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Geotechnical News • June 2013
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THESIS ABSTRACTS
In Situ Stress Magnitude and Core Disking
Seong Sik Lim
Seong Sik Lim, Alberta Environment, E: sslim@ualberta.ca
Accurate measurement of in situ stress using surface-drilled
deep boreholes is a challenge in high stress regimes in
which both horizontal stresses exceed the vertical stress. In
such cases, hydraulic fracturing and overcoring often yields
unreliable results. For such stress regimes core damage and
core disking is often observed and these indirect observa-
tions were used to constrain the stress state.
Microcracks in cored rock samples are classed as either
natural microcracks resulting from geological processes or
stress-induced microcracks that are caused by the drilling
and sampling process in a highly stressed rock mass. Micro-
cracks in these samples can be identified in laboratory tests
using a methodology related to volumetric strain. This meth-
odology was used to quantify stress-induced microcracks in
87 granite-cored samples, obtained in the depth range from
ground surface to 1000 m at Pinawa, Canada and Forsmark,
Sweden. Digital image analysis of 9 of these samples was
used to examine the characteristics of these mirocracks. The
results indicate that at depths of less than 200 m, the domi-
nant mode of microcracks can be classed as naturally occur-
ring. The volume of stress-induced microcracks was found
to increase linearly with sampling depth with the proportion
of grain-boundary, intragranular and transgranular micro-
cracks remaining relatively constant regardless of the depth.
However, when the mean in situ stress magnitudes normal-
ized by the laboratory tensile strength was higher than 4, the
proportion of intragranular and transgranular microcracks
increased significantly for Lac du Bonnet granite. Moreover
it was observed that most of the stress-induced transgranular
microcracks formed in a plane perpendicular to the core
axis. Thus the ratio horizontal to axial transgranular micro-
crack could be an indicator of the degree of stress-induced
core damage.
Core disking is an extreme form of stress-induced micro-
cracking and an indicator of elevated stress magnitudes.
Disked cores from boreholes drilled from underground
excavations in massive unfractured granite at AECL’s
Underground Research Laboratory, where the stress mag-
nitudes are known with confidence, were used to establish
a relationship between ore disk thickness and the stress
magnitude. Relationships were established for three disk
thickness categories; (1) thin (t/D<0.2), (2) medium (0.2<t/
D<0.4) and (3) thick (0.4<t/D,2.2) and partial disking. The
data suggests that core disking initiates when the maximum
principal stress normalized to the tensile strength is 6.5.
Stress path analyses indicated that tensile stress controlled
the onset of disking.
Tensile stress plays a critical role in core disking. Three
dimensional numerical analyses were carried out to deter-
mine the distribution of these tensile stresses in the vicinity
of the advancing drill bit. A methodology was developed to
examine the spatial distribution of the maximum, minimum,
and average, maximum tensile stress. The analyses were
also used to assess the influence of drill-bit geometry on
he magnitude and distribution of these tensile stresses. A
criterion based on the Averaged Maximum Tensile Stress
(AMTS) was found to give good agreement with the thick-
ness of core disks measured on core from 75-mm diameter
boreholes. This approach was applied to two sites and found
to be in agreement with field observations. According to
the criterion, approximately 40% higher horizontal in situ
stresses are required for solid core disking than for ring core
disking. Numerical analysis using standard drill bit geom-
etry demonstrated that larger, round bits may reduce stress-
induced core damage.
Supervisor: C. Derek Martin, University of Alberta, Civil &
Environmental Engineering, Edmonton, Alberta, Canada T6G 2G7,
T: 780-492-2176, F: 780-492-8198
Permeability of Porous Media in the
Presence of Gas Hydrates
Mohana Lakshme, Delli
Mohana Lakshme, Delli, Golder Associates Ltd., 2535 – 3
rd
Avenue
SE, Calgary, Canada T2A 7W5
Gas hydrates which form at high pressure and low tempera-
ture conditions, are solid crystalline compounds comprised
of a lattice of water molecules that encage gas molecules.
Gas hydrates impact society because of their potential as an
immense energy source and their role in submarine geohaz-
ards. Economical production of natural gas from hydrate
reservoirs crucially depends on the formation permeability
and the relative permeability of the sediment to fluid flow.
Permeability measurements from natural core samples are
difficult owing to core disturbance during retrieval and pro-
cessing. Laboratory synthesized samples provides a viable
alternative with the flexibility to form samples with desired
morphology and mineralogy. Unfortunately, very few
permeability and relative permeability measurements have
been performed and thus the range of media properties and
saturations expected in natural hydrate bearing sediments is
not available. In the absence of reliable experimental data,
numerical reservoir simulators employ theoretical models
for permeability prediction. However, experimental verifica-
tion of the theoretical models is still required.
This thesis focuses on understanding the effect of hydrate
formation on the relative permeability of porous media to
fluid flow. In doing so, a better way to evaluate the suitabil-
ity of the existing theoretical models has been developed.
It also explores if better permeability prediction can be
achieved using a combination of available theoretical mod-
els. Finally, an experimental program in which relative per-
meability of porous media in the presence of carbon dioxide