Module L: More Rock Mechanics Issues in Drilling

1
Module LMore 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.
2
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

3
Linear Poroelastic Borehole Model

• Eqn
• Where
• pwcr critical wellbore pressure, shear
  initiation
• pi pressure just inside the borehole wall
• s1, s3 largest, smallest ppl s in borehole plane
• A a(1-2?)/(1-?) (? Poissons ratio)
• a Biots coefficient (1.0 for soft rocks)
• N friction coefficient (1 sin?)/(1 - sin?)
• UCS, ? Unconfined Compressive Strength,
  friction angle (MC yield criterion)
• ?p drawdown pi - po

4
Discussion of Parameters
pw
pi

• pw pi is support pressure
• Usually, we ignore effects of a, except in low
  porosity, stiff shales (E gt 30-40 GPa)
• UCS and N are equivalent to the c, ? of the
  linear MC yield criterion for shear
• Poissons ratio for shales, 0.25 to 0.35
• s1, s3 are computed using equations converting
  3-D stress to stresses in the plane of the
  borehole (90 to hole axis)

po
radius - r
5
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

6
Defining Limits in Our Well Plan
Gradient
Pressure or stress
Predicted MW for onset of unmanageable sloughing
shmin, danger of LC
sv
Onset of ballooning in shale zones
sv
po, onset of blowout if in a sand zone
Depth
Depth
7
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

8
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

9
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

10
Underbalanced Stress Conditions
s stress
sq
High shear stress at the borehole wall
shmin sHMAX
sr
po
pw lt po
pw
radius
Some tensile stress exists near the hole wall in
underbalanced drilling because po gt pw
11
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!

Mud Rheology Diagram
Static condition
m mud viscosity
Shearing resistance
Yield point
YP
Dynamic conditions
Shearing rate
12
Effect of Mud Weight Increase
?, shear stress
MC failure line
??
yield
Mohrs circle of stresses
no yield
c?
??n, normal stress
??r
??a
Increasing MW (with good cake) reduces the
stresses on the wall
13
Effect of Loss of Good Filter Cake
?, shear stress
MC failure line
??
failure
Mohrs circle of stresses
c?
??n, normal stress
??r
??a
With loss of mudcake effect, radial support
disappears, shear stress increases
14
Stresses and Drilling
sv
sHMAX sv gtgt shmin
To increase hole stability, the best orientation
is that which minimizes the principal
stress difference normal to the axis
60-80 cone
sHMAX
shmin
sv
Favored hole orientation
sv
Drill within a 60cone (30) from the
most favored direction
sHMAX
sHMAX
shmin
shmin
sHMAX gtgt sv gt shmin
sv gtgt sHMAX gt shmin
15
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

16
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!

17
Some Diagnostic Hole Geometries
d.
a.
General sloughing and washout
Swelling, squeeze
b.
sHMAX
drill pipe
Keyseating
e.
shmin
Breakouts
Fissility sloughing
Induced by high stress differences
c.
c.
f.
Only breakouts are symmetric in one direction
with an enlarged major axis
18
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

19
ECD
Pressure gradient plot
15
16
17
18
19 ppg
PF (shmin)
MW 16.7 ppg (static value)
reamers and stabilizers
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 (shmin). When
the pumps stop, much of the mud comes back into
the hole!
BHA and collars
High ECD!
Depth
20
ECD

• pBH mud weight plus friction Dp loss
• High ECD values (gt0.5 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!

21
High ECD Effects
15
16
17
18
19 ppg
Gradient plot
PF shmin
po
Top of restrictive BHA
MW 16.7 ppg (static value)
reamers
mud rings
Dynamic pressure (ECD) because of friction, hole
restrictions, high mud m
Cannot reduce the MW much because of borehole
instability uphole or blowout danger on trips,
connections, gas cutting
BHA and collars
stabilizers
Large mud losses at hole bottom because of
fracturing
High ECD!
Depth
22
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

23
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
24
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

25
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
26
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
27
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

Keep the drillhole within this cone to avoid
severe fissility sloughing problems
100-120 cone
Normal to bedding planes
28
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 CURVES
PHOTOELECTRIC FACTOR CURVES
BOTTOM OF BOREHOLE
GR
Density Neutron Image
29
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 DV(t) measurements can be of great
  value to good drilling
• Cavings monitoring
• MWD (ECD, resistivity, bit torque)

30
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 s?n leads to slip
• BHA gets stuck on trip out
• 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

pw
s?n
31
Slip of a High-Angle Fault Plane
borehole
sv s1
casing bending and pinching in completed holes
sh s3
high pressure transmission
pipe stuck on trips
slip of joint surface
slip of joint
(after Maury, 1994)
32
Slip Affected by Hole Orientation!
OFFSET ALONG PRE-EXISTING DISCONTINUITIES
FILTRATE
TYPICAL MUD OVER-PRESSURE
Courtesy Geomec a.s.
33
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
34
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

35
Mud Volume Measurements

• Extremely useful, but, accurate DV/Dt 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
Time - min
0
5
6
7
8
9
B
Losses - gpm
20
15
10
5
Hole deepening rate
Filtration fluid loss
Time - min
0
5
6
7
8
9
36
A Precise Mud Volume Installation
Outlet mud line Precision flow meter
37
Actual Field Example of Analysis
Courtesy Geomec a.s
This information proved extremely valuable for
reservoir engineers in this case, as a gas
reservoir was found
38
Losses Identify Fractured Zones
Mud Loss Rate litres/min
Likely, each event involved filling a single
fracture
Depth - m
39
Problems in Coal Drilling

• OBM are worse than WBM in Coal
• Filtrate penetrates easily (oil wettability)
• Coal fractures open easily if pw gt po
• Coal 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

40
Drilling in Coal
stresses around wellbore
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
41
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

42
A Case History of Salt Diapir Drilling in the
North Sea
43
North Sea Case, Shallow Depth
Well A
1a
Shallow Gas
2000 m
Gas Pull Down
Courtesy Geomec a.s.
44
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 shmin)
• 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)

45
Deeper, Around the Diapir
This region avoided
Well A
1b
Gas Pull Down
2000 m
Mid-Miocene regional pressure boundary
Top Balder Top Chalk Intra Hod/Salt
3000 m
Courtesy Geomec a.s.
46
MWD RESISTIVITY LOG SIGNATURE (OBM)
Invaded Zone
Courtesy Geomec a.s.
Time-lapse and different spacing resistivity
logging data identified fractured zone clearly
47
INVADED ZONE Symmetry(O-B)
Courtesy Geomec a.s.
48
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

49
Statfjord Case North Sea
OVERBALANCED!
-800 psi
These wells were drilled with overbalance a MW
above the lowest estimated shmin in the zone
Courtesy Geomec a.s.
50
Conclusions

• Fracturing pressure can be increased by several
  100 psi by graded LCM, analysis
• Youngs 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

51
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
52
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 shmin data
• Hole condition from CRD scan on trips
• Weighted pills placed for trips
• Mud properties well maintained (ECD)

53
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
5000 m
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
54
Real-Time Wellbore Stability

• For deep, difficult, costly holes only
• Quality prognosis is needed po(z), shmin(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,,,)

55
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)

56
Mud Cooling to Increase Borehole Stability in
Shales
57
Heating and Cooling in the Hole
T
cooling in tanks
Heating occurs uphole, cooling downhole. The
heating effect can be large, exceptionally
30-35C in long open-hole sections in areas with
high T gradients. Heating is most serious at
the last shoe. The shale expands, and this
increases s?q, often promoting failure and
sloughing. At the bit, cooling, shrinkage, both
of which enhance stability. Commercial software
exists to draw these curves
mud up annulus
casing
heating
shoe
geothermal temperature
open hole
T
mud down pipe
drill pipe
BHA
mud temperature
-T
cooling
depth
bit
58
DT 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 10-40C 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 cross-over
  point (T diff. Is as large as 30-40C)
• 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

59
A Simple Quantitative Example

• Change in s?q at the wall is given by
• Ds?qri (DTbE)/(1-n)
• E Youngs modulus 1 to 5?106 psi
• b Thermal expan. coef. 10-15?10-6/C
• n Poissons ratio 0.30 0.35
• DT Temperature change
• Reasonable values are E 3?106 psi, b 12
  ?10-6/C, n 0.35, DT 25C
• This increases s?q at the wall by 1400 psi!
• Not good for shale stability!

60
Heat Also Reduces Strength a Bit
About 10 strength loss for this DT, so this is a
secondary effect
61
More Temperature Effects

• 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. DT
• Boy! Does this make modeling difficult!

62
Cooling the Mud Reduces DT
Cooling mud
T
The mud is cooled at surface through heat
exchangers and sea water. As much as -30C to
-40C is feasible in some cases. Now, the
amount of heating at the shoe is very small, only
a few degrees. Also, the shale remains stronger
by virtue of the cooling. There are other
benefits as well
mud up annulus
T
-T
cooling
depth
63
Benefits of Mud Cooling

• 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!

64
Lessons Learned

• 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