Final(1).docx

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学号: 19460324
常州大学
毕业设计（论文）
（2023 届）
题目 Research on Failure model of cap rocks' sealing for
gas storage in depleted petroleum reservoirs
学生 KHAMIS HAFIDH ABDALLA_ ___
学院石油工程学院专业班级 193
校内指导教师朱庆杰__ 专业技术职务
校外指导老师专业技术职务

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Research on Failure model of cap rocks' sealing for gas storage in
depleted petroleum reservoirs
Dissertation Submitted To
Changzhou University
By
(Petroleum And Natural Gas Engineering)
Dissertation Supervisor: JIAJIA
April, 2023

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Abstract
With the rapid development of the construction of deploy oil reserve-type
UGGS, the underground structures of UGGS are Becoming more and more
complex. Along them, the cases of damage and leakage of UGGS caused by
damage To the rock have upgraded, which has brought serious problems to the
safety of UGGS That's really important to study the sealing damage risk of the
rock During the operation of the gas storage, and the safety of the gas storage
also provides a guarantee For the sustainable development of the City.
The sealing failure of the Rock can be divested into two failure models:
Permeability failure and de-strictive failure. The sealing failure of the rock is
analyzed from Four asparts: The macroscopic characteristic factor of the
caprock, the microstructure factor, the geologic Activity factor and the
production condition factor Reservation of Wen'an slope as a typical example,
the responding final element geometry model of the gas storage is established
according to the characteristics of the reservation. The deformation
Characteristics of the gas storage in the process of gas injection were compared
and the experience Were carried out. Considering the effect of hidden faults in
the storage, the responding final Element model is established, and the
influence of faults on the damage of rock is analyzed Animation at the fluid-
structure coupling effect of porous media in the gas flow in the reserve During
gas injection and gas escape in the leakage process, a fine element calculation
model of Fluid-structure coupling in porous media was established, and the
coupling failure mechanism of the Caprock was analyzed. In order to explore
the crack propagation process of capping failure, the Mechanical properties of
capping cracks were analyzed from the perspective of fractures mechanisms
and the crack propagation was divested into the process of initial defect
initiative, macroscope Crack propagation and formation of leakage channels.
Mechanical analysis and finite element Calculation model was established to

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analyze the crack propagation mechanism of cap rock. The Research results
show that the caprock with a large Young's module and a small Poisson's ratio
has Strong sealing performance, and the porosity and permeability
characteristics of the reserve have Little effect. Cyclic injection and production
will cause fatigue damage to the rock in the water Stage; with the same activity
level, the reverse fault has the greatest impact on the capture; During gas
injection, the fluid-solid coupling effect increment with the increment of
reservations pore Pressure and performance, and gas leakage It will cause more
damage to the weak part at the bottom Of the capping layer; The larger the size
of the crack-type defect in the capping layer, the easier it is to expand. During
the expansion, the crack surface opens first, and then advances along the crack
tip to You can upgrade a leak channel. The research results in this paper provide
a method reference and Reference for the protection of rock leakage and the
formation of energy plans for deploy Reserved-type UGGS
Key words: Ground gas storage; rock seal; Faulty; fluid-structure interaction;
Crack propagation

Influence factor of cap rock failure
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Table of Contents
1 Introduction.................................................................................................................................................6
1.1 purpose and significance of the study ........................................................................................ 6
1.2 The current situation of research at home and abroad ....................................................... 9
1.2.1 Development overview of the depleted reservoir................................................................9
1.2.2 Analysis of the destruction mechanisms and leakage risks of the reservoir ....................12
1.2.3 Multi-hole media flow coupling for gas leakage...............................................................15
2. Influence factor of cap rock failure.......................................................................................................17
2.1 The effect of the failure of the seal of the cover......................................................................17
3. Finite seal failure analysis ......................................................................................................................18
3.1 finite element modeling.............................................................................................................18
3.2 Deform of cover.........................................................................................................................25
3.3 Analysis of the factors that influence the failure of the cover ................................................29
4. Fault influence analysis.............................................................................................................................29
4.1 Modeling with a tomographic reservoir...................................................................................29
4.2 Tomographic impact analysis ...................................................................................................32
4.2.1 The effect of tomographic mismovement on the cover.....................................................32
4.2.2 The effect of reverse fault motion on the cover.................................................................36
4.2.3 The effect of translational fault motion on the cover.........................................................39
4.3 Tomosynthesis of sublayer damage..........................................................................................41
4.4 This chapter is a summary ........................................................................................................43
Multi-hole media flow coupling calculation for 5-cover gas leakage........................................................43
5. Porous media coupling failure model of cap rock...................................................................................44

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5.1s Analysis of the flow coupling cooperation for the lid leakage process................................44
6. Result analysis and engineering protection..............................................................................................44
6.1 Basic theory of fracture mechanics ........................................................................................................44
6.1.1 Characteristics and classification of cracks........................................................................45
6.1.2 Basic breaking criteria.........................................................................................................46
6.1.3 ADINA finite element fracture theory................................................................................49
6.2 Guidelines for breaking cracked covers...................................................................................49
6.3 HSE and economic evaluation................................................................................................................50
1. HSE Evaluation:.........................................................................................................................................50
2. Economic Evaluation: ...............................................................................................................................50
References .....................................................................................................................................................51
1 Introduction
1.1 purpose and significance of the study
Natural gas is a high-quality clean energy source that is suited for China due to its quality,
clean energy, friendliness to the environment, and rapid social and economic development.
The proportion of energy consumed in China has been continuously rising as a result of the
development of a low-carbon ecological economy with benefits not offered by conventional
energy sources. It must also deal with rising supply and demand imbalances for natural gas
as well as peak adjustment, specifically the inequitable supply of winter and summer gas,
which necessitates the seasonal peaking of natural gas. Many nations have created
underground energy storage facilities, as well as oil and gas-depleted reservoirs, aquifers,
and gas reserves, to meet the need for natural gas peak-setting. The underground storage
system, which is mostly utilized as a natural gas storage facility, has the advantages of having

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vast reserves, a secure environment, and a lengthy operational life. It is now a successful
method for maintaining the strategic reserve of national energy as well as the supply of gas
in cities.Consequently, the safe operation of underground storage tanks is related to the state's
rapid development as well as the secure and healthy preservation of citizens' lives and
property. It also contributes to the environment's damaging air pollution. As a result, it is
vital and crucial to ensure the stability and safety of underground storage tanks in their
normal operations.
In Ohio, United States, the Leroy gas reservoir experienced a leak in September 1973. The
reservoir is an aquifer reservoir, and when the pressure of the gas was 12 MPa and the volume
of air was 110 million cubic meters, the 3# drilling barrel began to leak gas and spray out of
the ground,resulting in gas spills and migration across faults as a result of anaerobic bacteria
degradation in the gray rock formations at the 4# drilling site. When 4# drilling is blocked
to temporarily stop a leak in 3# drilling, the leak is temporarily stopped. The following year,
1981, the reservoir's 4A# well kept leaking because of flaws in the sleeve and gas leakage
after repair. In order to stop the leaks, the reservoir pressure had to be controlled. This proves
that drilling integrity and geological construction have a big impact on sealing.
A leak in the Yaggy gas reservoir in Kansas, United States, happened in January 2001.
The reservoir is a salt pit reservoir that resulted in two gas explosions in a small town nearby,
which led to the deaths of two individuals and major facility devastation. Gas leakage from
the reservoir was estimated to be 4 million cubic meters. According to the accident
investigation, gas was seeping from the damaged S1 gas well casing and was escaping to the
town through a break in the white cloud rock formations when it exploded from two
abandoned salt mine haline wells in the town. California-based AISO, October 2015 Canyon
reservoir has a leak. The gas reservoir serves as a repository for stored dried-up gas and oil.
The most serious methane leakage accident in US history occurred in this episode. From the
time of the leak to the end of the closure, it took over four months. 90,000 tons of methane
were released in total. Due to the occurrence, several adjacent residents became gas poisoned
and were forced to flee, which resulted in a scarcity of electricity in the nearby communities.

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Question [1].
If the closure is delayed, it will result in the project failing and a significant financial loss,
and eventually use salt rock. In 2015, China's gold altar salt pit reservoir revealed a
microporous layer. The reservoir leaking area is blocked by the process of recrystallization.
The International Gas Union (IGU) estimates that more than 700 underground gas reservoirs,
comprising depleted oil and gas reserves, had been created by the end of 2018. With an
average of 81%, underground gas reserves are the most prevalent. There have been 16
examples of leaks from depleted oil and gas reservoirs reported globally. During the injection
procedure, there were a total of 43.8% gas migration leaks from ground level, 31% filling
well or sleeve mishaps, and 18.8% leaks as a result of malfunctioning reservoir ground
facilities [2]. This demonstrates that the subterranean gas storage tanks, coupled with the
economy's rapid development, also constitute a threat to safety that cannot be ignored. By
the end of 2020, China will have installed more than 30 subterranean gas storage tanks. This
is the reservoir kind of gas and the greatest number of gas storage banks. It is more
practicable to use the exhausted oil reserves close to established areas to convert them into
storage tanks in order to offer natural gas to neighboring cities because the majority of the
gas deposits are mostly located in the western and northern sections of our country, far from
the metropolitan areas. It is an issue to investigate whether the reservoir, which is oil-
depleted and remodeling, can seal natural gas under challenging operating conditions. The
reservoir has a natural advantage in storing natural gas. The sealing capabilities of the
reservoir in particular are crucial for preventing natural gas loss.
Therefore, it is necessary to rigorously examine the danger of damage to the depleted oil
reservoir lid during normal operation, taking into account both the cover's internal elements
and exterior ones. Strengthening the system management and supervision of the reservoir,
establishing a system and system for testing, improving protective measures and emergency
means, doing everything in our power to reduce the reservoir leakage rate, ensuring the
reservoir's safe and stable operation, taking responsibility for the safety of citizens and the
state, and fostering the orderly development of society are challenging but important tasks.
the offering. To do this, we must create a workable model to examine the process of damage

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to the depleted reservoir's cover and implement the necessary safeguards to assure the
reservoir's secure and stable operation.
1.2 The current situation of research at home and abroad
1.2.1 Development overview of the depleted reservoir
The first underground gas storage test was carried out by the Welland Gas Field in
Canada in 1915, marking the start of the construction of natural gas storage facilities in
underground areas. However, the Zoar gas field in New York, which was transformed into a
reservoir in 1916, was the site of the first actual gas reservoir. It is an oil and gas reservoir
that has dried up. Following that, Europe and the former Soviet Union started to construct a
reservoir [3] out of depleted oil and gas sources. There are now more than 700 gas reservoirs
globally thanks to the last century, They are mostly produced from exhausted oil and gas
reservoirs. 76% of the total amount of them are in the reservoir, 5% are in the reservoir, and
11.8% are in the entire amount of gas consumed globally.
The building of the gas storage bank abroad has already begun, and it is currently moving
along steadily. The biggest producers and consumers of natural gas have already finished
building their Good reservoir systems. By the end of 2020, 716 distinct types of subterranean
gas reservoirs will have been built in 36 different nations globally,includes 478 dry-type
reservoirs, 45 dry-oil reservoirs, 107 salt-pit reservoirs, 82 water-containing reservoirs, and
4 abandoned mine-pit reservoirs [4]; the majority of them are concentrated in the US, the
EU, and Russia. The majority of reservoirs found overseas have straightforward geological
structures with shallow buried depths, are made of high-porous kinds [5], are easier to
develop, and have been matched with appropriate technologies [6]. includes 478 dry-type
reservoirs, 45 dry-oil reservoirs, 107 salt-pit reservoirs, 82 water-containing reservoirs, and
4 abandoned mine-pit reservoirs [4]; the majority of them are concentrated in the US, the
EU, and Russia. The majority of reservoirs found overseas have straightforward geological
structures with shallow buried depths, are made of high-porous kinds [5], are easier to
develop, and have been matched with appropriate technologies [6]. China started funding

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the construction of a gas storage reservoir at the turn of the century, and it is currently in a
stage of rapid expansion. 30 storage banks were built in 17 storage regions by the end of
2020. Of these, China Oil constructed and ran 26 of them. Sinopec and Huaihua Gas
managed the remaining ones. Due to their unique geological context, our country's reservoirs
differ from those of other nations in that they are deeply buried, have broken construction,
are of excellent quality, have a high operating pressure, and have a complex filling
mechanism [8].
The characteristics of domestic and foreign reservoirs are as shown in Table 1-1:
Table 1-1 Type and characteristics of domestic and foreign gas reservoirs
Table 1-1 types and Characteristics of UGGS at home and Abroad
The type of
reservoir
Storage media Features
Dry up oil and
gas storage
Raw oil and
gas porosity
permeates the
ground
Foreign: Large storage, shallow buried, simple
and easy to develop construction, mature
technology for building and storing
China: Deep buried, complicated construction,
broken development, low penetration of middle
holes, strong nonhomogeneous mass,
complicated flow
Water-
containing
version
Porosity
permeates the
ground layer
Overseas: Good aquifers, large storage capacity,
high stability of the ground, but high cost of
building and storing the base
China: No reservoir for aquifers at this time
Salt-cave type Salt caves that
are dissolved
in water in salt
formations
Abroad: Low level of cushion, high injection
rate, flexible peak adjustment, high construction
price, long time for building stock
China: Complex rock, multiple layers, thin salt

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layer, small air storage
Waste mine-
hole type
Underground
mine holes
formed after
mining
Abroad: Flexibility, low cost of building and
warehouse, small scale, high risk
China: No waste mine-cave type gas reservoir
The majority of the world's depleted oil reservoir reservoirs are located in the United States,
and Poland is one of the few other nations that still has a sizable number of these reservoirs.
This is currently obvious. The depleting oil reservoir and the reservoir's continued use
present technical challenges. China's petroleum essentially developed the depleted oil
resource in China. Rather than immediately in the depleted oil reservoir, it is more likely to
store gas at the reservoir's top.
The first dry-up reservoir in China is the Lamandian Gas Reservoir. It was constructed in
1975 after being transformed from a depleted ceiling oil reservoir, and thus far it has
accumulated. Simple and fault-free, the reservoir has minimal issues. The oil ring seals off
the gas, yet to keep the oil zone's pressure balance and assure the reservoir's steady operation,
it must do so.
The Beijing 58's storage tank was created in the North China oil field and is made of layers
of both gas and oil. The setting for the ground level is 10° to 20° The storage tank has a
maximum capacity of 1.15m m3, the maximum working pressure is 20.6MPa, the high spot
is buried deep at -1750m, the spill point is around -1950m, the closing range is about 200m,
the closing area is about 1.15mm2, and this helps to ease the conflict between supply and
demand of air for use in the winter in northern China.
In China, the use of existing storage and storage facilities utilizing depleted oil sources is
still in its early stages. Consequently, studies on storage and conversion of depleted oil
reserves is strengthened In order to improve the corresponding building and storage
technology and increase the utilization rate of the depleted oil reserves, this is of great
significance for alleviating the huge energy demand in eastern cities.

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1.2.2 Analysis of the destruction mechanisms and leakage risks of the reservoir
The reservoir is currently being built to a rather high degree abroad. Research on the
mechanism for stopping reservoir leakage has been crucial and results from the study on
sealing the reservoir have been highly significant. More successes were made. Pierre The
outcome of a mechanical simulation of the elastic nature of irreversible deformation brought
on by plastic reservoir compression of the depleted reservoir, such as Jeanne, demonstrates
that ground compaction significantly affects the size and direction of the stress yield and
may cause neighboring faults to become active and damage reservoir tightness [9]].
Muhammad and others forecast the reservoir's maximum operational pressure using fluid
dynamics models and surface displacements in porous media reservoirsTo apply to the
depleted gas reservoir in southern Germany and to ensure fault stability and cover integrity
[10]. BI When studying the minimal risk associated with a salt pit resource, Aorui et al.
provided a method for quantitative evaluation of the safety of the reservoir, established a
system of safety assessment indicators for the reservoir, and computed the combined weight
based on fuzzy sets [11]. Based on the BT model and BN model, Arun Agarwal and others
devised a method to assess the likelihood of a reservoir infraction., effectively calculating
the likelihood that the subsurface reservoir's seal may fail [12]. Kazemi Esmaeel Through
core experiments, such as TOOSEH, the effect of the interaction of air-water rocks during
the filling of the reservoir in the low-seepage water reservoir was studied, leading to an
increase in filling speed and pressure to increase the volume of gas injected and improve gas
storage in the reservoir [13]. The sealing of the reservoir lid has been the subject of extensive
investigation abroad. Increased stress, depth, and duration could lower penetration to restrict
potential leakage through the cover fracture, according to the findings of the three-axis
straight-cut approach, as demonstrated by Luke P. Frash, who evaluated the variations in
reservoir layer surface penetration under various ground conditions. [14] Bakhtiari Mohsen
and others performed rock mechanics testing and flow coupling simulations of the Sarajeh
oil fields' geological mechanics. To help with the forecast of the maximum amount of storage
that could be used, it was discovered that the earth was raised by approximately 6 cm as a

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result of injection and that the pressure for rock damage and fault activation was 11283 psi
due to the settlement of 16 cm as a result of gas extraction 9986 psi [15]. Petr Rapant, for
instance, discovered that the seasonal filling of the reservoir is more relevant to the periodic
changes in the upper terrain height, but the surrounding terrain has changed in contrast to
this, which was studied by the local fault structure [16] [Benedict Tos An integrated approach
to modeling fluid flow and geological mechanics values has been developed as a result of
research on the ground motion caused by seasonal cycle injection of depleted oil and gas
reservoirs in the Italian plains, including Christoforos. This approach reproduces the key
structural characteristics of the underlying formations and the injection process. Pore
pressure change has improved the management of subterranean resources and increased the
security of developed regions [17].
The last roughly two decades have seen a concentration of research on home gas storage
tanks. There are numerous variables to sealing since the geological environment where our
gas storage tanks are located is more complex. The issue was also thoroughly researched.
Models were created by Wang Lei and others to evaluate the likelihood that the mining and
reservoir fluid injection processes would cause nearby faults to be activated. They have
identified variations in the critical state fluid pressure inside the reservoir using the idea of
superposition of stresses on the surface around the fault, and they have identified the position,
length, and The rock's Poisson's ratio and pitch are extremely sensitive [18]. The basis for a
quantitative assessment of the risk of activity at the reservoir break level under an alternating
load is the application of uncertainty quantification methodologies, such as Zhang Shengyue,
to study the risk of failure close to the reservoir well area. [19] Shao Jixin and coworkers
conducted stress sensitivity experiments on gas flow sensitivity and load time while
sampling the carbonate rock formations in the XG reservoir. The study discovered that
greater microcracks were produced as a result of the rocks' plastic deformation, which also
made them more sensitive to stress and hence more likely to leak [20]. in the proper
location. Li Yinping and others have solved the scope of the seal in connection with the
project to fix the failure of the seal of the cylinder of the reservoir of the gold altar salt pit.

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The scope of the seal is affected by the parameters of the injection of pulp, the makeup of
the plasma, and the coupling of the flow. For the purpose of analysing the seal's short- and
long-term impacts, evaluation standards have been developed [21]. In order to produce a
significant impact on the dynamic performance of the reservoir [22] on molecular
proliferation and media deformation, Yu Fook and others built a model of drying up crack
oil storage and converting it into a reservoir, taking into account the proliferation of
molecules and the deformation of storage media. The model was solved using the finite
differential method. Guangquan Zhang, assesses whether the reservoir can operate safely by,
among other things, taking into account the impact of changes in the injection process storage
pore pressure on the fault, determining the likelihood of fault activation based on changes in
stress field, and determining the dynamic sealing of the reservoir closure [23]. By developing
a regional scale 3D dynamic geological mechanics model and a 3D fine geological model,
Liu Wei and others were able to analyze the dynamic changes in the sealing of the exhalation
wall reservoir in Xinjiang under high-intensity injection and provide guidance for
determining the safety of the reservoir's long-term alternating injection operation [24]. This
demonstrates that the fault function, the continual alternating and vigorous injection, and the
consequent microcracks have the greatest impact on the tightness of our reservoir.
China primarily concentrates on the types of oil and gas storage that are being depleted
while choosing a storage gas reservoir. The cover layer containment investigation is
therefore very important. Chinese academics have done extensive research in this regard.
Wang Dichin and colleagues examined the displacement of the reservoir's surface cover
using GPS observation data, and they discovered a high correlation between changes in
reservoir pore pressure and the horizontal displacement of the surface cover during injection.
Three-axis compression was carried out by Li Shuangjian aand dumping tests on the surface
of the reservoir's mud cover, resulting in a linear relationship between the depth of the cover
and the maximum level stress as the ground rises, and the hairpin and other horizontal lifting
tanks The analysis of the logging parameters and the breakthrough pressure test in
conjunction with the characteristics of the cover were based on theoretical analysis and the

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results of the in-room test, which proposed the criteria for the classification of the sealing
strength of the lift reservoir, and the lifting of the reservoir cover was a strong-strong
level [27]. Li GuoTao and others evaluated the sealing of the lid of the reservoir of aquifers
by conducting a comprehensive study of a large number of reservoir stores abroad, pointing
out that the change in pressure in the reservoir is an important indicator of the leakage of the
reservoir [28]. The analysis of the sealing of the salt-based mud cover in the reservoir at the
Cape has shown that the reservoir pores are poorly developed, have low permeability, have
high pressure breakthroughs, are continuously distributed, and have a certain strength of
sealing [29]. Zhang Guangquan and others have established a damage structure model for
the reservoir cover and reservoir through the three-axis cycle loading of the simultaneous
permeability test experiment, resulting in an increased permeability rate for the mud cover,
which decreases first as stress increases, until the sample is destroyed [30]. It is evident that
factors such as congenital geological conditions and pressure changes caused by gas
injection affect the reservoir lid's ability to seal gas.
1.2.3 Multi-hole media flow coupling for gas leakage
As technology develops and research develops, a large number of studies show that
the interaction between the flow and structural fields, namely, the use of the flow coupling,
is important in engineering issues In the early 20th century, Bourrierest first analyzed and
pushed for the use of fluid-line coupling Exports flow-coupled motion microequations [31].
In the mid-20th century, Paidoussis derived expressions of nonlinear dynamics equations
based on the flow-line motion microequations, taking into account the effects of gravity of
the fluid and pipe material damping. Lee and others have completed the maths calculation
of the flow-coupled nonlinear model for the first time based on Paidoussis's nonlinear
dynamic equation expression and simplified piping mechanics model, and Lee have refined
the model further, considering the axial Kelvin of the pipeline and the internal liquid
movement Centrifugal force, which raises the Poisson interaction of the coupling process
[34]. Gormen et al. studied the interaction of structural and flow fields in the event of a large

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variation in the pipeline and calculated complex flow coupling equations based on the results
of the study [35].
As research progresses, researchers have found that similar fluid-to-pipe interactions
exist between the permeate and structural fields in porous media structures. This is the effect
of the porous media flow coupling cooperation in the actual engineering on the porous
structure. AndräHeiko and others studied the effect of distortion of porous filtration water
equipment under high water pressure on its filtration capacity and presented a model
describing the coupling process based on numerical simulation results [36]. TitiSui et al.
analyzed monopiles affected by waves by numerical simulation, which showed that the depth
of monopiles greatly affected the extent of pile damage and that post-pile porosity pressure
in the same soil conditions was less than before piles, and pressure was reduced without
regard to the residual response of the seabed [37] In the right place. A computational
framework based on hydrodynamics methods such as Nguyen to optimize the flow coupling
study method in variable porous media [38]. By comparing the results of different mesh
calculations in finite element modeling, such as Haruhiko Kohno, the models under the
triangular node elements and the quadrennial node elements have a higher convergence,
suitable for calculating complex, flow-coupled models [39]. Dashtbesh Nargs et al. proposed
a sequential method of nonmixed-phase, multiphase flow simulation for the calculation of
nonhomogeneous porous media, which uses dynamic zone disaggregation and calculation [40]
in different flow areas of a porous media structure.
Scholars in China have also gradually begun to pay attention to the importance of research
for the use of streamlined coupling cooperation. In recent years, research in this area has
been continuously carried out, and Fan Yunpeng has also considered oil and gas development
The influence of factors such as changes in ground porosity pressure and fluid movements
during the mining process gives an overview of the current and development direction of the
research related to the theory of flow coupling in oil and gas extraction [41]. The lightning
has used a numerical simulation to model the river top tube, which shows that the tube has a

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large displacement and stress variation under the influence of the permeation field [42]. Zhu
Qingjie and others have.For the purpose of breaking up chunks of oil and gas in a deep sea
area, a model of porous medium-stream coupling finite elements was constructed. Huang
Chaqin and others created a mathematical model of the discrete oil reservoir that takes into
account the distortion of the storage medium. In order to create the porous media creep rock
stream-coupled calculation model [45] with the permeation parameters, Weicar and
colleagues studied the flow coupling cooperation between the permeation field and the rock
creep field. Mr. Hsing-wah and others have created mechanical models of water injection-
induced stress field changes using finite element methods and porous medium theory.
According to the analysis, superpore pressure created by ground fill water will boost ground
activity. [46] The effective permeability region in mesh cracks is the key to capacity impact,
according to Ma Dongwook and colleagues who developed multi-scale multi-level burst
horizontal well production models under the influence of hydraulic cracking and reservoir
coupling [47]. For the goal of linking the porous medium, Zhu Qingjie and coworkers have
developed a finite element model for the deformation of the ground near the lower casing.
Based on the findings, they study the casing's destruction mechanism under the combined
operation of the current coupling and forecast its position [48]. In order to prevent
greenhouse effect carbon dioxide from being injected into the aquifer process and prevent
excessive injection rates that could harm reservoir formation, Kang has examined the
combination of porous media streams [49]. A bigger impact on the stress variation of
permeability of the reservoir has been seen as a consequence of the analysis of deformation
and stress sensitivity of the reservoir cover during gas injection.
2. Influence factor of cap rock failure
2.1 The effect of the failure of the seal of the cover
The depleted oil reservoir has a significant storage depth, and the complexity of the
geological construction is what distinguishes it from other forms of storage tanks. The

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different oil and gas reservoirs have been created, and they all have very different cover
patterns. Even within the same cover, there are differences in the ability to close oil and gas,
which may be due to the thickness, rock properties, and nonhomogeneity of the cover during
formation. The pressure of the oil filling and the late-stage gas extraction will also be
employed to cause the cover to be in a changing environment after the dry-running oil
collecting, which will unavoidably damage the initial sealing capacity. Analysis of the
consequences of the closure's failure is thus very important. Four primary components,
including the macro- and micro-characteristics of the cover itself, outside geological activity,
and manufacturing conditions, are what determine the outcome for many of the factors.
3. Finite seal failure analysis
3.1 finite element modeling
Parasolid modeling with ADINNA software creates a dry reservoir of 1000 × 800 ×
800m size The gas reservoir model, which uses the fault that controls the storage function as
the boundary of the model, layers the model according to the actual reservoir rock and, by
properly simplifying the consolidation of several layers separated by the mud-rock layers
into one, dividing the reservoir into two areas, water and gas. Two holes of drilling are set
up above the reservoir using Boolean operation with an outer diameter of 219mm and
377mm as injection and extraction wells respectively. The bottom of the reservoir position
is set as a base rock, with the top cover and top coat, and the top of the model is used to
mount a portion of the top cover and to apply a pressure load over it to simulate the
overpressure on the ground, as shown in Figure 3-6.

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Figure 3-6 Storage Repository Geometry Model
The rock properties of the model are defined by the rock formations in the sand upper
oil reservoir, and the rock material in the reservoir geometry is based on the Mole-Kuren
material, material parameters as shown in Table 3-1.
Table 3-1 Model material Parameters
Ground Rock Young's
modulus
(MPa)
Density
(Kg/m3)
Poisson's
ratio
Internal
gathering
force
(MPa)
Friction
angle
(°)
Top coat Gray Rock 24000 2700 0.28 100 30
Cover Dark gray
mud
4380 2320 0.32 0.52 23
Storage
layer
Sandstone 25700 2510 0.12 21.28 31.45
Kiwam Granite 45,000 2790 0.17 14 52
The model is based rock, storage, The formation of the cover and the upper cover is
granite, sandstone, mud, and gray rock respectively, with the formation shown in Figure 3-
7.

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Figure 3-7 Storage reservoir strata division
The model requires a constraint, a full constraint is built into the system, and a custom
normal constraint is required for the X direction, Y direction, and XY direction. The full
constraint is applied to the bottom base rock, the lateral boundary of the model is restrained
in the direction, and the inside side of the two wells is restrained in the XY direction to
simulate the effect of the fixation hole, as shown in Figure 3-8:
Figure 3-8 XY direction Constraint setting
The model contains three types of loads, one is the gravity load of the model, which
simulates the effect of the weight of the ground, the other is the pressure load, which is buried
deep in the reservoir The drawing of a complete upper overlay will cause the model to be
too large to affect the calculation accuracy. Therefore, pressure is applied to a small part of
the upper overlay to simulate the effect of the upper layer pressure. Another is the pore
pressure, which can only be used if there is a porous media structure and applied to the
reservoir position to simulate the internal pore pressure of the reservoir The role of the . The
parameters are shown in Table 3-2.
Table 3-2 Model load Parameters

21
Load name Take the value size Direction
Gravity loading 9.8N/kg The model is vertical down as
a whole
Pressure load 14.9MPa Normal to top of top coat face
down
Pore pressure load 20.3 MPa The reservoir is internally full
outward
Set the model calculation to 10 time steps and complete the initial condition loading in
the first 3 time steps, i.e., the application completes the original reservoir ground state, at the
last 7 The time step applies the internal cavity pressure of the reservoir to simulate the
pressure change for each month during the reservoir injection period, as shown in Table 3-3.
Table 3-3 Gas storage Pressure
Month of injection Pressure value (MPa) Month of injection Pressure value (MPa)
Before filling 15.6 July 19.1
April 16.2 August 19.7
May 17.8 September 20.1
June 18.6 October 20.3
The load time function is set according to the reservoir pressure at the end of each month
during the injection period, and the time step setting and time function is shown in Figure 3-
9.
Figure 3–9-time step and Time function settings
After the creation of the geometry model, restraint and load application, consider the
ground displacement caused by the filling of the reservoir and set the kinematic analysis to
a large displacement. The initial temperature of the model is set to 73 degrees C, taking into

22
account the effect of the ground temperature on the rock properties, as shown in Figure 3-
10.
Figure 3-10 Analyze the assumptions settings
To facilitate calculation and analysis, you need to group the whole model. The division
of the unit groups provides a clear representation of the relationship between the various
strata of the reservoir model and the overall structure, as well as the ability to apply material
properties and loading conditions to a structure individually, and to analyze the status of a
group of units separately in the post-processing of the calculation. This article has four
formations of the reservoir model, divided into five groups of elements, where the reservoir
is divided into two groups. The model consists of a base rock element group, a water
reservoir element group, an air reservoir unit group, a cover unit group, and an upper
laminate unit group. Except for the 2 reservoir unit groups set to the porous media properties,
the remaining cell groups are set to three-dimensional entity elements, divided into groups
as shown in Figure 3-11.
Figure 3-11 Element group Division
Since the reservoir is a porous media material, the corresponding porous media
properties need to be set after the unit group is divided. According to the parameters of the

23
sand-first upper reservoir, the model reservoir porosity is set to 25%, and it is important to
note that the permeability parameters in the software are not permeability factors, the values
should be the permeability factor to a severe ratio, the axes are equal, and the volume
modulus of water in the reservoir is 2.6GPA, and the natural gas volume modulus is 0.05 GP
A, the porous media properties are set as shown in Figure 3-12.
Figure 3-12 Porous media Properties

24
The meshing of a finite element model essentially divides the whole model into mesh
forms, and only the parts that are meshed are involved the density of the grid is calculated
as the amount of thickness divided into the model. This model is modeled using Parasolid,
which is meshed by dividing the edges of the model into appropriate copies. Since the focus
is on the cover, the cover model needs to be divided more closely to achieve higher accuracy.
After you set the mesh density, divide it by four-node, three-dimensional solid elements. The
grid model is set as shown in Figure 3-13.
Figure 3-13 Gas storage Model Meshing
The distribution of the constraints and loading conditions applied can be visually
displayed on the reservoir grid model, as shown in Figure 3-14.
Figure 3-14 Model Constraints and loads

25
3.2 Deform of cover.
The simulation results of the FE software provide a simulated cloud map of the reservoir
cover over a single injection cycle, which calculates the completed reservoir cover profile
The result of the change is shown in Figure 3-15. The displacement cloud map shows that
for the entire area of the cover, the deformation is concentrated primarily in the upper cover
position of the reservoir reservoir area, and the deformation of the upper layer of the reservoir
is small, due to the deformation of the cover due to high pressure caused by the large filling
of the reservoir and the potential increase caused by the drop in the interface of the high
pressure gas driver layer This can have some effect on the cover. By studying the
characteristics of regional deformation, it is found that the area with the upper cover of the
reservoir is prone to damage due to large displacements at the high construction, at the air-
water interface and near the filling wells.
Figure 3-15 the reservoir cap layered displacement cloud map.
The loading of the reservoir pore pressure load in the calculation model is divided and
applied progressively according to the pressure at the end of each month during the injection
period, so it is possible To map the calculation of each step to the state of the reservoir cover
after each month of filling, analyze the variation in the degree of deformation of the cover at
different stages of the injection process, with the reservoir cover shift in each month as shown
in Figure 3-16.

26
Figure 3-16 the month-by-month layered shift cloud map.
According to the displacement cloud map of the cover of each month during the
injection period, the basic deformation area of the cover has been formed after the first
injection and the subsequent injection will gradually deepen this The degree of deformation
of the area, especially above the air receiver. After the two months with the highest volume
of air injection in April and May, the cover is gradually deformed to the maximum, and the
final calculation is subtracted from the initial condition value to achieve a maximum
settlement of 44.3mm, and the maximum lift is 15.3mm. Therefore, subsequent injections
need to be adjusted to maintain stability of the cover.
Figure 3-17 the reservoir cap stress cloud diagram
The effective stress cloud diagram of the reservoir cover is shown in Figure 3-17.
Effective stress distribution is like displacement, with high stress concentrations near the
cover, the construction high, the air-water interface and the injection wells, which are prone
to first destruction. Weak areas are prone to high pressure damage due to the small thickness
of rocks, low strength, high volume of gas gathering in high areas, strong pressure on the
cover, gas and water in the surface of the cover, large pressure values are easily destroyed,

27
and stability of the cover is affected by the stress redistribution caused by drilling, which
causes the drilling to be carried out in close proximity Stress concentrations are prone to
occur, increasing the risk of damage.
The effective stresses of the reservoir cover for each month are shown in Figure 3-18.
Figure 3-18 the monthly layer stress cloud map
According to the effective stress cloud map for each month of the cover, the first stress
concentration of the cover during the entire injection is around the well, due to drilling the
original stress distribution of the cover has been changed, causing initial defects at the top
well, which may be first damaged and leaking by high pressure gas. The locations of the
stress concentrations that follow are in turn at the water interface. Weak areas and high
construction areas, which are subject to increased stress as the volume of injection increases,
are finally calculated to show that the maximum effective stress on the cover is 11.39 MPa,
which appears in the vicinity of the well bore and the construction high, with a focus on the
closure sealing at that point.
The maximum lift and effective stress curves for each month of the cover are shown in
Figure 3-19.

28
(A) maximum lift curve
(B) effective stress curve
Figure 3-19 maximum lift and stress curve for cover
The rise of the cover is directly related to the volume of the reservoir injection. As can
be seen from the curve changes, the amount of injection in the pre-injection period is high
and the bottom of the cover is raised significantly, as well the volume of air injected in the
next few months has gradually decreased and the deformation of the cover has gradually
decreased. However, after the injection, the volume of air injected has decreased
significantly compared to the previous period. However, the deformation of the cover has
increased. It can be assumed that the reservoir pressure has reached the limit of the strength
of the cover. There is a risk of leakage The rapid trend requires control of the post-injection
volume.

29
3.3 Analysis of the factors that influence the failure of the cover.
To analyze the failure model of cap rock's sealing, a comprehensive understanding of these
factors is required. This involves geotechnical investigations, rock property assessments,
stress analysis, reservoir modeling, and monitoring of operational parameters. By
considering these factors collectively, engineers and geoscientists can develop strategies to
mitigate the risk of cap rock failure and ensure the long-term integrity of gas storage in
depleted petroleum reservoirs.
4. Fault influence analysis
The previous article examined the factors in the structure of the reservoir of depleted
oil and the failure of the seal of the cover under operating conditions The geological
environment in which the site is located is very complex, with many broken fractured belts
and widespread fault development. Near the reservoir in the deep underground section, there
is often construction that creates a large, concealed fault. Although these faults do not go
directly through the reservoir area and their own activity rate is low, and do not directly
damage the reservoir seal, the mechanical nature of the fault may be altered during
continuous gas injection, thereby affecting the reservoir's sealing. To study the effect of fault
activity on the cover, create a reservoir with faults finite element model, simulate changes in
the cover in different fault activity situations, and analyze the results of the calculation.
4.1 Modeling with a tomographic reservoir
The reservoir of depleted oil reservoir, which is studied in this paper, is located on the
Moon on slope with the middle and the middle of the Pass, is a cut-off field, with broken
ground and extensive fracture and development, and inside the reservoir There are early
faults. Considering that large faults through the reservoir area ground directly cause natural
gas to migrate along the fault line, this paper examines the effect of fault movement of the
internal fault in the reservoir on the sealing of the cover, sets up the concealed fault in the
reservoir on the basis of the FE model above, and builds a reservoir model with faults, as
shown in Figure 4-1.

30
Figure 4-1 Contains the tomographic reservoir geometry model
The reservoir is an underground rock structure using the Mor-Crunon material, with the
same material parameters as the previous chapter basic models, with a cover-layer of dark
gray mud and storage The porosity is 25%, the permeability is 500md, the reservoir is buried
deep 1540m, the pressure of the upper strata is 14.9MPa, and the maximum pressure of the
gas is 20.3MPa. The set fault is the north-south direction with a inclination of 70°.
To study the effect of slice activity on the cover, apply displacement loads on the slice
to simulate the tomographic mismovement and change the direction of displacement loading
And size to define the type and extent of activity of the slices. The lower slice faces are
simulated by applying an upward displacement load, a downward displacement load, and a
horizontal shift load to simulate a positive, reverse, and translational slice, respectively. The
loading on the slices is applied as shown in Figure 4-2.

31
(A) Tomography
(B) Reverse slices
(C) Pan the slice
Figure 4-2 Displacement Load application method for Faults
Simulate the size of the slice fault motion by setting the displacement load taking value,
with the tomographic error values set to 1m, 2m, 3m. When solving a finite element model
with a tomographic reservoir, set 10-time steps to load all slice shift values, and apply loads

32
that can be fast and slow first, which facilitates convergence of the model solution process,
as shown in Figure 4-3.
Figure 4-3 Displacement Load time function
4.2 Tomographic impact analysis
Mesh and calculate the model as 4-node solid elements, 1m for each of the normal,
reverse, and translational slice conditions. Error calculation for 2m, 3m, analyzing the effect
of fault motion on the cover of different forms of faults.
4.2.1 The effect of tomographic mismovement on the cover
When the lower slice is raised relative to the upper pan, the upper plate is fixed and the
lower plate movement is controlled, the other parameters are unchanged, and the full
displacement is gradually loaded in 10 time steps Quantity. In the calculation, to eliminate
interference from the results of the remaining large stress values in the area, the 1m stress
distribution at the top of the cover layer above the top of the slice is 2m analyzed, 3m as

33
(A) Fault fault fault action 1m
(B) Fault fault fault action 2m

34
(C) Fault fault fault action 3m
Figure 4-4 Normal fault variable amount lower cover stress distribution
Figure 4-4 the stress Distribution of rock under different location mum of Fault
The stress distribution diagram of the cover shows that the filling process is increasing
with small positive fault motion in the reservoir, directly above the cover. The stress values
of the are also increasing. The highest stress during the entire fault fault is 5.17 MPa, where
the stress is highest at the weaker position on the cover, where it is easy to first be damaged
by fault action and the risk of leakage when gas is injected into this position. Stress increases
are distributed basically along the slice path and peak at the end of the slice, where stress
concentrations are most likely to become leakage points on the cover, thus the fault action
has a significant effect on the reservoir cap seal.
Figure 4-5 the stress Change process of the rock
The process of changing the stresses of the cover under the three fault rates of the
tomographic faults is shown in Figure 4-5, with the stress values and ranges increasing in 10
time steps. Stress values are first increased at a thin layer, and are distributed in blocks, which
are striped along the slice as the loading process progresses. It is important to note that during
this process, the stress at both ends of the slice is still increasing, the stress growth range is
spreading all around, and the spread is greater than the upper plate in the lower part of the
slice, resulting in a dumbbell distribution with higher stress values at both ends and at the
middle. This phenomenon is mainly due to the tendency of distortion of the cover on the side
of the lower slice when the primary slice is moving, the cover on the upper part of the slice
is restricted by the ground, resulting in a strong shear stress on the top of the slice and a
stronger shear on both ends of the slice The situation.
To analyze the displacement and stress changes of the cover layer under the effect of a
normal slice, take a node closest to the slice on the cover with the displacement and stress at
that node The values change to analyze the sealing of the cover under fault conditions, and
the stress and displacement curves are shown in Figure 4-6.

35
(A) effective stress curve
(B) maximum shear stress curve
(C) Displacement curve
Figure 4-6 Normal Tomography lower cover stress and displacement curve
As can be seen from the diagram, the overall change in stress is in an unstable growth
trend. When the positive fault 1m is moved, the stress at the cover above the slice increases
steadily to 1.84 MPa in the previous period and remains for a period of time, with a sharp
increase in the latter; when the fault fault 2m is moved, the stress suddenly decreases after

36
the previous period increases to 1.91 MPa, and then increases sharply again when the stress
falls to 1.25MPa; break When lamination mismovement 3m, the stress growth trend is
similar to that of misacting 2m, the rapid growth of the previous period to 1.85 MPa is
followed by a sudden decrease in stress to 1.02 MPa, and the effect of the post-layer main
shear stress is noted.
This trend of stress change is due to the stress on the cover layer caused by the lower
plate rise in the early stages of the fault activity, and the stress of the cover layer rise to a
certain extent The damage to the rock, which is damaged by the overlayer, can be seen as a
dump that causes the strain of the cover to drop, and as the fault continues to move, the
broken rock is unsupported and under pressure under the upper overburden pressure, and
stress increases dramatically. As indicated by the displacement curve at the top layer above
the slice, the previous layer is raised by the lower slice rising, until the stress limit of the
cover rock is destroyed, a settlement is created by pressure of the upper overlay, and the
formation of the slice will break faster when the fault activity is high Break down and sink
down.
The simulation results can show that the tomographic activity has a large impact on the
cover. The small mis movement of internal fault faults within the storage tank is not sufficient
to damage the seal of the cover, but excessive fault activity will break the nearby rocks and
damage the seal of the cover causing gas leakage. Therefore, the intensity of the injection
gas needs to be controlled to prevent the reactivation of the faults in the reservoir.
4.2.2 The effect of reverse fault motion on the cover
When the lower slice is lowered relative to the upper pan, the upper plate is fixed and
the lower pan is controlled to move, the other parameters remain unchanged, and the error
of the reverse slice 3m is loaded in 10 time steps the dynamic force, which results in the
effective stress at the position of the cover above the slice, is shown in Figure 4-7.

37
Figure 4-7 Reverse fault: 3m lower cover effective stress distribution
As can be seen from the layer stress cloud, the stress distribution feature of the cover
under the reverse fault effect is more similar to the normal fault, and the stress is mainly
present at the higher point The stress distribution at each end of the slice is also greater than
the other, but slightly less than the stress increase range caused by the normal fault. This is
due to the reverse fault being misdirected, not directly against the cover arch, resulting in a
large stress change. The stress and displacement changes of the cover are shown in Figure
4-8.

38
Figure 4-8 The cover stresses and displacement curves are applied by the inverse slice
The stress variation curve shows that the stress increases with the gradual loading of
the inverse fault motion, and the increased trend is fast first, then slow, and the most effective
stress is available It reaches 7.72 MPa, which is similar to the time function curve of the
positive and erroneous volume loads, so it is assumed that the stress increase of the cover
under the back fault effect has a linear relationship to the increase in the error amount. The
displacement of the cover is small initially and then rising at the end, with a maximum
displacement of 14.9mm, which is caused by the degradation of the cover when the reverse
fault lower plate first starts to lower, but as the fault moves, the upper plate moves up against
the arch of the lower plate, causing the cover to be subjected to Continuous lifting of the
upper slice is carried out continuously, and will eventually damage the cover and affect the

39
sealing.
4.2.3 The effect of translational fault motion on the cover
When the two slices are moving toward relative error, the upper plate is fixed and the
lower slice is controlled to slide along, the other parameters are unchanged and 3 is loaded
in 10 time steps M, the calculated effective stress at the position of the cover above the slice
is shown in Figure 4-9.
Figure 4-9 Reverse fault: 3m lower cover effective stress distribution
As can be seen from the overlay stress cloud diagram under the translational slice, the
stress is concentrated primarily on the top of the slice misdirection because of the stored
position at the point of error The limitations of layers and cover rock, which continue to
mismove, result in large stress concentrations. The stress distribution at the end of the fault
is gradually spreading down the side of the disc, which is caused by the movement of the
lower plate by controlling the stationary position of the upper slice, so the side with more
fault motion will have a greater effect on the upper cover. The stress and displacement curves
are shown in Figure 4-10.

40

41
Figure 4-10 Translate the slice stress and displacement curve under the slice action
Figure 4-10 the stress and displacement curve of rock under translation Fault
The stress variation curve shows that the stress of the cover under the translational slice
is roughly a straight-line growth trend, up to a maximum of 5.59 MPa. The stress increases
faster during mis movement, and the translational fault will cause the cover to reach the
destroyed stress value more quickly. This is because the fault of the internal translation slice
is limited by the surrounding rocks and is not capable of producing large deformation and is
therefore prone to rapid increase in internal stresses. The displacement of the cover is also
increasing, raising a total of 2.8 mm, which is smaller than other types of faults.
4.3 Tomosynthesis of sublayer damage
By creating a reservoir finite element model with faults, the reservoir cover is analyzed
by a tomographic section. The effect of reverse and translational fault action has resulted in
the damage to the reservoir cover. Considering the different functions of the three faults, take
the example of fault fault motion 3m to analyze the extent of damage to the cover by the
fault movement of different types of faults, and the stress and displacement curve of the
cover by each type of fault is shown in Figure 5-11.

42
Figure 4-11 Different types of lower slice stress and displacement curves
Figure 4-11 stress and Displacement curves of rock under different types of Faults
The stress variation curve of the cover layer shows that with 3m of fault motion, the
cover layer under the reverse fault is the most stress and the effective stress is reached 7.72
MPa, maximum shear stress of 4.19 MPa. The lowest stress in the cover under the normal
fault effect, the effective stress is 5.03 MPa, the maximum shear stress is 2.84 MPa, and the

43
stress increase in the translational slice is slightly higher than the normal slice. Except for
the trend of descending and then rising stress changes in the normal slices, the activity of the
reverse and translational slices will gradually increase the stress values of the cover layer.
The curve of the shift of the cover shift shows that, also with fault fault motion of 3m,
the effect of the reverse fault is greatest, raising the cover 14 9mm, the translational slice has
a small effect, raising the cover by approximately 2.8mm. The tomographic section causes
the cover to settle and the settlement amount is 6.3mm. Therefore, different forms of damage
can occur to the cover under different fault types.
In summary, with the same degree of fault activity, the reverse fault has the greatest
effect on the cover, making it easier to destroy the cover near the fault and to affect the
sealing The effects of both the tomographic and translational slices are equally important.
Therefore, the distribution and operation of faults in the reservoir need to be explored in
advance to keep an eye on their activity during the injection process and to prevent damage
to the reservoir cover caused by fault activation.
4.4 This chapter is a summary
This chapter builds a reservoir finite element model with a concealed fault in the
reservoir, examines the effect of the reservoir internal fault activity on the seal of the cover
and analyzes the normal fault The stress and displacement changes of the cover under the
effect of the inverse and translational slices, and the degree of destruction of the three fault
activities. The following is understood:
(1) Normal Tomography lower layer stress is mainly distributed along the slice, and the
cover shift is a small rise before settling.
(2) the stress of the cover layer is distributed mainly along the slice by the reverse fault,
and the shift of the cover layer is small in settlement in the early stages of the fault activity,
and is raised continuously.
(3) Shift of the cover layer displacement and stress under the effect of the translational
slice are increasing continuously, stress is growing rapidly, mainly concentrated at the end
of the fault, displacement is shifted Smaller.
(4) Reverse fault activity poses the greatest threat to the closure sealing and requires a
focus during the injection process
Multi-hole media flow coupling calculation for 5-cover gas leakage

44
The flow of natural gas in the reservoir can cause some external force to the surrounding
rocks. When gas is injected into the reservoir, the gas enters the ground at higher pressure
and flow rates, rapidly spreading around the porous media reservoir, creating a stronger
coupling effect, resulting in deformation of the reservoir and cover, and deformation of the
reservoir acts on the lid, thus affecting the sealing of the cover. When the lid seal breaks the
passage of gas leakage, the area of the lid where gas leaks can be seen as porous media, the
gas leaks will attack the surrounding area, form a porous medium domain containing fluid
domains, and analyze the coupling damage mechanism of gas flow to the lid using a porous
medium stream coupling calculation, and explore the cover The effect of seal failure.
5. Porous media coupling failure model of cap rock
5.1s Analysis of the flow coupling cooperation for the lid leakage process
The effect of reservoir porous media flow coupling cooperation on the cover is
examined above, however, when the cover is damaged by external forces and a leakage path
occurs, The process of gas leakage from the cover is also considered as a flow coupling
cooperation between natural gas and the cover rock. Therefore, based on the previous FE
simulation, a gas leakage area is divided into the highest stress and displacement position of
the cover, and the area is set to a porous medium property, and the effect of the flow coupling
cooperation for gas leakage in the lid leakage area is studied.
6. Result analysis and engineering protection
6.1 Basic theory of fracture mechanics
To ensure safe operation of the reservoir, research can reflect the existence of a flaw in
the reservoir cover rock, based on the reality that there is a crack defect in the reservoir Break
criteria under the part to suit engineering needs. From the perspective of fracture mechanics,
the crack extension process can be divided into cracks of the initial defect, and the expansion
will be made to form macro cracks, and eventually become unstable until it breaks. First, it
is necessary to study the critical situation where cracks break under actual operating
conditions, i.e., cracking conditions, and the post-cracking expansion depends on the
increase in load, known as the steady expansion of the cracks, which, after a period of time,

45
is known as cracking at high speed without continuing to increase the load The resulting
unstable expansion of the thread, which eventually damages the structure.
6.1.1 Characteristics and classification of cracks
The geometric characteristics of the cracks are classified according to their location in
the component, making it easier to treat spatial cracks as shown in Figure 6-1.
(1) through cracks: Cracks that run through or over half the depth of the component
along the thickness of the component are generally ideal for the use of tip cracks.
(2) Surface cracks: Cracks are located on the surface of the component, and the depth
of the cracks is also considered to be very small relative to the thickness of the component,
and are generally ideal as semi-ovales Cracks.
(3) deep-buried cracks: Cracks are located inside the component and cannot be directly
observed, and are generally ideal as oval-shaped cracks.
Figure 6-1 Split
Geometry Classification

46
The mechanical characterization of the cracks is classified according to the pattern of
the cracks being loaded externally, allowing for mechanical analysis of complex cracks, as
(1) Open type (Type I): Under external loading, the cracks are shifted by opening under
the tensile stress perpendicular to the leading edge of the cracks and the crack surface,
causing the cracks to break Extends in the direction of the original crack cracking.
(2) Slide-open (Type II): Under external loading, the cracks are subjected to shear stress
perpendicular to the leading edge of the crack parallel to the crack surface, resulting in a
leading edge of the crack The vertical misdirection displacement is also called misopening
cracks.
(3) tear type (III): Under external loading, the cracks are subjected to shear stress
parallel to the leading edge of the cracks and the crack surface, resulting in a parallel edge
to the leading edge of the crack Slide displacement in direction to tear off the cracks.
Figure 6-2 Classification of the characteristics of crack mechanics
Figure 6-2 Cack Mechanical properties Classification
The most common type of crack is the open type (type I), which is the most dangerous
and the focus of engineering research. In fact, cracks in most structures do not usually exist
in the form of the single type of crack mentioned above, creating a composite crack of two
or more basic types under various complex loads, so analyzing the type of crack is the first
step in fracture mechanics analysis.
6.1.2 Basic breaking criteria
(1) stress strength factor K criterion
Cracks in components are often the cause of low stress brittle cuts. When a component
is subjected to external force under operating conditions, the pattern of the crack is directly
linked to the stress field changes at the crack tip, and therefore the stress strength factor K is
introduced as an indicator of the stress field strength of the crack tip attachment to determine
the extent of the crack expansion and the possibility of damage to the component under
external force.
In case of line elasticity, the stress field of the crack tip can be represented as:
( )
2 r
i
i j
j
K
F

 
 (6-2)

47
In: ij
 — Space stress components,
( )
ij
F  —corner distribution function.
By the way up, when r tends to 0, ij
 It is apparent that the stress field strength of the
crack tip is meaningless by means of stress size, known as the singularity of the split tip.
The test on the ideal brittle material found that the change in the stress strength factor
K was load-related and that the crack could be made when the load was increased to a
threshold Continue to expand with constant load, where the stress strength factor KC is
called material fracture toughness. The strength factor can be used as a criterion for breaking
the stress state of the crack.
(2) Energy Release rate G guidelines
The energy balance of the crack tip is considered to be the energy release rate G for the
area of the crack surface as the energy source for the crack to continue to expand to form a
new crack surface. According to Griffith's energy theory, energy released by the crack
extension unit area can be used as a parameter to reflect the state of the crack, and is
applicable to the break criteria describing brittle fracture, called energy release rate,
expressed in G, as follows:
0
lim
A
G
A A
 
 
 
  
 
 
 
(6-3)
In: — Total system potential,
A —the area of the crack.
The criterion can also be defined as: Surface energy consumed by the crack extension
unit area, i.e., the crack extension resistance GC, which is marked as:
0
lim
C
A
G
A A
 
 
 
 
 
 
 
(6-4)
In:  — Surface capability of the system.
(3) CTOD guidelines for spread displacement of the crack tip
In case of elastplastic, the leading edge of the crack is loaded to produce an open
displacement perpendicular to the crack surface. Wells conducted a large number of
theoretical calculations and fracture tests, proposing that the spread of the crack tip be shifted
as a breaking reference to reflect the stress strain field of the crack tip. However, the theory
is not universally applicable, the definition is unclear, the theory is not well founded, and the
establishment of the break criteria is based on empirical and test data In many imperfections,
problems are often not handled well when encountering complex cracks in shape and size.

48
Researchers have given several commonly used CTOD definitions.
CTOD for small Range yield: Split tip spread displacement is:
0 0 0
2
2
2
16 4 4
2
r ry K K K G
E E
 
       

   

Ⅰ Ⅰ Ⅰ Ⅰ
（） (6-5)
CTOD with banding: Assuming the center of the infinite slab runs through cracks, the
split tip is spread to:
0 0
2 2
0
a K G
E E
 

  
  
Ⅰ Ⅰ
(6-6)
In: 0
 — Material yield limit,
G
Ⅰ—the crack spread energy release rate.
(4) J Points criteria
Rice proposed the J-point concept and was seeking to solve the problem of cracks
because of the complexity of the plastic area near the crack tip under the plastic conditions
In this case, the crack tip stresses, To reduce computational difficulties in strain fields, the J-
integral method of studying surface cracks is proposed as an integral loop of the path of the
bottom surface of the crack, in the counterclockwise direction, to the top surface of the crack,
as shown in Figure 6-3:
Figure 6-3 Flat crack J integral Circuit
Figure 6-3 Plane crack J-integral loop
The loop definition for J points is given by the wire-fence points:
u
J dy T ds
x



 
 
 

 
 (6-7)
In:  — Integral circuit,

49
 — Strain energy density,
T — Stress vector,
u — Displacement vector,
ds —Arc element on the loop.
The mathematical definition of J-integral, i.e. the circuit definition, is capable of
representing the strength of the crack tip field, and the same value for J-integral on different
circuits, called the persistence of J-integral points.
The loop definition is a complex process for calculating J-integral, giving J-integral
variant power definitions for experimental and theoretical calculations:
p
U U
J G
a a a

  
     
    
     
  
     
Ⅰ= (6-8)
In: — Potential energy per thickness sample,
U Strain energy per unit thickness sample.
6.1.3 ADINA finite element fracture theory
(1) Line contour integration
The line contour integration method is used for 2D crack analysis, which uses loop
definitions to calculate J-integral parameters to characterize the displacement of the crack
tip. Changes in stress and strain fields. For linear elastic materials, the relationship between
J-integral and stress strength factor K is:
2 2 2
' '
K K K
J
E E G
  
Ⅰ Ⅱ Ⅲ
(6-9)
In: K —Stress intensity factor,
'
E — Material modulus of elasticity,
G —Energy release rate.
(2) Virtual crack extension
Virtual crack extension calculates the crack using the variant power definition of J-
integral by using a finite element model with the same structure and slightly different crack
length Strain energy release rate, which is also the basic method for fracture mechanics
analysis of most finite element software.
6.2 Guidelines for breaking cracked covers

50
6.3 HSE and economic evaluation
Research on the failure model of cap rock sealing for gas storage in depleted petroleum
reservoirs involves evaluating the health, safety, and environmental (HSE) aspects, as well
as conducting economic evaluations. Here's an overview of these evaluations:
1. HSE Evaluation:
a. Risk Assessment: Assess the potential risks associated with cap rock sealing failures,
such as gas leakage, environmental contamination, or health hazards. Identify the likelihood
and consequences of different failure scenarios.
b. Safety Measures: Develop guidelines for implementing safety measures and protocols
to prevent or mitigate sealing failures. This may include monitoring systems, early warning
indicators, and emergency response plans.
c. Environmental Impact Assessment: Evaluate the potential environmental impacts
resulting from cap rock sealing failures, including the release of greenhouse gases or other
pollutants. Assess the long-term consequences and develop strategies to minimize and
manage environmental risks.
2. Economic Evaluation:
a. Cost-Benefit Analysis: Assess the economic feasibility of gas storage operations
considering the potential risks and failure scenarios. Evaluate the costs associated with
sealing failures, such as loss of stored gas, repair and remediation expenses, and any
associated legal or regulatory costs.
b. Risk Management Strategies: Identify cost-effective risk management strategies to
minimize the likelihood and consequences of cap rock sealing failures. This may include
measures such as improved monitoring systems, periodic integrity assessments, or
alternative sealing techniques.
c. Decision Support: Provide decision-makers with economic insights to make informed
choices regarding gas storage operations. Evaluate the trade-offs between potential benefits
and costs, considering different risk scenarios and their financial implications.
The HSE and economic evaluation in the research on failure models of cap rock sealing aim
to provide a comprehensive understanding of the risks, impacts, and economic
considerations associated with gas storage operations. By considering both safety and
economic aspects, researchers can develop guidelines and recommendations that optimize
the balance between operational efficiency, risk management, and cost-effectiveness.

51
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