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M. Cao et al.
1 Introduction
Oil shale, also known as kerogen shale, is mainly composed of a large number of
inorganic minerals and solid organic kerogen contained in its skeleton, and kerogen
is pyrolyzed at a certain temperature and converted into shale oil and gas products. Consequently, to some extent, oil shale can be regarded as alternative energy for petroleum
[1–3]. It has been reported that there are almost 4.8 trillion barrels of oil shale reserves
in the world, and the reserves in China (around 354 billion barrels) are occupying 7%
of the world’s reserves [4]. As an unconventional energy source, oil shale has a broad
development prospect.
Generally, oil shale can be exploited by conventional ex-situ exploitation and in-situ
exploitation. In terms of the conventional ex-situ exploitation, oil shale is extracted to the
surface by the methods of opencast or underground, and then shale oil and hydrocarbon
gas can be obtained by distillation method at low temperature. However, for in-situ
exploitation, shale oil and gas products can be obtained through heating an oil shale
formation with different heating methods [5–7].
Up to now, although in-situ underground exploitation technology is still in the
research and development stage, it has a bright application prospect and value with
the merit of no large-scale mining, less occupation of land, and high exploitation rate.
In high-temperature conditions, the kerogen in the oil shale is converted into shale
oil and gas after pyrolysis, resulting in a large number of pores and cracks and providing
more penetration channels. Many researchers [7–12] have studied the pore and fracture
structures of oil shale before and after using the method of MIP or micro-CT et al.
The results demonstrated that the pyrolysis dramatically affected the pore structure, and
a larger number of fractures emerged when the temperature was higher than 350 °C.
Tiwari et al. [10] found that the distribution of kerogen was related to the value of
porosity and the pyrolysis led the generation of a larger fracture channel; Using the µCT
scan technology, Saif et al. [11] dynamically monitored the pyrolytic process of the oil
shale and observed that the number of interconnected pores and porosity significantly
increased.
The permeability is a key physical parameter to recover shale oil and gas products
from the oil shale seam. For a given rock, its permeability can be influenced by many
factors, including pore pressure, volumetric stress, and temperature. For example, the
permeability of natural rock decreases as the volumetric rises at room temperature, but
it may be enhanced by heating the rock to a high temperature. So, it is significant to
clearly understand the permeability sensitivity to temperature variation of the oil shale
formation. Kang et al. [13] investigated the permeability of the cylindrical oil shale
sample with the axial direction almost parallel to the original bedding at temperatures
up to 500 °C and found that the permeability dramatically grew at around 350 °C. Yang
et al. [14] reported that the oil shale permeability at 600 °C was 3.0 × 10 −8 m 2 , which is
almost 600 times that at room temperature. Dong et al. [15] investigated the permeability
evolution of oil shale under in-situ conditions and stated that the permeability of oil shale
dramatically increased for the temperature larger than the initial threshold, and threshold
temperature is related to the tri-axial stress state.
M. Cao et al.
1 Introduction
Oil shale, also known as kerogen shale, is mainly composed of a large number of
inorganic minerals and solid organic kerogen contained in its skeleton, and kerogen
is pyrolyzed at a certain temperature and converted into shale oil and gas products. Consequently, to some extent, oil shale can be regarded as alternative energy for petroleum
[1–3]. It has been reported that there are almost 4.8 trillion barrels of oil shale reserves
in the world, and the reserves in China (around 354 billion barrels) are occupying 7%
of the world’s reserves [4]. As an unconventional energy source, oil shale has a broad
development prospect.
Generally, oil shale can be exploited by conventional ex-situ exploitation and in-situ
exploitation. In terms of the conventional ex-situ exploitation, oil shale is extracted to the
surface by the methods of opencast or underground, and then shale oil and hydrocarbon
gas can be obtained by distillation method at low temperature. However, for in-situ
exploitation, shale oil and gas products can be obtained through heating an oil shale
formation with different heating methods [5–7].
Up to now, although in-situ underground exploitation technology is still in the
research and development stage, it has a bright application prospect and value with
the merit of no large-scale mining, less occupation of land, and high exploitation rate.
In high-temperature conditions, the kerogen in the oil shale is converted into shale
oil and gas after pyrolysis, resulting in a large number of pores and cracks and providing
more penetration channels. Many researchers [7–12] have studied the pore and fracture
structures of oil shale before and after using the method of MIP or micro-CT et al.
The results demonstrated that the pyrolysis dramatically affected the pore structure, and
a larger number of fractures emerged when the temperature was higher than 350 °C.
Tiwari et al. [10] found that the distribution of kerogen was related to the value of
porosity and the pyrolysis led the generation of a larger fracture channel; Using the µCT
scan technology, Saif et al. [11] dynamically monitored the pyrolytic process of the oil
shale and observed that the number of interconnected pores and porosity significantly
increased.
The permeability is a key physical parameter to recover shale oil and gas products
from the oil shale seam. For a given rock, its permeability can be influenced by many
factors, including pore pressure, volumetric stress, and temperature. For example, the
permeability of natural rock decreases as the volumetric rises at room temperature, but
it may be enhanced by heating the rock to a high temperature. So, it is significant to
clearly understand the permeability sensitivity to temperature variation of the oil shale
formation. Kang et al. [13] investigated the permeability of the cylindrical oil shale
sample with the axial direction almost parallel to the original bedding at temperatures
up to 500 °C and found that the permeability dramatically grew at around 350 °C. Yang
et al. [14] reported that the oil shale permeability at 600 °C was 3.0 × 10 −8 m 2 , which is
almost 600 times that at room temperature. Dong et al. [15] investigated the permeability
evolution of oil shale under in-situ conditions and stated that the permeability of oil shale
dramatically increased for the temperature larger than the initial threshold, and threshold
temperature is related to the tri-axial stress state.