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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
6
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
14
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
18
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
19
20
21
Gas Lift?Glass Beads?
How to HandleConnections?Trips?
Gas Lift?Glass Beads?
Rotating BOP
22
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
SEA WATER HYDROSTATIC
PRESSURE
PRESSURE
DE
PT
HSTATIC PRESSURE
CIRCULATING PRESSURE
PBIT
BOP
FLOATER
25
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
SEA WATER HYDROSTATIC
PRESSURE
PRESSURE
DE
PT
H
MUD HYDROSTATIC
PRESSURE SMD
MUD HYDROSTATIC
PRESSURE Conventional
BOP
FLOATER
RISER
CHOKE LINE
SMD
27
Static Wellbore Pressures
SEAFLOOR
SEA WATER HYDROSTATIC
PRESSURE
PRESSURE
DEPTH
MUD HYDROSTATIC
PRESSURE Subsea Mudlift
MUD HYDROSTATIC
PRESSURE Conventional
28
Static Wellbore Pressures
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?
29
Static Wellbore Pressures
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
30
Wellbore Pressures
SEAFLOOR
FRACTURE PRESSURE
PORE PRESSURE
SEA WATER HYDROSTATIC
PRESSURE
PRESSURE
DEPTH
MUD HYDROSTATIC
PRESSURE Conventional
31
SEAFLOOR
FRACTURE PRESSURE
PORE PRESSURE
MUD HYDROSTATIC
PRESSURE Subsea Mudlift
SEA WATER HYDROSTATIC
PRESSURE
PRESSURE
DEPTH
MUD HYDROSTATIC
PRESSURE Conventional
Wellbore Pressures
32
Casing Requirements - Conventional
SEAFLOOR
FRACTURE PRESSURE
PORE PRESSURE
SEA WATER HYDROSTATIC
PRESSURE
PRESSURE
DEPTH
MUD HYDROSTATIC
PRESSURE Conventional
33
Casing Requirements - Subsea Mudlift
SEAFLOOR
FRACTURE PRESSURE
PORE PRESSURE
MUD HYDROSTATIC
PRESSURE
Subsea Mudlift
SEA WATER HYDROSTATIC
PRESSURE
PRESSURE
DEPTH
34
Pressure Considerations
SEAFLOOR
FRACTURE PRESSURE
PORE PRESSURE
MUD HYDROSTATIC
PRESSURE
Subsea Mudlift
SEA WATER HYDROSTATIC
PRESSURE
PRESSURE
DEPTH
MUD HYDROSTATIC
PRESSURE
ConventionalSubsea Mudlift
35
Wellbore Pressures - Conventional
SEAFLOOR
SEA WATER HYDROSTATIC
PRESSURE
PRESSURE
DE
PT
H
STATIC PRESSURE
CIRCULATING PRESSURE
PBIT
BOP
FLOATER
36
Static Pressures- Subsea Mudlift Drilling
SEAFLOOR
SEA WATER HYDROSTATIC
PRESSURE
PRESSURE
DE
PT
H
ANNULUS AND RETURN LINE
BOP
FLOATER
37
Drillstring Circulating Pressures
SEAFLOOR
SEA WATER HYDROSTATIC
PRESSURE
PRESSURE
DEPTH
CONVENTIONAL
Subsea Mudlift
PBIT
38
Annulus Circulating Pressures
SEAFLOOR
SEA WATER HYDROSTATIC
PRESSURE
PRESSURE
DEPTH
CONVENTIONAL (13.2 lb/gal)
Subsea Mudlift (15.5 lb/gal)
PPUMP
39
Circulating Pressures - Subsea Mudlift Drilling
SEAFLOOR
SEA WATER HYDROSTATIC
PRESSURE
PRESSURE
DEPTH
ANNULUS AND RETURN LINE
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
41
MUDLIFT
U-Tubing in Subsea Mudlift Drilling
~SEAWATER HYDROSTATIC
PRESSURE
BOP
STATIC FLUID LEVEL
FLOATER
42
Static Pressures - Subsea Mudlift Drilling
SEAFLOOR
SEA WATER HYDROSTATIC
PRESSURE
PRESSURE
DEPTH
ANNULUS AND RETURN LINE
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
44
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
45
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)
46
Circulate Kick out of hole
Keep the BHP constant throughout
47
Circulate Old Mud out of hole
Keep the BHP constant throughout
48
MUDLIFTBOP
Subsea Mudlift Drilling System
SEAFLOOR
~SEAWATER HYDROSTATIC
PRESSURE
FLOATER
30,000’
10,000’
KICK
Surface Pump: Constant RateMudlift: Const. Inlet Press.
49
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
50
Influx has stopped andpressures have stabilized
Kick Detection and Control
51ATM
Kick Detection and Control
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
58
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
59
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.