geothermal reservoir geomechanics: the … · geothermal reservoir geomechanics: the application of...

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

Characterizing Geothermal Prospects:

The Problem

•

•

•

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

Geothermal Ultrasonic Fracture Imager

Beam Field Spot and Test at 300oC

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.


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


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.


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


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


Uncertainty - Well Productivity

41

Ratio ± 3%

To reduce uncertainty,

measure productivity

of an existing well

• Shmax magnitude

• Fracture strength

• Fracture flow properties


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


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


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


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