16. Dual Gradient Drilling

transcript

TAMU - PemexOffshore Drilling

Lesson 16 Dual Gradient Drilling

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?

Subsea Mudlift Drilling or “Riserless

Drilling”Drillstring

Mud Return

…sometimes called

“Dual Gradient Drilling”or

“Dual Density Drilling

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

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

Subsea Mudlift drilling in three phases

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

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)

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.

“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?

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

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

Apparent Pressures in the Wellbore

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

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

Apparent Pressures in the Wellbore

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

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

Gas Lift?Glass Beads?

How to HandleConnections?Trips?

Gas Lift?Glass Beads?

Rotating BOP

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.

Conventional Riser Drilling - Static Wellbore Pressures

SEAFLOOR

SEA WATER HYDROSTATIC

PRESSURE

MUD HYDROSTATIC

FLOATER

DRILLING RISER

CHOKE LINE

Conventional Riser Drilling - Wellbore Pressures

SEAFLOOR

PRESSURE

HSTATIC PRESSURE

CIRCULATING PRESSURE

FLOATER

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

Alternate SMD Configuration- Wellbore Pressures

PRESSURE

MUD HYDROSTATIC

PRESSURE SMD

MUD HYDROSTATIC

PRESSURE Conventional

FLOATER

CHOKE LINE

Static Wellbore Pressures

SEAFLOOR

PRESSURE

MUD HYDROSTATIC

PRESSURE Subsea Mudlift

MUD HYDROSTATIC

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?

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

Wellbore Pressures

SEAFLOOR

FRACTURE PRESSURE

PORE PRESSURE

PRESSURE

MUD HYDROSTATIC

SEAFLOOR

FRACTURE PRESSURE

PORE PRESSURE

MUD HYDROSTATIC

PRESSURE Subsea Mudlift

PRESSURE

MUD HYDROSTATIC

Wellbore Pressures

Casing Requirements - Conventional

SEAFLOOR

FRACTURE PRESSURE

PORE PRESSURE

PRESSURE

MUD HYDROSTATIC

Casing Requirements - Subsea Mudlift

SEAFLOOR

FRACTURE PRESSURE

PORE PRESSURE

MUD HYDROSTATIC

PRESSURE

Subsea Mudlift

PRESSURE

Pressure Considerations

SEAFLOOR

FRACTURE PRESSURE

PORE PRESSURE

MUD HYDROSTATIC

PRESSURE

Subsea Mudlift

PRESSURE

MUD HYDROSTATIC

PRESSURE

ConventionalSubsea Mudlift

Wellbore Pressures - Conventional

SEAFLOOR

PRESSURE

STATIC PRESSURE

CIRCULATING PRESSURE

FLOATER

Static Pressures- Subsea Mudlift Drilling

SEAFLOOR

PRESSURE

ANNULUS AND RETURN LINE

FLOATER

Drillstring Circulating Pressures

SEAFLOOR

PRESSURE

CONVENTIONAL

Subsea Mudlift

Annulus Circulating Pressures

SEAFLOOR

PRESSURE

CONVENTIONAL (13.2 lb/gal)

Subsea Mudlift (15.5 lb/gal)

Circulating Pressures - Subsea Mudlift Drilling

SEAFLOOR

PRESSURE

DRILLSTRING PRESSURE

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

MUDLIFT

U-Tubing in Subsea Mudlift Drilling

PRESSURE

STATIC FLUID LEVEL

FLOATER

Static Pressures - Subsea Mudlift Drilling

SEAFLOOR

PRESSURE

DRILLSTRING PRESSURE

U-Tubing Rate vs. Time after Pump OFF

0 5 10 15 20 25 30

Elapsed Time, min

pm 3-in ID

4.276-in ID6-in ID

Fluid Level vs. Time after Pump OFF

0 5 10 15 20 25 30

Elapsed Time, min

evel i

ipe, f

3-in ID 4.276-in ID6-in ID

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)

Circulate Kick out of hole

Keep the BHP constant throughout

Circulate Old Mud out of hole

Keep the BHP constant throughout

MUDLIFTBOP

Subsea Mudlift Drilling System

SEAFLOOR

PRESSURE

FLOATER

30,000’

10,000’

Surface Pump: Constant RateMudlift: Const. Inlet Press.

Kick Detection and Control

-30 -20 -10 0 10 20 30

Time, minutes

Kick begins

Kick Detected - - Slow down Seafloor Pump

SEAFLOOR PUMP

SURFACE PUMP

Influx has stopped andpressures have stabilized

Kick Detection and Control

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

Time, minutes

Kick begins

Kick Detected - - Slow down Seafloor Pump

SEAFLOOR PUMP

SURFACE PUMP

Standpipe Pressure vs. Time

0 200 400 600 800

Time, minutes

ConventionalStart circulating kick out of hole

Kill mud enters top of drillpipe

Kill mud at bit

Surface Choke Pressure vs. Time

0 100 200 300 400 500 600 700

Time, minutes

Start circulatingkick out

Top of kick at surface

Kick out of return line

Kill mud enters annulusSICP

CONVENTIONAL

Pit Gain vs. Time

0 100 200 300 400 500 600 700

Time, minutes

bls SMD

Conventional

Pressure at the Sea Floor vs. Time

0 200 400 600 800

Time, minutes

Conventional

SMDStart circulating kick out

Kick at seafloor

Kill mud enters annulus

Kill mud at seafloor

Pressure at inlet to Mudlift System

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

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

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

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.

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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