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Petroleum Engineering 661 Drilling Engineering

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In dual-gradient drilling the pressure profile in the annulus has two distinct ... Drilling: Circumventing the Size/cost Cycle in Deepwater,' by Allen D. Gault. ... – PowerPoint PPT presentation

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Title: Petroleum Engineering 661 Drilling Engineering


1
Petroleum Engineering 661 Drilling Engineering
  • Lesson 18
  • Dual Gradient Drilling

2
What is Dual Gradient Drilling? (DGD)?
  • In dual-gradient drilling the pressure profile
    in the annulus has two distinct pressure
    gradients
  • An example would be a heavy mud below the
    mudline and a seawater gradient above the mud
    line

3
Conventional Riser Drilling - Wellbore Pressures
FLOATER
SEA WATER HYDROSTATIC
DRILLING RISER
CHOKE LINE
MUD HYDROSTATIC
BOP
SEAFLOOR
DEPTH
PRESSURE
ATM
4
Static Wellbore Pressures
FLOATER
MUD HYDROSTATIC PRESSURE DGD
MUD HYDROSTATIC PRESSURE Conventional
RISER
CHOKE LINE
BOP
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PRESSURE
DGD
5
Dual Gradient Drilling Projects
  • Subsea Mudlift Drilling (SMD)
  • Hollow Glass Spheres
  • Deep Vision
  • Shell

6
Subsea Mudlift Drilling ( SMD ) Note Pump and
Separate Return Line
7
Subsea Mudlift Drilling
  • What is Subsea Mudlift 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?

8
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.
References
HW 10 (due 04-08-02)
9
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
  • Seawater gradient from surface to seafloor
  • Heavier drilling mud inside the wellbore

10
Current SMD Concepts - contd
  • 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

11
Static Wellbore Pressures
FLOATER
MUD HYDROSTATIC PRESSURE DGD
MUD HYDROSTATIC PRESSURE Conventional
RISER
CHOKE LINE
BOP
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PRESSURE
DGD
12
Current Problems
  • Deeper water results in longer 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
16
Equivalent Mud Density, ppg
Fig. 7.21 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
Conventional Riser Drilling - Wellbore Pressures
FLOATER
STATIC PRESSURE
CIRCULATING PRESSURE
BOP
DEPTH
SEAFLOOR
SEA WATER HYDROSTATIC PRESSURE
?PBIT
PRESSURE
ATM
20
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
21
Static Wellbore Pressures
P 0.052 MW Depth For conventional
drilling, Minimum mud wt. 21,000/(0.052
30,000) 13.5 lb/gal Seafloor pressure
0.0528.610,000 4,472 psig For SMD, Minimum
mud weight (21,000 - 4,472)/(0.052 20,000)
15.9 lb/gal
ATM
22
Solution Static Wellbore Pressures
15.1 lb/gal SMD
13.9 lb/gal Conventional
8.6 lb/gal SEA WATER HYDROSTATIC PRESSURE
DEPTH
4,472 psi
21,000 psi
ATM
23
Wellbore Pressures
MUD HYDROSTATIC PRESSURE Conventional
SEAFLOOR
FRACTURE PRESSURE
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PORE PRESSURE
PRESSURE
ATM
24
Wellbore Pressures
MUD HYDROSTATIC PRESSURE SMD
MUD HYDROSTATIC PRESSURE Conventional
SEAFLOOR
FRACTURE PRESSURE
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PORE PRESSURE
PRESSURE
ATM
25
Casing Requirements - Conventional
MUD HYDROSTATIC PRESSURE Conventional
SEAFLOOR
DEPTH
FRACTURE PRESSURE
SEA WATER HYDROSTATIC PRESSURE
PORE PRESSURE
PRESSURE
ATM
26
Casing Requirements - SMD
MUD HYDROSTATIC PRESSURE SMD
SEAFLOOR
FRACTURE PRESSURE
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PORE PRESSURE
PRESSURE
ATM
27
Pressure Considerations
MUD HYDROSTATIC PRESSURE SMD
MUD HYDROSTATIC PRESSURE Conventional
RISERLESS
SEAFLOOR
FRACTURE PRESSURE
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PORE PRESSURE
PRESSURE
ATM
28
Wellbore Pressures - Conventional
FLOATER
STATIC PRESSURE
CIRCULATING PRESSURE
SEAFLOOR
BOP
DEPTH
SEA WATER HYDROSTATIC PRESSURE
?PBIT
PRESSURE
ATM
29
Static Pressures - SMD
FLOATER
ANNULUS AND RETURN LINE
SEAFLOOR
BOP
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PRESSURE
ATM
30
Drillstring Circulating Pressures
CONVENTIONAL
SMD
SEAFLOOR
DEPTH
SEA WATER HYDROSTATIC PRESSURE
??PBIT
PRESSURE
ATM
31
Annulus Circulating Pressures
CONVENTIONAL (13.5 lb/gal)
SMD (15.9 lb/gal)
SEAFLOOR
??PPUMP
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PRESSURE
ATM
32
Circulating Pressures - SMD
DRILLSTRING PRESSURE
ANNULUS AND RETURN LINE
SEAFLOOR
?PPUMP
DEPTH
SEA WATER HYDROSTATIC PRESSURE
?PBIT
PRESSURE
ATM
33
Circulating Pressures - Summary
CONVENTIONAL
?PBIT
PRESSURE
?PML_PUMP
SMD
SEA WATER HYDROSTATIC
DISTANCE FROM STANDPIPE
ATM
34
Transients
  • Transient Behavior when Stopping 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
35
U-Tubing in SMD
FLOATER
SEAWATER HYDROSTATIC PRESSURE
STATIC FLUID LEVEL
MUDLIFT
BOP
ATM
36
Static Pressures - SMD
ANNULUS AND RETURN LINE
DRILLSTRING PRESSURE
SEAFLOOR
DEPTH
SEA WATER HYDROSTATIC PRESSURE
PRESSURE
ATM
37
ATM
38
ATM
39
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40
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
41
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42
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43
  • General Summary
  • Dual Gradient Drlng 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
44
Summary - contd A pumping system is generally
used to feed the return lines, thereby making a
dual-gradient 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
45
Well Control in DGD
  • Review of Conventional Well Control
  • Kick Detection in SMD
  • Kick containment in SMD
  • Behavior of Real Gases in SMD Wells
  • Well Control in SMD

46
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)

47
Circulate Kick out of hole
Wait and Weight Method
Old Mud
Kill Mud
Kick
Keep the BHP constant throughout
48
Gas Kick Behavior - Perfect Gas
Maximum Choke Pressure 1,000 psia. BHP 6,000
psia
0.02 deg F/ft
0.00 deg F/ft
0.01 deg F/ft
49
Shallow Gas Kick - Summary
0.02 deg F/ft
Real Gas Ideal Gas PV
constant
50
Gas Kicks in Deep SMD Wells
MudLine
MudLift Pump
As expected, most of the expansion occurs in the
top 3,000 ft or so
51
Gas Kicks in Deep SMD Wells
PV ZnRT
PV constant
52
Gas Kick Behavior - Z-Factor
Gas Specific Gravity 0.65 (S.G. of air
1)
53
Subsea Mudlift Drilling System
FLOATER
Surface Pump Constant Rate Mudlift Const. Inlet
Press.
SEAWATER HYDROSTATIC PRESSURE
10,000
SEAFLOOR
BOP
MUDLIFT
30,000
KICK
What next??
54
Kick Detection and Control
55
Kick Detection and Control
Influx has stopped and pressures have stabilized
ATM
56
Kick Detection and Control
ATM
57
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58
SICP
CONVENTIONAL
59
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60
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61
Kick Detection Method Still Valid for
DGD? Pit Gain Still valid Flow Rate Increase
(Returns) Much Better Well Flows With Rig Pump
Off … 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) Still valid
62
  • 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

63
  • Well Control Summary contd
  • 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 !!

64
General Summary Subsea Mudlift Drilling is a
method that offers potential for lowering
drilling costs in very deep waters. ( fewer
casing strings, smaller rigs, less time on
location, larger hole ) The method utilizes one
or more small-diameter return lines from the
seafloor to the surface. The drillpipe is not
inside a return line.
65
Summary - contd 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 is quite
similar to that in conventional drilling with a
riser - There are some real
advantages higher margins, choke line DP, etc.
66
Gas Lift? Glass Beads?
Pumps? Gas Lift? Glass Beads?
Dual Gradient Alternatives
How to Handle Connections? Trips?
Rotating BOP
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