1022
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
that failure mechanisms may occur at earlier temperature stages
than what is obtained by the classical macro (continuum) model.
9.5
0
C
19.1
0
C
57.1.6
0
C
66.7
0
C
33.3
0
C
71.4
0
C
Tensile
failures
One physical explanation for this earlier development of
tensile failure in the micro model compared to the macro model
is partly linked to the stress history prior to coring or drilling.
For cemented sandstone (i.e. Field A) the submerged weight of
the overburden is initially carried by the original grain
framework. When introducing cement to grain boundaries in the
pore space during chemical compaction, the cement will
initially be more or less stress free since the overburden
pressure is already carried by the original grain framework.
During unloading, since the stresses are smaller in the cement, it
will therefore first reach tensile failure. Also, the micro model
experience local stress concentrations which are not a part of the
global continuum model. Temperature changes affect this local
stress pattern differently compared to global unloading. This
might explain the higher amount of tensile failures in the micro
model also for the un-cemented Field B case. Further
understanding of these physical processes is important to be
able to explain and quantify core damage and formation damage
in general.
Tensile
failures
47.6
0
C 52.4
0
C
57.1
0
C
59.5
0
C
64.3
0
C
66.7
0
C
A
A
A
A
A
A
Figure 6. Development of tensile failures in formation predicted by
micro FE-model.
5 CONCLUSIONS
The paper presents an approach for modelling the process of
core and formation damages during drilling. Numerical
simulations show that fracturing on a local micro scale seems to
start at an earlier stage compared to macroscopic failure at the
macro continuum scale. The main hypothesis or physical
explanation for this earlier fracturing in the micro model
compared to the macro model is partly linked to the stress and
cementation history of sediments prior to coring or drilling, in
interaction with mechanical, thermal and flow stresses induced
during drilling, coring and production. This work is a promising
contribution to a better understanding of physical processes
resulting in core damage and formation damage in general.
However, the work was limited to 2D micro modelling.
Figure 5. Development of tensile failures in formation predicted by
macro FE-model.
Ideally, due to variability in possible grain structures and
material properties and the 2D idealization of the 3D problem,
the micro model should be extended to 3D and calibrated with
laboratory data. There are also major uncertainties related to
temperature history within the well and temperature distribution
from the well into the formation. Further, available
experimental data on the stress dependent anisotropy of
sandstones shows very complex interrelationship between stress
magnitudes and directions, and shear modulus. Future activities
should also address better constitutive models for stress induced
anisotropy in uncemented and cemented materials.
6 ACKNOWLEDGMENT
The authors would like to acknowledge Statoil and the
Norwegian Research Council for contributing to and sponsoring
this research and for providing data.
7 REFERENCES
Bjørlykke K.1989,
Sedimentology and Petroleum Geology
, Springer-
Verlag Berlin Heidelberg.
Sayers C.M., Nagy Z., Adachi J., Singh V., Tagbor K. and Hooyman P.
2009. Determination of in-situ stress and rock strength using
borehole acoustic data. Proceedings of the SEG International
Exposition and Annual Meeting, Houston.
Storvoll V. and Bjorlykke K. 2004. Sonic velocity and grain contact
properties in reservoir sandstones,
Petroleum geoscience
10 (3),
215-226.
