PETE 625 Well Control

1
PETE 625Well Control

• Lesson 19
• Offshore Operations

2
Offshore Operations

• Equipment Used in Floating Drilling
• Operational Considerations in Floating Well
  Control
• Shallow Gas Hazards
• Trends in Deepwater Drilling

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Assignments

• HW 11 Ch 6, Problems 1, 3, 5,
  11 and 19 
•            Due Friday, July 16
• Read Chapters 7 8

4
Schedule

• Project Abstracts are due on Friday, July
  16. Type-written, less than one page. 
  Drilling topic.
• Project Presentations Monday, July 19
• FINAL Wednesday, July 21, 9-11 a.m. in
  Room 511

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Sonats George Washington
A Semi-Submersible Rig
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Zapatas TraderA Drillship
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Floating Drilling Operations
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Floating Drilling Operations

• Step 1.
• Temporary guide base with guide lines lowered
  on DP to seafloor
• Guide base leveled.
• DP released and retrieved.
• Guideline-less bases sometimes installed

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Floating Drilling Operations

• Step 2
• Pilot bit and hole opener lowered to the
  temporary guide base.
• Slackoff force shears the drillstring from the
  guide arms.
• Bit passes through the guide base and first
  hole section drilled with returns taken at
  seafloor.
• Drillstring removed

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Conventional Riser Drilling - Install 30-in
Conductor -
FLOATER
DRILLPIPE
200
30
Jet 30-in Conductor to 200 ft below mudline No
riser - Mud returns to seafloor No annulus -
no cementing (in GOM)
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Floating Drilling Operations

• Step 3
• Structural casing and permanent guide base run
  along guide lines.
• Casing cemented through drillstring.
• Running tool sheared and DP retrieved.

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Floating Drilling Operations

• Step 4
• Conductor hole drilled taking returns at the
  seafloor.
• e.g., 26 hole
• for the 20 casing

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Floating Drilling Operations

• Step 5
• Conductor casing run.
• Wellhead installed.
• Assembly is lowered to the seafloor and into
  the wellbore.
• Conductor casing is cemented in place.

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Floating Drilling Operations

• Step 6
• BOP stack and riser are run to seafloor
• BOP stack is attached to the wellhead by
  Hydraulic connectors.
• BOP stack is tested.
• Surface hole drilled, e.g., 17.5
• All other casings run through riser and BOPE

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Equipment Used in Floating Drilling

• Subsea Wellheads
• Stack Equipment
• The Marine Riser and Associated Equipment
• Choke and Kill Lines
• Control Systems

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Typical sealing arrangement for subsea wells.
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Fig. 8.5Subsea Well-ControlEquipment
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Fig. 8.6Mandril-Type Hydraulic Connector
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The Marine Riser and Associated Equipment

• Primary functions of marine riser
• set and retrieve the stack
• return mud and cuttings back to the drill
  vessel
• guide the drillstring into the wellbore
• support the choke and kill lines

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From Petex, Marine Riser Systems and Subsea
Blowout Preventers
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Lower Marine Riser Package
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Choke and Kill Lines
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Fig. 8.12Common C and K Line Arrangement on a
Subsea Stack
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Fig. 8.13Cut-away View of a Fail-safe Gate Valve
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Control Systems
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psw 890 psig
pmax
atmospheric pressure.
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Operational Considerations in Floating Well
Control

• Kick Detection
• Shut-In and Hang-Off Procedure
• Effect of the Choke and Kill Lines
• Hydrates
• Post-Kill Considerations
• Disconnecting the Riser

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Kick Detection

• More difficult than with surface stacks
• Pitch and roll of vessel causes the floats in
  pits to show a periodic gain and loss.
• Heave of vessel effects flow detection.

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Effect of the Choke and Kill Lines

• Sometimes high friction pressures
• Affect the start-up procedures on kick
  circulation.
• Measure CLFP when KCP measured.

CLFP Choke Line Friction Pressure KCP Kill
Rate Circulating Pressure
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1,585
4,585
8,200
Fig. 8.15
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( gf 0.65 )
( SIDPP KCPriser )
?
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Removing effect of CLFP
Monitor pressure on gauge B and keep constant at
SI value during startup.
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Gas in CL
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Fig. 8-19Hydrates form at High Pressures and Low
Temps.
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Riser Margin
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( BHP/D )
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Disconnecting the Riser

• May occur due to
• storms
• impending blowouts
• station keeping equipment fails

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Shallow Gas Hazards

• Broaching Concerns
• Shallow Fracture Gradients
• Shallow Gas Kicks Causes and Prevention
• Managing a Shallow Gas Flow

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Broaching Concerns

• Broaching can be due to
• Vertical fractures
• Fault planes
• Poor cement jobs
• Can result in cratering under the rig

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Shallow Fracture Gradients

• Actual leak-off data are rare for formations
  less than 1,000 BML.
• Extrapolation of published fracture gradient
  data to the ML is not accurate

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Shallow Fracture Gradients

• Shallow fracture gradients are often higher
  than expected because of one or more of the
  following
• rock with a finite tensile strength
• the overburden stress being the least principal
  stress
• effect of water depth on sediment compaction
• plastic behavior in the younger clays and other
  rocks

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Depth, ft
( Fully plastic )
Fracture Gradient, lb/gal
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Sediment Depth, ft BML
Fracture Gradient, lb/gal
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Clean MW 9.0 ppg Hole diameter 26
10.0
800 GPM 1000 GPM
Mud Weight in Annulus, lb/gal
240
Penetration Rate, ft/hr
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Shallow Gas Kicks Causes and Prevention

• Pre-drill hazard assessment
• Many operators attempt to steer clear of
  possible shallow gas zones
• Pore pressure environment
• Abnormally pressured shallow gas sands are
  often sandwiched between normally pressured
  shales and are very difficult to anticipate

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Fig. 8.24 Drilled Gas and Effective Annular Mud
Density
BASE CASE Hole Size 26 in Circ. Rate
800 gal/min Clean mud density 9.2
lb/gal Drilling rate 200 ft/hr
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Riserless Drilling Concerns

• True riserless drilling.
• Often used in floating drilling operations
  prior to setting conductor.
• Seawater is utilized as drilling fluid.
• Returns taken at seafloor.

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( steel below mudline )
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Lost Circulation Trip Induced Pressures

• Low fracture gradients are of concern when
  drilling at shallow depth
• Low swab pressures can induce kicks

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Hydrostatic Pressure provided by a 0.5 ppg trip
margin
Higher trip margins might help, but, low frac
gradients often preclude this
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Procedure for avoiding swab conditions without a
riser
2. Disp mud in DS with SW to Float
3. DS will pull wet
5. DS pulls dry
4. Wellbore fills w/ SW
1. Spot densified mud in Annulus
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Underbalancing a Riser-Drilled Conductor Hole

• When disconnecting a riser to run casing, the
  mud in the riser will u- tube into the ocean.
• This results in a reduced BHP due to a dual
  density fluid system.
• Spotting a weighted pill on bottom can replace
  the riser margin

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Managing a Shallow Gas Flow

• Use of diverters
• no standard practice
• often fail

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Annular Sealing Element
Remote Actuated Valve
Vent Line
Hydraulic or Pneumatic control
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Diverter Failure

• Does not operate when activated
• Falls apart because of the dynamic loads
• Leaks
• Is cut by solids
• Becomes blocked
• Imposes enough back pressure to cause formation
  fracture and broaching

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