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The Real Causes of Borehole Misbehavior
and Operational Practices to Eliminate Them
Fred E. Dupriest, P.E.
Professor of Engineering Practices
Texas A&M University
Society of Petroleum Engineers
East Texas Chapter, Tyler
December 9, 2014
2
Three major performance limiters (cause of reduced ft/day)
• Whirl-induced borehole patterns
• Instability
• Hole cleaning
Borehole Misbehaviors That Limit Performance (ft/day)
3
Gulf of Mexico Sakhalin Australia South Texas
No
n-b
it L
imit
ers
(Co
ntr
ol D
rillin
g)
V
ibra
tio
ns
MSEadj MSEadj
MSEadj
MSEadj
Control Drilling (Efficient Bit, non-bit limiters)
Whirl and
Stickslip
Whirl is Common, no Matter What Your Downhole Tools Say
SPE 102210
Blue Curve: Mechanical Specific Energy (MSE)
SPE 67818
Borehole appears 1.5” undergauge every 4 ft
All BHAs that whirl attempt to create
patterns, even those with low levels of whirl.
The result is:
• Inability to transfer weight to steer bent motors
• Effects of reduce weight transfer on ROP
• Reaming on connections and trips for tight
hole
• Incorrect belief that short trips are required due
to drag
• Misdiagnosed as inadequate hole cleaning,
which leads to circulating time and control
drilling
• Inability to properly centralizer casing
• Cement channels and poor zone isolation
• Poor log evaluation
• Stuck BHA or casing
Tight Hole is Caused by “Borehole Patterns” Created by Whirl
3D
Image
5
Whirl is best thought of as a sine wave
that develops in the BHA when we rotate
The bit is part of the wave
All BHAs have some amplitude of wave,
so all bits are effected by whirl, no matter
what the LWD tools say
- Reduce ROP
- Reduce Bit life
- Borehole Patterns
LWD tools tell you what vibrations are
doing to the BHA. MSE tells you want
they are doing to the bit
Whirl Is a Sine Wave in the BHA with a Bit Screwed On the End
We eliminate patterns by
eliminating whirl
SPE 134580
Maximize WOB to Constrain Lateral Bit Motion
SPE 14ATCE 2328
Run step tests to maximize WOB. Increased rock exposure from
depth of cut increases resistance to the lateral force from BHA wave
Run RPM Step Tests to Identify Low-Whirl Rotating Speeds
“Fingerprint” whirl vs RPM as soon as you get your BHA out of the shoe. Use
MSE to avoid RPM ranges with higher whirl
Extend the Gauge Length
Gauge Length
Increased gauge length limits the rate of side-cutting. Patterns become
more likely to decay than gain in amplitude
If the gauge area does not touch the curvature of the wellbore, it
cannot be interfering with the build rate.
Run 4-6” gauge in wells with 0-5 deg/100ft of build curvature
Above 5 deg/100ft, work with vendor on specific gauge design. It
depends on the type of profile
SPE 119625
If the gauge does
not make contact
at the planned
curvature, the
gauge does not
effect steering
9
Engineering Redesign: MSE Declines with Gauge Length
0
200
11250 10750 11000 10500
MSEadj
ksi
Dept (ft)
0
200
MSEadj
ksi
11250 10750 11000 10500 Depth MD (ft)
2 inch gauge
6 inch gauge
0
250
MSEadj
ksi
0
250
MSEadj
ksi
6500 ft MD 9500 ft MD
4 inch gauge
6 inch gauge
MSE pattern is similar, but reduced
Less effect in softer formations where amplitude is lower to start with
Softer Harder
SPE 119625
Comparison of well with
different gauge lengths
Lower MSE means longer bit
life, significantly higher ROP
for the same WOB, and
reduced amplitude in spiral
patterns
Minimize AKO in Bent Motors, or Use RSS
0.78 AKO
Straight Motor
IPTC 10706
Sliding Rotating
Borehole Instability Sequence of Events
1. Hole is cut in stressed rock. Stress
attempts to close the hole in the
radial direction (reduce the diameter
a few thousandths)
2. Circumference is compressed
tangentially as the radius declines
3. Hole diameter stops declining when
the tangential stress (Hoop Stress)
balances the radial
4. If the Hoop Stress exceeds the rock
strength before the stresses come
to equilibrium, the rock breaks and
the hole enlarges
5. If the Hoop Stress does not exceed
the rock strength, there is no
breakout and the hole remains
gauge
6. We reduce hoop stress by
increasing the internal pressure
(MW), which expands the
circumference
Resisting Pressure
Increased Circumferential Compression
Length of circumference for bit size
12
Borehole Contraction is Not the Cause of Tight Hole
How much does the borehole shrink due to stress changes?
Dia x P = Dia Change
E
ECD Example:
8.5 x 500 psi = 0.00212 in
2,000,000
Extremely low FCS Example:
8.5 x 2500 psi = 0.0106 in
2,000,000
Stress Contraction?
Stress contraction may occur during many operations. The magnitude
is usually very small • ECD lost on connection after each stand down
• MW cut
• BHP falls to equal FCS after losses
• Very ductile evaporates may creep over time (salt, anhydrite)
• Shale hydration results in small shale volume change (but also reduced
strength and enlargement)
13 13
Stress Contraction is Limited by Failure
Slight contraction of the ID will occur as stress tries to close the hole. But this deformation is not enough to cause tight hole in most shales or sands. The amount of stress and contraction that would cause tight hole is almost always sufficient to break the rock. The hole gets larger not smaller
Hole Shrinkage Prior to Enlargement
x Ductile or “Plastic” Rock
Brittle Rock
x
Failure and Breakout
Str
ess
Deformation
Result of
moderate stress
Instability limits
stress contraction
14
Angular or Splintery
Raise MW 0.2 - 0.5 ppg
Photo-document and Analyze Your Shaker Material
Tabular or Blocky
Raise MW 0.5 - 1.0 ppg
σ1 From the side of the hole with the
lowest stress anisotropy
(i.e., top of high angle hole)
From side of the hole with
the highest anisotropy
(i.e., side of high angle hole)
Higher
Anisotropy
Lower
Anisotropy
Third stress (into the page) is
the same for both side
15
Higher MW Does Not Destabilize Fracture Shales
Original Hole
MW = 9.2 ppg OBM
Sidetrack
MW = 11.7 ppg OBM
0 10 20 30 40
2750
2800
2850
2900
2950
3000
BGT - Kaliber Log
Bohrung: Walsrode Z5 1.Loch
Kaliber (in)
B
o
h
r
t
e
u
f
e
(
m
)
0 10 20 30 40
2750
2800
2850
2900
2950
3000
EMM - Kaliber Log
Bohrung: Walsrode Z5 2.Loch
Kaliber (in)
B
o
h
r
t
e
u
f
e
(
m
)
Naturally fractured shale
confirmed with core
The 11.7 ppg fully stabilized the non-fractured shales and reduced the enlargement in the
fractured shales. Full stabilization of the fractured shale would have required 13.0 ppg,
but integrity in shallower zones was inadequate to use this MW. Instability may also be
time dependent in fractured shale, even with a NAF.
Hole Diameter (inch)
Dep
th (
m M
D)
Hole Diameter (inch) D
ep
th (
m M
D)
SPE 128728
9.2 ppg 11.7 ppg
Caliper 2,4 -------
Caliper 1,3 -------
Non-fractured shale
16
High Angle Physics
Equilibrium Bed Height
(Fluid Force > Force required to roll)
Dynamic
fluid force
Gravity Buoyancy
Dynamic
fluid force
Gravity
Buoyancy
No net lifting force,
falls rapidly
Once it hits bottom, how
does it get off? It doesn’t.
It rolls or skips.
SPE 28306
Fluid Shear
Rolling
High Angle (60-90) deg
Resistive force
17
Enlargements Dominate Safe ROP for Hole Cleaning
If drill rate is limited by hole cleaning, you probably have enlargements. The safe
ROP in gauge hole may be 3x that in even moderately enlarged hole
High angle boreholes fill to the same flow area as gauge hole, not the same bed
height. If this large stored mass is mobilized, it will not fit in the gauge hole
above
A1
A1 A2
Equilibrium
Bed Height
A2
SPE 134580
Manage transient events that mobilize the stored mass: 1) rapid changes in
pump rate, 2) rapid string movement, 3) pulling/washing BHA into stored mass
18
Redesign What is Preventing You from Raising MW
Effect of MW on ROP
Higher MW increases the effective stress in the rock exposed beneath the bit, which
causes the rock’s compressive strength to increase slightly. The resulting small loss of
depth of cut can be offset by raising WOB
Differential Sticking
Differential sticking can be eliminated, regardless of MW used. The industry continues
to have sticking events because they do not consistently use the practices required to
eliminate them. See SPE 128129
Lost Circulation
Effective practices exist for preventing or responding to lost circulation. In general,
losses can be stop, while instability is irreversible. See SPE 163481
Formation Damage
If filtrate is damaging to well productivity, you simply have the wrong fluid. If whole
mud is entering and particles are plugging pore throat use properly engineered
blocking solids to prevent fines migration.
19
1. Tight hole is caused primarily by whirl-induced patterns.
Reduce whirl
2. Hole enlargement is caused primarily by inadequate MW.
Raise the MW
3. Poor hole cleaning is caused by enlarged hole.
Raise the MW
Key Take-Aways
Recommended