Transcript Petroleum Engineering 411 Well Drilling Lesson 27 Dual Gradient Drilling

Petroleum Engineering 411
Well Drilling
Lesson 27
Dual Gradient Drilling
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DESIGN PROJECT
Your PETE 411 Design Project will be
assigned on Friday, November 15
The Project Report is Due in Dr. J-W’s
Office by 5 p.m. on Monday, Dec. 9
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What is Dual Gradient Drilling?
(DGD)?
 In dual-gradient drilling the pressure
profile in the annulus appears to have
two distinct pressure gradients
 An example would be a heavy mud
below the mudline and a seawater
gradient above the mud line
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Conventional Riser Drilling
- Wellbore Pressures
FLOATER
DRILLING RISER
SEA WATER
HYDROSTATIC
CHOKE LINE
MUD HYDROSTATIC
DEPTH
BOP
A
T
SEAFLOOR
PRESSURE
4
Static Wellbore Pressures
FLOATER
RISER
MUD
HYDROSTATIC
PRESSURE
CHOKE
LINE
DGD
MUD
HYDROSTATIC
PRESSURE
Conventional
BOP
DGD
SEA WATER
HYDROSTATIC PRESSURE
PRESSURE
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Dual Gradient Drilling Projects
 Subsea Mudlift Drilling (SMD)
 Hollow Glass Spheres
 Deep Vision
 Shell
 Gas Lift
 H.P. Riser
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Subsea
Mudlift
Drilling
( SMD )
Note Pump
and Return
Line
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Subsea Mudlift Drilling







What is Subsea Mudlift Drilling?
How does it work?
Why do we need it?
Pore pressures and fracture pressures
Mud weights and casing programs
What about connections and trips?
What about kicks?
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References
1. “Riserless Drilling: Circumventing the Size/cost
Cycle in Deepwater,” by Allen D. Gault.
May 1996, Offshore, p.49
2. “Subsea Mudlift Drilling JIP:
Achieving dual-gradient technology,”
by K.L. Smith et al., World Oil - Deepwater
Technology, August 1999, pp 21-28.
HW #15 (due 11-15-01)
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Current SMD Concepts

A water-filled drilling riser


One or more separate small-diameter
mud return line(s) from seafloor to
surface (e.g., two 4.5-in ID lines)
A “dual mud density” system (DGD)


Seawater gradient from surface to
seafloor
Heavier drilling mud inside the wellbore
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Current SMD Concepts - cont’d

A seafloor mud pump to lift mud to surface

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

Retains a Riser Margin
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Current Problems
Deeper water results in longer, larger diameter and
heavier drilling risers
High pore pressures and low fracture pressures
lead to more casing strings
This leads to larger wellheads, even larger and
heavier risers, and finally to bigger and more
expensive rigs
Well control is more difficult - because of the pore
pressure / fracture pressure proximity, and long
choke lines with high friction pressure drops
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Effect of Increasing Water Depth
Weight of drilling riser increases
with depth. In 10,000 ft of water:
– 21-inch riser has an internal capacity
of ~ 4,000 bbls! (value ~ $1 million)
– Weight of riser ~ 2 million lbs.
Weight of 16 lb/gal mud inside riser
~ 2.7 million lbs
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What About Subsea Mudlift Drilling?




Two 4.5” ID return lines with
~ 400 bbls capacity 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?
 SMD 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 two small diameter pipes separate from the
drillpipe (outside the main riser pipe)
 A Mudlift system is used in the Return Line
TM
A
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Equivalent Mud Density, ppg
Fracture
Gradient
0.5 ppg
0.5 ppg
Fig. 7.21 ADE
Pore
pressure
gradient and
fracture
gradient data
for Jefferson
Parish, LA.
Pore
Pressure
Gradient
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Conventional Casing Seat Selection
SEAFLOOR
Frac Pressure
Max Mud Wt
Min Mud Wt
Pore Pressure
Equivalent Mud Wt, lb/gal
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17.3 ppg
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.
11.5 ppg
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Conventional Riser Drilling
- Wellbore Pressures
FLOATER
STATIC PRESSURE
CIRCULATING PRESSURE
DEPTH
BOP
SEAFLOOR
Drill String
SEA WATER
HYDROSTATIC
PRESSURE
T
A
M
PBI
T
PRESSURE
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Static Wellbore Pressures
MUD
HYDROSTATIC
PRESSURE
SMD
SEAFLOOR
MUD
HYDROSTATIC
PRESSURE
Conventional
DEPTH
SEA WATER
HYDROSTATIC
PRESSURE
PRESSURE
TM
A
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Example: Static Wellbore Pressures
At 30,000 ft, in 10,000 ft of water, the
pore pressure is 21,000 psig.
For conventional drilling, what is the
minimum mud weight that can control this
pressure?
For SMD, what is the minimum mud weight
that can control this pressure?
TM
A
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Static Wellbore Pressures
P = 0.052 * MW * Depth
For conventional drilling, Minimum mud wt.
MWmin = 21,000/(0.052 * 30,000) = 13.5 lb/gal
Seafloor pressure = 0.052*8.6*10,000 = 4,472 psig
For SMD, Minimum mud weight
= (21,000 - 4,472)/(0.052 * 20,000) = 15.9 lb/gal
TM
A
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Solution: Static Wellbore Pressures
SEA WATER
HYDROSTATIC
PRESSURE
15.9 lb/gal
SMD
13.5 lb/gal
Conventional
8.6 lb/gal
DEPTH
4,472 psi
TM
A
21,000 psi
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Wellbore Pressures
MUD
HYDROSTATIC
PRESSURE
Conventional
SEAFLOOR
FRACTURE
PRESSURE
DEPTH
SEA WATER
HYDROSTATIC
PRESSURE
PORE PRESSURE
PRESSURE
TM
A
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Wellbore Pressures
SEAFLOOR
MUD
HYDROSTATIC
PRESSURE
MUD
HYDROSTATIC
PRESSURE
SMD
Conventional
FRACTURE
PRESSURE
DEPTH
SEA WATER
HYDROSTATIC
PRESSURE
PORE PRESSURE
PRESSURE
TM
A
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Casing Requirements - Conventional
MUD
HYDROSTATIC
PRESSURE
Conventional
SEAFLOOR
DEPTH
FRACTURE
PRESSURE
SEA WATER
HYDROSTATIC
PRESSURE
PORE PRESSURE
PRESSURE
TM
A
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Casing Requirements - SMD
MUD
HYDROSTATIC
PRESSURE
SMD
SEAFLOOR
FRACTURE
PRESSURE
DEPTH
SEA WATER
HYDROSTATIC
PRESSURE
PORE PRESSURE
PRESSURE
TM
A
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Pressure Considerations
SMD
MUD
HYDROSTATIC
PRESSURE
MUD
HYDROSTATIC
PRESSURE
SMD
Conventional
SEAFLOOR
FRACTURE
PRESSURE
DEPTH
SEA WATER
HYDROSTATIC
PRESSURE
PORE PRESSURE
PRESSURE
TM
A
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Wellbore Pressures - Conventional
FLOATER
STATIC PRESSURE
CIRCULATING PRESSURE
SEAFLOOR
DEPTH
BOP
SEA WATER
HYDROSTATIC
PRESSURE
PBIT
PRESSURE
TM
A
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FLOATER
Static Pressures - SMD
ANNULUS AND
RETURN LINE
SEAFLOOR
BOP
SEA WATER
HYDROSTATIC
PRESSURE
PRESSURE
TM
A
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Drillstring Circulating Pressures
CONVENTIONAL
SMD
SEAFLOOR
DEPTH
SEA WATER
HYDROSTATIC
PRESSURE
PBIT
PRESSURE
TM
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Annulus Circulating Pressures
CONVENTIONAL (13.5 lb/gal)
SMD (15.9 lb/gal)
SEAFLOOR
PPUMP
DEPTH
SEA WATER
HYDROSTATIC
PRESSURE
PRESSURE
TM
A
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Circulating Pressures - SMD
DRILLSTRING PRESSURE
ANNULUS AND RETURN LINE
SEAFLOOR
PPUMP
DEPTH
SEA WATER
HYDROSTATIC
PRESSURE
PBIT
PRESSURE
TM
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Circulating Pressures - Summary
CONVENTIONAL
PBIT
PML_PUMP
SMD
TM
A
DISTANCE FROM STANDPIPE
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Transients
Transient Behavior when Stopping Rig Pump
(U-tubing or Free-fall)



TM
A
Why does the drillpipe fluid level fall?
How fast does the Fluid Level in the
drillpipe drop?
How far does the Fluid Level drop?
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U-Tubing in SMD
STATIC
FLUID
LEVEL
~SEAWATER
HYDROSTATIC
PRESSURE
Annulus
Drillstring
FLOATER
BOP
TM
A
MUDLIFT
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Static Pressures - SMD
Static Fluid
Level in DP
ANNULUS AND RETURN LINE
DRILLSTRING PRESSURE
SEAFLOOR
DEPTH
SEA WATER
HYDROSTATIC
PRESSURE
PRESSURE
TM
A
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U-Tubing Rate vs. Time after Pump OFF
Mud Flow Rate, gpm
800
3-in ID
4.276-in ID
6-in ID
600
400
200
0
0
5
10
15
20
25
30
Elapsed Time, min
TM
A
10,000 ft Water Depth
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Fluid Level vs. Time after Pump OFF
Fluid Level in Drillpipe, ft
5,000
4,000
3,000
2,000
3-in ID
4.276-in ID
6-in ID
1,000
0
0
5
10
15
20
25
30
Elapsed Time, min
TM
A
10,000 ft Water Depth
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Well Control Considerations
How do you shut a well in after taking a kick?
 With a DSV this is almost routine
 Better still, it is not necessary to shut the well in.
The wellbore pressures can be increased by
temporarily slowing down the mudlift pump
 Friction in the choke line is handled by the
Mudlift Pump and is not seen by the
weak formations
TM
A
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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 separate from the return lines
TM
A
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Summary - cont’d
A mudlift system (pump) is used to feed the
return lines, thereby making a “dual-density”
mud system possible
Wellhead pressure is maintained at seawater
hydrostatic, so well is “dead” at all times
Well control is quite similar to that in
conventional drilling with a riser, but offers a
number of significant advantages
TM
A
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THE END
TM
A
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Dual Gradient
Alternatives
Rotating BOP
Gas
Lift?
Pumps?
Glass
Beads?
Gas Lift?
Glass Beads?
How to Handle
Connections?
Trips?
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Advanced SMD System
FLOATER
~SEAWATER
HYDROSTATIC
PRESSURE
SEAFLOOR
BOP
10,000’
MUDLIFT
Circulation Rate 650 gpm
Drillpipe OD
Return Line ID
Hole Size
TM
A
6 5/8 in
30,000’
6 in
12 1/4 in
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