16. dual gradient drilling

1

TAMU - PemexOffshore Drilling

Lesson 16 Dual Gradient Drilling

2

Dual Gradient Drilling

What is Dual Gradient Drilling? How does it work? Why do we need it? Pore pressures and fracture gradients. Mud weights and casing programs. What about connections and trips? What about kicks?

3

Subsea Mudlift Drilling or “Riserless

Drilling”Drillstring

Mud Return

Line

Pump

…sometimes called

“Dual Gradient Drilling”or

“Dual Density Drilling

4

Subsea Mudlift Drilling Concept

No conventional drilling riser A separate small-diameter mud

return line from seafloor to surface

A “dual mud density” system Seawater gradient from surface to

seafloor Heavier drilling mud inside the wellbore

5

Subsea Mudlift Drilling Concept

A seafloor mud pump on the return line

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

7Seafloor detail of Subsea Mudlift configuration

Rotating BOP (RBOP) with Pack-Off

ConventionalAnnular BOP

ConventionalRam BOPs

8

Subsea Mudlift drilling in three phases

9

Effect of Increasing Water Depth

Weight of drilling riser increases with depth:

21-inch riser has an internal capacity of ~ 400 bbls/ 1,000 ft

At 10,000 ft, wt. of riser ~ 2 million lbs. and wt. of 17.1 1b/gal mud

inside riser ~ 2.7 million lbs

10

Effect of Increasing Water Depth

More storage space required on drilling vessel - especially with flotation elements on the riser

A larger and more expensive drilling vessel is required

Few such vessels are available (14 EA. 4th generation?) (1996)

11

What About Subsea Mudlift Drilling?

A 6” return line with ~36 bbls/1000 ft 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.

12ATM

“Subsea Mudlift” Drilling, in the present context, 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 one or more small - diameter pipe(s) separate from the drillpipe.

A Mudlift system is used in the Return Line

What is “Subsea Mudlift” Drilling?

13

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

15

Casing Setting Depths

based on pore pressure grad.,

fracture grad. and mud wt.

5,000 ft water

0.5 ppg kick tolerance

Mud Weight (Gradient), ppg

16

(?)

17

Apparent Pressures in the Wellbore

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

8.00 9.00 10.00 11.00 12.00 13.00 14.00

Equivalent mud density, ppg

Ver

tica

l Dep

th,

ft

RISERLESSMud Weight = 15.5 lb/gal

RISERLESSSeawater Density = 8.6 lb/gal

CONVENTIONALMud Weight = 13.2 lb/gal

0 5,000 10,000 15,000 20,000 25,000

SEAWATER8.6 lb/gal

CONVENTIONAL8.6 lb/gal

RISERLESS15.5 lb/gal

Wellbore Pressure, psi

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Apparent Pressures in the Wellbore

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

8.00 9.00 10.00 11.00 12.00 13.00 14.00

Equivalent mud density, ppg

Ver

tica

l Dep

th,

ft

RISERLESSMud Weight = 15.5 lb/gal

RISERLESSSeawater Density = 8.6 lb/gal

CONVENTIONALMud Weight = 13.2 lb/gal

8 9 10 11 12 13 14

SEAWATER8.6 lb/gal

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Gas Lift?Glass Beads?

How to HandleConnections?Trips?

Gas Lift?Glass Beads?

Rotating BOP

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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.

23ATM

Conventional Riser Drilling - Static Wellbore Pressures

SEAFLOOR

SEA WATER HYDROSTATIC

PRESSURE

DE

PT

H

MUD HYDROSTATIC

BOP

FLOATER

DRILLING RISER

CHOKE LINE

24ATM

Conventional Riser Drilling - Wellbore Pressures

SEAFLOOR


PRESSURE

PRESSURE

DE

PT

HSTATIC PRESSURE

CIRCULATING PRESSURE

PBIT

BOP

FLOATER

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MUDLIFTBOP

Subsea Mudlift Drilling System

SEAFLOOR

~SEAWATER HYDROSTATIC

PRESSURE

FLOATER

30,000’

10,000’

BASE CASE

Circulation Rate 650 gpm

Drillpipe OD 6 5/8 in

Return Line ID 6 in

Hole Size 12 1/4 in

26

Alternate SMD Configuration- Wellbore Pressures


PRESSURE

PRESSURE

DE

PT

H

MUD HYDROSTATIC

PRESSURE SMD

MUD HYDROSTATIC

PRESSURE Conventional

BOP

FLOATER

RISER

CHOKE LINE

SMD

27

Static Wellbore Pressures

SEAFLOOR


PRESSURE

PRESSURE

DEPTH

MUD HYDROSTATIC

PRESSURE Subsea Mudlift

MUD HYDROSTATIC


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In a 30,000 ft well in 10,000 ft of water, the bottomhole pressure is 21,000 psig.

For conventional drilling, what is the minimum mud weight that can control this pressure?

For Subsea Mudlift drilling, what is the minimum mud weight that can control this pressure?

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P = 0.052 * MW * Depth

For conventional drilling, Minimum mud weight = 21,000/(0.052 * 30,000) = 13.5 lb/gal

Seafloor pressure = 0.052*8.6*10,000 = 4,472 psig

For Subsea Mudlift drilling, Minimum mud weight = (21,000 - 4,472)/0.052 * 20,000 = 15.9 lb/gal

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Wellbore Pressures

SEAFLOOR

FRACTURE PRESSURE

PORE PRESSURE


PRESSURE

PRESSURE

DEPTH

MUD HYDROSTATIC


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SEAFLOOR

FRACTURE PRESSURE

PORE PRESSURE

MUD HYDROSTATIC

PRESSURE Subsea Mudlift


PRESSURE

PRESSURE

DEPTH

MUD HYDROSTATIC


Wellbore Pressures

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Casing Requirements - Conventional

SEAFLOOR

FRACTURE PRESSURE

PORE PRESSURE


PRESSURE

PRESSURE

DEPTH

MUD HYDROSTATIC


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Casing Requirements - Subsea Mudlift

SEAFLOOR

FRACTURE PRESSURE

PORE PRESSURE

MUD HYDROSTATIC

PRESSURE

Subsea Mudlift


PRESSURE

PRESSURE

DEPTH

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Pressure Considerations

SEAFLOOR

FRACTURE PRESSURE

PORE PRESSURE

MUD HYDROSTATIC

PRESSURE

Subsea Mudlift


PRESSURE

PRESSURE

DEPTH

MUD HYDROSTATIC

PRESSURE

ConventionalSubsea Mudlift

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Wellbore Pressures - Conventional

SEAFLOOR


PRESSURE

PRESSURE

DE

PT

H

STATIC PRESSURE

CIRCULATING PRESSURE

PBIT

BOP

FLOATER

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Static Pressures- Subsea Mudlift Drilling

SEAFLOOR


PRESSURE

PRESSURE

DE

PT

H

ANNULUS AND RETURN LINE

BOP

FLOATER

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Drillstring Circulating Pressures

SEAFLOOR


PRESSURE

PRESSURE

DEPTH

CONVENTIONAL

Subsea Mudlift

PBIT

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Annulus Circulating Pressures

SEAFLOOR


PRESSURE

PRESSURE

DEPTH

CONVENTIONAL (13.2 lb/gal)

Subsea Mudlift (15.5 lb/gal)

PPUMP

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Circulating Pressures - Subsea Mudlift Drilling

SEAFLOOR


PRESSURE

PRESSURE

DEPTH


DRILLSTRING PRESSURE

PBIT

PPUMP

40

Pressures in the Wellbore

Transient Behavior when Stopping Pump (U-tubing or Free-fall)

How fast does the Fluid Level in the drillpipe drop?

How far does the Fluid Level drop?

Preview of Kick Detection and Well Control

Transients

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MUDLIFT

U-Tubing in Subsea Mudlift Drilling


PRESSURE

BOP

STATIC FLUID LEVEL

FLOATER

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Static Pressures - Subsea Mudlift Drilling

SEAFLOOR


PRESSURE

PRESSURE

DEPTH


DRILLSTRING PRESSURE

43

U-Tubing Rate vs. Time after Pump OFF

0

200

400

600

800

0 5 10 15 20 25 30

Elapsed Time, min

Mu

d F

low

Rate

, g

pm 3-in ID

4.276-in ID6-in ID

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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

Flu

id L

evel i

n D

rillp

ipe, f

t

3-in ID 4.276-in ID6-in ID

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Kick Detection & Well Control- Review - Conventional -

Kick Indication (what is a KICK?)

Kick Detection (confirmation)

Kick containment (stop flow into well)

Circulate Kick out of hole

Increase Mud Weight in hole (if necessary)

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Circulate Kick out of hole

Keep the BHP constant throughout

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Circulate Old Mud out of hole

Keep the BHP constant throughout

48

MUDLIFTBOP

Subsea Mudlift Drilling System

SEAFLOOR


PRESSURE

FLOATER

30,000’

10,000’

KICK

Surface Pump: Constant RateMudlift: Const. Inlet Press.

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Kick Detection and Control

640

650

660

670

680

690

700

-30 -20 -10 0 10 20 30

Time, minutes

Cir

cu

lati

on

Ra

te,

ga

l/min

Kick begins

Kick Detected - - Slow down Seafloor Pump

SEAFLOOR PUMP

SURFACE PUMP

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Influx has stopped andpressures have stabilized


51ATM


640

650

660

670

680

690

700

-30 -20 -10 0 10 20 30 40 50

Time, minutes

Cir

cu

lati

on

Ra

te, g

al/m

in

Kick begins

Kick Detected - - Slow down Seafloor Pump

SEAFLOOR PUMP

SURFACE PUMP

52

Standpipe Pressure vs. Time

0

1,000

2,000

3,000

4,000

5,000

0 200 400 600 800

Time, minutes

Pre

ssur

e, p

si

SMD

ConventionalStart circulating kick out of hole

Kill mud enters top of drillpipe

Kill mud at bit

53

Surface Choke Pressure vs. Time

0

200

400

600

800

1,000

1,200

1,400

0 100 200 300 400 500 600 700

Time, minutes

Pre

ssu

re,

psi

SIDPP

Start circulatingkick out

Top of kick at surface

Kick out of return line

Kill mud enters annulusSICP

CONVENTIONAL

54

Pit Gain vs. Time

0

10

20

30

40

50

60

70

0 100 200 300 400 500 600 700

Time, minutes

Pit

Volu

me

Gai

n, b

bls SMD

Conventional

55ATM

Pressure at the Sea Floor vs. Time

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

0 200 400 600 800

Time, minutes

Pre

ssu

re,

psi

Conventional

SMDStart circulating kick out

Kick at seafloor

Kill mud enters annulus

Kill mud at seafloor

Pressure at inlet to Mudlift System

56

Kick Detection Method Still Valid for RD?

Pit Gain Still valid

Flow Rate Increase (Returns) Much Better

Well Flows With Pump Off Not valid … U-tubing

Fluid Fill-up on Trips More complicated ...

Increased Hook Load Still Valid

Drop in Standpipe Pressure Better

Drop in BHP - MWD Still Valid

Drilling Break Still valid

Decreased Mud Weight (Gas Cut Mud) Still valid

57

Well Control Summary

Most Conventional kick detection methods are still valid

New differential flow rate method - better - use with computer monitoring

Lower standpipe pressure makes kick detection easier and faster

Conventional well control can be adapted - with modifications

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Well Control Summary cont’d

Not necessary to shut well in …

Seafloor pump controls the BHP

Surface choke pressure generally does not affect wellbore pressures

Well control training will be essential

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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 not inside a return line.

60

Summary - cont’d

A “mudlift” system is used in the return line thereby making a “dual-density” mud system possible.

Wellhead pressure is maintained at seawater hydrostatic, so well is “dead” at all times

Well control does not appear to present any serious difficulties, and is quite similar to conventional drilling with a riser.

16. dual gradient drilling

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