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Petroleum Engineering 411Well Drilling

Lesson 27Dual Gradient Drilling

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DESIGN PROJECTDESIGN 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?What is Dual Gradient Drilling?(DGD)?(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

4AT

Conventional Riser Drilling- Wellbore Pressures

SEAFLOOR

SEA WATER HYDROSTATIC

PRESSURE

DEP

TH

MUD HYDROSTATIC BOP

FLOATER

DRILLING RISER

CHOKE LINE

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Static Wellbore Pressures

SEA WATER HYDROSTATIC

PRESSUREPRESSURE

DEP

TH

MUD HYDROSTATIC

PRESSURE DGD

MUD HYDROSTATIC

PRESSURE Conventional

BOP

FLOATER

RISER

CHOKE LINE

DGD

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Dual Gradient Drilling ProjectsDual Gradient Drilling Projects

� Subsea Mudlift Drilling (SMD)

� Hollow Glass Spheres

� Deep Vision

� Shell

� Gas Lift

� H.P. Riser

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Subsea MudliftDrilling

( 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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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)

References

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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 ProblemsDeeper 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 increaseswith 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

15ATM

g SMD refers to drilling where mud returns DO NOT go through a conventional, large-diameter, drilling riser

g Instead the returns move from the seafloor to the surface through two small -diameter pipes separate from the drillpipe (outside the main riser pipe)

g A Mudlift system is used in the Return Line

What is Subsea Mudlift Drilling?

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Fig. 7.21 ADE

Pore pressure

gradient and fracture

gradient data for Jefferson Parish, LA.

Fracture Gradient

Pore Pressure Gradient

Equivalent Mud Density, ppg

0.5 ppg

0.5 ppg

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Pore Pressure

Frac Pressure

Max Mud Wt

Min Mud Wt

Equivalent Mud Wt, lb/gal

SEAFLOORConventional Casing Seat Selection

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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.

17.3 ppg

11.5 ppg

19ATM

Conventional Riser Drilling- Wellbore Pressures

SEAFLOOR

SEA WATER HYDROSTATIC

PRESSURE

PRESSURE

DEP

THSTATIC PRESSURE

CIRCULATING PRESSURE

∆∆∆∆PBIT

BOP

FLOATER

Drill String

20ATM

Static Wellbore Pressures

SEAFLOOR

SEA WATER HYDROSTATIC

PRESSURE

PRESSURE

DEPTH

MUD HYDROSTATIC

PRESSURE SMD

MUD HYDROSTATIC

PRESSURE Conventional

21ATM

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?

22ATM

Static Wellbore Pressures

P = 0.052 * MW * DepthFor 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

23ATM

Solution: Static Wellbore Pressures

SEA WATER HYDROSTATIC

PRESSURE

8.6 lb/gal

4,472 psi

DEPTH

15.9 lb/gal SMD

13.5 lb/gal Conventional

21,000 psi

24ATM

Wellbore Pressures

SEAFLOOR

FRACTURE PRESSURE

PORE PRESSURE

SEA WATER HYDROSTATIC

PRESSURE

PRESSURE

DEPTH

MUD HYDROSTATIC

PRESSURE Conventional

25ATM

SEAFLOOR

FRACTURE PRESSURE

PORE PRESSURE

MUD HYDROSTATIC

PRESSURE

SMD

SEA WATER HYDROSTATIC

PRESSURE

PRESSURE

DEPTH

MUD HYDROSTATIC

PRESSURE Conventional

Wellbore Pressures

26ATM

Casing Requirements - Conventional

SEAFLOOR

FRACTURE PRESSURE

PORE PRESSURE

SEA WATER HYDROSTATIC

PRESSURE

PRESSURE

DEPTH

MUD HYDROSTATIC

PRESSURE Conventional

27ATM

Casing Requirements - SMD

SEAFLOOR

FRACTURE PRESSURE

PORE PRESSURE

MUD HYDROSTATIC

PRESSURE SMD

SEA WATER HYDROSTATIC

PRESSURE

PRESSURE

DEPTH

28ATM

Pressure Considerations

SEAFLOOR

FRACTURE PRESSURE

PORE PRESSURE

MUD HYDROSTATIC

PRESSURE SMD

SEA WATER HYDROSTATIC

PRESSURE

PRESSURE

DEPTH

MUD HYDROSTATIC

PRESSURE Conventional

SMD

29ATM

Wellbore Pressures - Conventional

SEAFLOOR

SEA WATER HYDROSTATIC

PRESSURE

PRESSURE

DEP

TH

STATIC PRESSURE

CIRCULATING PRESSURE

∆∆∆∆PBIT

BOP

FLOATER

30ATM

Static Pressures - SMD

SEAFLOOR

SEA WATER HYDROSTATIC

PRESSURE

PRESSURE

DEP

TH

ANNULUS AND RETURN LINE

BOP

FLOATER

31ATM

Drillstring Circulating Pressures

SEAFLOOR

SEA WATER HYDROSTATIC

PRESSURE

PRESSURE

DEPTH

CONVENTIONALSMD

∆∆∆∆PBIT

32ATM

Annulus Circulating Pressures

SEAFLOOR

SEA WATER HYDROSTATIC

PRESSURE

PRESSURE

DEPTH

CONVENTIONAL (13.5 lb/gal)

SMD (15.9 lb/gal)

∆∆∆∆PPUMP

33ATM

Circulating Pressures - SMD

SEAFLOOR

SEA WATER HYDROSTATIC

PRESSURE

PRESSURE

DEPTH

ANNULUS AND RETURN LINE

DRILLSTRING PRESSURE

∆∆∆∆PBIT

∆∆∆∆PPUMP

34ATM

Circulating Pressures - SummaryPR

ESSU

RE

DISTANCE FROM STANDPIPE

CONVENTIONAL

SMD

∆∆∆∆PBIT

∆∆∆∆PML_PUMP

35ATM

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?

Transients

36ATM

MUDLIFT

U-Tubing in SMD

~SEAWATER HYDROSTATIC

PRESSURE

BOP

STATIC FLUID LEVEL

FLOATER

Dril

lstr

ing

Ann

ulus

37ATM

Static Pressures - SMD

SEAFLOOR

SEA WATER HYDROSTATIC

PRESSURE

PRESSURE

DEPTH

ANNULUS AND RETURN LINE DRILLSTRING PRESSURE

Static Fluid Level in DP

38ATM

U-Tubing Rate vs. Time after Pump OFF

0

200

400

600

800

0 5 10 15 20 25 30

Elapsed Time, min

Mud

Flo

w R

ate,

gpm

3-in ID 4.276-in ID6-in ID

10,000 ft Water Depth

39ATM

Fluid Level vs. Time after Pump OFF

0

1,000

2,000

3,000

4,000

5,000

0 5 10 15 20 25 30

Elapsed Time, min

Flui

d Le

vel i

n Dr

illpi

pe,

ft

3-in ID 4.276-in ID6-in ID

10,000 ft Water Depth

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41ATM

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

Well Control Considerations

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44ATM

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

45ATM

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

46ATM

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Gas Lift?Glass Beads?

How to HandleConnections?Trips?

Pumps?Gas Lift?Glass Beads?

Rotating BOP

Dual Gradient Alternatives

48ATM

MUDLIFTBOP

Advanced SMD System

SEAFLOOR

~SEAWATER HYDROSTATIC

PRESSURE

FLOATER

30,000’

10,000’

Circulation Rate 650 gpm

Drillpipe OD 6 5/8 in

Return Line ID 6 in

Hole Size 12 1/4 in