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Casing deformation occurs frequently in shale gas wells, resulting in the failure of some fracturing stages to complete normal operations, which seriously affects shale gas production. This paper firstly analyzes the casing deformation mechanism of shale gas wells and determines that the casing shear deformation caused by formation slip is the main factor of casing deformation. In order to solve the problem of casing shear deformation, a finite element numerical model including the formation, cement ring, and the casing is established. The casing shear deformation caused by formation slip is simulated, and the deformation results obtained are consistent with those measured in the actual wellbore. Based on the numerical model, a casing deformation governance method is proposed to optimize the wellbore trajectory and adjust the cement ring properties. It is calculated that reducing the dip angle á of the formation and casing vertical direction and reducing the formation slip is beneficial to reducing casing deformation. The use of low elastic modulus cement and uncementing operations can provide a buffer space for formation slip, effectively reduce casing deformation, and provide an effective treatment method for shear deformation of shale gas wells. [ABSTRACT FROM AUTHOR]

As a catenary riser, unbonded flexible pipe (UFP) has the advantages of convenient installation, recyclability, flexibility, anti-corrosion and large design space. To analyze the residual fatigue life of in-service unbonded flexible riser (UFR), a global model of riser is established in this paper. By analyzing the distribution of configuration, tension, bending moment and bending curvature along the riser under static and dynamic loads, the most dangerous position and dynamic response are determined. And then, the residual fatigue life of the UFR is calculated. The calculation results show that the fatigue life of the UFR is greatly affected by the pipe annulus condition, and the dry annulus condition is 4.6 times that of the wet annulus condition. It is concluded that when evaluating the remaining fatigue life of an in-service UFR, the S-N curve of the tensile amour layers must be calculated according to different annular conditions, so that the fatigue life can be accurately obtained. Summarily, this paper provides an effective method for calculating the residual fatigue life of in-service UFRs. [ABSTRACT FROM AUTHOR]

• Revealed the inherent mechanism of casing shear deformation. • Proposed measures to add a wellbore annulus buffer layer to reduce casing shear deformation. • Through experiments, it has been confirmed that adding a buffer layer can effectively reduce the shear deformation of the casing. • Optimized the structural parameters of the buffer layer through finite element method. In the process of exploiting shale gas, wells experience casing deformation at rates reaching 40%, leading to the failure of setting tools such as bridge plugs during completion operations. This failure results in cessation of fracturing operations, seriously affecting productivity. To reduce and reveal the mechanism of casing deformation, in this work, casing shear deformation experiments were designed and carried out simulating an 8.5-inch wellbore, which verified that the casing deformation was shear deformation caused by formation slip. Based on the mechanism of casing shear deformation, the measure of adding a wellbore annulus buffer layer to reduce casing shear deformation is proposed, and the finite-element method is used to verify that increasing the compressible space of the buffer layer and reducing the compression stiffness can further reduce the casing deformation. Finally, a casing shear deformation experiment with an added buffer layer was developed. The casing generated a few shear deformations under 40-mm formation slip, which verified the effectiveness of the measures proposed in this paper to reduce the shear deformation of the casing. [ABSTRACT FROM AUTHOR]

During the process of deep-sea oil and gas exploitation, unbonded flexible risers (UFRs) are corroded by CO 2 and other media over long periods of time, which causes serious damage to the tensile armor layers and threatens the safety of the UFRs in service. To study the effects of torsional response, ultimate loading, and failure mode on UFRs after corrosion, and to reveal the torsional failure mechanism of the UFR while considering corrosion defects, an 8-layer UFR torsional response model is established in this paper. Local bending of the UFR and lateral buckling failure of the inner tensile armor layer occur under counterclockwise ultimate torsion, whereas strength failure of the inner tensile armor layer occurs under clockwise torsion. When the degree of corrosion is 0.4 mm, the counterclockwise torsional stiffness of thickness corrosion, width corrosion, and combined corrosion decreases by 27.6%, 10.8%, and 28.4%, and the ultimate loading of counterclockwise torsion for thickness corrosion, width corrosion, and combined corrosion decreases by 37%, 13.4%, and 39.3%, respectively. Therefore, the influence of thickness corrosion and combined corrosion on the torsional response of UFRs is worthy of further attention. • Established a theoretical and numerical model for unbonded flexible risers, and compared and verified the correctness of the model. • Studied the influence of corrosion of the tensile armor layer on the torsional response of unbonded flexible risers. • Revealed the ultimate torsional failure mechanism of unbonded flexible risers. • Restored the lateral buckling failure mode of unbonded flexible risers. [ABSTRACT FROM AUTHOR]

Flexible pipes connect the two critical systems of the floating production platform and underwater production, which play an important role in the transportation of oil and gas as well as water injection. However, flexible pipes are often subjected to multiple loads that threaten their stability in the service process. In this study, a model of a flexible pipe containing practical cross section in all layers is established to study the effects of axial compression and wet collapse loads on clockwise and anticlockwise torsional responses. The results of calculation show that the axial compression and radial expansion of the flexible pipe are caused by the axial compression load, which reduces clockwise torsional stiffness. The wet collapse load induces the bending deformation of the flexible pipes in clockwise and anticlockwise torsion, where the bending deformation of clockwise torsion is significantly affected by the change in the Young's modulus of the tensile armor layers. This study shows that clockwise torsional stiffness is small and exhibits a trend of axial compression. Clockwise torsion has poor stability in combination with axial compression and wet collapse loads, respectively. Therefore, to prevent instability in flexible pipes, clockwise torsion load, or which coupled with axial compression and wet collapse load, should be avoided. [ABSTRACT FROM AUTHOR]

Unbonded flexible risers (UFRs) connect surface production platforms to subsea production systems, and are therefore vital for tasks such as the transportation of oil and gas and the injection of water. The tensile armor layer is the main load layer of UFRs. Once this layer is partially broken, the safety of offshore oil and gas gathering and transportation is seriously threatened. In this study, an 8-layer UFR model is developed to calculate the effects of damaged inner and outer armor wires and boundary conditions on the tensile stiffness, ultimate load capacity, and stress distribution and load ratio of the inner and outer tensile armor layers. Broken inner tensile armor wires are found to have a greater impact on the tensile properties and produce the same torsional buckling failure mode as observed in the field. The influence of the pressure load on the ultimate load capacity of the UFRs is analyzed, and the results demonstrate that the outermost pressure increases the interlayer action between the tensile armor layers and the adjacent layers, thereby reducing the torsional angle of the UFRs under the ultimate tension. Additionally, the innermost pressure is found to have a negligible effect on the ultimate load. • A finite element modeling method for ultimate tensile finite elements of unbonded flexible risers that takes into account efficiency and accuracy is proposed. • The torsional buckling failure mechanism under the ultimate tensile load of the unbonded flexible riser was revealed. • The influence of the fracture number of tensile armor wires, internal and external pressure on the ultimate tensile of unbonded flexible risers were analyzed. [ABSTRACT FROM AUTHOR]

Ovalization, as an important factor affecting the performance of flexible pipes, is unavoidable in the manufacturing process. In this paper, a model of a flexible pipe with initial ovalization is established numerically. The numerical model is verified by a case analysis, and the influence of ovalization on the torsional response of a flexible pipe is discussed. It is found that the ovalization always has a negative effect on the torsional stiffness of the flexible pipe. Then, the effective numerical model is used to calculate the torsional response of a flexible pipe with initial ovalization under dry and wet annulus pressure. It is found that the dry annulus pressure improves the torsional stiffness of the flexible pipe by advancing the interaction between the layers and hindering the radial expansion of the outer tensile armor layer. However, the dry annulus pressure also increases the stress of the tensile armor layers, which reduces the withstanding capacity of the flexible pipe. Especially with the increase of ovalization, the loading capacity decreases more obviously. The wet annulus pressure has little effect on the torsional response of the flexible pipe, but it extends the risk of radial collapse of the carcass layer. [ABSTRACT FROM AUTHOR]

• An effective equivalent model was proposed for flexible pipe. • The correctness of the FEM was verified by experimental. • The axial elongation of the flexible pipe in tension shall be less than 1.3%. • The influence of hydrostatic pressure on the tensile properties of flexible pipes with different initial ovality is discussed. Flexible pipes play an important role in the transportation of oil and gas from the seabed to floating production units. They are also employed to inject water or gas into offshore wells. In deep-sea oil and gas extraction, flexible pipes must be able to withstand very high tensile and hydrostatic pressures. In this paper, an equivalent method is used to represent the complex carcass layer and the pressure armor layer as a regular cylinder, and then a numerical model of a 2.5" flexible pipe was developed and verified by experiment. A tensile analysis of flexible pipes with different degrees of initial ovalization indicated that ovalization of 0–1% does not affect the tensile rigidity, but increasing the ovalization creates an uneven load and stress concentration on the tensile armor layer. When a flexible pipe is subjected to hydrostatic pressure, the outer pressure increases the tensile rigidity of the flexible pipe more than the annular pressure, but the reduction of the ultimate tensile load is clear. A high annular pressure can cause the collapse of the carcass layer. [ABSTRACT FROM AUTHOR]