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New inspiration on effective development of tight reservoir in secondary exploitation by using rock mechanics method

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Energy Exploration & Exploitation
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This study attempted to raise new significant inspiration on effective development of tight reservoir in secondary exploitation by using rock mechanics method. After refitting the conventional tri-axial rock mechanics testing instruments, a set of probing experiments on rock mechanics determination were performed. Results show that mechanism of the influence of confining pressure on the Young's modulus is different from that of the pore pressure. The physical significance of the slope and intercept value in Young's modulus changing curves were analyzed; two kinds of mechanisms which cause changes of Young's modulus were raised. Meanwhile, we put forward the concept of “microscopic” anisotropy and introduce the coefficient of “S” and “H” to quantitatively characterize the relative difference on impact extent, where pore pressure and confining pressure could lead to change in Young's modulus and the size of microscopic anisotropy of Young's modulus. We predicted the size of microscopic anisotropy of Young's modulus in Qijiagulong district of Daqing Oilfield by using the above method. The conclusion could provide scientific basis for judging the well pattern adaptability, preponderant direction of water flooding, the reservoir rock fracturing, and optimization design of fracturing parameters.
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Original Article
New inspiration on effective
development of tight reservoir
in secondary exploitation by
using rock mechanics method
Shuheng Du,
1
Yongmin Shi,
1
Ping Guan
1
and
Yuguang Zhang
2
Abstract
This study attempted to raise new significant inspiration on effective development of tight reser-
voir in secondary exploitation by using rock mechanics method. After refitting the conventional
tri-axial rock mechanics testing instruments, a set of probing experiments on rock mechanics
determination were performed. Results show that mechanism of the influence of confining pres-
sure on the Young’s modulus is different from that of the pore pressure. The physical significance
of the slope and intercept value in Young’s modulus changing curves were analyzed; two kinds of
mechanisms which cause changes of Young’s modulus were raised. Meanwhile, we put forward the
concept of ‘‘microscopic’’ anisotropy and introduce the coefficient of ‘‘S’’ and ‘‘H’’ to quantita-
tively characterize the relative difference on impact extent, where pore pressure and confining
pressure could lead to change in Young’s modulus and the size of microscopic anisotropy of
Young’s modulus. We predicted the size of microscopic anisotropy of Young’s modulus in
Qijiagulong district of Daqing Oilfield by using the above method. The conclusion could provide
scientific basis for judging the well pattern adaptability, preponderant direction of water flooding,
the reservoir rock fracturing, and optimization design of fracturing parameters.
Keywords
Rock mechanics, tight reservoir, microscopic anisotropy, secondary exploitation, pore pressure,
confining pressure, in-situ condition
1
School of Earth and Space Science, Peking University, Beijing, China
2
Daqing Oilfield Company, PetroChina, Daqing, Heilongjiang, China
Corresponding author:
Shuheng Du, School of Earth and Space Science, Peking University, Beijing 100871, China.
Email: dushuheng@pku.edu.cn
Energy Exploration & Exploitation
2016, Vol. 34(1) 3–18
!The Author(s) 2016
Reprints and permissions:
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DOI: 10.1177/0144598715623661
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Introduction
Compared with the initial state of reservoir, pore pressure and confining pressure would
change apparently during water flooding development (Zheng et al., 2013). As special mater-
ials with porous medium, factors which influence the mechanical properties of oil and gas
reservoir are various, including pore pressure, confining pressure, temperature, fluid satur-
ation, etc. (Cai et al., 2014). Change in pore pressure occurs within the rock, contacted with
the microscopic pore structure of the rock directly, which can vary with confining pressure.
As horizontal pressure is applied to the rock mass, confining pressure is closely related to the
tectonic stress. As confining pressure is not directly contacted with rock internal pore throat,
we believe that the mechanism of influence on rock mass mechanics parameters by confining
pressure will be different from the pore pressure. At the same time, due to dominant direc-
tion of deposition which could occur when reservoir is constructed, the main seepage channel
would also be produced, which could determine the microscopic anisotropy of pore structure
rock and also lead to variations in anisotropy of rock mass mechanics parameters.
Predecessors have done a lot of research on factors affecting mechanical properties of rock
mass in reservoir. Zhang et al. (2002) pointed out that the relationship between effective
confining pressure and Young’s modulus, compressive strength, is exponential through the
core measurement but did not construct continuous changing model of mechanics properties
in the process of water flooding; You (2003) believed that law of Young’s modulus changing
with the confining pressure reflects the rock internal damage state by measuring the cores with
different lithology, and he thought that average Young’s modulus value will be accurate
relatively when considering the heterogeneity of the rock at the same time; Zeng et al.
(2012) put forward the storage and seepage unit which consists of sedimentation, diagenesis,
and tectogenesis by using the method of clustering analysis to evaluate reservoir anisotropy of
low-permeability sandstones. Yu and Tian (2013) calculated the anisotropic coefficient when
rock mass mechanics parameters changed with confining pressure and predicted the relation-
ship between the elastic modulus and depth; Adebayo and Adetula (2013) determined the
hardness, brittleness, Rock Abrasivity Index (RAI), penetration rate, and bit wear rate to
study the condition of drilling categorization. Tang (2014) analyzed wave velocities and
modulus of the anisotropic rock under the condition of quasi static and dynamic state, he
believed that anisotropic characteristics was related to porosity, compaction history, and
particle composition; Nishimura (2014) used a new type of tri-axial test instrument to quantify
the small strain of rock material value in high precision, which could help to get more accurate
determination of anisotropic Young’s modulus; Kongkitkul et al. (2014) determined Young’s
modulus of two kinds of new asphalt materials, and concluded that the vertical of Young’s
modulus is greater than that of the horizontal. Actually, there are also lots of scholars at home
and abroad who have studied the relationship between natural resources and rock mechanics
method (Fan et al., 2014; Lee and Bobet, 2014; Liu et al., 2014; Markov et al., 2014; Song and
Hu, 2014; Wasantha and Ranjith, 2014; Wu et al., 2014; Yang et al., 2014).
Concerned about the above research status at home and abroad, the following three
aspects of problems could be referred (Xing et al., 2014; You and Su, 2003; Du et al.,
2015; Ghassemi, 2012; Kukudzhanov, 2011; Kurek et al., 2011; Markides and
Kourkoulis, 2012; Mene
´ndez and David, 2007; Turner, 2013):
(1) Most of the sampling methods in rock measurements are sampled from a certain single
direction, which is not from three directions of X, Y, and Z.
4Energy Exploration & Exploitation 34(1)
(2) Little measurements are based on synchronous measurement on structure of the reser-
voir rock, composition and so on, thus the factors that influence anisotropy of mech-
anical properties in rock mass are relatively poor.
(3) At present, most of the research and experiments are based on the determination of dry
samples, or just loading one or two conditions like the confining pressure, pore pressure,
temperature and fluid, etc. Little measurements were designed under specific under-
ground reservoir conditions of temperature, confining pressure, pore pressure, the deter-
mination of oil–water saturation, and so the results of most experiments could not reflect
reservoir accurately.
In this article, we select samples from wells ‘‘A’’, ‘‘Q’’, ‘‘G’’, drill three pieces of small
samples respectively from the horizontal X direction, Y direction and the vertical Z direction
to perform the rock mechanics parameters determination under the condition of changing
pore pressure or confining pressure, and do some research and analysis on the mechanism
which affects Young’s modulus and the anisotropy of rock mass mechanics parameters.
Geologic setting
The study area is located in the central depression belt of Songliao basin, China. The tec-
tonics is characterized by the juxtaposition of uplifted and depression units and the fault
systems are complex and various.
Delta was the main facies of this area, including distributary channel and inter-
distributary bay. Sedimentary sequence of distributary channel consisted of scour surface,
detainment sedimentation, large fine sandstone facies, small silty sand lithofacies, and hori-
zontal bedding phase composition. Inter-distributary bay is given priority to mudstone and
silty mudstone, horizontal bedding, plastic deformation bedding, and intermittent sand
layer. Sedimentary facies model of pay zone is shown in Figure 1.
Determination of anisotropic Young’s modulus
Samples of well ‘‘A’’
Rock sample of well ‘‘A’’ lies at a depth of 1883.0 m, logging interpretation results show that
the porosity is 13.4%, the permeability is 2.62 10
3
mm
2
, and lithology is fine-grained
feldspar lithic sandstone.
Experimental equipment used is the servo rock mechanics tri-axial stress test system
(Figure 2). The system mainly includes the axial pressure control system, pore pressure
control system (peak could be up to 40 MPa), confining pressure control system (peak
could be up to 70 MPa), and computer acquisition and control system.
We drill three pieces of small samples respectively from the horizontal X direction, Y
direction and the vertical Z direction to perform the rock mechanics parameters determin-
ation under the condition of changing pore pressure or confining pressure. Before determin-
ation, according to the actual sample of formation conditions, we set the fluid conditions to
oil and water saturation; the volume ratio is 6:4 and the temperature is set to 45. We keep
the confining pressure constant, increasing pore pressure in the interval of 2 MPa and then
keep pore pressure constant, increasing confining pressure in the interval of 2 MPa to per-
form the experiment. Finally we draw the cross-plot between effective confining pressure (the
difference between the confining pressure and pore pressure) and Young’s modulus, respec-
tively, the result is shown in Figure 3.
Du et al. 5
As can be seen in Figure 3, with the increase of effective confining pressure, Young’s
modulus increases linearly. As to samples of Y, Z direction, the slopes of Young’s modulus
for two pressures are basically the same but intercept values are different. Only the slopes of
the X direction sample are rather different. Values measured under changing pore pressure
are greater than those under confining pressure, but the slope values are the opposite.
Figure 1. Sedimentary micro-facies.
Figure 2. Experimental equipment and samples.
6Energy Exploration & Exploitation 34(1)
It could prove that there are differences between the mechanism of the changing pore pres-
sure and confining pressure, reflecting the anisotropy of rock mechanics properties.
Samples of well ‘‘Q’’
We select samples from well ‘‘Q’’ at a depth of 1926.5 m. The lithology is silty mudstone. We
select in-situ conditions to perform the similar experiments and draw the cross-plot, as
shown in Figure 4.
Discussion on mechanisms by which microscopic anisotropy
of Young’s modulus change
In fact, as a kind of force applied onto the outer rock, confining pressure will cause the
compression of pore fluid thus increasing the pore pressure inevitably. In the above experi-
ments, when increasing the confining pressure and keeping pore pressure constant it will lead
to the decrease of the Young’s modulus. Conversely, Young’s modulus is growing. It can be
seen that the degree of rock pore shape compression caused by the increase of confining
pressure without artificial change in pore pressure is greater than the degree of rock pore
morphology restoration caused by the increase of pore pressure induced by increasing con-
fining pressure.
As reaction of formation pressure, pore pressure is a kind of stress induced by gravity,
which is ‘‘hidden’’ within the rock particles and applied to the rock skeleton by fluid. By
keeping either the confining pressure or pore pressure constant and changing the other, we
could get the same effective confining pressure value, but results on pore structure under
these two different phenomena are apparently completely different.
Figure 3. Cross-plot between effective confining pressures (the difference between the confining
pressure and pore pressure) and Young’s modulus of ‘‘A’’.
Du et al. 7
The introduction of coefficient ‘‘S’’ and ‘‘H’’
Theory of ‘‘Biot’’ pointed that energy of seismic wave is lost partly due to the interaction
between fluid and solid skeleton; ‘‘Biot’s coefficient’’ could quantitatively characterize the
viscoelasticity of pore and fluid, which promote the development of the theory of viscoelas-
ticity (Liu et al., 2014; Song and Hu, 2014; Wu et al., 2014; Yang et al., 2014).
In this paper, based on the above experiments, considering that the mechanism of two
kinds of pressure is different, we introduce two new coefficients to quantitatively characterize
viscoelastic properties of particles and pore fluid, called the S coefficient and H coefficient,
respectively.
Calculation of S coefficient and H coefficient is based on slope of Young’s modulus’s
change called ‘‘S’’ and intercept of vertical axis called ‘‘B’’ (Figures 3 and 4), So these two
coefficients can be represented as:
S¼1
Kp
Kc
H¼1
Bp
Bc
Among them, the coefficient of Sand His dimensionless, K
p
—slope of pore pressure
change, K
c
—slope of confining pressure change, B
p
—intercept of pore pressure change, and
B
c
—intercept of confining pressure change.
Meanwhile, calculation results of well ‘‘Q’’ samples are shown in Table 2.
Physical significance of S coefficient and H coefficient
Rock slices taken from three directions of samples from well ‘‘A’’ and ‘‘Q’’ are shown in the
microscope (Figures 5 and 6). As can be seen from Figure 5, brittle minerals like quartz and
Figure 4. Cross-plot between effective confining pressures (the difference between the confining
pressure and pore pressure) and Young’s modulus of ‘‘Q’’.
8Energy Exploration & Exploitation 34(1)
feldspar accounted for more than 50% pore and throat is not obviously found. Seepage
channels of Y direction sample are not oriented, so that whether we change pore pressure in
a certain value or change confining pressure in the same value reversely the influence on
Young’s modulus could be substantially the same. Meanwhile, as to samples of Z and X,
particles and seepage channels oriented well, so that whether we change pore pressure in
some range or change confining pressure in the same range reversely, both the influence on
young’s modulus could be almost different. So S coefficient could be used to characterize the
Figure 5. Characteristics of horizontal and vertical rock slices of well ‘‘A’’. Note: A—X direction slice
under orthogonal light, B—X direction slice under single polarization direction, C—Y direction slice
under orthogonal light, D—Y direction slice under single polarization direction, E—Z direction slice
under orthogonal light, F—Z direction slice under single polarization direction.
Du et al. 9
size of ‘‘micro’’ anisotropy of Young’s modulus which is affected by characteristics of the
pore and throat. A large value could reflect that the degree of influence on ‘‘micro’’ anisot-
ropy of Young’s modulus caused by pore and throat structure is great. The ‘‘micro’’ anisot-
ropy referred to here were paid more attention to emphasize the respective different
characteristics of three different drill strings large core taken from X, Y, Z direction. This
is different from the traditional method that focuses on the whole anisotropy of a large core
without drilling strings from different directions. So this research method and perspective
can be more comprehensive to reflect the overall properties of the core.
As can be seen in Figure 6, for samples of X and Y directions in ‘‘Q’’, particles and
seepage channel, the arrangement is good. On the contrary, sample of the Z direction is the
opposite, which is also reflected in size of S coefficient which is X >Y>Z. Similar conclu-
sions could also be achieved.
Therefore S coefficient characterizes the relative difference of capability to refactor
pore throat structure by changing pore pressure or confining pressure. The coefficient
range is [0,1].
For H coefficient, as can be seen in Figure 5, difference in S coefficient is large between the
X and Y direction samples of well ‘‘A’’, but H coefficient values of these two samples are
nearly equal. Similar conclusion could be found in well ‘‘Q’’. As we know pore throat, fluid,
and skeleton materials (including solid material such as rock particles, filler content) could
be treated as the three major elements in rock. For well ‘‘A’’, take the example of X and Y,
since there are apparent differences on the properties of pore size and structure between
them, so we have enough evidence to believe that the reason which X and Y have the nearly
equal H coefficient value may be related to properties of their skeleton materials. It means
that the inherent properties of rock materials (such as rock stiffness) of these two samples are
nearly the same, resulting that if we changed pressure condition, the inherent nature of these
two samples may damage in the the same extent. Thus it would cause that when the pressure
is increasing, micro fracture would have the same possibility to occur.
Therefore H coefficient can be used as characterization of the extent of the damage in
inherent nature of these two samples by changing pressure conditions, and the possibility of
breaking to produce micro cracks are nearly the same in the process of changing pressure.
Large H coefficient reflects that this relative difference of influence on Young’s modulus
when changing pore pressure in a certain value or change confining pressure in the same
value reversely. The greater the H coefficient, the higher the extent of influence, where rock
skeleton material damage leads to change in microscopic anisotropy of Young’s modulus,
the value range of H coefficient is generally [0,1]. For well ‘‘A’’, H coefficient of sample in
Z direction is the highest among the three samples, which reflected that skeleton materials of
Z direction is relatively hard to be damaged. Similarly, skeleton materials of X direction
sample of well ‘‘Q’’ are relatively hard to be damaged.
For special low permeability reservoir, ‘‘dead pores’’ which are not connected to each
other could be commonly found in sandstone samples. As the compressibility of pore and
fluid is higher than that of the skeleton material, when the pore and fluid was not fully
compressed, it penetrated between the skeleton materials, and could buffer the occurrence of
fracturing the skeleton material caused by compression. Only if effective confining pressure
and degree of rock pore throat closed turned out to be high, skeleton material contacted with
each other. The function of ‘‘buffer’’ resulted from pore throat and fluid would be greatly
weakened; skeleton material may rupture into micro cracks due to compression, leading to
changes in Young’s modulus. Accordingly, there are two microscopic mechanisms which
10 Energy Exploration & Exploitation 34(1)
could influence Young’s modulus, which are ‘‘change in the pore throat and fluid property’’
and ‘‘skeleton materials crack in the process of compression’’. At a certain time during water
flooding development, one of these two mechanisms will be the dominant factor. In the
development process, the irregular change of pore fluid pressure and confining pressure, then
led to the irregular change of the effective confining pressure, and finally led to the dominant
factors which cause the change of Young’s modulus at different time to be different, result-
ing in unsteady change in microscopic anisotropy of Young’s modulus.
Figure 6. Characteristics of horizontal and vertical rock slices of well ‘‘Q’’. Note: A—X direction slice
under orthogonal light, B—X direction slice under single polarization direction, C—Y direction slice
under orthogonal light, D—Y direction slice under single polarization direction, E—Z direction slice
under orthogonal light, F—Z direction slice under single polarization direction.
Du et al. 11
Significance about difference of Young’s modulus under two kinds
of pressure changes
We treated difference of Young’s modulus under two kinds of pressure change as the Y
axis, and effective confining pressure value as X axis to draw the cross-plots (Figures 7
and 8).
Figure 7. Cross-plot for difference of Young’s modulus under two kinds of pressure change and
effective confining pressure value of well ‘‘A’’.
Figure 8. Cross-plot of difference of Young’s modulus under two kinds of pressure change and effective
confining pressure value of well ‘‘Q’’.
12 Energy Exploration & Exploitation 34(1)
In Figures 7 and 8, Ypx, Ypy, and Ypz represent the variable Young’s modulus of X, Y
and Z direction under the condition of changing pore pressure, respectively, Ycx, Ycy, and
Ycz represent the variable young’s modulus of X, Y and Z direction under the condition of
changing confining pressure, respectively.
Tectonic stress recovery simulation data on above study areas show that the value of
confining pressure and pore pressure could be up to 50 MPa and 25 MPa, respectively, so
value of effective confining pressure could be up to 50 MPa in theory. Comparing Figure 7
and Table 1, S coefficient of Y direction sample is 0.1143, which means microscopic anisot-
ropy is the weakest and Young’s modulus difference is on the decline with the increase of
effective confining pressure. But when the effective confining pressure is within 50 MPa, the
scope of the change in Young’s modulus is little which is about 0.3 GPa, which suggests that
on the premise of within rock skeleton material stiffness, when S coefficient value is small,
the degree of influence on pore and throat structure, caused by changing pore pressure or
confining pressure, is considerable. At the same time, when comparing Figure 7 with Table 2,
S coefficient of X and Y directions samples is 0.8771 and 0.7844, respectively, and micro-
scopic anisotropy for both are stronger. Young’s modulus difference is on the rise with the
increase of effective confining pressure. Meanwhile when the effective confining pressure is
within 50 MPa, the scope of the change in Young’s modulus is large which is about 6.5 GPa
and 2 GPa, respectively, which suggests that on the premise of within rock skeleton material
stiffness, when S coefficient value is large, the degree of influence on pore and throat struc-
ture, caused by decreasing pore pressure, in a certain value tend to be larger than increasing
confining pressure in the same value.
When pore throat property is the dominant factor to influence microscopic anisotropy
under the premise of within rock skeleton material stiffness, from the perspective of the
Table 1. S coefficient and H coefficient calculation on samples of well ‘‘A’’.
Experiment condition/samples Slope S Intercept H S coefficient H coefficient
Changing pore pressure/X 0.0161 13.6760 0.5785 0.0399
Changing confining pressure/X 0.0382 13.1510
Changing pore pressure/Y 0.0434 14.0720 0.1143 0.0362
Changing confining pressure/Y 0.049 13.5810
Changing pore pressure/Z 0.0658 10.7920 0.3237 0.1332
Changing confining pressure/Z 0.0973 9.5237
Table 2. S coefficient and H coefficient calculation on samples of well ‘‘Q’’.
Experiment condition/samples Slope S Intercept H S coefficient H coefficient
Changing pore pressure/X 0.0191 23.226 0.877091377 0.122354306
Changing confining pressure/X 0.1554 20.694
Changing pore pressure/Y 0.0778 36.052 0.78440367 0.020927137
Changing confining pressure/Y 0.0436 35.313
Changing pore pressure/Z 0.0511 20.932 0.195275591 0.025149031
Changing confining pressure/Z 0.0635 21.472
Du et al. 13
micro mechanism analysis, we found that influence on pore throat structure with obvious
grain orientation reconstruction by changing pressure could be reflected (Figure 9). First of
all, keep pore pressure constant in 8 MPa, increasing confining pressure by the step of
2 MPa from 15 MPa. When the confining pressure increased to 19 MPa, keep the confining
pressure constant, start to increase pore pressure by the step of 2 MPa to 12 MPa. As can
be seen from (a) and (f), (b) and (e) in Figure 9, effective pressure values of above two
groups are the same which is 7MPa and 9 MPa respectively. But according to the above
experimental conclusion, the degree of compression on rock pore and throat by increasing
confining pressure per 2 MPa is bigger than the degree of making rock pore and throat
reopen by increasing pore pressure per 2 MPa. So even under the condition of the
same effective stress, structures of rock particles are also not identical. It is totally related
to the absolute value of pore pressure and confining pressure, respectively, so that anisot-
ropy of petro physical parameters and mechanical properties of anisotropic rock mass are
closely related, the greater the S coefficient, the greater anisotropy of rock mechanics
properties.
Reversely, influence on irregular pore and throat structure’s reconstruction by changing
pressure could be reflected (Figure 10). Pore pressure and confining pressure values in dif-
ferent stages are the same as in Figure 9. According to the experimental conclusion, as a
Figure 9. Schema graph of influence on pore throat structure with obvious grain orientation by chan-
ging pressure. Note: each circle represents a low permeable reservoir rock section, black color shape
represents rock particles, shape filled with light gray in the background represents the filler content and
other skeleton materials, colorless part represents pore throat structure of special low permeability res-
ervoir rock.
14 Energy Exploration & Exploitation 34(1)
result of small S coefficient and no obvious directionality, the degree of compression on rock
pore and throat by increasing confining pressure per 2 MPa is nearly the same as the degree
of making rock pore and throat reopen by increasing pore pressure per 2 MPa. Anisotropy
reflected by the change of the rock mass mechanics parameters is not obvious.
Application
We select samples from well ‘‘G’’ at a depth of 2804 m in another area of Daqing oilfield, the
lithology is silty mudstone. We selected its in-situ conditions to perform the similar experi-
ments and draw the cross-plot as, as shown in Figure 11.
For samples of well ‘‘G’’, S coefficient and H coefficient are calculated, as shown in
Table 3.
As can be seen from Figure 11 and Table 3, order of S coefficient size is X >Y>Z, order
of H coefficient is Z >Y>X. So we could conclude as follows: when the pore and throat
properties turned out to be the dominant factor that result in the change of Young’s modu-
lus, order of anisotropy size is X >Y>Z; when the dominant factor turned out to be
inherent rock skeleton materials, order of anisotropy size is Z >X>Y. This conclusion
will play a significant role in fracture distribution in the process of hydraulic fracturing.
Figure 10. Schema graph of influence on irregular pore and throat structure by changing pressure.
Note: each circle represents a low permeable reservoir rock section, black color shape represents rock
particles, shape filled with light gray in the background represents the filler content and other skeleton
materials, colorless part represents pore throat structure of special low permeability reservoir rock.
Du et al. 15
Conclusion
(1) Change in the pressure conditions will play a significant role on rock mass mechanics
parameters of the reservoir, like Young’s modulus. Generally, amplitude and mechanism
of leading to the change of Young’s modulus by pore pressure or confining pressure are
rather different.
(2) Based on the fact that influence mechanisms of two kinds of pressure are different, we
introduce two new coefficients to quantitatively characterize viscoelastic properties of
particles and pore fluid, called the S coefficient and H coefficient, respectively, which can
better reflect pore and throat and viscoelastic fluid properties, fracture properties of rock
skeleton materials and the microscopic anisotropy size of Young’s modulus. Large S
coefficient reflects that the relative difference of influence on Young’s modulus, when
changing pore pressure in a certain value or change confining pressure in the same value
reversely, is large. It could also reflect that the degree of influence on microscopic
anisotropy of Young’s modulus caused by pore and throat structure is great. Large H
coefficient reflect that the relative difference of influence on Young’s modulus, when
Figure 11. Cross-plot between effective confining pressures (the difference between the confining pres-
sure and pore pressure) and Young’s modulus of well ‘‘G’’.
Table 3. S coefficient and H coefficient calculation on samples of well ‘‘G’’.
Experiment condition/samples Slope S Intercept H S coefficient H coefficient
Changing pore pressure/X 0.0422 31.252 0.2423698 0.0030942
Changing confining pressure/X 0.0557 31.349
Changing pore pressure/Y 0.0836 15.202 0.1236559 0.0070884
Changing confining pressure/Y 0.0744 15.095
Changing pore pressure/Z 0.1119 23.845 0.1147152 0.0569124
Changing confining pressure/Z 0.1264 22.561
16 Energy Exploration & Exploitation 34(1)
changing pore pressure in a certain value or change confining pressure in the same value
reversely, is large. The greater the H coefficient, the higher the extent of influence which
rock skeleton material damage leads to change in microscopic anisotropy of Young’s
modulus.
(3) At a certain time during water flooding development, one of above two mechanisms will
be the dominant factor, which should be analyzed and considered separately.
(4) Judgment on microscopic anisotropy of Young’s modulus plays a crucial role on
research about change rule of in-situ stress in secondary exploitation, which should be
highly valued in the process of fracturing parameters design.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or
publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or
publication of this article: Chinese national key basic research development plan (973 program, No.
2009CB219302).
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18 Energy Exploration & Exploitation 34(1)
... Young's modulus is the ratio of strain to stress generated per unit area of rock under uniaxial tension. Young's modulus reflects the rigidity of a material, and the larger its value, the less likely the rock is to deform (Du et al., 2016;Yin et al., 2022). The calculation formula for Young's modulus is shown in Eq. 2. The compressive strength is the limit at which a rock can withstand pressure, and its calculation formula is shown in Eq. 3. ...
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... Recently investigators observed that there was a significant difference in rock's young modulus caused by changes in confining pressure and changes due to pore pressure. 23 The substantial deterioration of the mechanical properties of the specimens tested under slick water and DI water conditions could mainly be attributed to the physical and chemical interaction between the fluid and the rock mineral particles. Typically, the hydration and swelling of the clay minerals are the main reason for the observed difference in subcritical crack propagation behavior. ...
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... For example, the degree of core sample saturation, drainage conditions, and loading rates can cause undrained poro-elastic effects that can significantly affect testing results (Cheng, 1997;Puzrin, 2012;McKean, 2017). Numerous other factors can also significantly affect results, including confining pressure, pore pressure, core plug diameter and aspect ratio, and the presence of discontinuities (King, 1983;Jaeger, Cook & Zimmerman, 2009;Asef and Najibi, 2013;Holt et al., 2013;Blocher, 2014;Du et al., 2015Du et al., , 2016. Compilation of a dataset of 133 public triaxial tests data from 20 Duvernay cores in the Kaybob area (Figs. 2 and 8) revealed that confining pressure for the triaxial tests varies between 0 MPa and 41.4 MPa (0-6000 psi) (Fig. 9). ...
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Shales are important source rocks and sealing rocks with intrinsic anisotropy. The character of stress-strain response and ultrasonic speed response for shales are obtained by experimental measurement, and the features of static and dynamic moduli on anisotropic rock are studied. The dynamic modulus of anisotropic rock is gained by measuring vertical and horizontal wave velocities of core under different directions, and the static modulus is obtained through the measurement of stress-strain characteristics in loading process. Characteristics of wave velocity and elasticity modulus of anisotropic rock under quasi-static and dynamic conditions are analyzed. Except the difference of loading frequency, the strain amplitude difference are obtained respectively under static and dynamic measurements. Study of geophysical response characteristics for anisotropic rocks is of great significance.
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We implement a method of 25-point frequency-space domain finite difference obtain numerical solutions of the equations for two-phase isotropic media based on BISQ model. The modeling results of Biot theory and BISQ theory are compared and analyzed for providing insight into the coupling influence of Biot-flow (macroscopic) and Squirt-flow (microcosmic) on seismic waves. The numerical simulation is implemented within seismic frequency bandwidths, and the numerical results show that: for the two cases of ideal and viscous fluid, the Squirt-flow mechanism yields much higher velocity dispersion and attenuation than the Biot-flow mechanism. That is, under the coupling of Biot-flow and Squirt-flow, velocity and amplitude of fast P-wave are smaller than those only considering the Biot-flow, and attenuation of slow P-wave is higher. Meanwhile, we also investigate reflection and transmission at an interface separating two layers of two-phase isotropic media. The results are similar to the case for single-phase media. The study in this paper justifies that proposed method is effective and feasible to simulate wave propagation in two-phase porous media, and can be further applied in full waveform inversion in porous media.