Mechanical behaviour, natural
permeability, and stimulation of
fractured reservoirs
Agust Gudmundsson, Ivar Grunnaleite*, Stefan Hoffmann,
Belinda Larsen**, Christian Müller, Sonja L. Philipp
Department of Structural Geology and Geodynamics Geoscience Centre,
University of Göttingen, Germany
*The Carbonate Group, International Institute of Research, Stavanger,
Norway
**Department of Earth Science, University of Bergen, Norway
For a natural or man-made geothermal field there must be: a heat source,
fluid (water), and sufficient permeability. Example: a high-temperature
field in North Iceland.
Main high-temperature fields (>200C at 1 km depth) as red dots and
low-temperature fields (<150 C at 1 km depth) as brown areas.
Volcanic zones (white), volcanic systems (red), and central
(composite) volcanoes (black dots).
High-temperature fields are mostly confined to central volcanoes (composite
volcanoes and calderas), such as the Hengill Volcano close to the capital Reykjavik.
A geothermal power plant now about 200 MWe (electricity) but
perhaps 300 MWe in the near future. The Hengill Volcano, SW
Iceland, is 25 km from Reykjavik.
A. T. Gudmundsson, 2001
In a central volcano the main heat
sources are swarms of inclined sheets
and the shallow magma chamber.
Low-temperature fields occur outside
central volcanoes; their heat sources
are regional dyke swarms and the
general heat flow through the crust.
Palaeogeothermal fields can be studied in deeply eroded, extinct
central volcanoes such as the Geitafell Volcano, Southeast Iceland.
S. Burchardt 2006
A narrow depression marks the contact between 100% sheets (right) and the
source chamber (gabbro, left) in the 5-6 Ma old Geitafell Volcano, SE Iceland.
Gabbro
Sheets
Part of the Slaufrudalur magma chamber, at 2 km depth. Walls and the roof are
well exposed, the latter being dissected by dykes (yellow arrows).
Geothermal electricity production in Iceland to 2002: already obsolate figures.
Fractured reservoirs can be of various types. In igneous rocks columnar
(cooling) joints may generate highly permeable layers.
Fingal‘s cave, Scotland
Dverghamrar, Iceland
Giant‘s Causeway, Ireland
(Hatcher
Fractures (Bruch) tend to become arrested, or at least offset, at weak
contacts between (particularly mechanically dissimilar) layers.
Fractured sandstone
layers in Germany:
Buntsandstein
at
Reinhausen, Lower
Saxony
1m
A common feature in layered rocks is that many fractures become arrested at
contacts between layers. The percolation threshold may thus not be reached.
Limestone (stiff) and shale (soft) layers, Wales, UK
About 20 degrees change in trend of joints between limestone layers
Mattinata Fault in Southeast Italy is an active strike-slip fault.
There are many quarries (carbonate rocks) along the fault trace.
Grunnaleite, Carbonate Group, 2006
Whereas other quarries are in the damage zone of the fault.
Grunnaleite, Carbonate Group, 2006
To use or make a fractured, fluid-filled reservoir, we must know how its
permeability is generated and maintained. Also, how the fractured rock responds
to a drillhole, hydraulic fracturing, and/or stimulation?
Grunnaleite, Carbonate Group, 2006
Fault Zones, Here Dip-Slip, are Composed Mainly of Two
Mechanical Units; a Damage Zone (with Subzones) and a Core
Gently Dipping Normal Fault Cutting Limestone and Shale, Kilve
Normal Fault, Kilve, England; the main permeability is in the hanging wall.
In a subsurface reservoir, a fault contributes to the permeability
depending on the infrastructure of the fault, its trend in relation to
the current stresses, and its trend in relation to the “hydraulic
gradient”.
Grunnaleite, Carbonate Group, 2006
Damage zone and core increase in thickness as the fault develops:
Normal faults in sandstone and shale, Sinai (Walsh et al. 2002).
Most faults have a core and damage zone; in the damage zone the fracture
frequency increases towards the core, or the fault plane. Here: a microfault.
TaFractures per metre
16
14
12
10
8
6
4
2
0
1
4
7
10 13 16 19 22 25 28 31 34 37
Metre along profile
An extinct geothermal system, consisting of mineral veins, in a
fault zone in Iceland
One can model fluid transport in a fractured reservoir using the
program Fred (Fracman); is primarily kinematic. Stumo (Bergen) 2002.
Young’s Modulus Decreases Towards the Core of a Fault
Core and damage zone tend to have their own local stresses, different from teh
surrounding (regional) stress field. Trajectories of σ3. Fault zone with E =1, 5, 10
GPa; host rock E=40 GPa; loading -5MPa
Hydraulic fracturing
(Smith & Shlyapobersky 2000)
Hydraulic fractures are primarily extension fractures, although in
detail…
(modified from Twiss & Moores 1992)
(Brenner 2003)
…they are mixed-mode fractures that tend to use the existing
weaknesses and fractures in the target layer.
Which is of course one reason why we have numerous earthquakes
during any type of reservoir stimulation.
(Asanuma
et al. 2002)
Ideally, a hydraulic fracture is a horizontal extension fracture.
u W  pe
Qx  
12  x
3
Qx = volumetric fluid flow rate of a water-filled
fracture in the horizontal xy-plane
For a lateral flow in a fluid-driven fracture along a
“neutral buoyancy” layer, the only available driving
pressure is the excess pressure in the drill hole.
A hydraulic fracture may propagate partly vertically, say along an
existing fault in which case buoyancy contributes to the driving pressure.
 pe 
W





g
sin


r
m


12 
L 
e u
QL 
3
Qze = Volumetric flow rate through an elastic hydraulic fracture
u

= Aperture (opening) of the fluid-filled fracture
= Dynamic (absolute) viscosity
W = Width perpendicular to flow direction
 m = density of the magma (ρr = density of the host rock)
g = acceleration due to gravity
sin
= dip of the hydraulic fracture
 pe = excess-pressure gradient in flow direction
z
Stiff lava flows and soft pyroclastic layers, Iceland
Soft
Stiff
A.T. Gudmundsson, 2001
Hydraulic fracture aperture and the induced stress at the
surface depend on mechanical layering.
An arrested dyke in the Holocene rift zone, Iceland
The mechanical contrast at a contact, rather than layer thickness,
decides if a fracture becomes arrested. Shale/limestone contact, Wales.
When a fracture meets a multilayer or a weak contact, it
commonly becomes arrested (arrows) or, at least, offset.
Fracture arrest of this kind is a universal feature of
heterogeneous materials, including layered rocks and composite
materials, and much studied in fracture, rock, and micromechanics. Carbonate rocks, Mattinata, Italy.
Many thin layers
A fluid-driven extension fracture follows the trajectories of the
maximum principal compressive stress.
Stress concentration s3
Stress trajectories s1
Hydraulic fracture tends to open up weak discontinuities
such as fault zones. Comsol modelling.
Opening depends on attitude of fault in relation to the attitude
of the hydraulic fracture.
Main reason for opening of discontinuities is fracture-parallel tensile stress.
Cook and Gordon, 1964
Beasy model: hydraulic fractures meets a fault zone.
Horizontal model Sigma 3 undeformed Host rock 20GPa weak contact 1GPa
Overpressure 5MPa
Beasy model: hydraulic fractures meets a fault zone.
Beasy model: hydraulic fractures meets a fault zone.
60° model undeformed Sigma 3 Host rock 20GPa weak contact 1GPa
Overpressure 5MPa
Beasy model: hydraulic fractures meets a fault zone.
An extension fracture changes to a shear fracture in a soft layer
Fault attitude changes from 323/85 in the stiff dyke
to 333/65 in the soft pyroclastic rock
Hydraulic fracture meeting a weak fault plane tends to…
…open up the fault plane and become arrested.
So, what do we need to model a fractured fluid-filled geothermal
reservoir? Here: Gypsum Veins in Mudstone at Watchet, SW England
For example of this type: 1 = injection drillhole, 2 = stimulated
fracture system, 3 = production drill holes, and 5-8 = surface
installations, e.g. power plant, monitoring and maintenance
equipment. (From Geothermische Vereinigung, 2003).
Geometry/
Type
Geometry/
Type
Hydraulics
Geomechanics
Reservoir
Characterisation
Geomechanics
Heat Flow
Answer: We need to know or be able to infer the geometry and
types of fractures, its geomechanical behaviour, the fluid
transport and heat flow, and the state of stress.
How should we make successful man-made geothermal reservoirs?
• Make detailed studies of the fault systems, state of stress, and target
rocks exposed at the surface, preferably but not necessarily nearby
the potential reservoir site.
• Combine results on natural geothermal fields (e.g., Iceland) and
permeability of subsurface reservoirs (partly studied at the surface;
e.g., petroleum companies) to develop permeability models.
• Success in use of man-made geothermal energy depends to a large
degree on forecasting the potential reservoir-rock fractures, their
interconnectivity, and how they respond to stresses (and pressures).
• Rock mechanics, fracture mechanics, damage mechanics,
micromechanics contain well-developed principles that can help us
(with field data) to make accurate forecasts. Results from these
fields should be used as principles when making numerical models
of fluid-filled fractured reservoirs.
Thank you!