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uAbstract
uFigure
captions
uIntroduction
uMethods
uLogging
u3D
 model
uResults
uConclusions
uReferences
uAcknowledgments
uAbstract
uFigure
captions
uIntroduction
uMethods
uLogging
u3D
model
uResults
uConclusions
uReferences
uAcknowledgments
uAbstract
uFigure
captions
uIntroduction
uMethods
uLogging
u3D
model
uResults
uConclusions
uReferences
uAcknowledgments
uAbstract
uFigure
captions
uIntroduction
uMethods
uLogging
u3D
model
uResults
uConclusions
uReferences
uAcknowledgments
uAbstract
uFigure
captions
uIntroduction
uMethods
uLogging
u3D
model
uResults
uConclusions
uReferences
uAcknowledgments
uAbstract
uFigure
captions
uIntroduction
uMethods
uLogging
u3D
model
uResults
uConclusions
uReferences
uAcknowledgments
uAbstract
uFigure
captions
uIntroduction
uMethods
uLogging
u3D
model
uResults
uConclusions
uReferences
uAcknowledgments
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The
well Gross Schönebeck 3/90 (GrSk 3/90; NE of Berlin, Germany;
Figure 1) was drilled in 1990 assigned to
gas exploration in East Germany. The borehole was been closed due to
lack of gas. In 2000 the well was re-opened to use the borehole as
in-situ laboratory for geothermal low enthalphy reservoir studies. The
geothermal reservoir is situated in Rotliegend II strata in part of the
NE German Basin (NEGB). The NEGB is part of an extensive basin system
which extends from the North Sea towards Poland. It is bounded in the
north by the Baltic Shield and in the south by the Variscan fold belt.
Initial basin extension occurred between the latest Carboniferous and
the Early Permian and was accompanied by the deposition of volcanic
rocks which were subsequently covered by a siliciclastic sequence of
alluvial fans, ephemeral stream, playa deposits, and eolian sands (Rieke
et al., 2001). Thick cyclic evaporites and carbonates, deposited during
the Zechstein, are overlain by a thick section of Mesozoic and Cenozoic
sediments. The basic scientific function of the in-situ laboratory
consists in the development of hydraulic stimulation techniques in order
to enhance the permeability in siliciclastic and volcanic formations of
the Rotliegend. In order to assess the permeability of both the
sandstone matrix and the fracture porosity, an extensive logging program
has been conducted, while the geometry of the reservoir is revealed by
3D structural model building procedures.
The
first geophysical logging program was carried out in 1991 to explore a
potential gas reservoir in the Rotliegend sediments. Erdöl Erdgas GmbH
(EEG) provided logging data in digital format for this project (caliper,
spectral gamma ray, resistivity, neutron, density, sonic, and dipmeter).
The GeoForschungsZentrum (GFZ) logging operations were performed by the
Operational Support Group (OSG) implementing caliper, electric, spectral
gamma ray, resistivity, and acoustic measurements. The last logging
campaign was executed by Schlumberger during the winter 2003. Two new
logging tools were used in addition to the GFZ logging program. The
latest porosity measurement was achieved by Reservoir Saturation Tool
logs (RST, mark of Schlumberger) and is used for a comparison with
laboratory data.
The
basic 3D structural model is calculated, based on data of 6
pre-existing, reprocessed 2D seismic sections and 15 wells (locations in
Figure 1). This modeling procedure comprises
the development of a 3D conceptual model interpreted from a
time-thickness map of the Rotliegend II, and the calculation of a final
3D structural model of the reservoir integrating the seismic section and
well data with the conceptual model (Figure 2;
see also Figure 4, track lithology).
The
calculation of grids has been made with an iterative minimum tension
algorithm, the fault model has been developed according specific fault
hierarchies and the horizon model was processed according geological
intersection rules. Lithofacies data are provided by the well data,
resulting in following 5 lithotypes: (1) mudstone, (2) siltstone, (3)
fine-grained sandstone, (4) medium-grained sandstone, (5) coarse-grained
sandstone/conglomerates. To obtain both the structural and the
lithofacies information in one coherent model, an adequate workflow is
specified for the 3D lithofacies modeling. (I) Faulted property grids of
each facies type are calculated over the entire model range, based on
the initial structural reservoir model. Each property grid is the result
of the catenation of property grids of each fault block; effectively the
fault throw is taken into account (Figure 3a).
(II) Each facies grid is normalized between values of 1.0 and 0.0 in
order to delimit the property grid closely to the input data (Figure
3b). (III) The normalized grids are calculated pair-wisely against
each other to obtain 10 sub-grids. (IV) The sub-grids are reconciled
according to their lithofacies type inventory in order to create
0-isoenvelops; that clearly defines the different lithofacies bodies (Figure
3c). (V) The five lithofacies isoshell grids are assigned to each
fault block of the initial structural reservoir model, enabling the
calculation of a comprehensive 3D structural lithofacies model.
Figure 4 shows the lithology distribution
and stratigraphic classification of the well Gross Schönebeck 3/90. The
volcanic formation of the Lower Rotliegend consists of two different
magmatic rock types. The upper series shows higher thorium contents than
the lower series and is suggested to be more highly differentiated with
a trachydacitic or trachyandesitic character. The lower series is
characterized by geochemical properties of a more primitive source and
is classified as basaltic andesite. Both of the volcanic rock suites are
intersected by a crossbedded tuffaceous or tuffitic layer. The
interbedded sediments consist of marls, marly limestones, and mudstones,
subordinately interstratified by thin anhydritic evaporite layers.
The
sandstones and conglomerates of the Havel Subgroup show decreasing gamma
ray values attributed to the depletion of mechanically unstable,
chemically altered volcanic rock fragments within a fining-upward cycle
of the siliciclastic sediments. The nearly clean sandstones of the Lower
Dethlingen Formation (4130-4175m/Lower Elbe Subgroup) show the lowest
gamma ray emissions of all measured clastic sediments, owing to their
low clay content. The gamma ray values increase continuously upward
within the interval of interbedded siltstones/sandstones and reach a
maximum value in the mudstones of the Upper Elbe Subgroup.
The
petrophysical properties of the target horizons are important for the
characterization of the geothermal reservoir. Figure 4 shows a composite
log of the sandstones of interest. We used the neutron porosity
measurement from the RST data (track TPHI RST) to estimate
permeabilities (calculated permeability/logarithmic scale) using the
empirical formula from Pape et al. (1999, eq. 22) and observed
permeability values from 0.04 up to 110 mD. We repeated this procedure
with porosity data calculated from density and sonic measurements and
received transmissibilities in the range from 0.25 to 0.70 Dm for a 80m
sandstone interval (4100-4180m).
The
comparison of logging data (Tphi RST) with measured core porosities as
well as a comparison of calculated
permeabilities with core permeabilities is shown in tracks 3 and 4 (core
porosity: n =290; core permeability: n=109). The results indicate a good
correlation between logging data, permeability estimation, and core
data. The right-hand track illustrates a borehole temperature
measurement representing the current state after the last stimulation
experiment. Temperature logs record changes of the temperature field due
to injection and production of brines during hydraulic experiments. The
bright yellow areas mark the stimulated intervals. Three temperature
minima are recognized in identifying productive zones. The upper two
temperature signals prove the existence of pay sand horizons. The
productivity of the lowermost reservoir horizon indicates a cumulative
flow out of porous sandstones (Havel Subgroup) and of the naturally
fractured volcanic formation (Lower Rotliegend).
The 3D
lithofacies model reveals the spatial distribution of the five
lithofacies types that are part of the Rotliegend II siliciclastics (Figure
5). The conglomerates and the medium-grained sand were only
deposited south of a NW-SE-striking strike-slip fault and in
NE-SW-trending graben structures. The thickness of these facies types
increases toward the S to SW. The fine-grained sand type is distributed
over the whole model area and shows increasing thickness toward the WSW.
The facies type mudstone shows increasing thickness toward the NW and
SSW.
The
target horizon is in the fine- and medium-grained facies type at an
average 4150 m TVD due to its efficient porosity.
Recent
investigations on the state of stress in the NEGB have revealed a
constant ± N-S trend of the direction of the maximum horizontal stress SH
in the area north of Berlin (Röckel and Lempp, 2003). The mean
direction of SH was determined in
the open-hole section of Gross Schönebeck from borehole breakouts and
vertical hydraulically induced fractures to 18.5° ± 3.7° (Holl et al.,
2004). Thus only a small acute angle exists between SH and the NNE-trending faults. Röckel and Lempp (2003) have shown
that the current state of stress in the NGB is generally a
normal-faulting state; however, transitions to strike-slip
faulting-states may not be excluded.
The
recent stress regime in the Gross Schönebeck area is not fully known;
however, in both possible cases (SH
= s1
or SV =
s
1) the potential kinematic behavior of the NNE-trending faults is
transtensional, whereas the NW-trending faults may suffer frictional
blockade. The N- to NNE-trending faults are considered to be
hydraulically conductive due to its shear stresses and resulting high
tendency of transtensional slip (see also Zoback and Healy, 1992). Based
on this knowledge, a 3D hydrotectonic model is developed for the
reservoir (Figure 6), indicating
hydraulically conductive structures. This should be taken into account
when new potential drilling sites are going to be localized.
The
combined use of pre-existing and newly generated data sets provides new
understanding for the characteristics of the geothermal reservoir in the
Lower Rotliegend formation of the North East German Basin. The
multidisciplinary approach included implementation of existing well
data, reprocessing of pre-existing 2D seismic sections, interpretation
of various newly generated well log data, and reconciliation of all data
to a coherent 3D geological model .
The
petrophysical properties of the target, the depositional environment,
and the reservoir geometry are now well known. These data are crucial
for modeling the thermo-hydraulic conditions within the geothermal
reservoir during production. Future work will focus on the use of
geostatistical models, combining porosity and permeability distributions
with sedimentary facies architecture.
Baltrusch, S., and S. Klarner, 1993, Rotliegend-Gräben in
NE-Brandenburg: Zeitschrift der deutschen geologischen Gesellschaft, v.
144, p. 173-186.
Holl, H.-G., I. Moeck, and H. Schandelmeier, 2004,
Geothermal well Großschönebeck 3/90: A low enthalpy reservoir (Rotliegend,
NE Germany): Proceedings, 66th EAGE Conference,
F032, Paris, France.
Pape, H., C. Clauser, and J. Iffland, 1999, Permeability
prediction based on fractal pore-space geometry: Geophysics, v. 64, no.
5, p. 1447-1460.
Rieke, H., D. Kossow, T. McCann, and C. Krawczyk, 2001,
Tectono-sedimentary evolution of the northernmost margin of the NE
German Basin between uppermost Carboniferous and Late Permian (Rotliegend):
Geological Journal, v. 36, p. 19-38.
Röckel, T., and C. Lempp, 2003, Der Spannungszustand im
Norddeutschen Becken: Erdöl Erdgas Kohle, v. 119, no. 2, p. 73-80.
Ziegler, P.A., 1988, Evolution of the Arctic-North
Atlantic and the Western Tethys: AAPG Memoir, v. 43, 198 p.
Zoback, M.D., and J.H. Healy, 1992, In-situ measurements
to 3.5 km depth in the Cajon Pass scientific research borehole: Journal
of Geophysical Research, v. 97, p. 5039-5057.
This
multidisciplinary project is a joint research project of GFZ Potsdam,
BGR Hannover, GGA Hannover, TU Berlin, Ruhr University Bochum,
University Stuttgart and industrial partners (GTN Neubrandenburg, BWG
Neubrandenburg, MeSy Bochum). It is funded by BMWi, BMBF, BMU, MWI and
MWFK. The 3D models are processed with EarthVision®,
Dynamic Graphics Inc. (DGI). Our thanks are addressed to Erdöl Erdgas
GmbH (EEG) for data release, and to DGI (namely P. Luxey) for support
and advise
.
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