25-26 August 1999 Well Control Conference of the Americas
Greater Kick Tolerance and Fewer Casing Strings Make Dual
Gradient Drilling a Winner
Schubert, J. J., Seland, S.,
Johansen, T. J., Juvkam-Wold, H. C.
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Introduction
• Problems Associated With Deepwater Drilling
• Single Gradient vs. Dual Gradient Concept
• Methodology
• Results
• Conclusions
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Problems Associated With Deepwater Drilling
• Longer, heavier marine risers requires– Larger drilling vessels w/ greater storage
requirements– Large volume of mud just to fill the riser– Large forces imposed on the riser by currents– Large tension forces on the riser
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Problems Associated With Deepwater Drilling
• Narrowing of the Window between the Pore Pressure and Fracture Pressure– Increase in number of casing strings– Near elimination of kick tolerance
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Single vs. Dual Gradient Concept
• Single Gradient Wells– Wellbore contains a
single density fluid
– Single pressure gradient
• Dual Gradient Well– Wellbore feels
seawater gradient to the seafloor, and mud gradient to bottom
Pressure, psi
Depth
ft
Seafloor @ 10,000’
Seawater HSP
23,880 psi
@ 37,500’
12.4 ppg mud
13.5 ppg mud
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Dual Gradient Achieved by:
• Taking returns at the seafloor
• Gas lift at the seafloor
• Inject hollow gas spheres
• Seafloor mud pumps
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Assumptions
• Water depth of 10,000’
• Total depth of 37,500’
• For dual gradient drilling:– annulus pressure at the seafloor is kept equal to
seawater HSP during normal drilling operations– during kicks seafloor pressure adjusted to
control BHP equal to formation pressure
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Pore and Fracture Gradients0
5000
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15000
20000
25000
30000
35000
40000
8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 19.0
Pressure Gradient (ppg)
Dep
th (
TV
D s
s)
Dual Density: Pore Gradient Fracture Gradient
Conventional: Pore Gradient Fracture Gradient
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Methodology
• Conventional Riser Drilling– Picked casing points graphically with with a 0.5
ppg stand off– Looked at the effect of 750 psi SIP– Looked at the effects of circulating pressures
for both:• 0 bbls influx
• 50 bbls influx
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Methodology
• Compared results to a dual gradient system:– Statically– Dynamically
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Methodology
• Dual Gradient System, we picked casing points:– Graphically (static wellbore pressures)– Dynamically:
• 0 bbl influx
• 50 bbl influx
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Graphical Casing Seat Selection0
5000
10000
15000
20000
25000
30000
35000
40000
8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0
Pressure Gradient (ppg)
Dep
th (
TV
D s
s)
30" 26" 20" 16" 13 3/8" 113/4" 9 5/8" 7 5/8"
5 1/2" to TD
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Kick w/ 750 psi SIP
Pore Pressure
Fracture Pressure
Mud HSP (13.1 ppg)
Mud HSP + 750 psi
Formation has fractured
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Casing Seat Pressures
15000
15200
15400
15600
15800
16000
16200
16400
16600
010000200003000040000
Location of Bubble, ft. from Surface
Pre
ss
ure
, ps
ig
no gain
50 bbl gain
Fracture Pressure of 15,672 psi
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Static Pressure - Dual Gradients
Pore Pressure
Fracture Pressure
Mud HSP (15.5 ppg)
Mud HSP = 750 psi
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Choke Pressures1000 psi underbalanced, no pit gain
-1000
-800
-600
-400
-200
0
200
400
600
800
1000
1200
0500010000150002000025000300003500040000Location of Bubble, ft. from Surface
Pre
ss
ure
, ps
ig
Dual Density
ConventionalKill rate of 650 gpm
Indicates Choke is Wide Open CLFP imposed downhole
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
14000
14500
15000
15500
16000
16500
17000
0500010000150002000025000300003500040000Location of Bubble, ft. from Surface
Pre
ssu
re, p
sig
Dual Density
Conventional
Fracture Pressure of 15,672 psi
Formation has Fractured
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Bottom Hole Pressure, 1000 psi underbalanced, no gain
27200
27400
27600
27800
28000
28200
28400
28600
010000200003000040000Location of Bubble, ft. from Surface
Pre
ssur
e, p
sig
Dual Density
Conventional
Formation Pressure
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Graphical Casing Seat Selection - Dual Density
0
5000
10000
15000
20000
25000
30000
35000
40000
8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 19.0
Pressure Gradient (ppg)
Dep
th (
TV
D s
s)
0.5 ppg trip and kick margin1.0 ppg trip and kick margin
11 3/4"
13 3/8"
16"
20"
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Dynamic Casing Seat Selection
D
e
p
t
h
ft.
Pressure, psi
1.0 ppg kick
0 bbl influx volume
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Dynamic Casing Seat Selection
1.0 ppg kick
50 bbl influx volume
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Conclusions
• The narrow gap between pore pressure and fracture pressure results in excess number of casing strings for conventional riser drilling.
• The large number of casing strings may result in such a small production string that high flow rates may not be possible.
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Conclusions
• The narrow gap between pore pressure and fracture pressure results in virtually no kick tolerance for conventionally drilled wells.
• The dual gradient system drastically reduces the number of casing strings required to reach total depth.
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Conclusions
• The dual gradient system widens the gap between pore pressure and fracture pressure resulting in a much higher kick tolerance.
• The higher kick tolerance increases the probability of a kick being killed successfully.
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Conclusions
• The dual gradient system will increase the probability of reaching the geologic objective in ultra-deep water.
• Dual gradient drilling will allow large enough production casing to be able to install production tubing as large a 7” in diameter.
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
6000’ WDType Well 6000
Water Depth = 6000 feet0
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20000
25000
8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 19.0 20.0 21.0
Pressure Gradient (ppg)
Dep
th (
TV
D s
s)
Riserless Fracture Gradient
Riserless Pore Pressure
Conventional FractureGradientConventional Pore Pressure
20"
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
6000’ WDConventional Drilling - Type WellWater Depth = 6000 feet
0
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15000
20000
25000
8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 19.0 20.0 21.0
Pressure Gradient (psi)
Dep
th (
TV
D s
s)
Conventional Fracture Gradient
Conventional Pore Pressure
.5 PPG Standoff FractureGradient1 PPG Standoff FractureGradient
30"
11 7/8"
20"
16"
13 5/8"
9 7/8"
7 5/8"
Best kick tolerance line
but impractical
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
6000’ WDSMD Drilling Type Well 6000Water Depth = 6000 feet
0
5000
10000
15000
20000
25000
8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 19.0 20.0 21.0 22.0
Pressure Gradient (ppg)
Dep
th (
TV
D s
s)
Conventional FractureGradientConventional Pore Pressure
.5 PPG Standoff FractureGradient1 PPG Standoff FractureGradient
30"
11 7/8"
20"
16"
13 5/8"
9 7/8"
Full 1.0 PPG Fracture Gradient
Margin
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
6000’ WD
Kick Tolerance is Back520 psi Underbalance from Kick
Formation has Fractured
Formation Intact
SMDCONVENTIONAL
15.5 PPG MUD
15.5 ppg mud + 520 psi
18.4 ppg mud
18.4 ppg mud + 520 psi
25-26 August 1999 Well Control Conference of the Americas
Houston, Texas
Kick Tolerance
30” @ 6300’20” @ 7000’
16” @ 10,000’
13 3/8” @ 16,500’
11 3/4” @ 17,500’
9 5/8” @ 20,000’
0.5 ppg kick - 90 bbl gain
0.5 ppg kick - 35 bbl gain
0.5 ppg kick - 50 bbl gain
0.5 ppg kick - 80 bbl gain
SMDConventional
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