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The mechanical properties of gas shale under uniaxial stress
Daobing Wang, Hongkui Ge, Xiaoqiong Wang, Jianbo Wang and Fanbao Meng
Unconventional Natural Gas Research Institute,China University of Petroleum,102249 Beijing China
ABSTRACT
The matrix of shale gas play is extremely low permeable. It must be conducted effective hydraulic
fracturing to form its economical development. Its mechanical properties and heterogeneity have a
significant impact on shale gas drilling, stimulation and oil recovery etc, almost throughout the petroleum
engineering process. In this study, we investigated the mechanical properties and elastic properties of two
adjacent shale samples under uniaxial stress conditions.
Two adjacent shale samples were cored from Chongqiong gas well in the southwest China at depth of 1145
meters (shale01 sample and shale02 sample). The diameter of the rock specimens were 50mm and their
length about 100mm. These were then compared to one outcrop granite specimen. For each rock sample,
the wave velocities along vertical direction and diametric direction were first measured at room pressure,
which is to investigate the anisotropy of the samples. Then a uniaxial test was conducted at a constant
stress rate of 2MPa/min until brittle failure occurred. Four strain gauges were mounted on each rock
sample in orthogonal pairs to measure directly the axial and radial strains locally. Twelve P wave
piezoelectric ceramic transducers were also glued directly on each sample to record the acoustic
waveforms at 10MHz sampling rate. After failure, the TOC (total organic carbon) , mineral component
analysis and SEM have been conducted. Our main preliminary results are follows:
1 Although the two shale samples are from the same core and at the same depth, however their mechanical
properties are different (figure 1). The brittle strength of Shale01 was one order of magnitude less than that
of shale02 sample.
2 The SEM results showed that the two shale sample with different microstructure. It again demonstrated
that shale sample has strong heterogeneity.
3 The number of AEs of shale samples is less that of granite. However, at the beginning of loading, the
AEs are more than that of granite. Combined with the analysis of SEM, maybe shale samples have more
natural microcracks than granite. Anyway, acoustic emission has an important guiding role for the
evaluation of shale fracability.
4 The main factors of shale heterogeneity maybe are mineral composition, TOC and natural microcracks.
More experiments need to be conducted in the laboratory.
KEYWORDS: shale gas play, acoustic emission(AE), heterogeneity, brittleness, mechanical property
90
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-axial strain
-volumetric strain
-radial strain
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-axial strain
-volumetric strain
-radial strain
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strain(%)
strain(%)
a)shale_01
b)shale_02
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Figure 1 stress-strain curves of two shale samples
(a) shale_01
(b) shale_02
Figure 2 The SEM results of shale_01 sample and shale_02 sample
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