greater kick tolerance and fewer casing strings make dual gradient drilling a winner

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.


Houston, Texas

Introduction

• Problems Associated With Deepwater Drilling

• Single Gradient vs. Dual Gradient Concept

• Methodology

• Results

• Conclusions


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


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


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


Houston, Texas

Dual Gradient Achieved by:

• Taking returns at the seafloor

• Gas lift at the seafloor

• Inject hollow gas spheres

• Seafloor mud pumps


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


Houston, Texas

Pore and Fracture Gradients0

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)

Dual Density: Pore Gradient Fracture Gradient

Conventional: Pore Gradient Fracture Gradient


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


Houston, Texas

Methodology

• Compared results to a dual gradient system:– Statically– Dynamically


Houston, Texas

Methodology

• Dual Gradient System, we picked casing points:– Graphically (static wellbore pressures)– Dynamically:

• 0 bbl influx

• 50 bbl influx


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


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


Houston, Texas

Kick w/ 750 psi SIP

Pore Pressure

Fracture Pressure

Mud HSP (13.1 ppg)

Mud HSP + 750 psi

Formation has fractured


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


Houston, Texas

Static Pressure - Dual Gradients

Pore Pressure

Fracture Pressure

Mud HSP (15.5 ppg)

Mud HSP = 750 psi


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


Houston, Texas

14000

14500

15000

15500

16000

16500

17000


Pre

ssu

re, p

sig

Dual Density

Conventional

Fracture Pressure of 15,672 psi

Formation has Fractured


Houston, Texas

Bottom Hole Pressure, 1000 psi underbalanced, no gain

27200

27400

27600

27800

28000

28200

28400

28600


Pre

ssur

e, p

sig

Dual Density

Conventional

Formation Pressure


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


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"


Houston, Texas

Dynamic Casing Seat Selection

D

e

p

t

h

ft.

Pressure, psi

1.0 ppg kick

0 bbl influx volume


Houston, Texas

Dynamic Casing Seat Selection

1.0 ppg kick

50 bbl influx volume


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.


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.


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.


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.


Houston, Texas


Houston, Texas

6000’ WDType Well 6000

Water Depth = 6000 feet0

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


Dep

th (

TV

D s

s)

Riserless Fracture Gradient

Riserless Pore Pressure

Conventional FractureGradientConventional Pore Pressure

20"


Houston, Texas

6000’ WDConventional Drilling - Type WellWater 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

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


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


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


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


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




SMDConventional

greater kick tolerance and fewer casing strings make dual gradient drilling a winner

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