1. Petroleum Engineering 411 Well Drilling
Lesson 27
Dual Gradient Drilling

2. 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

3. 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

4. Conventional Riser Drilling - Wellbore Pressures
FLOATER
SEA WATER HYDROSTATIC
DRILLING RISER
CHOKE LINE
MUD HYDROSTATIC
BOP
SEAFLOOR
DEPTH
PRESSURE
ATM 16

5. Static Wellbore Pressures
FLOATER
MUD HYDROSTATIC PRESSURE DGD
MUD HYDROSTATIC PRESSURE Conventional
RISER
CHOKE LINE
BOP
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PRESSURE
DGD 10

6. Dual Gradient Drilling Projects
Subsea Mudlift Drilling (SMD)
Hollow Glass Spheres
Deep Vision
Shell
Gas Lift
H.P. Riser

7. Subsea Mudlift Drilling ( SMD ) Note Pump and Return Line

8. 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?

9. References HW #15 (due 11-15-01)
1. “Riserless Drilling: Circumventing the Size/cost Cycle in Deepwater,” by Allen D. Gault. May 1996, Offshore, p “Subsea Mudlift Drilling JIP: Achieving dual-gradient technology,” by K.L. Smith et al., World Oil - Deepwater Technology, August 1999, pp
HW #15 (due )

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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
ATM 5

16. Equivalent Mud Density, ppg Pore Pressure Gradient
Fig ADE
Pore pressure gradient and fracture gradient data for Jefferson Parish, LA.
Fracture Gradient
0.5 ppg
0.5 ppg
Pore Pressure Gradient

17. Conventional Casing Seat Selection
SEAFLOOR
Frac Pressure
Max Mud Wt
Min Mud Wt
Pore Pressure
Equivalent Mud Wt, lb/gal

18. 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.
17.3 ppg
11.5 ppg

19. SEA WATER HYDROSTATIC PRESSURE
Conventional Riser Drilling - Wellbore Pressures
FLOATER
STATIC PRESSURE
CIRCULATING PRESSURE
BOP
DEPTH
SEAFLOOR
Drill String
SEA WATER HYDROSTATIC PRESSURE
PBIT
PRESSURE
ATM 16

20. Static Wellbore Pressures
MUD HYDROSTATIC PRESSURE SMD
MUD HYDROSTATIC PRESSURE Conventional
SEAFLOOR
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PRESSURE
ATM 10

21. 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?
ATM 10

22. Static Wellbore Pressures
P = * MW * Depth
For conventional drilling, Minimum mud wt.
MWmin = 21,000/(0.052 * 30,000) = lb/gal
Seafloor pressure = 0.052*8.6*10,000 = 4,472 psig
For SMD, Minimum mud weight
= (21, ,472)/(0.052 * 20,000) = lb/gal
ATM 10

23. SEA WATER HYDROSTATIC PRESSURE
Solution: Static Wellbore Pressures
SEA WATER HYDROSTATIC PRESSURE
8.6 lb/gal
15.9 lb/gal SMD
13.5 lb/gal Conventional
DEPTH
4,472 psi
21,000 psi
ATM 10

24. MUD HYDROSTATIC PRESSURE Conventional SEA WATER HYDROSTATIC PRESSURE
Wellbore Pressures
MUD HYDROSTATIC PRESSURE Conventional
SEAFLOOR
FRACTURE PRESSURE
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PORE PRESSURE
PRESSURE
ATM 11

25. Wellbore Pressures DEPTH PRESSURE ATM MUD HYDROSTATIC PRESSURE
SMD
MUD HYDROSTATIC PRESSURE Conventional
SEAFLOOR
FRACTURE PRESSURE
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PORE PRESSURE
PRESSURE
ATM 12

26. MUD HYDROSTATIC PRESSURE Conventional SEA WATER HYDROSTATIC PRESSURE
Casing Requirements - Conventional
MUD HYDROSTATIC PRESSURE Conventional
SEAFLOOR
DEPTH
FRACTURE PRESSURE
SEA WATER HYDROSTATIC PRESSURE
PORE PRESSURE
PRESSURE
ATM 13

27. MUD HYDROSTATIC PRESSURE SMD SEA WATER HYDROSTATIC PRESSURE
Casing Requirements - SMD
MUD HYDROSTATIC PRESSURE SMD
SEAFLOOR
FRACTURE PRESSURE
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PORE PRESSURE
PRESSURE
ATM 14

28. Pressure Considerations
MUD HYDROSTATIC PRESSURE SMD
MUD HYDROSTATIC PRESSURE Conventional
SMD
SEAFLOOR
FRACTURE PRESSURE
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PORE PRESSURE
PRESSURE
ATM 15

29. SEA WATER HYDROSTATIC PRESSURE
Wellbore Pressures - Conventional
FLOATER
STATIC PRESSURE
CIRCULATING PRESSURE
SEAFLOOR
BOP
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PBIT
PRESSURE
ATM 16

30. ANNULUS AND RETURN LINE SEA WATER HYDROSTATIC PRESSURE
Static Pressures - SMD
FLOATER
ANNULUS AND RETURN LINE
SEAFLOOR
BOP
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PRESSURE
ATM 17

31. SEA WATER HYDROSTATIC PRESSURE
Drillstring Circulating Pressures
CONVENTIONAL
SMD
SEAFLOOR
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PBIT
PRESSURE
ATM 19

32. CONVENTIONAL (13.5 lb/gal) SEA WATER HYDROSTATIC PRESSURE
Annulus Circulating Pressures
CONVENTIONAL (13.5 lb/gal)
SMD (15.9 lb/gal)
SEAFLOOR
PPUMP
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PRESSURE
ATM 20

33. ANNULUS AND RETURN LINE SEA WATER HYDROSTATIC PRESSURE
Circulating Pressures - SMD
DRILLSTRING PRESSURE
ANNULUS AND RETURN LINE
SEAFLOOR
PPUMP
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PBIT
PRESSURE
ATM 21

34. Circulating Pressures - Summary
CONVENTIONAL
PBIT
PRESSURE
PML_PUMP
SMD
DISTANCE FROM STANDPIPE
ATM 21

35. Transients Transient Behavior when Stopping Rig Pump
(U-tubing or Free-fall)
Why does the drillpipe fluid level fall?
How fast does the Fluid Level in the drillpipe drop?
How far does the Fluid Level drop?
ATM 22

36. ~SEAWATER HYDROSTATIC PRESSURE
U-Tubing in SMD
FLOATER
~SEAWATER HYDROSTATIC PRESSURE
Drillstring
STATIC FLUID LEVEL
Annulus
BOP
MUDLIFT
ATM 23

37. ANNULUS AND RETURN LINE SEA WATER HYDROSTATIC PRESSURE
Static Pressures - SMD
Static Fluid Level in DP
ANNULUS AND RETURN LINE
DRILLSTRING PRESSURE
SEAFLOOR
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PRESSURE
ATM 18

38. 10,000 ft Water Depth
ATM 24

39. 10,000 ft Water Depth
ATM 25

41. 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
ATM 22

44. 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
ATM 36

45. 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
ATM 37

46. THE END
ATM 38

47. Dual Gradient Alternatives
Gas Lift?
Glass Beads?
Pumps?
Gas Lift?
Glass Beads?
Dual Gradient Alternatives
How to Handle
Connections?
Trips?
Rotating BOP

48. ~SEAWATER HYDROSTATIC PRESSURE
Advanced SMD System
FLOATER
~SEAWATER HYDROSTATIC PRESSURE
10,000’
SEAFLOOR
BOP
MUDLIFT
Circulation Rate 650 gpm
Drillpipe OD 6 5/8 in
Return Line ID in
Hole Size /4 in
30,000’
ATM 6