TAMU Pemex Offshore Drilling

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TAMU - PemexOffshore Drilling

• Lesson 16
• Dual Gradient Drilling

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Dual Gradient Drilling

• What is Dual Gradient Drilling?
• How does it work?
• Why do we need it?
• Pore pressures and fracture gradients.
• Mud weights and casing programs.
• What about connections and trips?
• What about kicks?

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Subsea Mudlift Drilling or Riserless Drilling
Drillstring
sometimes called Dual Gradient
DrillingorDual Density Drilling
Mud Return Line
Pump
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Subsea Mudlift Drilling Concept

• No conventional drilling riser
• A separate small-diameter mud return line
  from seafloor to surface
• A dual mud density system
• Seawater gradient from surface to seafloor
• Heavier drilling mud inside the wellbore

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Subsea Mudlift Drilling Concept

• A seafloor mud pump on the return line
• Pressure inside wellbore at seafloor is the
  same as the pressure in the ocean at
  seafloor
• Theoretically the well is always dead
• Important in case of drive-off

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Rotating BOP (RBOP) with Pack-Off
ConventionalAnnular BOP
ConventionalRam BOPs
Seafloor detail of Subsea Mudlift configuration
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Subsea Mudlift drilling in three phases
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Effect of Increasing Water Depth

• Weight of drilling riser increases with
  depth
• 21-inch riser has an internal capacity of
  400 bbls/ 1,000 ft
• At 10,000 ft, wt. of riser 2 million lbs.
  and wt. of 17.1 1b/gal mud inside riser
  2.7 million lbs

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Effect of Increasing Water Depth

• More storage space required on drilling vessel
  - especially with flotation elements on the
  riser
• A larger and more expensive drilling vessel is
  required
• Few such vessels are available (14 EA. 4th
  generation?) (1996)

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What About Subsea Mudlift Drilling?

• A 6 return line with 36 bbls/1000 ft can do
  the job
• Requires much less weight and volume for
  storage!
• A smaller vessel can do the job.
• A smaller vessel can easier be upgraded to do
  the job.

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What is Subsea Mudlift Drilling?

• Subsea Mudlift Drilling, in the present
  context, refers to drilling where mud returns
  DO NOT go through a conventional,
  large- diameter, drilling riser.
• Instead the returns move from the seafloor to
  the surface through one or more small -
  diameter pipe(s) separate from the drillpipe.
• A Mudlift system is used in the Return Line

ATM
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Equivalent Mud Density, ppg
Fig. 7.21 ADE Pore pressure gradient and fracture
gradient data for Jefferson Parish, LA.
Fracture Gradient
0.5 ppg
0.5 ppg
Pore Pressure Gradient
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Casing Setting Depths based on pore pressure
grad., fracture grad. and mud wt. 5,000 ft
water 0.5 ppg kick tolerance
Mud Weight (Gradient), ppg
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(?)
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CONVENTIONAL 8.6 lb/gal
SEAWATER 8.6 lb/gal
RISERLESS 15.5 lb/gal
0 5,000 10,000 15,000
20,000 25,000
Wellbore Pressure, psi
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SEAWATER 8.6 lb/gal
8 9 10 11
12 13 14
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Gas Lift? Glass Beads?
Gas Lift? Glass Beads?
How to Handle Connections? Trips?
Rotating BOP
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Typical Overburden Pressure grad.vs. Depth Ref
Fracture gradient prediction for the new
generation, by B.A. Eaton and T.L. Eaton. World
Oil, October 1997.
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Conventional Riser Drilling - Static Wellbore
Pressures
FLOATER
SEA WATER HYDROSTATIC
DRILLING RISER
CHOKE LINE
MUD HYDROSTATIC
SEAFLOOR
BOP
DEPTH
PRESSURE
ATM
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Conventional Riser Drilling - Wellbore Pressures
FLOATER
STATIC PRESSURE
CIRCULATING PRESSURE
SEAFLOOR
BOP
DEPTH
SEA WATER HYDROSTATIC PRESSURE
?PBIT
PRESSURE
ATM
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Subsea Mudlift Drilling System
FLOATER
SEAWATER HYDROSTATIC PRESSURE
10,000
SEAFLOOR
BOP
MUDLIFT
BASE CASE Circulation Rate 650 gpm Drillpipe OD
6 5/8 in Return Line ID 6 in Hole Size
12 1/4 in
30,000
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Alternate SMD Configuration - Wellbore Pressures
FLOATER
MUD HYDROSTATIC PRESSURE SMD
MUD HYDROSTATIC PRESSURE Conventional
RISER
CHOKE LINE
BOP
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PRESSURE
SMD
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Static Wellbore Pressures
MUD HYDROSTATIC PRESSURE Subsea Mudlift
MUD HYDROSTATIC PRESSURE Conventional
SEAFLOOR
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PRESSURE
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Static Wellbore Pressures
In a 30,000 ft well in 10,000 ft of water, the
bottomhole pressure is 21,000 psig. For
conventional drilling, what is the minimum mud
weight that can control this pressure? For
Subsea Mudlift drilling, what is the minimum mud
weight that can control this pressure?
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Static Wellbore Pressures
P 0.052 MW Depth For conventional
drilling, Minimum mud weight 21,000/(0.052
30,000) 13.5 lb/gal Seafloor pressure
0.0528.610,000 4,472 psig For Subsea Mudlift
drilling, Minimum mud weight (21,000 -
4,472)/0.052 20,000 15.9 lb/gal
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Wellbore Pressures
MUD HYDROSTATIC PRESSURE Conventional
SEAFLOOR
FRACTURE PRESSURE
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PORE PRESSURE
PRESSURE
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Wellbore Pressures
MUD HYDROSTATIC PRESSURE Subsea Mudlift
MUD HYDROSTATIC PRESSURE Conventional
SEAFLOOR
FRACTURE PRESSURE
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PORE PRESSURE
PRESSURE
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Casing Requirements - Conventional
MUD HYDROSTATIC PRESSURE Conventional
SEAFLOOR
DEPTH
FRACTURE PRESSURE
SEA WATER HYDROSTATIC PRESSURE
PORE PRESSURE
PRESSURE
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Casing Requirements - Subsea Mudlift
MUD HYDROSTATIC PRESSURE Subsea Mudlift
SEAFLOOR
FRACTURE PRESSURE
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PORE PRESSURE
PRESSURE
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Pressure Considerations
MUD HYDROSTATIC PRESSURE Subsea Mudlift
MUD HYDROSTATIC PRESSURE Conventional
Subsea Mudlift
SEAFLOOR
FRACTURE PRESSURE
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PORE PRESSURE
PRESSURE
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Wellbore Pressures - Conventional
FLOATER
STATIC PRESSURE
CIRCULATING PRESSURE
SEAFLOOR
BOP
DEPTH
SEA WATER HYDROSTATIC PRESSURE
?PBIT
PRESSURE
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Static Pressures - Subsea Mudlift Drilling
FLOATER
ANNULUS AND RETURN LINE
SEAFLOOR
BOP
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PRESSURE
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Drillstring Circulating Pressures
CONVENTIONAL
Subsea Mudlift
SEAFLOOR
DEPTH
SEA WATER HYDROSTATIC PRESSURE
??PBIT
PRESSURE
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Annulus Circulating Pressures
CONVENTIONAL (13.2 lb/gal)
Subsea Mudlift (15.5 lb/gal)
SEAFLOOR
??PPUMP
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PRESSURE
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Circulating Pressures - Subsea Mudlift
Drilling
DRILLSTRING PRESSURE
ANNULUS AND RETURN LINE
SEAFLOOR
?PPUMP
DEPTH
SEA WATER HYDROSTATIC PRESSURE
?PBIT
PRESSURE
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Transients

• Pressures in the Wellbore
• Transient Behavior when Stopping Pump
• (U-tubing or Free-fall)
• How fast does the Fluid Level in the
  drillpipe drop?
• How far does the Fluid Level drop?
• Preview of Kick Detection and Well Control

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U-Tubing in Subsea Mudlift Drilling
FLOATER
SEAWATER HYDROSTATIC PRESSURE
STATIC FLUID LEVEL
MUDLIFT
BOP
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Static Pressures - Subsea Mudlift Drilling
ANNULUS AND RETURN LINE
DRILLSTRING PRESSURE
SEAFLOOR
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PRESSURE
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Kick Detection Well Control- Review -
Conventional -

• Kick Indication (what is a KICK?)
• Kick Detection (confirmation)
• Kick containment (stop flow into well)
• Circulate Kick out of hole
• Increase Mud Weight in hole (if necessary)

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Circulate Kick out of hole
Keep the BHP constant throughout
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Circulate Old Mud out of hole
Keep the BHP constant throughout
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Subsea Mudlift Drilling System
FLOATER
Surface Pump Constant Rate Mudlift Const. Inlet
Press.
SEAWATER HYDROSTATIC PRESSURE
10,000
SEAFLOOR
BOP
MUDLIFT
30,000
KICK
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Kick Detection and Control
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Kick Detection and Control
Influx has stopped and pressures have stabilized
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Kick Detection and Control
ATM
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SICP
CONVENTIONAL
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ATM
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Kick Detection Method Still Valid for
RD? Pit Gain Still valid Flow Rate Increase
(Returns) Much Better Well Flows With Pump
Off Not valid U-tubing Fluid Fill-up on
Trips More complicated ... Increased Hook
Load Still Valid Drop in Standpipe
Pressure Better Drop in BHP - MWD Still
Valid Drilling Break Still valid Decreased
Mud Weight (Gas Cut Mud) Still valid
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• Well Control Summary
• Most Conventional kick detection methods are
  still valid
• New differential flow rate method - better
  - use with computer monitoring
• Lower standpipe pressure makes kick detection
  easier and faster
• Conventional well control can be adapted -
  with modifications

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• Well Control Summary contd
• Not necessary to shut well in
• Seafloor pump controls the BHP
• Surface choke pressure generally does not
  affect wellbore pressures
• Well control training will be essential

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General Summary Dual Gradient Drilling is a
method that offers potential for lowering
drilling costs in very deep waters. (fewer
casing strings, smaller rigs, less time on
location) The method utilizes one or more
small-diameter return lines from the seafloor to
the surface. The drillpipe is not inside a return
line.
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Summary - contd A mudlift system is used in
the return line thereby making a dual-density
mud system possible. Wellhead pressure is
maintained at seawater hydrostatic, so well is
dead at all times Well control does not appear
to present any serious difficulties, and is quite
similar to conventional drilling with a riser.