In the complex fracturing process of shale gas wells, casing is subjected to serious deformation, which can easily to the failure of wellbore integrity and the reduction of well construction productivity. It is particularly important to clarify the casing deformation mechanism and carry out effective control. Based on the logging data of casing deformation from well and full-scale indoor tests, the casing deformation mechanism is mainly considered to be shear and non-uniform extrusion deformation caused by formation slip and displacement control, i.e., the ultimate working conditions. The slip displacement boundary (<40 mm) under complex fracturing conditions is quantified to provide the design and optimization basis. Then, the influence laws of steel grade and wall thickness on the shear and non-uniform extrusion bearing characteristics are studied, using unconventional oil and gas well casing simulation test systems for 110 ksi (φ139.7 × 10.54 mm) and 125 ksi (φ139.7 × 12.7 mm) casings. Furthermore, combined with the full-scale simulation tests and finite-element simulation, the effects of elastic and modified cement slurry with hollow glass beads on casing deformation are compared and studied. The results show that the deformation capacity mitigation of casing is limited by reducing the cement elastic modulus and increasing the elastic cement thickness. By reasonably adding hollow glass beads of modified cement slurry, the maximum geological movement absorption of cement slurry is up to 27 mm. This new method can obviously decrease casing deformation and have an excellent control effect. Combined with the cementing technology of Luzhou blocks, the formula of modified cement slurry is optimized, and the optimization window of the casing deformation control process is formed, which can ensure the smooth progress of engineering fracturing.

With the escalating intricacy of downhole operational scenarios, encompassing frequent well cycling, acidification, multi-stage fracturing, steam injection, and intensive extraction, the efficacy of traditional casing-string-design methods rooted in strength considerations is progressively unveiling its limitations. Henceforth, it becomes imperative to establish string-design method standards that embrace the entirety of a well’s lifecycle, encompassing the phases of drilling, completion, fracturing, and production operations. Beginning with an analysis of the advantages and limitations of traditional casing-string-design methods, this paper introduces the features of strain and sealing design methods developed for the full lifecycle of the string. The strain design method, a departure from conventional design philosophies, enables the design concept of the controllable deformation of the pipe string. The sealing design method currently stands as the sole standard method for the design of tubing strings. Simultaneously, this paper proposes the establishment of a time dimension-based lifecycle pipe string-design method standard. This approach considers the trend of pipe strength degradation, effectively addressing the safety concerns related to pipe string design in production and operation.

In the increasingly complex environment of downhole working conditions such as frequent opening and closing of oil and gas wells, acidification/multi-stage fracturing, steam huff and puff, and strong injection and production, the traditional casing string design method standards based on strength design have gradually highlighted their limitations. It is necessary to formulate string design method standards that consider the full life cycle of wells such as drilling and completion, fracturing, and production operations. Starting from the analysis of the advantages and disadvantages of traditional casing string design methods, this paper presents the characteristics of the strain design methods and seal design methods that have been formulated for the life cycle of the string. The strain design method breaks through the traditional design concept and allows the design concept of controllable deformation of the pipe string. The sealing design method is currently the only standard method for the design of tubing strings. At the same time, it further proposes to consider the trend of pipe strength deterioration and establish a time dimension-based life cycle pipe string design method standard, which effectively solves the safety problem of pipe string design in production and operation.

During the formation testing in high-temperature (HT) and ultra-high-pressure (UHP) wells, one of the major challenges is packer failure in the downhole caused by high-rate fracturing. In such a case, the axial shrinkage trend of the tubing string could be caused by the sudden drop in temperature, but the actual axial length of the tubing string would not change because of the constraints at wellhead and packer. Therefore, this could lead to the upward pull-out of the packer that is due to excessive load from the tubing string. This out-of-control downhole pressure often leads to irreversible consequences, even well abandonment. An expansion joint, as a movable splicer, has the characteristic of mitigating packer load, which can theoretically enhance packer safety. To study the protective effect of an expansion joint on the packer quantitatively, the microscopic characteristics and macroscopic properties of the tubing material (13Cr110) are obtained through experimental tests. Moreover, the mechanical properties of the material at different temperatures are also tested. Then, the testing results are extended to modeling the finite element model (FEM) of the whole section of tubing string with the expansion joint—casing and simulating its internal force changes under fracturing conditions with different injection rates. Our simulation results indicated that an expansion joint can significantly change the distribution of the internal forces in the tubing string, and this change can effectively reduce the load on the packer. Eventually, a tubing string buckling identification plate that considers the injection rate and expansion joint–packer length is obtained to optimize the placement of an expansion joint in the tubing string. Our work can provide a detailed theoretical reference and basis for an expansion joint in field application.

CO₂ injection composite fracturing is an effective method for shale oil and gas well development. The downhole casing is prone to uniform corrosion, pitting, perforation, and even corrosion fracture in the CO₂ environment. Therefore, it is particularly important to reveal the physical characteristics of CO₂ under actual geological conditions and the impact of CO₂ corrosion on the performance of casing. A mathematical model for the temperature and pressure field of CO₂ in the wellbore under fracturing conditions is established in this paper, and the temperature and pressure distribution along the depth of the well is calculated. By optimizing the CO₂ state equation and using the S-W equation, Lee model, and RK model to calculate the CO₂ density, viscosity and compression factor, respectively, the phase distribution pattern of CO₂ along the actual wellbore is obtained. Through CO₂ corrosion tests on the casing, the influence of temperature and CO₂ concentration on the corrosion rate of the casing is clarified. The peak corrosion rate of Q125 steel corresponds to 80 °C, and the corrosion rate increases with the increase in CO₂ concentration. Finally, a prediction model for the uniform corrosion rate of casing under different temperatures and CO₂ concentration conditions is obtained, which can provide technical support for the design of CO₂-enhanced fracturing technology.

Unconventional oil and gas reservoirs, characterized by low porosity and permeability, often require multistage fracturing techniques for development. The high-pressure fracturing fluids with large volumes can easily cause alternating changes in both temperature and pressure within the casing. Using a theoretical model and field data from hydraulic fracturing operations, this paper calculated the alternating ranges of axial loads and temperatures in the reservoir section. Based on the calculation results, the temperature–load alternating coupling test of the P110 casing was carried out, and the tensile test was performed to analyze the yield strength variation law of the casing material. The results indicate that the yield strength, ultimate strength, and elastic modulus of casing materials are decreased under alternating thermal–mechanical coupling conditions. As the number of alternating cycles increases, there is an initial rapid decrease followed by a slower declining trend. Moreover, the tension–tension (T–T) cycles induce greater reductions in yield strength and ultimate strength than tension–compression (T–C) cycles. Meanwhile, under the same axial load condition, the higher the circulating temperature, the more significant the reduction in yield strength and ultimate strength. In essence, this is the result of the coupling effect of low-cycle fatigue and temperature aging. Finally, based on the experimental data, a yield strength prediction model of the P110 casing under the alternating thermal–mechanical coupling condition was established. The research results provide theoretical guidance for the safe design and material selection of a casing string under multistage volumetric fracturing conditions of shale gas exploration.