset until the 20” casing has been set and cemented.

2. Mooring winches will be manned while the 8 1/2” pilot holes for the 30” and 20” casings are being drilled.

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Shallow Gas Plan

3. Mooring winches will be manned while the 8 1/2” pilot holes for the 30” and 20” casings are being opened up or under-reamed.

4. The moonpool and seafloor will be observed for gas bubbles until the 20” casing is set and cemented.

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36” Hole Plan

1. Premix 600 barrels of 11.5 ppg kill mud prior to spudding the well.

2. PU and TIH with an 8 1/2” bit, 6 - 6 1/2” drill collars, 6 jts of 5” Hevi-Wate drill pipe, and sufficient 5” drill pipe.

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36” Hole Plan

3. Tag bottom with the pilot bit, and note and report the following:

a. RKB to water levelb. RKB to mud linec. Water depthd. Time of day (tide allowance)

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36” Hole Plan

4. Lower TV camera, and observe bit entering guide base. Retrieve universal guide frame back to surface.

5. Upon spudding, space out drill string with pup joints so that it will not be necessary to pull the bit above the guide base to make the first connection.

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36” Hole Plan

6. Drill an 8 1/2” hole to +/- 30’ below the setting depth of the 30” casing (estimated at 180’ BML).

Circulate returns to the sea floor, and monitor returns with the TV camera.

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36” Hole Plan

7. If there are no problems with shallow gas, pull out of hole, PU 26” bit and 36” hole opener, 6-9 1/2” DC’s, 6 jts 5” Hevi-Wate DP, and sufficient 5” DP.

Drill 36” hole to set 150’ (4 joints) of 30” OD structural casing.

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36” Hole Plan

Drill with sea water as follows: a. Circulate viscous sweeps as required to clean the hole. b. Survey hole at 30’, 60’, and 150’ BML. c. At TD of 36” hole, displace hole to the mud line with viscous mud.

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36” Hole Plan

Drill with sea water cont.: d. Make a wiper trip. e. Circulate the hole to the mud line with viscous mud. f. Penetration rate should not exceed 100 ft/hr overall.

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36” Hole Plan

8. Run 30” structural casing per procedure.9. If there are problems with shallow gas,

displace the 8 1/2” hole with kill mud until the gas stops or the hole is full of kill mud. Monitor returns with the TV camera for evidence of gas or flow, and if after one hour the hole is stable, proceed as in steps 7 and 8.

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36” Hole Plan

10. If the kill mud in step 9 does not stabilize the well and it appears that heavier mud will not stabilize the well or will break down the formation, then prepare to cement. Mix and pump, sufficient 15.8 ppg cement slurry to circulate cement to the mud line, and monitor returns for gas with the TV camera.

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36” Hole Plan

10. Make sure that the hole is stable

POH with BHARetrieve TGB Move rig as required

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26” Hole Plan

1. Have 600 barrels of 11.5 ppg kill mud prior to drilling out below the 30” casing.

2. PU and TIH with an 8 1/2” bit, 9-6 1/2” DC’s, 9 jts of 5” Hevi-Wate DP, and sufficient 5” DP.

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26” Hole Plan

3. Drill an 8 1/2” hole to +/- 40’ below the setting depth of the 20” casing (estimated at 1040’ BML).

Circulate returns to the rig shakers, and monitor returns for indications of gas or flow.

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26” Hole Plan

4. Displace the hole with viscous spud mud, make a wiper trip, displace the hole with viscous spud mud, POH, and log well as required.

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26” Hole Plan

5. If there are no problems with shallow gas, pull the riser, PU & TIH with a 17 1/2” bit, 26” hole opener, monel DC, 6-9 1/2” DC’s, 6-8” DC’s, 9 jts 5” Hevi-Wate DP, 26” stabilizer at 60’, qand sufficient 5” DP. Drill a 26” hole to set 1040’ of 20” OD conductor casing as follows:

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26” Hole Plan

a. Circulate viscous pills as required to clean the hole.b. Circulate returns to the sea floor with sea water.c. Maintain inclination at less than three degrees.d. Spot viscous mud at TD of 26” hole.

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26” Hole Plan

e. Make a wiper trip.f. Spot viscous mud as required.g. Drop multishot and POH.

6. Run 20” OD conductor casing and cement per procedure.

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26” Hole Plan

7. If there are problems with shallow gas in Step 5, circulate the hole with viscous spud mud and slowly increase the weight until the flow has stopped or until the active system is depleted. If the flow continues, pump the kill mud at the maximum rate until the active system is depleted.

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26” Hole Plan

7. (Cont.) Then pump sea water at the maximum rate until the hole bridges.

8. If the flow rate is significant, and the hole will not bridge, prepare to move the rig.

Cement the hole to just below the sea floor with 15.8 ppg cement. POH with the BHA. Cut or shoot the 30” casing, and pull the TGB and PGB. Move rig as required.

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26” Hole Plan

9. If the gas in step 7 depletes or the density is sufficient to control the well, then casing can be run or the well can be drilled ahead.

10. Drill 8 1/2” hole to +/- 40’ below the setting depth of the 20” casing (estimated at 1040’ BML).

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26” Hole Plan

11. Circulate and condition for logs. Pull out of hole, and log well per procedure.

12. PU & TIH with 17 1/2” bit, Monel DC, 6-9 1/2” DC’s, stabilizers at 60’ amd 90’, 6-8” DC’s, jars, 9 jts 5” Hevi-Wate DP.

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13. Drill a 17 1/2” hole to sufficient depth to set 1040’ of 20” conductor casing. Drop multishot, and POH.

14. PU & TIH with 17 1/2” bit and 26” underreamer, 6-9 1/2” drill collars, 6-8” drill collars, 9 jts 5” Hevi-Wate DP, and 26” stabilizer at 60’.

26” Hole Plan

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15. Underream to sufficient depth to set 1040’ of 20” conductor casing.

16. Circulate and condition the hole for casing. Care must be taken to have a balanced mud weight all the way around with no heavy slugs.

26” Hole Plan

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17. Displace hole from TD to the sea floor with sufficient weight mud to balance the hydrostatic when the riser is removed. Again, care must be taken to have a balanced mud weight while displacing, and the riser may have to be voided with sea water as the heavier mud is circulated.

26” Hole Plan

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18. POH, run the 20” casing and 18 3/4” - 10,000 psi wellhead housing, and cement per procedure.

19. If there is evidence that the hole cannot be drilled deeper safely in step 9, the well will be underreamed at the depth reached in step 9 and 20” casing will be set.

26” Hole Plan

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20. It will then be determined whether future casing settings need to be changed.

etc. etc. etc.

26” Hole Plan

101

102