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Analyses
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RACOS®
RACOS® (Rock Anisotropy Characterization
On Samples) can be used to determine the 3D in situ stresses, and
their changes resulting from modification of pore pressure, and also the
related deformations and the changes in significant rock parameters (pore
volume, hydraulic conductivity etc.). The procedure involves special tests on a
small quantity of core. It requires no on-site activity, can be carried out at
any time after the core was taken and makes no prior assumptions about the rock
behaviour.
The analyses are based on the measurement
of the directional propagation characteristics of shear and compression
waves in loaded samples of rock to determine the magnitudes and directions
of the principal propagation velocities.
If oriented coring was not used the core to be tested can be geographically
reorientated using various procedures. One relies on viscous remnant magnetism
in the rock and another on the comparison of scans of the
core surface with images of the geographically reorientated wellbore wall.
The information can also be checked in RACOS® using geological
and tectonic data for the cored area.
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Stresses
In RACOS® the 3D effective stresses are determined directly using special
analyses of the changes in the rock structure which occur at the time of coring. The total
stresses are then calculated using the pore pressure and the pore pressure effectiveness
(derived from the deformation analysis). Effective paleo loading conditions can also be
determined.
With RACOS ® it is also possible to calculate in advance the in situ stress changes
resulting from modification of the pore pressure.
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Deformations
For these, in
addition to the elastic wave velocities on loaded samples, the density
and porosity on unloaded samples are also measured.
The loading-dependent axial deformations of a loaded plug are measured
in order to calibrate and check the analysis results.
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Elastic deformation and pore pressure effectiveness
Loading-dependent elastic deformation parameters are determined in RACOS®
on the basis of elastic theory with the measured characteristics of the elastic waves and the
density of the samples. From these parameters the elastic components of deformation and the pore
pressure effectiveness can be calculated directly.
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Complete deformation
To evaluate complete deformations in RACOS® the measured loading-dependent compression
wave velocities and the initial density and porosity of the relevant samples are used to determine
the loading-dependent changes in porosity. The 3D deformations of the rock and of its pore space are
then determined using the deformation parameters for the solid components derived from the measured
characteristics of the elastic waves.
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Further information to download 
For more information on
RACOS® see:
Braun, R.; Jahns, E. & Stromeyer, D.: Ermittlung von Gebirgsstrukturen
und -belastungen. Erdöl Erdgas Kohle, Vol 115, No.4, S. 191-197, 1999
Braun, R.: Predicting Production Induced Changes in Reservoirs. OIL
GAS European Magazine, 3/2006, pp OG124 - OG129
Braun, R.: A Commonly Neglected Factor in Rock Mass and Borehole Stability.
OIL GAS European Magazine, 2/2007, pp. OG79 - OG82

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