Module F: Drilling in Unusual Stress Regimes Part I

1
Module FDrilling in Unusual Stress Regimes Part
I Overpressured Cases
Argentina SPE 2005 Course on Earth Stresses and
Drilling Rock Mechanics Maurice B.
Dusseault University of Waterloo and Geomec a.s
2
Drilling in Overpressured Zones

• For practical purposes (), reducing the number
  of casings or liners is desirable
• However, drilling in OP zones carries
  simultaneous risks of blowouts and lost
  circulation that are difficult to manage.
• There now exist new options that help us
• Drilling slightly above shmin with LCM in the mud
• Bicentre bits and expandable casings
• Understanding overpressure and also the deep zone
  of stress reversion will help

3
Pressures at Depth
Fresh water 10 MPa/km 8.33 ppg
0.43 psi/frt Sat. NaCl brine 12 MPa/km
10 ppg 0.516 psi/ft
pressure (MPa)
10 MPa
Hydrostatic pressure distribution p(z) rw?g?z
1 km
Underpressured case underpressure ratio
p/(rw?g?z), a value less than 0.95
Overpressured case overpressure ratio
p/(rw?g?z), a value greater than 1.2
underpressure
overpressure
Normally pressured range 0.95 lt p(norm) lt 1.2
depth
4
Some Definitions

• For consistency, some definitions
• Hydrostatic po weight column of water above
  the point, r 8.33 ppg to 10 ppg in exceptional
  cases of saturated NaCl brine
• Underpressure is defined as po less than 95 of
  the hydrostatic po, usually found only at
  relatively shallow depths (lt2 km) or in regions
  of very high relief (canyons)
• Mild overpressure po of 10 ppg to 60 sv
• Medium overpressure po of 60 to 80 sv
• Strong overpressure po gt 80 of sv

5
Abnormal Pressure, Gradient Plot
1.0
2.0

• Typically, po is close to hydrostatic in the
  upper region
• shmin is close to sv in shallow muds, soft shale,
  but lower in stiff competent deeper shale
• A sharp transition zone is common (200-600 m)
• The OP zone may be 2-3 km thick
• A stress reversion zone may exist below OP

0
1
16.7 ppg
po
?hmin
2
?v
thick shale sequence
3
po
4
Target A
5
Target B
6
Target C
depth - kilometres
6
GoM The Classic OP Regime
7
Other Well-Known Strong OP Areas

• Iran, Tarim Basin (China), North Sea, Offshore
  Eastern Canada, Caspian
• In many thick basins, OP is found only at depth,
  without a sharp transition zone
• Most common in young basins that filled rapidly
  with thick shale sequences
• Good ductile shale seals, undercompaction
• Watch out for OP related to salt tectonics!
• These are most common offshore
• Land basins have often undergone uplift
• Tectonics have allowed pressures to dissipate

8
Eastern Canada Overpressured Areas
Nova Scotia Gas Belt
Importance of Geomechanics
Exports
9
Porosity vs Depth Overpressure
0
0.25
0.50
0.75
1.0
porosity
sands sandstones
mud
clay
Anomalously high f, low vP, vS, and other
properties may indicate OP
clay shale, normal line
mud- stone
In some cases, 28 f at depths of 6 km!
shale
effect of OP on porosity
4-8 km
T
depth
slate (deep)
10
Permeability and Depth
Permeability k Darcies

• Muds and shales have low k, lt 0.001 D, and as low
  as 10-10 D
• Exception in zones of deep fractured shale, k
  can approach 0.1-1 D
• Sands decrease in k with z
• Exception, high f sands in OP zones can have high
  k
• Anhydrite, salt k 0!
• Carbonates, it depends

0
1
2
3
4
5
Muds and Shales
Sands and Sandstones
Intact muds and shales have negligible k
Depth z 1000s ft
High porosity OP sands have anomalously high
porosity permeability
Fractured shales at depth may have high fracture
permeability
11
Abnormal po Causes

• Delayed compaction of thick shale zones
• Water is under high pressure
• Leak off to sands is very slow (low k)
• Thermal effects (H2O expansion)
• Nearby topographic highs (artesian effect)
• Hydrocarbon generation (shales expel HCs, they
  accumulate in traps at higher po)
• Gypsum dewatering (? anhydrite H2O)
• Clay mineral changes (Smectite ? Illite H2O
  SiO2)
• Isolated sand diagenesis (Df, no drainage)

12
Mechanisms for OP Generation
Mud, clays
Compaction H2O expelled to sand
bodies, especially from swelling clays
0-2000 m
Sand
H20
H20
H20
Shale
2000-4000 m
Montmorillonite much H2O
Sandstone
Diagenesis
4000-6000 m
Illite Kaolinite Chlorite
Compaction and Clay Diagenesis
Free H2O SiO2
13
Mechanisms for OP Generation

• Artesian effect (high elevation recharge)
• Thrust tectonics (small effect)
• Deep thermal expansion

rain
clays and silts
Artesian charging
3-10 km
Artesian charging is usually shallow only
Thrusting can lead to some OP
DT DV of H2O thermal expansion at depth
20-100 km
14
Offshore Trapping of OP
Listric faults on continental margins lead to
isolated fault blocks, good seals, high OP in the
isolated sand bodies from shale compaction
down-to-the-sea or listric faults
sea
stress
?v ?h po
shale
slip planes
shale
Sand bodies that have no drainage because of
fault seals, OP is trapped indefinitely
depth
Stress reversion zone
15
HC Generation and OP
HCs generated in organic shales
sv
T, p, s increase
shale
Semi-solid organics, kerogen, po lt sh lt sv
kerogen
sv
po sh lt sv, Fractures develop and grow
high T, p, s
sands
Pressured fluids are expelled through the
fracture network, po stored in OP sands
oil and gas
fluid flow
generation of hydrocarbon fluids
16
OP From Gas Cap Development
Thick gas cap development, perhaps charged from
below, can generate high OP
A
pressures along A-A?
stress
gas cap, low density
gas cap effect
oil, density 0.75-0.85
A?
Gas migration along fractured zones, faults, etc.
Fractured rock around fault
sh
po
Deep gas source
depth
Gas rises gravitational segregation
17
Abnormal Pressure Sand-Shales

• Overpressure is often generated due to shale
  compaction and clay diagenesis
• Montmorillonite (smectite) changes to
  lllite/Chlorite at depth. H20 is generated and
  is a source of OP.
• Pressure is generated in shales, sands accumulate
  pressure
• PF commonly higher in shales than sands
• Sand-shale osmotic effects (salinity differences)
  can also contribute to OP

18
PF in GoM Sand-Shale Sequences
Absolute stress values
Stress gradient plot
stress
shmin
sv
PF in sand line
shmin
sv
z
z
shale
sandstone
shale
sandstone
limestone
shale
depth
depth
Pore pressure distribution, top of OP zone
19
Some Additional Comments

• Casing shoes are set in shales (98)
• The LOT value reflects the higher shmin in the
  shales, therefore a higher PF
• As we drill deeper, through sands, the actual
  shmin value is less! By as much as 1 ppg in some
  regions
• Can be unsafe, particularly when we increase MW
  rapidly at the top of the OP zone
• You should test this using FIT while drilling

20
Examination of a Typical Synthetic OP Case
21
Particularly Difficult OP Case
2.0 (16.7 ppg)
1.0 (8.33 ppg)

• Deep water drilling, mud heavier than H2O
• Thick soft sediments section, PF sh sv
• Thin, shallow, gas-charged sand
• Zone where sh is roughly unchanged
• Sharp transition zone
• High OP, 90 of sv
• Deep zone of stress and pressure reversion

0
Sea water depth 800 m
800 m soft sediments
1
2000 m medium stiff shales and silts
2
sv
sh
po
3
seal
sharp transition
4
1400 m OP zone
5
Reversion zone
6
Z kilometers (3279 ft/km)
22
Upper Part of Hole
2.0 (16.7 ppg)
1.0 (8.33 ppg)

• The vertical lines are several MW choices
• Riser and first csg. MW
• 9.16 ppg does not control gas, but only fractures
  above 950 m
• 10.0 ppg controls gas, but losses above 1200 m
  will be a problem. It does allow deeper drlg.
• Solution, riser seat at 1000 m
• Casing shoe at 1400 m

0
Sea water - 800 m
9.16 ppg
10.0 ppg
800 m soft sediments
1
2
Medium stiff shales and silts
Z kilometers (3279 ft/km)
23
Riser Issues in this Example

• Sea water is 1.03 8.6 ppg
• At great depth, MW may be as high as 2.02 (17
  ppg) if the riser is exposed fully
• The D-pressure at the riser bottom is very large
  800m ? 9.81 ? (2.02 1.03) 7.8 MPa
• The riser must be designed to take this
• Or, special sea-floor level equipment must be
  installed
• Special mud lift systems from the sea floor, etc.

24
Approaching the Transition Zone
2.0 (16.7 ppg)
1.0 (8.33 ppg)
0

• LOT of 1.3, 10.83 ppg
• This limits us to 3.6 km for the next casing
• However, this will require a liner to go through
  transition zone
• Liner from 3600 m to 3750 3800 m
• If it is possible to drill 100 m deeper
  initially, to 3700 m, we may save the liner
  (1,000,000)

Sea water - 800 m
800 m soft sediments
1
2000 m shales and silts
2
sv
sh
po
3
sharp transition
4
OP zone
Z kilometers (3279 ft/km)
25
Solution A Casing or Liners
2.0 (16.7 ppg)
1.0 (8.33 ppg)
0

• This is the most conservative, safest, and the
  most costly
• Black line is MWmax
• If shale problems occur in the 1.6-3.6 km shale
  zone, requiring an extra casing (i.e., little
  margin for error)

Sea water
1
2000 m shales and silts
2
sv
sh
po
3
4
OP zone
Z kilometers (3279 ft/km)
26
Soln B Drill OB With LCM?
2.0 (16.7 ppg)
1.0 (8.33 ppg)
0

• Dashed line is from the previous slide
• Drilling with the purple line, saves a liner!
• This is 1.2 ppg OB at the shoe (quite a bit!)
• Place upper casings deeper if possible
• Drill with LCM in mud (see analysis approach in
  Additional Materials)
• Place a denser pill at final casing trip
• (Approach with caution)

Sea water
1
2000 m shales and silts
2
sv
sh
po
3
4
OP zone
Z kilometers (3279 ft/km)
27
Solution C Deeper Upper Casings
2.0 (16.7 ppg)
1.0 (8.33 ppg)
0

• 300 m subsea primary casing depth
• Casing at 1850 m depth
• Drill long shale section with MW shown as dashed
  black line
• Increase MW only in last 100 m (LCM to plug
  ballooning at the shoe)
• Slight OB of 0.2-0.3 ppg needed
• Casing may be saved (?)

Sea water
1
2
Slight OB needed
3
sv
sh
po
OP zone
4
Z kilometers (3279 ft/km)
28
Deeper Upper Casing Shoes

• Depending on the profile of OP stresses and
  pressures, this approach can be effective, but in
  some cases it is not
• Of course, the best approach is always to place
  the shoes as deeply as possible
• This may give us a one-string advantage deeper in
  the well if problems encountered
• At shallow depths (mudline to 4000 ft), use
  published correlations with caution because there
  are few good LOT data

29
Comments on the Approaches

• There is risk associated with saving a casing
  string risks must be well-managed
• The stress/pressure distribution sketched is a
  particularly difficult case
• Shallow pressured gas seam at 1500 m subsea
• PF (sh) is quite low around 3000 m subsea
• Transition zone is very sharp (250 m)
• OP is high (88-90 of sv)
• However, it could even be worse!
• More gas zones, depleted reservoirs at 3.6 km
• Etc

30
Drilling Through a Reversion Zone

• Below OP, usually a zone where po, sh (PF)
  gradually revert to normal values. This is
  rarely a sharp transition as at top of OP
• This is related to fractured shales that bleed
  off OP (i.e. lower OP seal is gone)
• Also, when shales change and shrink, the sh value
  (PF) drops as well
• Reverse internal blowout possibility
• Blowout higher in hole
• Fracturing lower in hole

31
Stress Reversion at Depth
stress (or pressure)
Note that ?hmin can become gt ?v
Region of strong overpressure
depth
Stresses revert to more ordinary state
Higher k rocks (fractured shales)
Z
32
Same Example
2.0 (16.7 ppg)
1.0 (8.33 ppg)

• OP casing was set at 3800 m depth
• Drill with 16.7 ppg MW
• At 5.5 km, large losses
• If we reduce MW, high po at 4.6 km can blow out,
  flow to bottom hole at 5.5 km (reverse internal
  BO)
• Set casing at 5450 m
• Drill ahead with reduced MW

4
1400 m OP zone
5
Reversion zone
sv
sh
po
6
Z kilometers (3279 ft/km)
33
Real Deep Overpressure Drilling

• Watch out for shallow gas sands
• Dark black line MWmax for the interval
• Dashed black line is the actual drilling MW
• Red stars excessive shale caving, blowouts
• Green stars ballooning and losses
• Surface casing string not drawn on figure

This is a deep North Sea case, west of Shetlands
34
Detecting OP Before Drilling

• Seismic stratigraphy and velocity analysis
• Anomalously low velocities, high attenuations
• Can often detect shallow gas-charged sands
  (unless they are really thin, lt 3-5 m)
• Geological expectations (right conditions, right
  type of basin and geological history)
• Offset well data, good earth model, so that
  lateral data extension is reliable

35
Detecting OP While Drilling

• Changes in the Dr exponent, penetration rate
  may increase rapidly in OP zone
• Changes in seismic velocity (tP increases)
• Changes in porosity of the cuttings (surface
  measurements or from MWD)
• Changes in the resistivity of shales from the
  basin trend lines
• Changes in the SP log
• Changes in drill chip and cavings shapes, also
  volumes if MW lt po
• Mud system parameters, etc

36
Comments on LWD

• Methods of data transmission
• Mud pulse 2 bits/s _at_ 30,000, 12-25 b/s is good
  at any depth
• Issues in data transmission
• Long wells, extended reach
• OBM, electrical noise, drilling noise
• ID changes in the drill string
• Pump harmonics, stick/slip sources
• Wire pipe extremely expensive
• High rate on out-trip, then download on rig
• New technologies will likely emerge soon

37
Reasons for Pore Press. Prediction

• Drilling Problems Due to Pressure Imbalance
• Overbalance Slow drilling, Differential
  Sticking, Lost circulation, Masked shows,
  Formation damage.
• Underbalance Imprudently fast drilling, Pack-
  offs, Sloughing shales, Kicks, Blowouts.

38
Pore Pressure Prediction Basics I

• Data from offset wells
• Logs, Dr data, sonics, neutron porosity,
  resistivity, etc.
• Transfer data to new well stratigraphy, z
• Plot sv gradient, sonic transit time, Dr,
  resistivity, porosity, etc. with depth
• Use trend analyses and published methods, to
  determine the normal compaction line
• Use an Eaton correlation chart if you have it for
  this area (use offset and other data)
• This is the prognosis profile for new well

39
Pore Pressure Prediction Basics II

• With seismic data and geological model of the new
  well region, assess
• Existence of OB conditions (seals, sources)
• Existence of faults, salt tectonic features
• Plot depth corrected velocities on profile
• Carefully compare the two
• Lower velocities greater OP risk
• Explain existence of any undercompacted zones and
  anomalies you have identified
• You now have as good a prognosis as you can
  develop with existing data

40
Sonic Transit Time Differences
2.0 (16.7 ppg)
1.0 (8.33 ppg)
Log of sonic transit time
0
Sea water depth 800 m
650 ms/m
1
Normal trend from the basin, offset data
Soft seds.
Seismic velocity model
Stiff shales and silts
2
sv
po
3
Sonic transit time from offset wells
seal
Expected OP transition
PROGNOSES FROM OFFSET WELL DATA, CORRECTED FOR Z,
ETC
4
Critical region
OP zone
5
Reversion zone
6
Z kilometers (3279 ft/km)
41
Prognoses Based on Seismics
Normal compaction line for the basin General
seismic profile data, depth corrected for new
well Corrected sonic transit time, calibrated
with the general seismic velocity data Regions
of substantial deviation are highlighted as
critical, experience used to choose likely top
of OP OP magnitude estimated, based on
correlations
OP beginning
Large OP expected
42
Seismic Cross-Sections

• Depth Converted
• 11 Horizontal / Vertical Ratio
• Offset Well Ties (Regional)
• Planned Wellbore (Local)
• Full Structural Picture
• Fully Annotated
• Radial Animation

43
North Sea Seismic Section - Diapir
Courtesy Geomec a.s.
44
Other Trend Line Approaches

• Methods exist for using trend analysis for many
  different measures, including
• Drilling exponent data
• Resistivity trends lines (salinity of strata)
• Deviations from expected porosity (less
  sensitive)
• SP log characteristics
• Perhaps some others
• Shale data are used because sand porosity is less
  predictable in general

45
Gas Cutting of the Drilling Mud

• Shale behaves plastically at elevated pressure
  and temperature gradients.
• Significance (and insignificance) of gas cut mud
  (GCM). Gas from CH4 in shales?
• Very large gas units 2,000 to 4,000 units ?
• Connection gas (CG) - better indicator. Use it
  for well to talk. Ineffective when too much
  overbalance.
• CG increase from 20, 40, 60 to 80 points. Yes,
  you are underbalanced.

46
Is MW a Pressure Indicator?

• No. The lower limits of MW in most OP regimes
  are related to shale stability, rather than to
  pore pressure
• Usually, in difficult shales, 1 to 2 ppg above po
  is needed to control excessive shale problems
• HOWEVER! MW limits from offset well drilling
  logs are useful to estimate MWmin
• Of course, this can change as well
• More inhibited WBM, using OBM instead, etc
• Faster drilling, less exposure, etc

47
MWmin Prognosis

• Offset well pressure, stress, drilling data
• Estimate target MWmin for new well prognosis
• If this generates too narrow a MW window, assess
  approaches
• Will OBM allow a lower MWmin? (on the plot, the
  dashed blue line is the estimated OBM MW for
  shale stability)
• Other factors?

48
MWmin, MWmax Well Prognosis
2.0 (16.7 ppg)
1.0 (8.33 ppg)

• Use a rock mechanics borehole stability model,
  calibrated, to estimate MWmin from geophysical
  logs and lab data
• Use offset well losses, ballooning, LOT, etc. to
  estimate MWmin
• This defines the local safe MW window
• Now, combine with casing program prognosis to
  plan the MW for the well

0
Sea water depth 800 m
1
Soft seds.
Weak rocks
Stiff shales and silts
2
sv
3
po
Expected OP transition
PROGNOSES FROM OFFSET WELL DATA, CORRECTED FOR Z,
ETC
4
OP zone
5
Reversion zone
Strong rocks
6
Z kilometers (3279 ft/km)
49
During Drilling

• Remember, in OP drilling we are trying to push
  the envelope to reduce casings
• Update the well prognosis regularly with actual
  LOT, MWD, ECD data
• Monitor, measure, observe
• Kick tolerances, ballooning behavior, gas cuts
• Chip morphology and volumes
• Flow rate gauges on flowline, pumps
• Mud temperature monitoring MWD temperature
• Sticky pipe, torque, ECD, mud pressure
  fluctuations
• Cuttings analyses vP, Brinnell hardness are used

50
Increasing Depth of Casing Shoe
(2.0 16.7 ppg)
density, g/cm3
prognosis for shmin
MW 1.92
Previous casing string
prognosis for po
sv
XLOT shmin value
shoe
overpressure transition zone
deeper shoe for casing string!
area indicates possible MW
strong overpressure zone
depth
Using high weight trip pills and careful
monitoring, the lower limit can be extended
51
High Weight Trip Pills

• Drill ahead beyond limit (if shales permit)
  with MW LOT at the shoe PF
• Some gas cutting of the mud and shale sloughing
  If too severe, casing
• For trip, set a pill of higher weight
• This creates a change in slope of the mud
  pressure line in the window (see figure)
• Pull out carefully, no swabbing please
• Set casing (best with top drive and some ability
  to pump casing down a bit)
• Unlikely to succeed with gas sands present

52
An OP Well Prognosis
PORE PRESSURE (PPG)
WELL DESIGN - HI 133 No. 1
EXPECTED MW (PPG)
FRAC GRAD. (SAND)
MW, PF, EST. po
FRAC. GRAD (SHALE)
DEPTH - ft
MUD WEIGHT - ppg
53
Same Overpressured Well, GoM
WELL DESIGN - HI 133 No. 1
MW, PF, ESTIMATED po
0
1000
2000
PORE PRESSURE (PPG)
EXPECTED MW (PPG)
3000
FRAC GRAD. (SAND)
FRAC. GRAD (SHALE)
4000
5000
6000
DEPTH
7000
8000
9000
10000
11000
12000
13000
14000
8
9
10
11
12
13
14
15
16
17
18
19
MUD WEIGHT
54
Approach for this Well - I

• From 8600to 9400po goes from 9.5 ppg to 15.7
  ppg (1.14 ? 1.89 g/cm3)!
• A liner over a 800-1200length is necessary, but
  we dont want to install a second liner
• Strategy
• Below the 3000 shoe, drill as close to po as
  possible, as fast as possible to avoid shale
  issues
• Below 8200, weight up while drlg. to as high as
  possible (upper part of hole will be
  overbalanced)
• This is a case where we may add carefully graded
  LCM to help build a stress-cage higher in the
  hole
• Drill as deep as possible, hopefully to 9100

55
Approach for this Well - II

• Strategy (contd)
• Push the envelope for depth, managing your ECD
  carefully, living with a bit of ballooning
• To trip out and case, place a high density pill
  for safety (e.g. 18 ppg mud for bottom 1500)
• Set casing (partly cemented only) at 9100-9200
• Mud up to MW slightly higher than po, drill out,
  do XLOT, advance carefully, gradually increasing
  MW
• Set a liner as deep as possible, 9900 if
  possible
• Mud up before drilling out with 16.5 ppg mud with
  carefully designed LCM to strengthen the hole
• Do a precision XLOT, drill ahead to TD,
  increasing MW only as required

56
Deep Water Drilling Stability

• Narrow operating window is common
• Circulating risks, ECDs, monitoring.
• Special mud rheology low T, riser cools the mud
  massively, down to 5-10 is common
• Casing design often requires many short casing
  strings, shallow muds, overpressure, and the zone
  of pressure reversion
• Well control is tricky because of the narrow
  window, long risers, etc
• Rig positioning and emergency disconnect critical
  for safety (no circulation for days)

57
Gullfaks
North Sea case Overpressure Reversion
zone Depletion effect
58
Franklin Field, UK West Sector

• 120-130 MPa po in deep Triassic zones
• T to 200-211C measured
• 6300 m deep (20,000 feet)
• Mud weights of 18-19 ppg required
• Very narrow MW window near reservoir
• Retrograde condensate field, liquids are
  generated near the well, reducing k
• Surface pres. up to 101 MPa (15000 psi)!
• Reservoir experienced rapid depletion and this
  led to very high effective stresses, as well as
  massively reduced lateral stresses

59
Lessons Learned

• OP drilling a major challenge, particularly
• In young offshore basins
• In deep water (riser length issues)
• Careful well prognoses are critical (PF, po)
• Prognoses must be updated while drilling
• The envelope can be pushed!
• Living with breakouts for lower MW
• Using LCM to generate somewhat higher PF
• Special trip practices, special equipment
• In OP drilling, vigilance is absolutely critical
• Increase your observations, understand them

60
Additional Materials

• Also, visit the following website for a
  comprehensive list of formulae for your pressure
  calculations in drilling
• http//www.tsapts.com.au/formulae_sheets.htm

61
Fracture Pressure Enhancement in Drilling Through
Use of Limited Entry Fracturing and Propping

• Courtesy of
• Francesco Sanfilippo
• Geomec a.s., Norway

Courtesy Geomec a.s.
62
The Concept
To enhance fracturing pressure by drilling
slightly overbalance and, at the same time, by
effectively plugging and sealing the induced
hydraulic fractures
Courtesy Geomec a.s.
63
How Can this be Analyzed?

• Find a simple description of this process
• First-order physics
• Estimate the fracturing pressure enhancement
• Evaluate the importance of the involved factors
  and identify the first-order parameters

Courtesy Geomec a.s.
64
Methodology

• Estimate the enhancement through the classical
  results (England and Green equation)
• Modify the Perkins-Kern-Nordgren model to take
  into account the effect of progressive plugging

Courtesy Geomec a.s.
65
Classical results

• England and Greens equation can be used once the
  geometrical parameters of the fracture are known.
• It estimates the hoop stress increase from the
  mechanical properties of the rock and and the
  geometrical parameters of the fracture
• Two shapes have been considered
• Penny shape-like fractures
• PKN-like fractures (lengthgtgtheight)
• Base case for the parametric study
• Young modulus 40 GPa
• Poissons ratio 0.2
• Fracture width 3 mm
• Fracture height/radius 10 m

Courtesy Geomec a.s.
66
Classical results effect of the Young modulus
Courtesy Geomec a.s.
67
Classical results effect of the Poisson
coefficient
Courtesy Geomec a.s.
68
Classical results effect of the fracture width
Courtesy Geomec a.s.
69
Classical results effect of the fracture height
Courtesy Geomec a.s.
70
Modified PKN model

• With this model the geometrical parameters of the
  fracture are estimated according to the
  measurements while drilling
• Plugging is considered through a reduction of the
  fracture permeability with time up to complete
  sealing
• Base case for the parametric study
• Youngs modulus 40 GPa
• Poissons ratio 0.2
• Mud viscosity 5 cP
• Mud loss rate 1 bbl/min
• Time required to plug the fracture at a given
  depth 30 min
• Rate Of Penetration 10 m/hr

Courtesy Geomec a.s.
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Modified PKN model fracture aperture vs. time
Courtesy Geomec a.s.
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Modified PKN model effect of Young modulus
Courtesy Geomec a.s.
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Modified PKN model effect of Poisson coefficient
Courtesy Geomec a.s.
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Modified PKN model effect of mud viscosity
Courtesy Geomec a.s.
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Modified PKN model effect of mud loss rate
Courtesy Geomec a.s.
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Modified PKN model effect of plugging time
Courtesy Geomec a.s.
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Modified PKN model effect of Rate of penetration
Courtesy Geomec a.s.
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Role and Design of Plugging Material

• The plugging material is a mixture of mud clay,
  barite, formation debris (cuttings), plus
  carefully sized LCM
• It plugs the induced fracture rapidly, and sq is
  increased permanently by propping
• The effect is limited in extent, but the sq
  stress does not relax during drilling
• The LCM is designed (concentration, size range)
  based on the mud parameters
• www.geomec.com for further details