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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