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The Coso EGS Project – Recent Developments
Pete Rose1, Joe Moore1, Katie Kovac1, Mike Adams1, Jess McCulloch2,
Paul Spielman2, Judith Sheridan3, Steve Hickman4, Nick Davatzes4,
Bruce Julian4, Gillian Foulger4, Ralph Weidler5
1Energy and Geoscience Institute, University of Utah
2Coso Operating Company
3Geomechanics International
4U.S.G.S. Menlo Park
5Q-Con
Great Basin Geothermal Workshop
Reno, Nevada
November 5, 2004
Coso/EGS Objectives
To create an Enhanced Geothermal System
on the margin of the Coso field through the
hydraulic, thermal, and/or chemical
stimulation of one or more tight injection wells
To increase the productivity of the Coso field
by 10 MWe
To develop and calibrate models of the
geomechanical/geochemical/fluid flow
processes in order to extend the Coso/EGS
concepts to wherever appropriate tectonic
and thermal conditions apply.
The Coso/EGS Concept
Wellbore stimulation produces permeability
enhancements due to a combination of hydraulic,
thermal and chemical effects.
First order effects are hydraulic and thermal.
Fractures re-open through shear failure.
Fractures that fail in shear are self-propping.
Second order effects are thermal and chemical.
Fracture apertures increase due to rock thermal contraction.
Fracture apertures change due to mineral dissolution and/or
precipitation.
These concepts can be extended to other geologic
settings where appropriate tectonic and thermal
conditions exist.
The East Flank of the Coso Geothermal
Reservoir is an Excellent Setting for an
Enhanced Geothermal System due to:
High reservoir temperatures
Often exceeding 300oC at less than 10,000 ft
A high degree of fracturing
Many fractures are optimally orientated for shear failure
Existing reservoir stresses are close to critical
Good candidate wells for testing of stimulation techniques
83-16
34A-9
34-9RD2
EGS R&D objectives that run parallel to COC’s reservoir-
management (injection) needs
EGI
The
Coso
Project
The Coso
East Flank
EGS Study
Area
Two Stimulation Targets
34A-9
Successful
stimulation history
Limited wellbore
data
Low injection
pressures
34-9RD2
Extensive data
and analysis
High injection
pressures planned
The Coso/EGS Organizational Structure
Fracture/Stress Analysis
Judith Sheridan and Steve Hickman
Objective:
To characterize reservoir fracturing and
stresses in order to model and predict
fracture shear failure and the subsequent
increases in permeability that result from
hydraulic stimulation
Analysis Technique
Fractures with Significant Apparent Aperture
Coso 38A-9
All Fractures Fractures with
Significant Apparent Aperture
Dip Dip Azi Dip Dip Azi
Depth, feet MD
Depth, MD
Good
data
Fair
data
Poor
data
Stress, psi
0 5000 10,000 15,000
0
Coso Well 38C-9
1000 Pp and Failure Envelope
drawn for surface hydrostat
2000 (pre-production)
Depth (TVD), ft
3000 SHmax bounds from general
EMI Co =15 22 kpsi absence of breakouts,
4000 Log S assuming minimum Co shown
hmin S
Hmax
(22 kpsi preliminary Coso
5000 Pp strength tests, 15 kpsi typical
Sv Strike-Slip granites [Lockner, 1998])
6000 Faulting
1.0
m = 0.6
7000
EMI
Co =15 22 kpsi EGS Target Depth
Log
8000 S (Well 34-9RD2)
S hmin
Hmax
extrapolation
9000 TD
Transitional NF SS Stress Model Strik
Ambient +1000 psi Am
Mode Strike-slip Faulting Stress Model
00 psi Ambient +500 psi
Advanced Logic
Technology’s (ALT)
High-Temperature
Borehole Televiewer
Prototype
•Funded by DOE and NAVAIR
Geothermal programs
•Needed to provide formation
and fracture information for
geothermal well characterization
•Upgraded low-temperature
televiewer system capable of 10
hour operation at +275°C
(excursions to +300°C)
Acoustic and
Resistivity Image Log
Comparison
•High-temperature borehole
televiewer log (on left) showing
images of ultrasound signals
reflected from the borehole wall
•Formation Microscanner (FMS) log
(on right) showing images of
resistivity contrast on the borehole
wall surface
Magnetotellurics
Objectives:
•To determine the
resistivity structure of
the Coso EGS study
area
•To evaluate the
effectiveness of using
MT techniques to
monitor porosity
changes in response
to hydraulic stimulation
MT
Survey of
Coso/EGS
East Flank
Study
Area
Petrologic/Petrographic Analysis
Joe Moore and Katie Kovac
Objectives:
To correlate rock type and alteration
mineralogy with permeability enhancements
resulting from various stimulation approaches
To characterize the vein-mineral paragenesis
and thermal history of the east-flank
compartment
Lithologic Characterizations at Coso
(Lutz and Moore, 1997)
Biotite Quartz Diorite
Hornblende-Biotite Quartz Diorite
Biotite Schist / Metamorphics
Biotite Granodiorite
Muscovite-Biotite Granite
Microgranite
Petrography and Petrology of 34A-9 from Wellbore Cuttings
Petrography and Petrology of 34-9RD2 from Wellbore Cuttings
Stimulation Experiments
34A-9 Stimulation History
Drilled in 1993, 34A-9 had
temperatures exceeding 300 C
but very low injectivity.
After a series of condensate
injections totaling 72,000 bbls,
the injection rate was 800 gpm
at 0 psi WHP.
A flow test indicated moderately
high productivity.
The well was used for injection, but damage in the
shallow casing required that it be shut in.
Recent 34A-9 Stimulation Results
After a ‘tie-back’ repair of the shallow casing,
34A-9 was placed on injection.
2000 gpm of hot, separated brine
60 psi WHP
Microseismicity was monitored during and
after the stimulation
Cold condensate injections planned
Tracer test initiated
Tracer Returns from Stimulated Well 34A-9
FY-2005 Plans:
Preparation, Reworking, Drilling, Stimulation, and
Evaluation of the EGS Injector 34-9RD2
FY-2005 Plans (Continued):
Preparation and Drilling
Rig up.
Pull 7” liner.
If possible, conduct FMS log.
Cement 5400’ zone.
Run and cement 7’ casing to 7600’.
Drill 6.125” hole, 7600’ to 8100’.
Take spot core.
Conduct mini-hydrofrac.
Run wireline image logs.
FY-2005 Plans (Continued):
Stimulation
Conduct injection tests
Design experimental conditions (fluids, flow
rates, pressures, geophysics, etc.)
Conduct hydraulic stimulation experiment at
pressures below Shmin
Microseismics monitoring
Magnetotellurics (MT) survey
Run post-stimulation wireline image logs,
PTS logs
Preliminary (Summary) 34-9RD2
Stimulation Plan
Phase Pmax (psi) V (bbl) Description
Fracture Initiation 1,500 2,000 Initiate fracture shearing at
upper bound of target P
PTS log 200 -- Check flow profile
Fracture propagation-- 1,500 30,000 Propagate fractures,
Phase I monitoring with microseismics
PTS log 1,500 -- Check flow profile
Stepwise hydrofrac--if 3,000 2,000 Create hydraulic fractures
needed
Fracture propagation-- 1,500 50,000 Continue to propagate
Phase II fractures, increasing
connectivity
Modeling Hydraulic, Thermal, and Chemical
Stimulation Processes
Fracture propagation using a poro-thermoelastic
code: University of North Dakota
Aperture changes due to thermal contraction using
T2STR (stress/strain added to TOUGH2 ): Kansas
State University
Aperture changes due to chemical dissolution and/or
precipitation using TOUGHREACT: Lawrence
Berkeley Laboratory
Self-propping vs. fracture sealing upon shear failure
using outcrop, core, cuttings, and borehole image
data: USGS, Menlo Park
Coso/EGS website:
http://egs.egi.utah.edu
Acknowledgements
This work was supported by the U.S. Department of
Energy, Assistant Secretary, Energy Efficiency and
Renewable Energy under grant DE-FC07-00ID14186.
Such support does not constitute an endorsement by the
U.S. Department of Energy of the views expressed in this
publication.
EGI
