The permeability (j[m2]) of fractured crystalline basement of the upper continental crust is an intrinsic property of a complex system of rocks and fractures that characterizes the flow properties of a representative volume of that system. Permeability decreases with depth. Permeability can be derived from hydraulic well test data in deep boreholes. Only a handful of such deep wells exist on a worldwide basis. Consequently, few data from hydraulically tested wells in crystalline basement are available to the depth of 4–5 km. The permeability of upper crust varies over a very large range depending on the predominant rock type at the studied site and the geological history of the drilled crystalline basement. Hydraulic tests in deep boreholes in the continental crystalline basement revealed permeability (j) values ranging over nine log-units from 10 -21 to 10 -12 m2. This large variance also decreases with depth, and at 4 km depth, a characteristic value for the permeability j is 1015 m2. The permeability varies with time due to deformation-related changes of fracture aperture and fracture geometry and as a result of chemical reaction of flowing fluids with the solids exposed along the fractures. Dissolution and precipitation of minerals contribute to the variation of the permeability with time. The time dependence of j is difficult to measure directly, and it has not been observed in hydraulic well tests. At depths below the deepest wells down to the brittle ductile transition zone, evidence of permeability variation with time can be found in surface exposures of rocks originally from this depth. Exposed hydrothermal reaction veins are very common in continental crustal rocks and witness fossil permeability and its variation with time. The transient evolution of permeability can be predicted from models using fictive and simple starting conditions. However, a geologically meaningful quantitative description of permeability variation with time in the deeper parts of the brittle continental crust resulting from combined fracturing and chemical reaction appears very difficult.
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Hydraulic conductivity of fractured upper crust: insightsfrom hydraulic tests in boreholes and fluid-rock interactionin crystalline basement rocks
I . STOBER1 AND K. BUCHER2
1Karlsruhe Institute of Technology KIT, Institute of Applied Geosciences, Karlsruhe, Germany; 2Mineralogy and Petrology,
University of Freiburg, Freiburg, Germany
ABSTRACT
The permeability (j[m2]) of fractured crystalline basement of the upper continental crust is an intrinsic property
of a complex system of rocks and fractures that characterizes the flow properties of a representative volume of
that system. Permeability decreases with depth. Permeability can be derived from hydraulic well test data in deep
boreholes. Only a handful of such deep wells exist on a worldwide basis. Consequently, few data from hydrauli-
cally tested wells in crystalline basement are available to the depth of 4–5 km. The permeability of upper crust
varies over a very large range depending on the predominant rock type at the studied site and the geological his-
tory of the drilled crystalline basement. Hydraulic tests in deep boreholes in the continental crystalline basement
revealed permeability (j) values ranging over nine log-units from 10�21 to 10�12 m2. This large variance also
decreases with depth, and at 4 km depth, a characteristic value for the permeability j is 10�15 m2. The perme-
ability varies with time due to deformation-related changes of fracture aperture and fracture geometry and as a
result of chemical reaction of flowing fluids with the solids exposed along the fractures. Dissolution and precipita-
tion of minerals contribute to the variation of the permeability with time. The time dependence of j is difficult to
measure directly, and it has not been observed in hydraulic well tests. At depths below the deepest wells down
to the brittle ductile transition zone, evidence of permeability variation with time can be found in surface expo-
sures of rocks originally from this depth. Exposed hydrothermal reaction veins are very common in continental
crustal rocks and witness fossil permeability and its variation with time. The transient evolution of permeability
can be predicted from models using fictive and simple starting conditions. However, a geologically meaningful
quantitative description of permeability variation with time in the deeper parts of the brittle continental crust
resulting from combined fracturing and chemical reaction appears very difficult.
Key words: brittle–ductile transition, deep well, hydrothermal, permeability, veins, well test
Received 13 January 2014; accepted 20 August 2014
Corresponding author: Ingrid Stober, Karlsruhe Institute of Technology KIT, Institute of Applied Geosciences,
flow may be absent in some areas because permeability of
the basement is low or because pressure gradients gradu-
ally diminish (with the exception of the cyclic tidal
forces).
Temperature profiles of km-deep boreholes, data, and
observations from hydraulic tests and tidal water level fluc-
tuations all consistently show that fluids in the continental
crust occupy an interconnected communicating pore space
(A)
(B)
Fig. 11. Well test design and test response data from hydraulic tests in fractured continental basement in the KTB pilot hole at 4 km depth (Oberpfalz, Ger-
of j cannot normally be recorded by well test methods.
The variation of the permeability structure of the upper
continental crust is related to neotectonic processes and
the chemical interaction of fluid with the rocks it comes in
contact to along the flow path. Reactive fluid flow causes a
number of chemical effects. Dissolution and precipitation
of minerals on the fracture walls are relevant for the per-
meability of the system. These reactions tend to be kineti-
cally slow at temperatures below 200°C, corresponding to
about 5 km depths from where well test T/H data are
available. Thus, indications of permeability changes in nor-
mal continental crust from that depth range are scarce and
mainly inferential (e.g., long-term temperature changes of
hot springs). An analysis and discussion of permeability–
time relationships related to coupled thermal–hydrological–
mechanical–chemical processes can be found in Rudqvist
(2014).
In the depth range 5–15 km brittle deformation and
Darcy fluid flow still dominates. Evidence for transient per-
meability and its variability with time can be studied at
outcrops from this depth range exposed at the present-day
erosion surface. The evidence indicates that hydrothermal
fluid–rock interaction tends to first increase permeability
after initial fracture formation and then later reduce perme-
ability by depositing solid reaction products on the fracture
surfaces until the fracture is completely sealed and impervi-
ous to fluid flow. The typical time dependence of the con-
ductivity of a single fracture is mostly related to the
variation of the aperture with time. Flow at a given instant
in time can be approximated by the cubic law for fluid
flow, where the flow rate per hydraulic head difference is
proportional to the cube of fracture aperture. The fracture
aperture is a function of time due to mechanical aperture
variations such as extension, compression, shearing, and
other deformational effects, in addition to the progressing
chemical reactions. The permeability of a representative
volume in the lower part of the upper crust above the duc-
tile transition zone and below the reach of deep boreholes,
at a given instant in time, comprises the integral conduc-
tive property of all fractures that contribute to flow.
Although each single fracture probably has its conductiv-
ity–time relationship, the permeability of the representative
volume may not necessarily vary considerably with time.
The time dependence of the permeability in this part of
the upper continental crust is not realistically accessible
from hydraulic data, geophysical data, or from numerical
models. This rather pessimistic view is well grounded in
the extreme complexity of the processes controlling disso-
lution–precipitation reactions in hydrothermal environ-
ments. One often neglected complexity is the effects of the
Earth tidal forces, which keep even deep fluids in continu-
ous motion despite the lack of obvious forces for flow such
as topography in continuous motion. The tidal pumping
of fluids into reactive microporosity increases reaction
kinetics of fluid–rock interaction in a complex manner that
is difficult to quantify and to study experimentally. The
effect of tidal fluid motion in the upper crust is a feature
that deserves more attention in the geoscience community.
ACKNOWLEDGEMENTS
We thank Steve Ingebritsen and Tom Gleeson for taking
the initiative for this exciting special volume of Geofluids
and for taking the burden for all the editorial work this
involves. We acknowledge the very constructive reviews of
all five reviewers, which helped to improve the manuscript
substantially. We again thank Steve Ingebritsen for out-
standing editorial work on our manuscript, which he
greatly improved. Of course, all remaining errors remain in
the responsibility of the authors.
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