The Mancos shale
core sample investigated in the present research
has been extracted from the late Cretaceous (upper cretaceous) geologic
formation of USA. Shale gas is usually obtained by horizontal drilling
which induces fractures to increase the flow ability of hydrocarbons.
Therefore, it is important to understand the mechanical properties,
heterogeneity, and their complexities associated with elastic properties
of shale. An experimental study was conducted to examine the morphological
characteristics of the Mancos shale core sample both pre- and post-treatment
with cryogenic liquid nitrogen (LN2) for various immersion
times, namely, 30, 60, and 90 min. The atomic force microscopy technique
is used to understand the surface roughness, irregularities in core
samples, and for more accuracy. Scanning electron microscopy (SEM)
results were employed to visualize the formation of cracks caused
by cryogenic liquid nitrogen. Results from SEM showed an increase
in the fracture size from 2 to 25 μm with an increase in the
aging time up to 90 min under the atmosphere of cryogenic LN2. Nano-indentation measurements revealed that the nano-indentation
moduli of the Mancos samples subjected to applied forces of 50 and
200 mN underwent a decrease from 24.6 to 16.8 and 15.6 GPa, respectively,
with an increase in cryogenic liquid nitrogen treatment time to 90
min. The permeability of the shale samples after LN2 treatment
showed a significant increase, whereas increasing net confining stress
from 1000 to 7000 psi for all untreated and treated rock samples exhibited
a decrease in permeability, which is attributed to increased compaction
between the pore spaces. Moreover, the porosity of the Mancos shale
increased from 3.78 to 6.92% for pretreated and treated rock samples.
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