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Paper Number: 95
Stress
field and natural fracture systems in Lower Paleozoic shale belt in
Poland based on borehole images
Zacharski, J.1, Buniak, A.1,Hansen,
B.2,Ruehlicke, B.2,Porebski,
S.3
1Orlen
Upstream Sp. z o.o., Warszawa, Poland
2Eriksfiord
sprl., Jumet, Belgium, birger.hansen@eriksfiord.com
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Borehole images from wells at Syczyn, Berejow, Gozdzik, Streczyn,
Uscimow and Stoczek along the East European margin in Poland were
investigated for structural features. Wellbore instability features were
used to calculate earth stress, borehole stability and fracture
stress.
At Uscimow, basinal tilt to SW during Cambro-Ordovician was followed
by straight subsidence during Silurian, and post-Silurian (Hercynian?)
tilt en-bloc as seen at Syczyn and Berejow. At Stoczek, a N
basin tilt before Silurian deposition is likely. WSW dipping faults were
found in all 4 wells. At Streczyn, a conjugate shallow ENE set was also
found. These faults are not seen on seismics. They can be assigned a
Hercynian origin with general tectonic transport towards ENE, but
recently reactivated, as we note stress-deflection at Streczyn.
Densely spaced NNE-SSW trending subvertical fractures are found in
Uscimow, Streczyn and Berejow wells. Steep ENE dipping fractures were
found in addition at Berejow and Streczyn, whereas at Syczyn and
Stoczek, this set dominates. NNE dipping steep fractures were also found
in lower part of the Silurian at Uscimow. All steep fractures seem to
constitute a strike-slip set in response to a general N-NNE horizontal
stress; alternatively, they formed by hydraulic tension and later
slipped tectonically. The fractures remain sensitive to strike-slip in
the current stress field with the exception that at Stoczek, the stress
is aligned with the fracture strike.
Both principal horizontal stresses are high, placing the area in
strike-slip regime. There is high horizontal stress anisotropy but due
to rock strength and low fluid pressure, the vertical wells are quite
stable to drill. The regime is favourable for drilling horizontal wells
along the minimal stress axis. An FEM model with anisotropic material
further explains the compressional and tensional stability of Syczyn and
Berejow laterals.
There are no obvious low-ν zones in Silurian to constrain upward
escape of artificial fractures apart from some thin tuff horizons, but
the strong anisotropy of shale should favour horizontal propagation. The
Ordovician limestone is also very stiff, hence fracture prone, so
fractures may also escape downwards. These facts are supported by
microseismics from fracturing of a Syczyn bed-parallel lateral. There is
some evidence of overpressure in the lower part of the Silurian, which
would make it more prone to fracturation and help constrain artificial
fractures to the lower (prospective) horizon.
