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Geothermal energy..and modeling of permeability enhancement in
geothermal reservoirs by shear-dilation stimulation
Inga Berre
Department of Mathematics, University of Bergen, NorwayYAE, EASAC/JRC, AE Bergen June 15, 2015
Conventional hydrothermal resources
Early Development
First experimental work in Lardello, Italy, by Prince GinoriConti in 1904, steam power plant, 5 light bulbs from 10 kWe dynamo.
World Geothermal Power
[Bertani, WGC 2015]
• 21 new power plants came online in 2014 adding about 610 MW of new capacity (highest since 1997)
• The global market is at about 12.8 GW of operating capacity (January 2015) in 24 countries.
• The World Bank estimates that 40 countries can meet a large proportion of their electricity demand by geothermal power.
Conventional resources:• Natural hydrothermal (<3000m depth)• Shallow (ground source heat pumps)
Unconventional resources: • Deep natural hydrothermal (>3000m depth)• Deep conduction dominated• Geopressured• Co-produced fluid (petroleum industry)• Supercritical/volcanic• Offshore
Resource types
0 km
3 km
6 km
100°
200°[Adapted from Geothermal Explorers Ltd.]
Geothermal potential
• More than half of the projected increase from EGS resources• Substantially more research, development and demonstration needed to
ensure that EGS becomes commercially viable by 2030, but EGS could potentially provide base-load power from a large energy resource that is well-distributed globally
Regional geothermal heat and power production and shares of cumulative global production
Enhanced geothermal systems
• Engineered reservoirs to produce energy from geothermal resources that are otherwise not economical due to lack of natural permeability and fluid.
• Reservoir performance depend on the presence of open, interconnected and distributed fracture networks or the ability to create such networks.
Enhanced Geothermal System (Soultz, France)
Source: Figures from BINE projektinfo 04/09Photo: I. Berre
Enhanced geothermal system (EGS) technology
[US DOE, 2008]
Enhanced geothermal system (EGS) technology
Key numbers for a commercial projectPower: 3-10 MWLifetime: 25 yrs
Rate: 50-100 kg/sTemperature:
>150°CBorehole dist.: 500-2000 mFracture area: 5-10 km2
[US DOE, 2008]
Hydraulic stimulation
Hydraulic fracturing • Single tensile fracture
propagates in direction perpendicular to least principal stress (p>σ3)
σ1
σ2σ3
Hydraulic stimulation
Hydraulic fracturing • Single tensile fracture
propagates in direction perpendicular to least principal stress (p>σ3)
Shear dilation • Induced slip on preexisting
fractures (p<σ3)
σ1
σ2σ3
Hydraulic stimulation
Hydraulic fracturing • Single tensile fracture
propagates in direction perpendicular to least principal stress (p>σ3)
Shear dilation • Induced slip on preexisting
fractures (p<σ3)
Mixed mechanism stimulation • shear and tensile fractures
Stimulation mechanisms are not sufficiently understood
?
σ1
σ2σ3
Induced seismicity
Different stages of injection for the ParalanaEGS project, Australia. Color events during injection, gray events after shut-in. Size of events correspond to size of circles. Largest event 2.3.
Location of microseismic events proxy for where fracture slip occurs [Albaric et al., 2013]
Induced seismicity
• Induced seismicity up to magnitude 3.4 observed in EGS projects
• Below 3.0 is considered as microearthquake
• Basel EGS project in Switzerland closed after 3.4 event (see Fig.)
[Häring et al, 2008]
Injection rate
Wellhead pressure
Trigger event rate
Earthquake magitudes
Large induced seismicity after shut-in? Explanation [Parotidis et al., 2004; Baisch et al., 2010]:• pressure diffusion may lead to local pressure increase
even in the shut-in period after an injection has been terminated
• concentration of shear stress develops due to induced slip
? Explanation [Dahm et al., 2010; Jung, 2013]:• induced by large tensile wing fractures that develops
d i ti l ti
Pressure in fault zone
Radial distance from well
At time ofshut-in
At time of largestmagnitude event
Environment• Large difference between
maximum and minimum horizontal stress
• Natural fractures already exist• Hard rock
Consequences• Fracture slip results in enhanced
fracture aperture due to contacting asperities or irregularities of the rock surfaces
• Its magnitude can be much larger than normal stress-induced opening
Shear dilation
shear slippage
in situ
final dilation
σ1
σ2
Shear dilation
e0 initial aperture
dilation angleE0 Us
ϕdilΔEs
initial aperturedilation
shear displacement
Shear displacement -> permeabilityenhancement
Shear displacement Us (mm)
Frac
ture
per
mea
bilit
y (c
m2 )
Nor
mal
dis
plac
emen
t ΔE s
(m
m)
Shear displacement Us (mm)
[Lee and Cho, 2002]
Mechanical aperture E (mm)
Hyd
raul
ic a
pertu
re e
(mm
)
[Lee and Cho, 2002][Lee and Cho, 2002]
Hydraulic stimulation - Coupled processesHydro-mechanical coupling1) fluid flow in the dominating fractures and
the surrounding fractured rock2) opening, closing, shear slippage and
dilation of fractures due to fluid pressure, normal and shear stresses
3) rock matrix deformation
Linear poroelasticity/stress
response
Linear poroelasticity/stress
response
Joint deformation
model
Joint deformation
modelFlow (matrix and
fractures)Flow (matrix and
fractures)
Fractured crystalline rock
≈100m ×100m
Fractured crystalline rock
≈25m×25m
≈100m ×100m
Fractured crystalline rock
≈5m x 5m
≈25m×25m
≈100m ×100m
Fractured crystalline rock
≈5m x 5m
≈25m×25m
≈100m ×100m
Fractured domains – modeling concepts
Discrete Fracture Matrix Model
Discrete Fracture Network Model
Continuum model
Enables dynamically changing apertures without re-gridding.
DFM model – hybrid representation of fractures
Fractures in geometric grid Computational fracture domain
Flow simulations - Previous work
• DFM model with hybrid representation of fractures
Applied in • Multiscale method for
pressure equation • Improved upscaling for
heat transport where transfer functions between continua are based on fine-scale descriptions
[Sandve, et al.,2013]
Final Aperture After Stimulation 2
m
2
3
4
5
6
7
x 10−5
Stimulation of fracture network - B Final Aperture After Stimulation 1
Time of Flight Before Stimulation
Time of Flight After Stimulation 1
Aperture Before Stimulation
Time of Flight After Stimulation 2
1
2
3
4
5
6
7
8
9
10
11x 105
Coupled processes - Outlook
• fluid flow in the dominating fractures and the surrounding fractured rock
• opening, closing, shear slippage and dilation of fractures due to fluid pressure, normal and shear stresses
• stress transfer due to slip and opening
• rock matrix deformation• hysteresis effects• propagation of splay fractures• possible phase change of the fluid• reservoir cooling due to the
injection of cold fluid into the formation
current work
next phaseof project
more effects…
EGS – future useDrivers• Climate change and negative impact of conventional power
and heat production• Energy independence and security • Need of base-load power• Increasing costs and growing energy demand• Limited land use
Barriers• High up-front investment costs• Resource development risk• Limited awareness and information• Lack of incentive schemes• Health, safety and environmental issues (mainly induced
seismicity)
Technological challenges• Improved geological data and exploration methods• Improved Enhanced Geothermal Systems technology• Larger number of demonstration projects
National policy support from employees in the petroleum sector
www.medborger.uib.no 33Ivarsflaten and Tvinnereim (University of Bergen)
www.medborger.uib.no
Acknowledgements