Module L:
More Rock Mechanics
Issues in Drilling
Argentina SPE 2005 Course on
Earth Stresses and Drilling Rock Mechanics
Maurice B. Dusseault
University of Waterloo and Geomec a.s.
“Predicting” Onset of Instability





Now, we have methods of estimating in situ
stress conditions
Also, we have methods of measuring or
estimating strength
Furthermore, we have methods of
calculating stresses around a circular
opening, subject to several assumptions…
Putting this together allows prediction of
shearing initiation on the borehole wall
…An estimate of “breakouts initiation”
Linear Poroelastic Borehole Model…
p w ]cr
Eqn:
 Where:

 pw]cr
31  3  A  p  UCS  (N  1)pi

N1
critical wellbore pressure, shear initiation
 pi
pressure just inside the borehole wall
 σ1, σ3
largest, smallest ppl σ in borehole plane
A
= α(1-2)/(1-) ( = Poisson’s ratio)
α
Biot’s coefficient (1.0 for soft rocks)
N
friction coefficient = (1 + sin’)/(1 - sin’)
 UCS,  Unconfined Compressive Strength,
friction angle (MC yield criterion)
 Δp
“drawdown” = pi - po
Discussion of Parameters

pw
pi
pw – pi is support pressure
po
radius - r
Usually, we ignore effects of “α”, except in low
porosity, stiff shales (E > 30-40 GPa)
 UCS and N are equivalent to the c’, ’ of the
linear MC yield criterion for shear
 Poisson’s ratio for shales, 0.25 to 0.35
 σ1, σ3 are computed using equations converting
3-D stress to stresses in the plane of the
borehole (90° to hole axis)

Control Parameters in Drilling
Mud weight, mud rheological properties,
the geochemistry of the filtrate, cake
quality, mud type (WBM, OBM, foam, etc.)
 LCM content, type and gradation
 Tripping and connection practices:

 Surging

(run-in), swabbing (pull-out) pressures
Drilling parameters:
 ROP,
bit type…
Hydraulics and hole cleaning
 ECD (BHA characteristics, mud properties)
 Well trajectory, and maybe a few others

Defining Limits in Our Well Plan
Gradient
Pressure or stress
Predicted MW for onset of
unmanageable sloughing
hmin, danger of LC
v
Onset of ballooning
in shale zones
v
po, onset of blowout if
in a sand zone
Depth
Depth
How are the Limits Defined?

Lower MW limit
 Pressure
control
 Rock Mechanics stability, experience, use of
correlations to predict stability line, etc.
 How much sloughing can we live with?
 Underbalanced Drilling is a good example of RM

Upper MW limit
 Avoiding
massive lost circulation
 Fracture gradient, earth stresses analysis
 Effects on ROP
 The new concept of overbalanced drilling is an
example of RM extending this envelope
Are All Limits Absolute?






No, and here are examples:
Drilling underbalanced? OK as long as it is
shales or lower permeability sands, and if
the shales are strong (little sloughing)
Drilling overbalanced? OK for up to ~1000
psi with properly designed LCM in mud!
Drilling below sloughing line? OK if good
hole cleaning, use increased MW for trips…
Pushing the envelope is typical in offshore
drilling, HPHT wells… (e.g. mud cooling…)
Vigilance and RM understanding needed…
Example: Drilling Underbalanced

It is a Rock Mechanics issue, a pore
pressure issue, and a fluids type issue
 If
the shale is strong enough to be self
supporting in a bore hole with a negative r
 If the pore pressure is not so high that it
“blows” sand and shale into the borehole
 If the fluids that enter the hole are “safe”, i.e.,
not oil and gas in large quantities

Excellent for drilling through depleted
zones, fast drilling through good shale,
entering water sensitive gas-bearing
strata, reservoirs that are easy to damage
Underbalanced Stress Conditions
 – stress
q
High shear
stress at the
borehole wall
hmin = HMAX
r
po
pw
pw < p o
radius
Some tensile stress exists near the hole wall
in underbalanced drilling because po > pw
Mud Rheology
High gel strength can
cause mud losses on
connections, trips
 Increases surge and
swab effects when BHA
is in a small dia. Hole
 Also affects ECD
 Mud rheology & density
can be changed for trips
to sustain hole integrity
 Hydraulics is a vital part
of borehole stability!

Shearing resistance
Mud Rheology Diagram
YP
Static condition
m – mud
viscosity
Yield
point
Dynamic conditions
Shearing rate
Effect of Mud Weight Increase
, shear stress
MC failure line

yield
Mohr’s circle
of stresses
 max  c   n tan  
no yield
c
r
a
n, normal stress
Increasing MW (with good cake) reduces the stresses on the wall
Effect of Loss of Good Filter Cake
, shear stress
failure
MC failure line

Mohr’s circle
of stresses
 max  c   n tan  
c
r
a
n, normal stress
With loss of mudcake effect, radial support disappears, shear stress increases
Stresses and Drilling
To increase hole stability, the
best orientation is that which
minimizes the principal stress
difference normal to the axis
Favored hole
orientation
v
60-80° cone
HMAX
hmin
v
Drill within a 60°cone
(±30°) from the most
favored direction
HMAX
hmin
v >> HMAX > hmin
HMAX ~ v
>> hmin
v
HMAX
hmin
HMAX >> v > hmin
Uncontrollable Parameters
Constrained trajectory (no choice as to the
wellbore path)
 Sequence of rock types (stratigraphy)
 Rock strength and other natural properties

 Fractured
shales
 Clay type in shales (swelling, coaly, fissile)
 Salt, etc.
Formation temperatures and pressures,
plus other properties such as geochemistry
 Natural earth stresses and orientations

Can You Live with Breakouts?
Yes, in most cases the breakouts are a
natural consequence of high stress
differences, and can be controlled
 In exceptional cases, the breakouts are so
bad that massive enlargement takes place
 If hole advance is necessary, there are
special things that can be done:

 Some
new products, silicates, polymers that set
in the hole and can even be set and then drilled
 Increase MW, even to the point of overbalance
 Gilsonite and graded LCM can help somewhat
 In desperation, set casing!
Some Diagnostic Hole Geometries
General
sloughing and
washout
a.
Swelling,
squeeze
b.
drill
pipe
d.
HMAX
Keyseating
hmin
e.
Breakouts
c.
c.
Fissility
sloughing
Induced by high
stress differences
f.
Only breakouts are symmetric in one
direction with an enlarged major axis
Equivalent Circulating Density





Viscous resistance increases the apparent
mud weight at the bottom of the hole
This is a kinematic (viscosity) effect, and
takes place only as the mud is circulating
ECD can lead to fracture at the bit though
static pressure of mud column is below PF
As high as 2.0#/gal recorded in 4¾” hole!
Real-time BHP pressure data allow it to be
measured and managed (offshore drilling)
 This
leads to early warnings of high ECD
 This leads to better control and mitigation
ECD
Pressure gradient plot
15
16
17
18
19 ppg
PF (hmin)
MW = 16.7 ppg
(static value)
mud rings also increase ECD
Dynamic pressure (ECD)
because of friction, hole
restrictions, high mud m
A hydraulic fracture is
induced at the base of
the hole where the ECD
exceeds PF (hmin).
When the pumps stop,
much of the mud comes
back into the hole!
Depth
BHA and collars
reamers and stabilizers
High ECD!
ECD
pBH = mud weight plus friction p loss
 High ECD values (>0.5 ppg) are related to:

 High
mud viscosities and gel strengths (evident
on connections and trips as “breathing” of hole)
 Rapid slim hole drilling leading to large cuttings
loads in the drilling fluids near the bit
 Limited clearance with BHA (MWD system),
reamer system, extra large collars…
 Sloughing of shales leading to partial mud rings
or high cavings loads in the mud

Reducing ECD is the same as expanding
your safe MW window for drilling!
High ECD Effects
15
16
po
17
18
19 ppg
PF = hmin
Gradient
plot
Top of restrictive BHA
MW = 16.7 ppg
(static value)
reamers
Cannot reduce the MW much
because of borehole instability
uphole or blowout danger on
trips, connections, gas cutting…
Dynamic pressure (ECD)
because of friction, hole
restrictions, high mud m
stabilizers
Large mud losses at hole bottom
because of fracturing
Depth
BHA and collars
mud rings
High ECD!
Reducing High ECD Values
High ECD: excessive ballooning, high losses,
increased risk, reducing the drilling window
 The high ECD values can be reduced in
several ways, here are a few examples:

 Reduce
the mud weight (careful about gas cuts!)
 Reduce the viscosity and gel strength
 Avoid sloughing above bit (increases ECD)
 Circulate out cavings and cuttings as needed
 Use less restrictive BHA, reduce ROP
 Use an off-center bit (lower friction losses)
 Redesign well plan (one less casing, larger hole)
 OBM probably somewhat better than WBM
North Sea ECD Example
Serious ECD problems,
but extra depth needed
 Very long & restrictive
BHA was being used
 Drill (mud motor) to Z
with 8.5” hole size
 Trip out, replace bit
with eccentric 9¾” bit
 Ream to bottom & trip
 Drill to TD with the 8.5”
drill bit size
 Set 7” casing to TD

10¼”
casing
High ECD Underream
Drill to TD
Some Other Comments on ECD





If high drill chip loads from rapid ROP are
contributing to ECD, reduce ROP
Lower viscosity and gel strength during
drilling, but increase it a bit for trips
Break the gel strength of the mud during
trips by pumping, rotating pipe as you are
breaking circulation
Be careful in inclined and horizontal holes
where pipe is not being rotated much,
better to rotate more aggressively
Use LCM in mud to plug fractures
ECD Services
Example of output
from BHI service
 MWD gauges used
 Gives ECD, MW,
annular pressure,
connection effects…
 This data can be used
in a diagnostic manner
during drilling to
manage ECD and aid
well performance
 This website gives
many useful formulae

http://www.tsapts.com.au/formulae_sheets.htm
Drilling and Shale Fissility

If a hole is within 20°
of strong fissility…





Sloughing is more likely
Shale breaks like small
brittle beams
Breakouts can develop
deep into strata
In this GoM case, in
the “tangent” section,
the hole angle was 61°
Vertical offset hole,
no problems
bedding
direction
Courtesy Stephen Willson, BP
Coping with Fissile Shale Sloughing
If possible, stay at least 30° away from
the fissility dip direction (see sketch)
 Otherwise, keep your mud properties
excellent, keep circulation rate & ECD low,
gilsonite and fn-gr LCM in mud may help…

100-120° cone
Keep the drillhole within this
cone to avoid severe fissility
sloughing problems
Normal to bedding planes
DENSITY NEUTRON IMAGE OF
12500’ MD SHALE BREAK OUT
From: Bruce
Matsutsuyu
SECTION OF
SHALE
BREAKOUT
Note that the
majority of the
shale sloughing
appears to be
from the top of
the borehole.
Density Neutron Image
PHOTOELECTRIC
FACTOR CURVES
BOTTOM OF
BOREHOLE
GR
DENSITY
CURVES
Drilling Through Faults

The fault plane region is often:
 Broken,
sheared, weak shales and rocks
 It may have a high permeability
 It can be charged with somewhat higher po

First, expect the faults from your data:
 Seismic
data analysis
 Near salt diapirs, especially shoulders
Accurate mud V(t) measurements can be
of great value to good drilling
 Cavings monitoring
 MWD (ECD, resistivity, bit torque…)

Borehole Shear Displacement

High angle faults, fractures can slip and
cause pipe pinching
 Near-slip
earth stresses condition
 High MW causes pw charging
 Reduction in n leads to slip
 BHA gets stuck on trip out
n
pw
Can be identified from borehole wall sonic
scanner logs (profile logs)
 Raising MW makes it worse! Lowering MW
is better…
 Also, LCM materials to plug the fault or
joint plane are effective

Slip of a High-Angle Fault Plane
borehole
v = 1
h = 3
high pressure
transmission
slip of joint surface
slip of joint
(after Maury, 1994)
casing bending
and pinching in
completed holes
pipe stuck on trips
Slip Affected by Hole Orientation!
OFFSET ALONG PRE-EXISTING DISCONTINUITIES
FILTRATE
75
70
Effective normal stress (bar)
Azimuth:
0
10
20
30
40
50
60
70
80
90
65
60
55
50
45
40
0
10
20
30
40
50
60
Inclination () (deg)
TYPICAL
MUD
OVER-PRESSURE
Courtesy Geomec a.s.
70
80
90
100
Diagnostics for Fault Slip Problems
In tectonic areas, near salt diapirs…
 On trips, BHA gets stuck at one point
 Easy to drop pipe, hard to raise it
 Borehole scanner shows strange shapes:
not the same as keyseating or breakouts

drill pipe
Start of keyseat
Serious keyseat
Evidence of fault plane slip
Curing Fault Plane Slip Problems





Usually occurs up-hole in normal faulting
regimes that are highly faulted, jointed, as
MW is increased to control po downhole
May occur suddenly near the bit when a fault
is encountered
Back-ream through the tight zone
High pw contributes to the slip of the plane,
thus reduce your MW if possible
Condition the mud to block or retard the
flow of mud pressure into the slip plane:
 Gilsonite,

designed LCM in the mud
Use an avoidance trajectory for the well
Mud Volume Measurements
Extremely useful, but,
accurate V/t needed
 Case A: fracture/fault
encountered, quickly
blocked, now analyze
data for k and aperture!
 Case B: fractured rock
not healed by LCM
 Other cases have their
own typical response
curves (ballooning, slow
kick…)
 Diagnostics!

Losses - gpm
20
A
15
10
5
Hole deepening rate
Filtration
fluid loss
0
5
Time - min
6
7
8
9
B
Losses - gpm
20
15
10
5
Hole deepening rate
Filtration
fluid loss
0
5
Time - min
6
7
8
9
A Precise Mud Volume Installation
Outlet mud line
Precision flow meter
(taken from SPE 38177 - Agip well)
Actual Field Example of Analysis
Hydraulic Aperture (mm)
Depth (m)
0
0.5
Average permeability (D/m)
0
1
2890
2890
2910
2910
2930
2930
2950
2950
2970
2970
2990
2990
3010
3010
3030
3030
3050
3050
20
40
This information proved extremely valuable for reservoir
engineers in this case, as a gas reservoir was found
60
Courtesy
Geomec a.s
Losses Identify Fractured Zones
(L/min)
MudQLoss
loss Rate – litres/min
70
60
35 liters
22 liters
50
19 liters
25 liters
40
30
20
10
0
-10
Likely, each event involved filling a single fracture
-20
4101.5
4101.7
4101.9
4102.1
Depth- (m)
Depth
m
4102.3
4102.5
4102.7
Problems in Coal Drilling

OBM are worse than WBM in Coal
 Filtrate

Coal fractures open easily if pw > po
 Coal

penetrates easily (oil wettability)
is extremely compressible
Difficult to build a filter cake on the wall
 Fissure
apertures open with surges
Sloughing on trips, connections, large
washouts, …
 Packing off of cuttings and sloughed Coal
around the pipe, even during trips

Drilling in Coal
stresses around wellbore
q
r
Mud rings and pack-off caused by
slugs of cavings and cuttings
Deep pore pressure penetration
because of coal fractures
Massive sloughing
fracture-dominated coal
Drilling Fractured Coal Safely





Keep jetting velocities low while drilling
through the coal (avoid washouts)
Keep MW modest to avoid fractures
opening and coal pressuring, low ECDs while
the BHA is opposite the coal seams
Drill with graded LCM in the mud to plug
the fractures and build a cake zone
Avoid swabbing and surging on trips
See Appendix to Module H for some
results on drilling overbalanced with LCM
A Case History of Salt Diapir
Drilling in the North Sea
North Sea Case, Shallow Depth
Well A
1a
2000 m
Shallow Gas
Gas Pull Down
Courtesy Geomec a.s.
Above a Deep Diapir, North Sea





Normal faulting observed well above the
top of the diapir, these will likely be zones
of substantial mud losses (low hmin)
Beds are distorted, likely shearing has
occurred along the bedding planes (weaker)
Seismic data show strong “gas pull-down
effect”, lower seismic velocities because of
free gas in the overlying shales and high po
Free gas zones are noted in the strata, and
these will increase gas cuts
(Gas “pull-down” refers to the effect of
free gas on seismic stratigraphy)
Deeper, Around the Diapir
This region avoided
1b
Well A
Gas Pull Down
2000 m
Mid-Miocene regional pressure boundary
Top Balder
Top Chalk
Intra Hod/Salt
3000 m
Courtesy Geomec a.s.
MWD RESISTIVITY LOG SIGNATURE (OBM)
Well A
Depth (m MD-RKB)
MWD Resistivity (Ohm.m)
2540
100
2560
2580
2600
2620
2640
2660
10
Invaded Zone
SESP
SEDP
1
0.1
Time-lapse and different spacing resistivity
logging data identified fractured zone clearly
Courtesy Geomec a.s.
INVADED ZONE Symmetry(O-B)
Well A
1.6
Ratio SEDP/SESP (Ohm.m)
1.4
1.2
1
0.8
0.6
0.4
0.2
0
2540
2560
2580
2600
2620
2640
2660
Depth (m MD-RKB)
Courtesy Geomec a.s.
What Was Done to Improve Drlg?

A trajectory was chosen to avoid the worst
of the crestal faulting and gas pressures
 Shales
also intersected at ~ 90 to fissility
Mud losses were carefully monitored with
depth in the critical zones, then analyzed
 Designed LCM in the mud allowed a bit of
overbalance in a critical region
 Of course, gas cuts, shale chip geometry,
total cutting volumes, etc., and many other
things were monitored in “real-time”

Statfjord Case: North Sea
STATFJORD
OVERBALANCED!
Mud Pressure minus
stress in MegaPascals
-800 psi
6
5
4
3
2
1
0
B-06B
B-23AT2
B-39A
B-39BT2
Well
These wells were drilled with overbalance: a MW
above the lowest estimated hmin in the zone
Courtesy Geomec a.s.
Conclusions
Fracturing pressure can be increased by
several 100 psi by graded LCM, analysis
 Young’s modulus (E) is the control parameter
 Induced fractures or even natural fractures
encountered open up almost immediately to
their final width:

 This
aperture controls LCM design
The plugging happens rapidly with right LCM
 The effect is enhanced with high viscosity
mud and slower ROP
 Design tools are available for this

A Well Plan, North Sea
• classical mud weight window is too narrow; cannot avoid
instability
• low mud weight  breakouts
• high mud weight  destabilized fractured zones & losses
• breakout problems are controllable by good hole cleaning;
fracture zones are uncontrollable
Strategy:
• keep mud weight low
• manage breakouts with good hole cleaning before
increasing mud weight during trips
• monitor cavings and mud losses for warning of
fractured zones
Courtesy Stephen Willson, BP
Executing this Difficult Well









Background gas controlled by ROP, not MW
Monitoring greatly reduced “wiper trips”
Continuous ECD and mud volume monitoring
to avoid destabilization (+”charged” faults)
Chip analysis to identify fractured shales
Strength profile modified “on-the-fly”
using ISONIC MWD + behavior + prognosis
Ballooning analysis refined hmin data
Hole condition from CRD scan on trips
Weighted pills placed for trips
Mud properties well maintained (ECD…)
Trajectory Variations Example





Erskine HPHT field
Deviated holes need
MWD, better control,
the dashed line path
was abandoned
Instead, reach was
established above
HTHP zone, then the
well turned vertical
No MWD used, hole
cleaning was better,
lower ECD, etc…
Also, low flow rates,
low surge-swab, etc…
S-profile trajectory
Reach section
Top of HTHP zone
A vertical trajectory in the
HTHP zone proved to be
cheaper and faster,
rather than steering an
inclined well trajectory
5000 m
Real-Time Wellbore Stability
For deep, difficult, costly holes only
 Quality prognosis is needed – po(z), hmin(z)
 Diagnostic tools used:

 Real-time
pressures (ECD management)
 Caliper and resistivity data, D-exponent data
 Borehole imagery (on trips)
 Accurate mud loss gauges & ballooning analysis
 Cuttings volumes and visual classification

Prevention and and remediation options:
 Mud
properties and special chemicals
 Hydraulics, drilling parameters, reamers…
 Special cures… (pills, LCM,,,)
Tests on the Rig Floor on Chips







Performed on “intact” cuttings
Brinnell hardness is related to strength
The dielectric properties can be related to
the shale geochemical sensitivity
Sonic travel time can be related to
strength and stiffness empirically
You can use dispersion tests in water of
different salinities to assess swelling
Even some others can be used
These can be taken regularly and plotted
as a log versus depth (very useful)
Mud Cooling to Increase Borehole
Stability in Shales
Heating and Cooling in the Hole
T
cooling
in tanks
mud up
annulus
Heating occurs uphole, cooling
downhole. The heating effect can
be large, exceptionally 30-35°C in
long open-hole sections in areas
with high T gradients.
casing
heating
+T
mud
down
pipe
mud
temperature
open
hole
drill
pipe
-T
At the bit, cooling, shrinkage, both
of which enhance stability.
BHA
cooling
depth
Heating is most serious at the last
shoe. The shale expands, and this
increases q, often promoting
failure and sloughing.
shoe
geothermal
temperature
bit
Commercial software exists to draw
these curves
T Effects in the Borehole






Mud goes down the drillpipe fast: ~5 to 10
 faster than it returns up the annulus
It picks up heat from rising mud in annulus
At the bit, still 10°-40°C cooler than rock
in HT wells with long open-hole sections
Rising uphole, the mud picks up heat from
formation, and heats rapidly till the crossover point (T diff. Is as large as 30°-40°C)
Then, it cools all the way to the surface
It gets to the tanks hot, and loses some
heat, but usually goes back in quite warm
A Simple Quantitative Example…

Change in q at the wall is given by:
q]ri ~ (T·b·E)/(1-)
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E = Young’s modulus = 1 to 5106 psi
b = Thermal expan. coef. = 10-1510-6/°C
 = Poisson’s ratio = 0.30 – 0.35
T = Temperature change
Reasonable values are: E = 3106 psi, b = 12
10-6/°C,  = 0.35, T = +25°C
This increases q at the wall by ~1400 psi!
Not good for shale stability!
Heat Also Reduces Strength a Bit
Deviatoric stress (MPa)
80
3 = 2.5 MPa
Temperature = 20°C
Temperature = 60°C
60
40
20
Mancos shale
0
0
0.5
1
1.5
2
2.5
3
3.5
Strain (%)
About 10% strength loss for this T, so this is a secondary effect
More Temperature Effects
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+T reduces strength, increases stress
+T also makes adsorbed water more mobile
Absorbed water layer thickness is reduced
Either water is expelled, or stresses must
change because the pore pressure changes
In either case, additional V takes place, in
addition to thermoelastic effects
Furthermore, reaction rates change w. T
Boy! Does this make modeling difficult!
Cooling the Mud Reduces +T
Cooling mud
T
The mud is cooled at surface
through heat exchangers and sea
water. As much as -30°C to -40°C
is feasible in some cases.
mud up
annulus
Now, the amount of heating at the
shoe is very small, only a few
degrees.
+T
Also, the shale remains stronger by
virtue of the cooling.
-T
There are other benefits as well…
BHA
cooling
depth
Benefits of Mud Cooling
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Increases shale stability throughout hole!
Low temperature reduces the rate of
negative geochemical reactions between
the mud filtrate and the shale
Generally, mud properties are far easier to
maintain with cooler mud, lower cost
Tanks are less hot (in some areas, mud can
exit the hole almost boiling!)
BHA is “protected” from high T
Use it when appropriate!
Lessons Learned
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Stability in drilling involves many factors
 Rock
mechanics information, cavings and
cuttings information, rig site tests…
 Hydraulics management
 Lithostratigraphic knowledge
 MWD in difficult offshore cases (ECD)
 Temperature management
 MW and rheology management
The key is rock mechanics behavior, as
stability is mainly a stress issue
 But… All factors must be considered
together in difficult wells
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