Transcript Slides

MagnetoTelluric
in combination
with seismic data
for geothermal exploration
A. Manzella1
V. Spichak2
Research Council – Institute of Geosciences and Earth Resources(CNRIGG), Pisa, Italy
1National
2GEMRC
IPE RAS, Troitsk, Russia
Why resistivity?
Geothermal
waters have high
concentrations of
dissolved salts
which provide
conducting
electrolytes within
a rock matrix
The conductivities of both the electrolytes and the rock
matrix are temperature dependent in a manner that
causes a large reduction of the bulk resistivity with
increasing temperature.
From Anderson et al., WGC2000
From Pellerin et al., 1996
The resulting
resistivity is also
related to the
presence of clay
minerals, and can
be reduced
considerably when
the clay minerals
are broadly
distributed.
Resistivity should be
always considered with
care. Experience has
shown that the apparent
one-to-one
correlation
between low resistivity
and the presence of fluids
is not correct, since
alteration
minerals
produce comparable, and
often higher reduction of
resistivity with respect to
fluid flow.
From Flovenz et al., WGC2005
Moreover, although the
geothermal
systems
have an associated lowresistivity signature, the
converse is not true.
Why MT?
• Easy, light (now) field layout with respect to
geolectrical soundings
• obtains a MT transfer function, from which not
only resistivity as a function of depth may be
computed, but also the maximum and minimum
resistance (anisotropy) as f.d.
• allows estimation of electromagnetic strike
• may penetrate at any depth, provided the
necessary frequency
• Disadvantage: being based on a weak natural
signal it cannot be used everywhere (EM noise
problem). Modern data processing is required
Various targets can be imaged by
MT and seismic geophysical
methods
• Regional structure (geothermal system)
• Fracture detection
• Monitoring
Regional exploration
Seismic
(reflection more
often used)
Advantages
• good geometrical
resolution of main
lithological units
Disadvantages
• expensive
• small response
from more
permeable zones
Magnetotelluric
Advantages
• cheap
• recognize fluid filled volumes
Disadvantages
• difficulty to distinguish
alteration clays from actual
fluid circulation (frozen
condition)
• poor geometrical resolution
(volume sounding). Improved
with dense spacing
Regional exploration:
MT examples
Minamikayabe Geothermal field, Japan
Takigami Geothermal Area, Japan
From Spichak 2003
From Ushijima et al., WGC 2005
“the low resistivity zone in the northeastern
part is intensive and shallower than that in
the southwestern par, in good agreement
with the geological feature”
Highly conductive areas with apparent
resistivity values not exceeding 6 Ohm⋅m
Las Tres Virgenes Geothermal Area,
Mexico
From Romo et al., WGC 2000
The results suggest the presence of a highly
attenuating and conductive zone along El Azufre
Canyon, which corresponds with the production
interval of wells LV-2 and LV-3/4. A graben
structure is outlined.
Mt. Amiata Geothermal Area, Italy
From Volpi et al., 2003
The interpretation revealed a good correlation
between the feature of the geothermal field
and the resistivity distribution at depth
Ogiri geothermal zone, Japan
From Uchida, 2005
3-D view of the resistivity model, from south.
Shallow blocks to a 200m depth are stripped out
and approximate locations of three faults are
overlaid.
Fracture/fault detection
Seismic
(2D and 3D reflection more often used)
Advantages
• good geometrical resolution
• advanced techniques developed for oil exploration
Disadvantages
• very expensive
• small response from productive fracture
• high cost/effective
Fracture/fault detection
Seismic
(advanced methodology)
• Amplitude Versus Offset (AVO)
• Amplitude Variation with Azimuth and offset
(AVAZ)
• shear wave splitting
Fracture/fault detection
Magnetotelluric
Advantages
• cheap
• resistivity changes are sensible
• EM strike direction may define azimuth
Disadvantages
• low geometrical resolution (lateral resolution
improved when using short site spacing)
Fracture/fault detection:
MT examples
Takigami Gothermal Area, Japan
Mt. Amiata Geothermal Area, Italy
From Tagomori et al., WGC 2005
“the large lost circulation occurred at the
depth of 1300 m BSL for the well TT-14R (90
t/h) when the well crossed through the
electrical discontinuity Fb”
From Fiordelisi et al., WGC 2000
Note the very steep conductor and its
correspondence in location to the fault
defined by seismic reflection data.
Monitoring
Seismic
• It is very effective for gas or for oil investigation
(water flood). Very expensive
• Not so easy to manage for geothermal since
resolution is lower (VP and VS change is smaller
than for oil)
Monitoring
Magnetotelluric
• Phase change of pore fluid (boiling/condensing) in
fractured rocks can result in resistivity changes that
are more than an order of magnitude greater than
those measured in intact rocks
• Production-induced changes in resistivity can
provide valuable insights into the evolution of the
host rock and resident fluids.
• No examples or applications found in literature
• Some examples from SP (electric field) showing
interesting results: is it possible to use the same
kind of information in MT? To be defined
Monitoring
SP monitoring
From Marquis et al., 2002
“the correspondence between the start (and the end) of the stimulation and the
increase (and decrease) in ΔV suggests a casual relationship between the two”
Integration of seismic and MT
data
It can be done
• quantitatively (joint inversion)
• qualitatively (by comparison and separated
inversion constraining the a priori conditions)
• semi-quantitative (joint interpretation)
Example of joint inversion
When resistivity and VP changes depends on the same
effect (e.g., permeability/porosity change) a resistivityvelocity cross-gradients relationship can be established
and incorporated in a joint inversion scheme.
This approach requires a
strong assumption: could be
valid only for limited
volumes and depths
From Gallardo and Meju, 2004
“Evolution of the joint inversion process. Shown are the
resultant resistivity and velocity models for each iteration.
Note the gradual development of common structural
features in both sets of models during the process.”
Example from comparison of
results
Example of using constrained
a priori model in MT inversion
Travale Geothermal Area, Italy
Quality of inversion results improves
when external data are used. Here we
show inversion results using an
homogeneous a priori model (above) or a
detailed a priori model where shallow
lithological units have been identified
from a resistivity point of view. The
resulting models appear like out-of-focus
in the first case, whereas it provides useful
information for comparison with known
geological structure in the second case.
Joint interpretation by postprocessing simulation
Needs:
• geological data
• seismic inversion data (VP, VS)
• MT inversion data (true resistivity)
• rock physics data joining VP, VS, resistivity to
lithology, temperature, pressure, permeability...
The key element in the joint interpretation is the use
of geothermal reservoir simulators to obtain a final
model complying with all available data, both
geophysical and thermo-hydraulic. To be evolved!
Conclusions
MT provides a useful contribution to geothermal
exploration and exploitation, through careful data
acquisition, processing, modeling and interpretation.
Its integration with
other geological and
geophysical data, in
particular seismic,
will improve the
imaging of static
and
dynamic
processes
of
geothermal systems.