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Geothermal Reservoir Geomechanics: The Application of High
Temperature / High Pressure Borehole Logging Technologies
to Reservoir Characterization and Production Prediction
Taupo, New Zealand 25-26th May 2011
HADES - Hotter And Deeper Exploration Science Workshop
Colleen Barton, PhD
Senior Technical Advisor
Baker Hughes | RDS | GMI
Sv
Shmin SHmax
UCS
Pp
The geomechanical model of a
reservoir involves detailed
knowledge of:
• In situ stress orientations
• In situ stress magnitudes
• Pore pressure
• Effective rock strength
• Fracture patterns
• Structure
The Geomechanical Model
Geomechanical Data
UCS Lab measurements, geophysical
logs,
Parameter Data Source
Vertical Stress
Shmin minifrac (sand), XLOT (shale)
Pp RFT, MDT, sonic, seismic, DST
S v z 0
( ) = r g dz 0
z 0
Density logs
seismic interpretation
Pore Pressure –
Stress Path -
SHmax magnitude Constraint based model
using observations of wellbore failure
Stress Orientation & Orientation Anisotropy
SHmax azimuth Image data interpretation
Rock strength constraint based model using
observations of wellbore failure
Horizontal Stress
Magnitudes –
Fracture and Fault
Strike and Dip Wellbore image data interpretation,
3
Open Hole HP/HT Logging Instruments
23.7°C
50.6°C
25
30
35
40
45
50
LI01
SP01
SP02
AR01
Nautilus UltraTM
500F and 30,000 psi
Current:
Formation
Evaluation
Cross-Dipole ( XMAC-F1)
Induction Resistivity (HDIL)
Porosity (CN)
Density (CDL)
Natural GR (GR/SL)
Borehole Geometry (XY-Cal)
Future:
Acoustic Borehole Imaging
Nautilus Ultra Formation Evaluation Tool O.D.
(in.)
Temperature
Rating (oF)
Pressure Rating
(psi)
Gamma Ray GR-HP/HT 4 1/8 500 30,000
Digital Spectralog DSL-HP/HT 4 1/8 500 30,000
High Definition Induction Log HDIL-HP/HT 4 1/8 500 30,000
Compensated Neutron CN-HP/HT 4 1/8 500 30,000
Compensated Density CDL-HP/HT 4 1/8 500 30,000
Cross-Multipole Array Acoustilog XMAC-HP/HT 4 1/8 450 30,000
Geothermal Ultrasonic Fracture Imager
• Geothermal Imager
– Goal is the development of wireline borehole televiewer
which can operate at 300oC with DOE in U.S.
– Demonstrated 300oC transducer
– Tool, Pressure & Temperature test by the end of Q1, 2012
– Efforts will directly feed into Nautilus-Ultra Borehole Imager
for the Oil & Gas industry
© 2010 Baker Hughes Incorporated. All Rights Reserved. 6
Case Histories – Successful HP/HT operations
Alice, Texas: HT Gas well logged using Nautilus UltraSM tools. Maximum BHT
recorded was 450°F (232°C) at a depth of 21,053 ft (6,417 m).
Central Oregon: Nautilus Ultra tools were deployed in a very difficult
geothermal, where BHT was expected to exceed 450° F (232° C). Data was
recorded successfully and max. temp. recorded was 474° F (246° C).
Moomba, Australia: Baker Hughes successfully logged this HT land well that
reached a depth of 16,109 ft (4,910 m) with 39°of deviation and a bottom hole
temperature of 470°F (243°C).
US Gulf of Mexico: Nautilus Ultra deployed at the McMoRan Exploration Co.
Davy Jones ultra-deep gas discovery on the shelf in the Gulf of Mexico. Nautilus
Ultra was deployed to a depth exceeding 28,000 feet, using our HP/HT Pipe
Conveyed Logging system. Maximum pressure exceeded 25,000 PSI .
8
Maximum temperature run to date: 474°F (246° C)
Maximum pressure to date: ~28,000 PSI
Conventional Gas
Geothermal
Ultra Deep Gas
Geothermal Exploration Cooper Basin
Images courtesy of Geodynamics Ltd.
Estimated crustal temperature at 5 km depth
(Holdgate and Chopra, 2004)
• The Cooper Basin is characterized by
high heat flow arising from the presence
of Paleozoic granites.
The HFR Reservoir Concept
Challenges:
• Granitic basement (>3,500m)
• Deep wells (~ 5,000 m)
• Temperatures > 240 °C
• Overpressures ( >1.7 SG)
• Thick sedimentary cover (>3,000m)
• Weak shale and coal intervals
An accurate geomechanical model will help to manage
wellbore stability in this hostile environment
Image courtesy of Geodynamics Ltd.
DITFs
Breakout
Natural fractures
Images are the key…
Continuous breakouts
develop in the granitic
basement.
Drilling induced tensile
fractures (DITF) develop in
quartz-rich intervals.
Breakouts and DITFs occur
within the same depth
intervals.
DITFs develop as vertical
features under a reverse
faulting stress regime.
Ap
lite
Observations
Acoustic image data (CBIL).
(after Fernández-Ibáñez, et al., 2009, IPA)
Observations of Breakout Rotations
13
Drilling experiences and stress
rotations suggest that the high Pp in
the basement is mostly confined to
natural fractures.
UNPERTURBED BREAKOUT
ROTATED BREAKOUT
ΔTmin = -60 °C
ΔTmax = -100 °C
BOs
DITFs
trips
11/11/2007
17/11/2007
25/11/2007
18/12/2007
1/1/2008
Dri
llin
g ti
me
sca
le
Thermoelastic Effects on Wellbore Stress
The effect at the wellbore wall of a
temperature difference DT between the
wellbore fluid and the wall rock is given
by the equation:
sqqDT = (a E DT)/(1-n)
Cooling increases the tensile stresses
(and decreases the compressive
stresses) at the wellbore wall.
DITFs form while drilling due to the
temperature contrast between the
drilling fluids (cool) and the formation
(hot).
Breakout development will be also
affected by the temperature contrast.
As the hole warms up, compressive
stresses will be increased and breakouts
will increase in severity.
(after Fernández-Ibáñez, et al., 2009, IPA)
Stress Model Summary
Transitional Strike-Slip to Thrust Faulting
Stress Regime
Overburden (SV) determined from integrated
pseudo density log data.
Minimum horizontal stress (Shmin) is
presumed to be comparable to Sv based on
wellbore failure observations and available
FITs.
Pore pressure (Pp) determined from drilling
experiences (gas readings), wireline logging
data, and mud weights used to drill wells in
the area. Overpressure → confined by
natural fractures
Maximum horizontal stress (SHmax)
magnitudes based on modeling of observed
wellbore failure. It is found to be >> Sv.
SHmax azimuth ~E-W.
SHmax modeling points
FITs
Pp ramp
(after Fernández-Ibáñez, et al., 2009, IPA)
Collapse pressure at
thermal equilibrium
Top granite
Collapse pressure
while drilling
Temperature-dependent Borehole Collapse
Pressure
Collapse pressure can be thought of as
the required bottom hole pressure to
prevent breakouts exceeding 90° width.
During drilling ΔT~100°C borehole
collapse pressure was generally lower
than the pore pressure.
As the hole warms up after drilling the
compressional stresses increase and so
does the collapse pressure.
Over time the hole falls into thermal
equilibrium (ΔT~ 0°C) and the collapse
pressure increases meaning a higher mud
weight is required to control excessive
breakout development (>90°).
Collapse pressure increases more rapidly
within the first 5-10 days after drilling the
well. After this period, the increase in
collapse pressure is less pronounced.
(after Fernández-Ibáñez, et al., 2009, IPA)
Top granite
Coal
Breakout width while
drilling
Breakout width at thermal
equilibrium
Temperature-dependent Breakout Width
An increase in the collapse pressure will directly
impact breakout width.
Breakouts form at the time of drilling and get wider
with time (i.e. warming).
There is an important temporal and thermal
component to breakout development in this hostile
environment.
A higher DT and less well exposure time could inhibit
breakout development or at least decelerate their
growth.
(change in temp. contrast / hole warming)
(after Fernández-Ibáñez, et al., 2009)
Fault and Fracture Dominated Basin and
Range Geothermal Reservoir
An accurate geomechanical model will help to
determine the optimal well placement to
enhance production in this hostile environment
(after Moos and Ronne et al., 2010, GRC)
Selecting the optimal logging suite for geothermal
reservoir evaluation
(after Moos and Ronne et al., 2010), GRC
General Properties of Fracture Zones
A fracture zone is usually a zone of intensely deformed rock
(cataclastic rocks), with a long history of deformation and
fluids circulation, high permeability, and associated mineral
deposits.
These are general indicators, but it does not necessarily imply in all cases.
Fracture zone signature while drilling:
• Losses
• Well inflow
• Drilling breaks
• Gas peaks
• Changes in cuttings mineralogy
• Reduction drillstring vibrations P
robabili
ty
Logging response in fracture zones:
• Reduction in Vp
• Reduction in resistivity
• Increase in porosity
• Reduction in density
• Increase in GR
• Shear waves anisotropy
• Reduction in Vp/Vs ratio
Pro
babili
ty
Crossed Dipole versus Image Analysis
• Fractures are more compliant because they are hydraulically conductive (i.e., losses)
• In hard, stiff, fractured rock, anisotropy is useful to identify zones of aligned fractures
that can provide a directionally consistent permeable pathway for fluid migration
Bedding (blue x)
Breakouts (black)
Anisotropy (green)
SHmax = bedding
strike
Circulation losses
where anisotropy
aligned in strike
direction steeply
dipping fracture
set
UPPER SEDIMENTS LOWER GRANITES
(after Moos and Ronne et al., 2010, GRC)
Propagation of “Critically Stressed” Shear
Fractures
26
So = Cohesion
m = Coefficient of Friction (sliding friction)
sn = Effective normal stress = Sn - Pp
Sliding occurs when:
t - msn - So > 0
Coulomb Criterion – Frictional Sliding
Why does slip enhance permeability?
28
Mode 1 (extensile) crack:
sn = Sn - Pp
“opens” only if sn < 0 Does not “self-prop”
Mohr Diagram
Shear (Coulomb failure model) crack:
Slips, creating opening, if t - msn - So > 0 Wall offset stiffens the crack
Shear fractures “self-prop”
t
sn = Sn - Pp sn>0
t
sn
Final stress state
Pp<<S3
Pp
Pp<S3
Pp>S3 S3
Pp<S3
Normal
Strike-slip
Reverse
Map V iew Stereonet Cross-section
SHmax Shmin
SHmax Shmin
X
Shmin
b
SHmax
a.
c.
b.
Sv> SHmax > Shmin
SHmax
Shmin
Normal
Strike-Slip
Reverse
SHmax
Shmin
SHmax
SHmax > Sv> Shmin
SHmax > Shmin> Sv
Sv
Shmin
Sv
Sv
Shmin SHmax
Sv
Sv
Different Stress Regimes Activate Different Fractures
Fractured Reservoirs
29
Predicting the Best Well Orientation
30
SHmax
Best well…
• Fracture distribution is clustered
• Stresses are not the same as
when the fractures formed
Therefore…
• The best well is not always
oriented perpendicular to the
most fractures
• The best well can be oblique to
the stress field
• The best well intersects the
maximum number of stress
sensitive fractures.
Perpendicular to SHmax
Perpendicular to the most
fractures
State of Stress Along Natural Fractures While Drilling
Pre-drill: fractures has a very low critical
injection pressure.
Effect: High pore pressure is confined
within the fractures.
While drilling: MW+ECD effect
increases BHP and fractures become
critically-stressed.
Effect: fractures release pressure to
the borehole; well flows.
Circulating kill mud: Extremely
high BHP; more fractures become
critically-stressed.
Effect: BHP>>Pp; losses.
(1)
(2)
(3)
Injection or Depletion is a Dynamic Process!!
35
• Permeability is a tensor
• Stimulation / depletion change magnitudes and principal component orientations
of the permeability tensor
Initial Conditions Injection Test
(lower pressure)
Injection Test
(higher pressure) Fall off
Best well
Best well
Best well
Best well
Characterizing the Dynamic Process
• How do we verify the initial conditions of statistically
based discrete fracture network models, how do we
capture changes in the permeability tensor?
• How can we determine how a naturally fractured
reservoirs will perform with stimulation?
• What are the required stimulation pressures?
• How much production improvement can be attained
with stimulation?
• What will be the reservoir performance with cooling?
36
Permeability Enhancement with Stimulation
37
PAperture
Q =12
3
Pre-slip:
Small open aperture (ao)
Soft (small ) closure
ns
Post-slip:
Larger open aperture (ao)
Stiffer (larger ) closure
ns
),( closure
noafAperture s=
Effective normal stress
Rela
tive p
erm
eabili
ty
oa closure
ns
oa
closure
ns
Sh
ea
r s
lip
Injectivity Test and Calibration
38
Four injection stages were
carried out using increasing
total injection rates
Injection rates were
converted to an effective
injectivity (red dots)
Fracture properties are determined
by fitting a computed injectivity to
this data (blue curve)
Key model parameters:
So = Cohesion
sn = Effective normal stress
ms = Sliding Friction
a0 = Fracture aperture
Barton and Moos, 2009, AAPG
Initial injection reduces
the stress holding
fractures closed but
does not open anything
Injectivity Predictions
39
Pressure fall-off or flow-back…due to permanent
enhancement induced by slip, the productivity
after stimulation is higher than before stimulation
permanent
productivity
enhancement
A rapid increase in
potential productivity
only occurs after
fractures begin to slip
Onset of microseismicity
Best Well Analysis Calibrated with Injection
40
3000 PSI injection S0 = 522 psi
Best Well = 63°/356°
Slipped fractures = 501
1000 PSI injection S0 = 522 psi
Best Well = 78°/356°
Slipped fractures = 116
Using analyses from injectivity tests the fracture flow parameters are established.
The best match to the increased production is cohesion, S0 = 522 psi and
coefficient of sliding friction, m=0.6. The increase in permeability using all modeled
fracture flow parameters is a factor of five.
WELL
PRODUCTIVITY
WELL
PRODUCTIVITY
Barton and Moos, 2009, AAPG
Uncertainty - Well Productivity
41
Ratio ± 3%
To reduce uncertainty,
measure productivity
of an existing well
• Shmax magnitude
• Fracture strength
• Fracture flow properties
Barton and Moos, 2009, AAPG
Open hole breakdown vs.
orientation
SHmax
SHmax
Mud weight vs. orientation
SHmax
Productivity vs.
orientation
SHmax
SHmax
What is the “best” wellbore orientation?
42
Orthogonal
to
hydrofracs
Easiest wells
to break
down
Easiest
wells to
drill
Most
productive
wells
Lessons Learned
Drilling in HT environments may benefit wellbore stability if a proper
drilling strategy is planned.
The high temperature contrast temporarily reduce the
compressional stresses around the wellbore.
If a higher ΔT and a shorter well exposure time can be achieved
it is possible to control breakout development, drill with lower
mud weights, and therefore, minimize the risk of formation
damage.
Cooling of the rock will induce thermal cracking that may
eventually work as a pre-stimulation test of the reservoir.
Lessons Learned
• Optimizing production in geothermal fractured reservoirs requires
characterization and verification of the fracture network geometry, the
fracture flow properties, the response of fractures to the pre-drilled
reservoir stress state and the response of fractures to the changes in
the reservoir state of stress with production
• The most effective way to reduce prediction uncertainties in planning
“Best Well™ trajectories and in predicting stimulation pressures to
enhance natural fractures is to calibrate fracture flow properties,
cohesion (So), sliding friction (ms), effective normal stress (sn ), and
fracture aperture (a0), against the productivity of a pre-existing well
• Applying geomechanics and the reservoir fracture distributions to
model permeability at both ambient pressure conditions and with
shear-enhanced permeability under injection pressure conditions
appears to be a promising improvement to existing fractured
reservoir flow models
Summary of Geothermal Logging Tools for Geomechanics
Application
45
Log Information Supplied Comments Density Bulk density computed from
electron density Estimate of content of Fe, Ca, and Mg relative to Si, from PEF
Required to compute overburden stress by integration Important for porosity Important constraint on gravity modeling PEF can help differentiate limestone, dolomite, and mafic-rich rocks from those with high quartz content, and help with clay mineralogy
Spectral natural gamma
Separately measures the contributions of K, Th, and U to the total GR
Total volume of clay and K-rich minerals U is mobile; high values could indicate paleo-flow zones Th and K help with clay mineralogy Recommended over standard GR
Neutron porosity Porosity from hydrogen content Volume of clays or hydrated alteration products
Important for porosity Useful for clay volume if non-clay minerals are radioactive Not sensitive to porosity variations for porosities below a few percent
Resistivity A measure of the volume of conductive fluids (i.e., porosity) In low porosity rock, a measure of the volume of conductive minerals
Important constraint on magnetotelluric modeling Low resistivity indicates higher porosity, or the presence of electrically conductive minerals e.g. clays, oxides, or pyrite May provide estimates of total dissolved solids of fluids
(after Moos and Ronne et al., 2010, GRC)
Summary of Geothermal Logging Tools for Geomechanics
Application
46
Acoustic Compressional and shear elastic-wave velocities With density, dynamic elastic moduli Stoneley-wave reflections and attenuation
Calibration for seismic or vertical seismic profile Measure of degree of consolidation (stiffness) Can be used to compute rock strength to help constrain stress from observations of wellbore failure Detecting compliant / conductive fractures Estimate of matrix permeability by Stoneley-wave inversion
Crossed dipole acoustic
Azimuthal shear-wave anisotropy Combined with Stoneley modeling, TI elastic moduli Hole ellipticity and its orientation, from centralizer calipers
Useful for better seismic ties using transversely anisotropic velocities Sensitive to stress to determine orientations of maximum and minimum horizontal stresses Sensitive to intrinsic anisotropy (steep open fractures; dipping bedding) Independent information is required to differentiate stress-induced from intrinsic anisotropy Can substitute for 4-arm dipmeter to detect breakouts Processing is carried out off-site
Electrical images Centimeter-scale image of wall rock resistivity Fine-scale fractures, resistive vs. conductive, NOT “open” vs. “closed” Subtle stratigraphy
Important for structural analysis Can be so sensitive to fine scale features it obscures useful information Identifying drilling-induced tensile fractures for stress In contrast to an acoustic image, does not provide complete wellbore coverage Cannot be used alone to detect “permeable” fractures Pads can be damaged by high temperature
(after Moos and Ronne et al., 2010, GRC)
Summary of Geothermal Logging Tools for
Geomechanics Application
47
Acoustic images Several cm-scale image of wellbore wall reflectivity Several cm-scale image of wellbore radius
Excellent to identify mechanically “weak” fractures Less resolution than electrical image logs Provides 100% wellbore coverage Excellent for breakout tensile fracture analyses
Azimuthal resistivity
Dip of electrically-anisotropic materials Resistivity perpendicular and parallel to bedding
Additional structural constraint May help separate structural from stress-induced elastic anisotropy Structural information available post-acquisition
Nuclear magnetic resonance
Porosity Estimate of permeability
Pad-type tools are sensitive to wellbore roughness Permeability estimate requires calibration May have temperature limitations
Wireline straddle packers
Pore pressure Stress from micro-fracturing tests
The stress and pore pressure data can be very important to supplement or replace an extended leakoff test May not be available from all service providers Have severe temperature constraints and pressure limitations
Pulsed neutron Mass fraction of individual elements
Detailed mineralogy Carbon content Precise depth delineation of lithologic contacts
Caliper Hole size, using one or more independently articulated arms
Detects weak fractures and faults that cause wellbore enlargements Single-arm caliper provides information to correct other logs for hole size Multi-arm caliper allows determination of hole shape; if oriented can be used to detect and orient wellbore breakouts for stress determination
(after Moos and Ronne et al., 2010, GRC)