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A microannulus (MA) is the primary reason for sustained casing pressure in multi-stage fractured-shale gas wells. However, the effect of the casing eccentricity on the long horizontal section has not been considered. In this study, a full-scale integrity tester for cement sheaths is adopted to measure the cumulative plastic deformation. Numerical models are applied to evaluate the development of the cumulative plastic deformation and quantify the MA width considering casing centralization and eccentricity in the context of multiple loading and unloading cycles. Subsequently, the influences of the eccentricity distance and angle, cement-sheath mechanical variables, and different well depths on the cumulative sheath plastic deformation and sheath MA development are explored. The research results demonstrate that casing eccentricity significantly increases the cumulative sheath plastic deformation compared with that of the casing-centered condition. Consequently, the risk of sealing integrity failure increases. The accumulated plastic deformation increases when the eccentricity distance increases. In contrast, the initial plastic deformation increases as the eccentricity angle increases. However, the cumulative plastic deformation decreases after a specific loading and unloading cycle count. Affected by the coupled influence of the internal casing pressure and fracturing stages, the width of the MA in the horizontal section increased from the toe to the heel, and the casing eccentricity significantly increased the MA width at each stage, thus increasing the risk of gas channeling. Finally, an engineering case is considered to study the influence of casing eccentricity. The results show that cement slurries that form low and high elastic moduli can be applied to form a cement sheath when the fracturing stage is lower or higher than a specific value, respectively. The results of this study offer theoretical references and engineering support for the integrity control of cement sheath sealing. [ABSTRACT FROM AUTHOR]

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On the basis of "Carbon Peak and Carbon Neutral" goals, carbon sequestration projects are increasing in China. The integrity of cement sheaths, as an important factor affecting carbon sequestration projects, has also received more attention and research. When CO2 is injected into the subsurface from sequestration wells, the cement sheath may mechanically fail due to the pressure accumulated inside the casing, which leads to the sealing of the cement sheath failing. The elasticity and strength parameters of the cement sheath are considered in this paper. The critical bottom-hole injection pressures of inclined well sections under anisotropic formation stresses at different depths were calculated for actual carbon-sealing wells in the X block—the CO2 sequestration target block. The sensitivity factors of the critical bottom-hole injection pressure were also analyzed. It was found that the cement sheath damage criterion was tensile damage. The Young's modulus and tensile strength of the cement sheath are the main factors affecting the mechanical failure of the cement sheath, with Poisson's ratio having the second highest influence. An increase in the Young's modulus, Poisson's ratio, and tensile strength of the cement sheath can help to improve the mechanical stability of cement sheaths in CO2 sequestration wells. This model can be used for the design and evaluation of cement in carbon sequestration wells. [ABSTRACT FROM AUTHOR]

Objective The cement sheath of CCUS well is exposed to CO2 corrosion environment for a long time, and the cement sheath will be corroded, leading to CO2 leakage. Therefore, it is necessary to predict the corrosion rate. However, the current corrosion prediction models mainly rely on wellbore and semi empirical formulas, which leads to inaccurate prediction of cement sheath corrosion and restricts the research on the prevention and control of cement sheath corrosion in CCUS wells. Methods In response to this issue, a CO2 corrosion depth prediction model was established based on the conservation laws of CO2 and calcium mass. This model analysed the impact of corrosion time, temperature, CO2 partial pressure, water-cement ratio, and corrosion-resistant material dosage on corrosion depth. An evaluation model was also established to rank the degree of influence of these factors. Results The research results indicated that the CO2 corrosion depth increased with the corrosion time, temperature, chloride ion content, CO2 partial pressure, water-cement ratio, and water saturation, and decreased with the cement sheath density and corrosion-resistant material dosage. Additionally, the CO2 concentration within the cement sheath showed a non-linear decrease with the increase in corrosion depth. The strength of the influencing factors was: water saturation > corrosion resistant material > water-cement ratio > cement ring density > CO2 partial pressure > corrosion time > chloride content > temperature. Conclusion The research results have guiding significance for controlling CO2 carbonization corrosion of cement sheath in CCUS well to ensure well integrity. [ABSTRACT FROM AUTHOR]

Copyright of China Petroleum Machinery is the property of China Petroleum Machinery Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In situ observations are a suitable method for studying the hydraulic fractures in a coal reservoir. The cement sheath and propped fractures can be directly observed by the naked eye. In addition, water samples collected in the roadway can be used to analyze the infiltration of the coal reservoir by fracturing fluid. The results indicate that the thickness of the cement sheath in coalbed methane (CBM) well M-1 is extremely uneven. Three failure modes have been observed on the cement sheath, namely, radial cracking, plastic deformation, and debonding between the cement and formation. The poor integrity of the cement sheath is related to the circumferential stress. When the casing pressure is large and is greater than the formation stress and the thickness of the cement sheath is small, the circumferential stress can generate tensile stress in the cement sheath. Tensile failure can occur in the cement sheath when the tensile stress is greater than the tensile strength of the cement. Based on in situ observations, natural fractures around the wellbore can induce the initiation of hydraulic fractures. The ability of different natural fractures to induce hydraulic fracture extension in order from strongest to weakest is as follows: exogenetic fractures > horizontal fractures > cleats. The flow capacity of the fracturing fluid is the best in exogenetic fractures, followed by horizontal fractures, and it is the worst in small-scale channels. This study provides evaluation evidence of the failure behavior caused by hydraulic fracturing. Highlights: Evidence of hydraulic fracture behavior near CBM wellbore was investigated from in situ observations. The relation between the poor integrity and uneven thickness of cement sheath was discussed. The mobility of proppant and fracturing fluid in different natural fractures was analyzed. The ion concentration was tested to determine range of fracturing fluid. [ABSTRACT FROM AUTHOR]

The damage of the cement sheath is inevitable during the perforation process, and the physical parameters of the cement sheath have a great influence on the damage of the cement sheath. A three-dimensional numerical model, which was composed of perforating charge, casing-cement sheath, and formation and consistent with the engineering practice, was established, considering the dynamic characteristics of materials in the process of explosion impact. The numerical simulation results are verified by the physical model test, which shows that the compression and shear of the cement sheath caused by the penetration of the metal jet in the perforation operation is the main reason for the failure of the cement sheath. The compressive strength, tensile strength, shear modulus and density of the cement sheath directly affect the damage degree of the cement sheath. Increasing the compressive strength and tensile strength of the cement sheath, or reducing the shear modulus and density of the cement sheath, can reduce the damage of the cement sheath. The study revealed the influence of the physical properties of the cement sheath on the impact damage of the cement sheath during the perforation process, which has a certain reference value for the optimal design of the cement sheath. [ABSTRACT FROM AUTHOR]

In geologically sequestrating CO2, the reaction between the CO2 and the water in the confining rocks produces a chemical that causes the cement sheath to corrode, the coupling of the corrosion damage and the pressure (stress) inside the casing string greatly affects the seal integrity of the cement sheath. In CO2 corrosion test, the mechanical parameters of the set cement with different corrosion severities are obtained. The stress-strain behavior of the cement sheath before and after corrosion is described using the concrete damaged plasticity (CDP) constitutive model and the Mohor-Coulomb criterion. Using the ABAQUS software, a finite element analysis model for the wellbore assembly (casing-cement sheath-confining formation rocks) is established taking into account the coupling of CO2 corrosion and stress. The effects of internal pressure of the casing and corrosion time on the integrity of cement sheath are investigated. It is found that at high internal pressure of casing, the cement sheath undergoes elastic-plastic deformation, structural damage is found in the cement sheath, and micro-gaps are easy to be generated in the interfaces between the casing and the cement sheath. The coupled action of corrosion and internal pressure of casing causes the integrity of the cement sheath to fail easily. Compared with the cement sheath that undergoes no corrosion, the cement sheath with corrosion damage after being pressurized has higher radial stress, higher equivalent plastic strain, wider micro-gap and more serious tensile and compressive damages. On the other hand, the plastic radius decreases, and the corrosion time does not significantly affect the micro-gap and the tensile and compressive damage. [ABSTRACT FROM AUTHOR]

The sustained casing pressure (SCP) caused by the sealing failure of cement sheaths has become one of the main threats to the safe and efficient production of high-temperature and high-pressure (HTHP) gas wells. An actual HTHP gas well was used as an example, and the temperature calculation models were established. Then, the temperature–pressure coupling finite element coupling models of casing-cement sheath-formation assembly for the whole well were established, and the factors affecting the integrity were examined. Finally, the maximum production and wellhead annular pressure based on the constraints of cement sheath integrity was analyzed. The results indicate that the natural gas would transfer energy to the wellbore assembly during production; the greater the production, the greater the temperature fluctuation of the cement sheath, which was unbeneficial to the integrity, the radial and tangential stresses of the cement sheath increase and decrease, respectively, with depth. The cement sheath near the ground was at risk of tensile failure, which could result in tangential tensile cracks. The cement sheath near the packer was susceptible to compression failure, resulting in radial compression cracks, reducing production and wellhead annular pressure were beneficial to relieve the stress state. Compared to the production, the wellhead pressure has a greater impact, it is necessary to control the wellhead pressure in a reasonable range to ensure integrity.

On the basis of “Carbon Peak and Carbon Neutral” goals, carbon sequestration projects are increasing in China. The integrity of cement sheaths, as an important factor affecting carbon sequestration projects, has also received more attention and research. When CO2 is injected into the subsurface from sequestration wells, the cement sheath may mechanically fail due to the pressure accumulated inside the casing, which leads to the sealing of the cement sheath failing. The elasticity and strength parameters of the cement sheath are considered in this paper. The critical bottom-hole injection pressures of inclined well sections under anisotropic formation stresses at different depths were calculated for actual carbon-sealing wells in the X block—the CO2 sequestration target block. The sensitivity factors of the critical bottom-hole injection pressure were also analyzed. It was found that the cement sheath damage criterion was tensile damage. The Young’s modulus and tensile strength of the cement sheath are the main factors affecting the mechanical failure of the cement sheath, with Poisson’s ratio having the second highest influence. An increase in the Young’s modulus, Poisson’s ratio, and tensile strength of the cement sheath can help to improve the mechanical stability of cement sheaths in CO2 sequestration wells. This model can be used for the design and evaluation of cement in carbon sequestration wells.

Abstract This article mainly studies the influence of high temperature, high pressure, and ultra‐deep well reservoir renovation construction on wellbore barriers, and explores the impact of reservoir renovation on cement sheath under high temperature and high‐pressure conditions. Combining the coupling effect of casing, cement, and formation rock, a three‐dimensional numerical model is established to analyze the stress changes and distribution of the cement sheath during the volume fracturing process. By establishing a numerical model for the analysis of the integrity of the wellbore during the fracturing process of the ultra‐deep well vertical section, the stress changes and distribution of the cement sheath during the fracturing process are analyzed. A numerical calculation model for crack propagation during perforation and cement sheath reservoir renovation is constructed, and the model results are verified through laboratory experiments and computed tomography scans. The shear failure of the cement sheath interface under different conditions such as different casing pressure, cement sheath thickness, elastic modulus, and Poisson's ratio is analyzed during the fracturing process. The research results can provide technical support for the integrity of the wellbore and the design of the well structure in similar ultra‐deep well development processes.

A significant concern in oil and gas well construction is well integrity. The inability to obtain proper zonal isolation during cementation can lead to severe consequences threatening the integrity of the wellbore. Micro-annulus in the bulk cement or behind the casing causes leakage when the cement bonds fail. Thus, the bonding between the cement and the casing is crucial in preserving the wellbore integrity. The bond mechanically supporting the casing pipe in the cement sheath is the cement-casing shear-bond. Many factors have a crucial part in strengthening the shear-bond are discussed in detail. This review systematically summarizes the efforts made on this topic in the recent past. The primary objective of this review is to discuss the factors affecting the strength of the cement-casing shear-bond by describing the different perspectives. It also explains the laboratory and the various bond evaluation methods applied in the field. In addition, it also discusses the challenges and the research gap along with the future scope in this domain. [ABSTRACT FROM AUTHOR]

The integrity of cement–salt rock interfaces (CSI) in salt cavern gas storage wells may be damaged during circulating gas injection, production, and long-term gas pressure maintenance. In this study, permeability tests were conducted on cement–salt rock samples under cyclic loading confining pressure at different holding times. The cracks and micro-annuli at the interface were visualised using X-ray computed tomography. The bonding strength of the interface after the permeability test was measured in a triaxial environment. The results showed that with an increase in the number of cycles, the cumulative volumetric strain rate and permeability first decreased rapidly and then tended to stabilise, and the decreasing amplitude was positively correlated with the holding time. When the holding times were 10, 30, and 60 min, the permeability change ratios were 0.844, 0.762, and 0.638, respectively, after one cycle and 0.722, 0.444, and 0.420, respectively, after five cycles. The permeability of CSI is mainly influenced by the holding time. The mechanical properties of the CSI specimens with similar permeabilities showed differences at different holding times. Cyclic loading easily degrades the mechanical properties, which can be improved by extending the holding time. The CSI is the main channel for gas leakage, extending the holding time, and increasing the number of cycles can effectively improve the pore structure of CSI and enhance the cement sheath integrity. This study provides a reference for long-term safe operation of salt cavern gas storage wells. Highlights: Permeability tests and triaxial shear bond strength tests after permeability tests were performed on a cement–salt rock interface. The effects of cyclic loading and holding time on the permeability and mechanical properties of the cement–salt rock interface were investigated. Microcracks' distribution within the cement–salt rock interface under cyclic loads was characterised using computed tomography. [ABSTRACT FROM AUTHOR]

Within deep and ultra-deep wells, the casing-cement sheath-stratum rock assembly(CCS) bears thermal, mechanical, and chemical multi-field coupling, causing some damage or creating micro-annular gaps at the interface between the casing and the cement sheath as well as the interface between the cement sheath and the stratum. Radial and axial cracks in the cement sheath occur, and even channeling-leakage channels form due to shear failure. These are critical reasons for sustained annular pressure and the risk of wellbore sealing integrity. This paper establishes an evaluation device for channeling-leakage properties of a compact-size CCS assembly. It observes the microstructure and morphology characteristics of cement sheath integrity, tests the channeling-leakage pressure and paths of two cement slurries, and explores the relationships between the channeling-leakage properties of the assembly and the damage to the cementing interfaces as well as the cracking characteristics of cement sheath. Results showed that the proposed method was suitable for evaluating unpredictable security risks of CCS assembly caused by a complex load in deep and ultra-deep wells, such as overload-induced cracking, cementing interface unsealing, and channeling-leakage of the cement sheath. The findings revealed the evolution law from deformation points to cracks and channeling-leakage channels inside the cement sheath under the action of load and proposed technical measures to avoid channeling-leakage of the cement sheath. This research provides a theoretical basis and testing method for evaluating the wellbore sealing integrity and formulating technical safety measures for deep and ultra-deep wells. [ABSTRACT FROM AUTHOR]

The stress concentration could easily aggravate the damage of perforated cement sheath due to the perforation hole in horizontal wells during staged fracturing. In addition, the wall of perforation hole was directly pressed by the high-pressure fracturing fluid, making cement integrity more challenged. A fully coupled thermo-hydro-mechanical model was set up to simulate the temperature and the stress variation of the cement sheath during fracturing, and the influence of casing pressure, fracturing fluid displacement, cement elastic modulus, hole diameter and density on the cement integrity was analyzed. The results show that there is shear failure on casing / cement interface, cement / rock interface and the hole wall during fracturing process. Considering the coupling of transient temperature and pressure, the risk of failure at the cement hole is increased. The fracturing fluid displacement, hole diameter and density have little effect on the tangential stress of the cement sheath, although a lower casing pressure and cement elastic modulus can reduce the shear failure coefficient to some extent, it is difficult to avoid the shear failure at the initial stage of hydraulic fracturing. [ABSTRACT FROM AUTHOR]

The integrity of oil well cement sheaths is closely related to the long-term production safety of oil and gas wells. The primary material used to form a cement sheath is brittle. In order to reduce the brittleness of oil well cement and improve its flexibility and resistance to stress damage, nano-silica was used to modify polymer elastic particles, and their properties were analyzed. The influence of the modified polymer particles on the properties of oil well cement-based composite materials was studied, and the microstructure of the polymer particle cement sample was analyzed. The results showed that nano-silica effectively encapsulates polymer particles, improves their hydrophilicity, and achieves a maximum temperature resistance of 415 °C. The effect of the modified polymer particles on the compressive strength of cement sample is reduced. Polymer particles with different dosages can effectively reduce the elastic modulus of cement paste, improve the deformation and elasticity of cement paste, and enhance the toughness of cement paste. Microstructural analysis showed that the polymer particles are embedded in the hydration products, which is the main reason for the improvement in the elasticity of cement paste. At the same time, polymer particle cement slurry can ensure the integrity of the cement sample after it is impacted, which helps to improve the ability of oil well cement-based composite materials to resist stress damage underground. [ABSTRACT FROM AUTHOR]

This article mainly studies the influence of high temperature, high pressure, and ultra‐deep well reservoir renovation construction on wellbore barriers, and explores the impact of reservoir renovation on cement sheath under high temperature and high‐pressure conditions. Combining the coupling effect of casing, cement, and formation rock, a three‐dimensional numerical model is established to analyze the stress changes and distribution of the cement sheath during the volume fracturing process. By establishing a numerical model for the analysis of the integrity of the wellbore during the fracturing process of the ultra‐deep well vertical section, the stress changes and distribution of the cement sheath during the fracturing process are analyzed. A numerical calculation model for crack propagation during perforation and cement sheath reservoir renovation is constructed, and the model results are verified through laboratory experiments and computed tomography scans. The shear failure of the cement sheath interface under different conditions such as different casing pressure, cement sheath thickness, elastic modulus, and Poisson's ratio is analyzed during the fracturing process. The research results can provide technical support for the integrity of the wellbore and the design of the well structure in similar ultra‐deep well development processes. [ABSTRACT FROM AUTHOR]

The stability of cement sheath under high temperature and high pressure is one of the most critical issues for the durability of geothermal well systems. In this study, a two-dimensional plane-strain finite element code was developed to investigate the coupled thermo-mechanical behaviors of the casing-cement-formation system. Different from previous linear elastic analyses, a thermoelasto-plastic constitutive model based on the thermodynamic theory was adopted for the cement sheath. It is shown that the finite element simulations using the proposed model provide a more accurate and realistic prediction of stress–strain responses of the cement sheath under high temperature. The results demonstrate that the radial stress concentration and the tensile strain concentration occur at both the cement–casing interface and the cement–formation interface, where the cement sheath is most likely to fail. High strength and low stiffness in the cement sheath and the formation are preferred for the integrity of the system. Both large thermal cycles and large differences between the internal fluid pressure and the external pressure should be avoided during operation. The new code is an alternative tool for guiding the geothermal well design. The finite element framework described herein is universal for other thermo-mechanical applications, such as energy foundations and energy tunnels.

Casing shear deformation caused by fault slip is a serious problem that restricts shale gas development. As yet, there is no effective method for controlling casing shear deformation. In this study, a cement sheath with hollow glass microspheres was used to prevent and mitigate casing deformation caused by a fault slip. An equivalent outer-diameter model was established. This model analyzes the mechanism by which casing shear deformation is controlled by a cement sheath with hollow glass microspheres under the condition of fault slip. The hollow glass microspheres in the cement sheath were broken owing to the strong shear forces of the fault slip. The internal volume of the hollow glass microspheres was released, which reduced the distance of the fault slip across the casing. The casing deformation was prevented or slowed down. A 3D assembly numerical model was established to evaluate the control effect of the hollow glass microspheres on casing shear deformation. Furthermore, the influences of slip distance, casing thickness, mechanical parameters of formation, mechanical parameters of the cement sheath, and crustal stress difference were analyzed. The mathematical calculation results showed that casing deformation increased with an increase in the fault slip distance, an increase in the stratum elastic modulus, and a decrease in the in situ stress difference. Therefore, the mass content of the hollow glass microspheres in the cement sheath also needs to be increased to ensure that the casing does not deform. The numerical analysis results showed that the change in casing thickness, Poisson’s ratio of formation, and mechanical parameters of the cement sheath had little effect on the mass content of hollow glass microspheres. Finally, the model was used to analyze the test wells in the Weirong Block, Sichuan, China. The calculation results showed that under the same condition of casing deformation of 8.23 mm in adjacent wells, casing shear deformation could be avoided by using a cement sheath with 11.9% hollow glass microspheres in the test well. This study provides a theoretical basis and reference for controlling the casing shear deformation caused by fault slip in shale gas wells.

Generally, the so-called expansion agent is very effective in eliminating all the micro-annuli that exist between the casing and the cement sheath or between the cement sheath and the formation. However, this approach can detrimentally affect the sealing ability of cement sheath if the expansion agent is used in an unreasonable way. For these reasons, in the present work, numerical simulations have been conducted to analyze the effect of elasticity modulus of cement sheath, the elasticity modulus of formation, the expansion rate of cement, the geo-stress on the micro-annulus caused by cement expansion, and the cement sheath expansion on the integrity of cement sheath and formation. The micro-annulus between the casing and the cement sheath has been found to decrease according to the ratio between the elasticity modulus of formation and the elasticity modulus of cement sheath. A positive correlation has been observed between the micro-annulus and the cement expansion ratio. The microannulus decreases as the geo-stress increases, but the effect of the geo-stress on the micro-annulus is much smaller. In conclusion, the expansion agent is suitable for the formation in which the elasticity modulus is higher than the cement sheath. [ABSTRACT FROM AUTHOR]

In order to study the variation of heat transfer capacity of cement sheath at different temperatures, the thermal conductivity of pure cement and hollow glass bead cement was measured by plate heat flow meter. At the same time, the numerical relationship between test temperature and thermal conductivity was determined by model fitting. The results show that the temperature on both sides of cementing cement increases with the extension of thermal recovery time, and the variation range is considerable. The thermal conductivity of cement sheath has a linear relationship with porosity, and the greater the porosity, the smaller the influence of test temperature on thermal conductivity. The test results of pure cement with different water cement ratio and hollow glass bead cement show that the influence of test temperature on their thermal conductivity is the same. That is, the increase of the temperature of the hot and cold plate can improve the thermal conductivity, but the influence of the temperature change on the thermal conductivity is significantly stronger than that of the hot plate. What􀆳s more, when the average temperature is constant, the higher the cold plate temperature, the greater the thermal conductivity. And several simulation results show that the numerical relationship between thermal conductivity and the test temperature can be expressed by Parabola2D model. The research results can provide some reference for the calculation of heat loss in geothermal wells and the prediction of wellhead water temperature. [ABSTRACT FROM AUTHOR]

A large amount of casing deformation has occurred in shale gas wells during the complex fracturing process, which affects the fracturing construction and single well production. Based on the statistical analysis of casing deformation wells and caliper logging interpretation, the main mechanism of casing deformation of shale gas wells is revealed as formation slip. By comprehensively considering the rotating speed under casing running condition, the cement solidification heat release under cementing condition, and the fracturing fluid temperature drop under fracturing condition, the safe service margin of the casing is large. Moreover, simply increasing the casing steel grade and wall thickness has no obvious effect on casing deformation prevention, so full wellbore casing deformation prevention measures should be considered. By using the method of unconventional oil and gas well casing string simulation test and numerical simulation, the mechanical response of wellbore and the mechanism of bridge plug resistance are analyzed. By analyzing the influence of elastic modulus and wall thickness of cement on the casing minimum drift diameter after shear deformation, an analytical model of the minimum drift diameter of shale gas casing under the effect of fracture slip is established and verified, which provides technical support for parameter selection of cement and measures to prevent casing deformation. [ABSTRACT FROM AUTHOR]

One of the main obstacles hindering the exploitation of high-temperature and high-pressure oil and gas is the sealing integrity of the cement sheath. Analyzing the microstructure of the cement sheath is therefore an important task. In this study, the microstructure of the cement sheath is determined using a CT scanner under different temperature and pressure conditions. The results suggest that the major cause of micro-cracks in the cement is the increase in the casing pressure. When the micro-cracks accumulate to a certain extent, the overall structure of the cement sheath is weakened, resulting in gas channeling, which poses a direct threat to the safe production of oil and gas. A change in the casing temperature has a limited effect on the microstructure of the cement sheath. [ABSTRACT FROM AUTHOR]

Micro-capsule has been considered the most promising self-healing material, while the complex and high temperature polymerization process, and environmental unfriendly raw materials limits its wide application. Herein, the green micro-capsule composed of epoxy resin and calcium alginate were prepared at room temperature via needle drop method，O/W/O emulsion method and spraying method. The effects of preparation methods on the water absorption, encapsulation rate of micro-capsules were revealed, and the differences in the self-healing efficiency of cement containing these green micro-capsules were also studied. Among all samples, the micro-capsules prepared by O/W/O emulsion method (MCE-2) owned the highest water absorption 54.71 % and the highest epoxy resin content 48.1 %. Compared with the blank samples, adding micro-capsules improved the liquidity of cement slurry and facilitated the hydration process, related to the release of extra water. While, the micro-capsules caused additional defects in the cement matrix and weakened the compressive strength of cement. In the self-healing measurement, the composites with MCE-2 showed the biggest mass transfer pressure and least permeability before and after repair. The higher water absorption of MCE-2 significantly increased greater mass transfer resistance to block the erosion of external fluids before repair, and the higher epoxy resin content could ensure stronger sealing effect after repair, achieving the best self-healing performance. [Display omitted] [ABSTRACT FROM AUTHOR]

Based on the elastoplastic model of cement sheath considering the influence of three-dimensional principal stress and the stress field model of interface crack, a mechanical performance design method of cement sheath is established to meet the wellbore sealing requirements during fracturing. This method takes the failure types of the cement sheath, such as tensile failure, plastic yield, interface crack propagation along interface and zigzag propagation into account. Meanwhile, the elasticity modulus and Poisson's ratio quantitative design charts of cement sheath are constructed based on this method, and the safety and risk areas of wellbores are defined, which quantify the yield strength and tensile strength indexes of cement sheath. The results show that decreasing elasticity modulus, increasing yield strength and Poisson's ratio of cement sheath can avoid plastic deformation of cement sheath; increasing the tensile strength of cement sheath can prevent its tensile failure; increasing elasticity modulus and Poisson's ratio of cement sheath is good for shortening the length of the interface crack, but will increase the risk of interface cracks zigzagging into cement sheath. The model calculation and case verification has proved that the method in this paper can give accurate calculation results and is convenient for field application.

The integrity of oil well cement sheaths is closely related to the long-term production safety of oil and gas wells. The primary material used to form a cement sheath is brittle. In order to reduce the brittleness of oil well cement and improve its flexibility and resistance to stress damage, nano-silica was used to modify polymer elastic particles, and their properties were analyzed. The influence of the modified polymer particles on the properties of oil well cement-based composite materials was studied, and the microstructure of the polymer particle cement sample was analyzed. The results showed that nano-silica effectively encapsulates polymer particles, improves their hydrophilicity, and achieves a maximum temperature resistance of 415 °C. The effect of the modified polymer particles on the compressive strength of cement sample is reduced. Polymer particles with different dosages can effectively reduce the elastic modulus of cement paste, improve the deformation and elasticity of cement paste, and enhance the toughness of cement paste. Microstructural analysis showed that the polymer particles are embedded in the hydration products, which is the main reason for the improvement in the elasticity of cement paste. At the same time, polymer particle cement slurry can ensure the integrity of the cement sample after it is impacted, which helps to improve the ability of oil well cement-based composite materials to resist stress damage underground.

The complex temperature and pressure conditions of deepwater wells have a serious impact on cementing quality. Therefore, the integrity of the cement seal becomes a critical factor that can restrict the safety of the wellbore, especially for wells ranging from the conventional deep water to deep water with more prominent deep stratum problems. The operating conditions of deep wells in deep water (referred as the dual-deep well) are complex and harsh. For the conventional evaluation device, it is difficult to accurately simulate the alternating HTHP conditions and to clarify the location and situation of the cement sheath leakage, which directly leads to the deviation of the evaluation results from the engineering reality. To solve this defect, based on the condition and structure characteristics of dual-deep wells, a device and method for casing/cement sheath/formation seal integrity evaluation for dual-deep wells at HTHP has been developed and established. Combined with the analysis of microstructure and micromorphology, the sealing ability and integrity of different cement slurry systems were evaluated. Through this study, a set of cement slurry systems suitable for dual-deep wells is selected that can satisfy the requirements of dual-deep wells with excellent isolation ability and seal integrity under temperature and stress changes. The research results provide a reference for deepwater wells' wellbore integrity evaluation and research. [ABSTRACT FROM AUTHOR]

Pressurized fluids may invade into the cement sheath of hydrocarbon wells during hydraulic fracturing and other construction operations in shale gas and oil exploitation. Excessive fluid pressure provides a driving force for small cracks to grow and tunnel through the length of the well, leading to the loss of zonal isolation and severe consequences. This work studies fluid-driven cracks tunneling in the bulk or along the interfaces of the cement sheath. We calculate the energy release rate of the tunneling crack as a function of the width of the tunnel. As long as the maximum energy release rate is below the fracture energy of the cement or the interfaces, no tunnels will form. This failsafe criterion predicts that the interfaces are much more vulnerable to tunneling cracks than the bulk of cement sheath. It is further shown that cement sheath with high Young's modulus and high Poisson's ratio can better resist fluid-driven cracks. The influence of casing pressure is also examined. Different loading conditions may have conflicting requirements on the properties of cement sheath. Compromises need to be made in engineering practice. [ABSTRACT FROM AUTHOR]

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Abstract The mechanical damage and failure of cement sheaths in gas storage wells under cyclic loading have been studied extensively. However, because the test device cannot restore the wellbore condition, most studies have been theoretical or regular experimental. If the load‐bearing mode and stress environment in a test device differ from those in a wellbore, then the damage and failure modes will deviate from what occurs in the actual wellbore. Therefore, it is necessary to explore a method that restores the wellbore condition and design a wellbore simulation device that reveals the deformation and damage of the cement sheath. In this study, the development laws of microannulus and microcracks in the cement sheath were studied by triaxial cyclic loading, low‐field nuclear magnetic resonance imaging, and scanning electron microscopy. A device to evaluate cement sheath integrity was then developed. A stress equivalence method was proposed, based on the principle that the stresses at the first and second interfaces of the device are equal to those of the wellbore cement sheath. This method was used to design material, size, and experimental conditions to simulate the load‐deformation law of a cement sheath in a gas storage well. The damage failure mechanism was investigated using the simulation results and computed tomography. Micropores and microcracks in the cement sheath increased continuously under cyclic loading. Damage accumulated, causing strength failure as the period of the cyclic loading increased. Plastic deformation of the cement sheath occurred under cyclic loading, but interfacial peeling did not. The reason is that the cement sheath is under compressive stress during loading and unloading, and the interface between the cement sheath and the outer wall is not damaged. This study provides a new method for studying the mechanical failure of a cement sheath under complex wellbore conditions.

The attempt to replace concrete aggregates and oil well cement additives with waste tires accumulated year by year is considered sustainable and waste recycling. Notably, introducing fiber material can eliminate the deterioration effect of waste tire rubber on the mechanical performance of rubberized cement material. However, the mechanical strength and deformation properties of rubberized cement sheath barrier reinforced with inorganic hybrid fibers has not attracted much attention. This study focused on the mechanical behavior of rubberized cement sheath barrier reinforced with inorganic hybrid fibers. The uniaxial compressive strength of the cubic cement samples increased gradually with the increase of curing time and the cement stone samples maintained the structural integrity after compressive testing. Triaxial compressive strength test results showed that the cement with the waste tire rubber, SBR latex and inorganic hybrid fiber showed good ductility characteristics from the stress-strain curves, and the energy-based ductility indexes were enhanced by 37.9%-64.6%. In addition, it was found that the low elastic modulus of the samples coupled with a large single cycle loading strain resulted in a large dissipation energy and strain. The three aspects of space skeleton, crystal dislocation and slippage, and bridging effect of fibers can be used to elucidate the mechanism of increasing deformation capacity. The results of SEM-EDX and XRD analysis showed that the inorganic hybrid fibers of different sizes randomly distributed in the rubberized cement matrix, and the composition of the cement slurry was improved. This study is intended to provide reference for improving the mechanical integrity of rubberized cement sheath barrier under multiple cyclic loading/unloading conditions. [Display omitted] • Inorganic hybrid fibers can reinforce rubberized cement sheath strength. • The deformation behavior of rubberized cement sheath under cyclic loading was studied. • An energy-based ductility index was proposed to discuss the ductility of cement stone. • The developed cement slurry can be used for cementing 3500–5000 m horizontal wells. • Adding waste tire rubber into oil well cement is a way to recycle waste. [ABSTRACT FROM AUTHOR]

This paper proposes an evaluation system for the sealing ability of cement–casing interface and cement matrix, which was developed based on a permeability testing device and a model that predicts the breakthrough pressure of cement sheath matrix and interfacial transition zone (ITZ). It was found that the breakthrough pressure of ITZ was much smaller than that of cement matrix. Moreover, compared with water-based drilling fluid, oil-based one led to lower breakthrough pressure of ITZ even after the flushing treatment. Meanwhile, latex, resin and elastic materials enabled a substantial rise in the breakthrough pressure of cement matrix. However, compared with the latex, resin and elastic cement, the expansive cement had higher breakthrough pressure of ITZ, indicating an improvement on the interface sealing ability. Additionally, a small enlargement rate of the hole diameter and long effective bond were able to prevent gas storage wells from leakage. [ABSTRACT FROM AUTHOR]

This paper investigates the fatigue damage of the wellbore cement sheath under different cyclic loading conditions. According to the irreversible principle of thermodynamics, a damage evolution model considering both tension and compression fatigue damage is established. With acoustic emission monitoring, uniaxial compression and Brazilian disc splitting cyclic loading failure tests were performed. Based on the test results, the damage theory is verified using the cumulative strain, Young's modulus and acoustic emission characteristics. The research gives a numerical algorithm of the fatigue damage theory to develop as a constitutive equation. With reference to the structural and operating parameters of a gas storage salt cavern in Jintan, a casing-cement sheath-stratum numerical model is established. By inserting the damage constitutive equation into the numerical model calculation, the damage characteristics of the cement sheath are shown. Finally, the gas storage operating parameters that meet the requirements of high injection-production efficiency and long service life are given. [ABSTRACT FROM AUTHOR]

Cement sheath provides zonal isolation and structural support during oil/gas well exploitation. Mechanical behaviours of cement sheath are of increasing concern with the exploitation of shale gas. In order to overcome the drawbacks of continuum assumption of cement sheath, a comprehensive thermo-poroelastic model, accounting for the pore water pressure resulted from hydraulic pressure in casing, the thermal loading induced by the temperature difference between the injected water in casing and underground formation, and thermo-osmotic effect, is developed to determine the evolution of effective stress of cement sheath. Moreover, failure criteria for the cement sheath, whose parameters are temperature dependent, are proposed to assess failure modes of cement sheath. A case study is also carried out to assess the tendencies of the failure modes of cement sheath during loading. Application of the proposed model on a triaxial test of undrained heating of oil well cement paste shows the capability of the model with k p T = 5 × 10 − 12 m 2 / (° C.s) to well reproduce the experimental results. [ABSTRACT FROM AUTHOR]

In large-scale multi-section hydraulic fracturing, the stress environment of wellbore is extreme complex, often causing the unbalanced stress distribution around the wellbore. That poses great challenges to the integrity of the sheath. In this paper, firstly, triaxial compression test and triaxial cyclic test are carried out at 130 °C to study the deformation characteristics of the cement under high temperature. Then based on that, an appropriate plastic mechanics model is established. Finally, the shakedown theory is applied to analyze the model and acquires a maximum cyclic loading under asymmetric stress. The result shows that (1) the well cement, with the increase of load, shows the plastic flow characteristics and can be regarded as an ideal elastic–plastic material under high temperature. (2) During the cyclic loading and unloading process, the "hysteresis loop" becomes denser, which indicates that the accumulation rate of plastic deformation is continuously declining. The main plastic strain appears in the phase of the first loading. (3) The external pressure Pz plays a positive role in the deformation control of the sheath. With the growth of Pz, the maximum cyclic loading Pmax will also increase. (4) Asymmetric stress distribution can significantly affect the bearing capacity of the sheath. If stress difference coefficient λ = 0.3, the Pmax tends to decrease nearly by 50%. With the growth of λ, the negative influence of stress asymmetry reduces gradually. High external pressure is beneficial to reduce the negative impact of the asymmetry. With the growth of λ, the benefit tends to enhance. (5) In engineering practice, if the geology around wellhole showcases the strong asymmetry (the value of λ is large), some steps need to be adopted to reduce the stress concentration. [ABSTRACT FROM AUTHOR]

Abstract Continuous gas leakage paths of cement sheath during hydraulic fracturing were investigated in this study. The finite‐element models of a casing–cement‐sheath–formation assembly for the whole wellbore were established, the integrity of the cement sheath of the whole wellbore during single‐stage fracturing was analyzed firstly, and the factors affecting the integrity were examined. Furthermore, a cyclic compression test was conducted. Then, test results for the accumulative plastic strain of the cement sheath under the intermediate casing shoe during multistage fracturing were analyzed. Finally, the mechanism of dilatancy of cement stone in cyclic compression tests was analyzed. The results show that the radial and tangential stresses of the cement sheath increase and decrease, respectively, with depth. The cement sheath above the intermediate casing shoe had a risk of tensile failure, which could result in tangential tensile cracks. The cement sheath below the intermediate casing shoes had cumulative plastic strain, resulting in a microannulus. Reducing Young's modulus, increasing the Poisson ratio of the cement sheath, and reducing the pump pressure were beneficial to relieve the stress state. When the cyclic compressive stress exceeded the dilatation yield stress, the permeability increased with cycle numbers, and the increase in permeability provided another leakage path for gas migration.

The integrity of cement sheath seals in casing connection sections is an important part of preventing air channelization. It is of great significance to evaluate the failure mechanism of cement sheath seals in casing connection sections under complex temperatures, pressures, and casing eccentricities. Based on the self-developed high-temperature and high-pressure full-size casing connection period of the cement sheath seal integrity evaluation device and the casing loading/unloading casing eccentricity influence on the cement sheath seal failure under the experimental conditions, it is concluded that the casing pressure changes, annulus preset pressure, and casing eccentricity influence the law of cement sheath seal integrity. The results show that the following: ① Casing eccentricity has an important influence on the failure of the cement sheath seal when the casing is loaded, and to ensure the integrity of the cement sheath seal the casing should be greater than 75%; ② in the case that the annular pressure is not set, casing eccentricity has little influence on the integrity of the cement annular seal during casing unloading, and sealing failure occurs when pressure unloading is 15–20 MPa; ③ when there is a preset pressure in the annulus, casing eccentricity has a great influence on seal failure during unloading. The greater the preset pressure in the annulus is, the better the seal integrity of the cement annulus. The research results provide an important basis for ensuring the integrity of cement sheath seals in casing connection sections.

Modeling of stress state of a perforated cement sheath in a well with hydraulic fracture is performed. The incompressible fluid flow model is used to calculate the pore pressure of a fluid. The linear-elastic body model and finite volume method with multipoint stress approximation are used to calculate the stress state of the cement sheath and production casing. The numerical model was verified by comparing the calculation results with a calculation in the Fenics open-source computing platform. It is shown that the maximum value of von Mises stress falls on the perforation zone at the junction of the cement sheath and the production casing. The presence of a hydraulic fracture can reduce the stress of the cement sheath.

The annulus pressure is more common in deep shale gas horizontal wells, which is mainly caused by the micro-annulus at the interface between casing and cement sheath. To address this problem, the generation and propagation of micro-annulus under prestressed cementing were analyzed by mechanical experimental means and numerical simulation methods, and the number of fracturing-resistant sections of cement sheath under different pre-stress was determined. The results showed that the lower the pressure inside the casing, the more times that the cement sheath withstood the cyclic loads on the premise of keeping the seal integrity; the micro-annulus width of 30. 89µm under cyclic load was the critical value for gas channeling. The prestressed cementing significantly reduced the initial plastic deformation and increased the plastic deformation increment; taking into account the radial prestrain generated by the casing under prestress, the prestressed cementing technology significantly reduced the micro-annulus width and increased the number of fracturing-resistant sections for cement sheath seal integrity in the multi-stage fracturing process. The higher the prestress value, the more fracturing-resistant sections before the micro-annulus appeared; with the same number of fracturing sections, the higher the prestress the smaller the micro-annulus of cement sheath. The field application results indicated that the annulus pressure of casing in deep shale gas horizontal wells was effectively reduced by prestressed cementing technology and cement slurry with low elasticity modulus. The results of the study provide technical support for the cementing of shale gas horizontal wells. [ABSTRACT FROM AUTHOR]

Once there are micro-annulus at cementing interfaces in the process of reservoir stimulation, it is easy to form fluid channeling in the later development. Fluid channeling reduces the efficiency of reservoir development and even the service life of oil and gas wells. Based on the characteristics of 'multi-stage' in stimulated reservoir volume, a mathematical model was set up to calculate the interface debonding length driven by hydraulic pressure. The propagation process of micro-annulus in stimulated reservoir volume was also analyzed. The results show that the propagation speed of the micro-annulus gradually slows down with the progress of fracturing. Meanwhile, the debonding azimuth of the micro-annulus and the cement elastic modulus have dominant effects on the propagation length. It is recommended that the setting position of the packer be outranged from the debonding area of the cementing interface in the design of the stimulated reservoir volume scheme to avoid fluid channeling between the stages. This study can predict the unsealing length of cement sheath during hydraulic fracturing as a reference for the optimization of fracturing parameters. [ABSTRACT FROM AUTHOR]

The complex temperature and pressure conditions of deepwater wells have a serious impact on cementing quality. Therefore, the integrity of the cement seal becomes a critical factor that can restrict the safety of the wellbore, especially for wells ranging from the conventional deep water to deep water with more prominent deep stratum problems. The operating conditions of deep wells in deep water (referred as the dual-deep well) are complex and harsh. For the conventional evaluation device, it is difficult to accurately simulate the alternating HTHP conditions and to clarify the location and situation of the cement sheath leakage, which directly leads to the deviation of the evaluation results from the engineering reality. To solve this defect, based on the condition and structure characteristics of dual-deep wells, a device and method for casing/cement sheath/formation seal integrity evaluation for dual-deep wells at HTHP has been developed and established. Combined with the analysis of microstructure and micromorphology, the sealing ability and integrity of different cement slurry systems were evaluated. Through this study, a set of cement slurry systems suitable for dual-deep wells is selected that can satisfy the requirements of dual-deep wells with excellent isolation ability and seal integrity under temperature and stress changes. The research results provide a reference for deepwater wells’ wellbore integrity evaluation and research.

The high temperature and high pressure in ul-tra-deep wells pose great challenges to the cement sealing integrity in southern margin of Junggar ba-sin. Therefore, a numerical model of casing-cement-formation was set up by characterizing elastoplastic constitutive relationship of cement with concrete plastic damage model in this paper. Then the Mohr-Coulomb criterion was used to evaluate the failure of cement, and the influence of the cement elastic modulus, Poisson's ratio, and cement sheath thickness on the failure coefficient was analyzed. The numerical results show that the cement integrity of Well Gao-tan-1 scores good during development. The cement slurry after solidification should have a lower elastic modulus and a higher Poisson's ratio, which is con-ducive to the cement integrity. When designing the well structure, the thickness of the cement sheath should always be kept above 11mm to ensure the wellbore sealing integrity. As the decreasing ampli-tude of failure coefficient gradually decreases with an increasing thickness, the upper limit of thickness was recommended as 38mm with the consideration of drilling costs. This research can be used to analyze the failure mechanism of the cement sheath in HTHP wells and provide technical support for the wellbore integrity of deep/ultra-deep wells in the southern margin of the Junggar basin. [ABSTRACT FROM AUTHOR]

The mechanical damage and failure of cement sheaths in gas storage wells under cyclic loading have been studied extensively. However, because the test device cannot restore the wellbore condition, most studies have been theoretical or regular experimental. If the load‐bearing mode and stress environment in a test device differ from those in a wellbore, then the damage and failure modes will deviate from what occurs in the actual wellbore. Therefore, it is necessary to explore a method that restores the wellbore condition and design a wellbore simulation device that reveals the deformation and damage of the cement sheath. In this study, the development laws of microannulus and microcracks in the cement sheath were studied by triaxial cyclic loading, low‐field nuclear magnetic resonance imaging, and scanning electron microscopy. A device to evaluate cement sheath integrity was then developed. A stress equivalence method was proposed, based on the principle that the stresses at the first and second interfaces of the device are equal to those of the wellbore cement sheath. This method was used to design material, size, and experimental conditions to simulate the load‐deformation law of a cement sheath in a gas storage well. The damage failure mechanism was investigated using the simulation results and computed tomography. Micropores and microcracks in the cement sheath increased continuously under cyclic loading. Damage accumulated, causing strength failure as the period of the cyclic loading increased. Plastic deformation of the cement sheath occurred under cyclic loading, but interfacial peeling did not. The reason is that the cement sheath is under compressive stress during loading and unloading, and the interface between the cement sheath and the outer wall is not damaged. This study provides a new method for studying the mechanical failure of a cement sheath under complex wellbore conditions. [ABSTRACT FROM AUTHOR]

The mineralized collagen fibril is the basic building block of bone, and hence is the key to understanding bone structure and function. Here we report imaging of mineralized pig bone samples in 3D using the focused ion beam-scanning electron microscope (FIB-SEM) under conditions that reveal the 67 nm D-banding of mineralized collagen fibrils. We show that in adult pig osteons, the lamellar bone comprises alternating layers with either collagen fibrils predominantly aligned in one direction, and layers in which fibrils are predominantly aligned in two directions. The cement sheath contains thin layers of both these motifs, but its dominant structural component comprises a very complex layer of fibrils predominantly aligned in three or more directions. The degree of mineralization of the cement sheath is comparable to that of the osteon interior. The extent of alignment (dispersion) of the collagen fibrils in the osteonal lamellar bone is significantly higher than in the cement sheath. Canaliculi within the cement sheath are mainly aligned parallel to the cement sheath boundary, whereas in the lamellar bone they are mainly aligned perpendicular to the lamellar boundaries. This study further characterizes the presence of two types of collagen fibril arrangements previously identified in demineralized lamellar bone from other species. The simple sample preparation procedure for mineralized bone and the lower risk of introducing artifacts opens the possibility of using FIB-SEM to study more samples, to obtain automatic quantitative information on collagen fibril organization and to evaluate the degrees of mineralization all in relatively large volumes of bone. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The current research on the stability of the formation - cement sheath - casing combination is mainly based on the plane modelling, with established 3D model as just an example. There are no systematic studies of the three-dimensional model under triaxial ground stress when the influence of physical parameters on stability of the composite cementing interfaces is evaluated. By establishing a three-dimensional formation - cement sheath - casing combination, it was shown that the two cementing interfaces are separated under the action of triaxial ground stress. The effects of physical properties (elastic modulus, Poisson's ratio, and density) of the cement sheath on the contact pressure and spacing of the cemented interfaces are studied. According to the established theoretical model, the influence of different physical parameters on well integrity after cementing in Qinghai Oilfield was analyzed, which provided theoretical support and guidance for the occurrence of out-of-tube turbulent flow and instability mechanism, and the direction for subsequent construction operations such as profile control and water plugging are pointed out. The results show that the physical properties of the cement sheath are critical in ensuring long-term sealing of the combination. [ABSTRACT FROM AUTHOR]

• Defects in the evaluation method of cement integrity were corrected and improved. • Integrity evaluation method of casing-cement-formation combination was proposed • Integrity of cement-formation interface can be controlled by adjusting the properties of cement. • This evaluation method is suitable to predict and control the failure of sealing integrity. The sealing integrity of cement sheath directly affects the service life of oil and gas wells, and plays an important role in oil well operation and production. In this paper, the evaluation of sealing integrity of cement sheath was divided into two parts: the integrity of formation and cement sheath, and the integrity of cement sheath and casing. Based on the convergence-constraint theory, the effect of convergence stress generated by formation on cement slurry during the hardening process was considered. And the evaluation method for casing-cement-formation sealing integrity was established. In addition, a quantitative relationship between compressive strength of cement sheath and formation characteristic parameters was proposed to determine the cement-formation sealing integrity. Besides, the integrity evaluation method of casing-cement-formation combination was verified by tensile strength, compressive strength and plastic zone of cement. The field application analysis showed that this method is valid to predict the cement failure. During the cementing operation, the integrity of casing-cement-formation interface can be controlled by adjusting the properties of the cement sheath. [ABSTRACT FROM AUTHOR]