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.

During the development of shale gas, one of the major challenges is the casing deformation (CD) in the horizontal section due to the geological activity. Recently, the casing deformation before hydraulic fracturing (CDBF) occurred in multiple shale gas wells in L block in China. In this paper, based on the theory of tubular mechanics, the relationship between casing buckling and CDBF caused by casing running in is analyzed qualitatively and quantitatively. It is found that the buckling deformation caused by running the casing string process is not sufficient to prevent the tool (Φ99 mm bridge plug). On the other hand, the mechanism of CDBF is systematically analyzed based on the actual field data and finite element (FE) method. In order to analyze the CDBF mechanism, the comparison between the actual casing trajectory and the reservoir rock (S#1, S#2, S#3 and M#1) in horizontal section in H2 platform is carried out, and the preliminary CDBF mechanism is proposed. Then, two groups of FE models are established to reduce the CD process to verify the preliminary mechanism. It is found that the numerical simulation results (high stress section length from FE model) are in good agreement with the actual CD logging results. Based on the analytical results, the corresponding mitigation measures are proposed based on the analysis of the CDBF mechanism. Our work could offer a detailed theoretical basis and reference of CDBF for shale gas well application.

During the process of natural gas development, sustained casing pressure (SCP) frequently occurs within the annulus of the gas wells; we specifically referred to the “A” annular space located between the tubing and the production casing in this paper. SCP in an annulus poses a paramount safety challenge, universally acknowledged as a significant threat to gas field development and production, jeopardizing well integrity, personnel safety, and environmental protection. There are multiple factors that contribute to this issue. Due to the multitude of factors contributing to SCP in an annulus and the unclear mechanisms underlying the pressure buildup in wells, an early assessment of downhole leakage risks remains challenging. Hence, this study focused on a comprehensive analysis of the SCP in the annulus of gas wells. A detailed experimental study on the pressure buildup in an annulus due to tubing leakage below the liquid level was conducted, and the variation patterns of the annulus pressure under various leakage conditions were explored. The findings indicated that the equilibrium attainment time of annulus pressure at the wellhead subsequent to tubing leakage decreases with the increase in the pressure difference between the tubing and the casing, the liquid level height, the leakage orifice diameter, and the quantity, while it increases with the increase in the leakage position and gas temperature. According to the theory of gas fluid dynamics, a predictive model of the annulus pressure buildup with sustained gas leakage below the liquid level was proposed, which was well-validated against experimental results, achieving a model accuracy of over 95%. This study provided a theoretical framework for diagnosing SCP in the annulus of gas wells and developing mitigation strategies, thereby contributing to the advancement of the research field and ensuring the safety of industrial operations.

This paper investigates the fatigue behavior of S135 high-strength drill pipe steel under tension–torsion multiaxial loading. Based on the concept of critical plane during fatigue, the fatigue model under the combined loading of tension–torsion is established. The proposed model is validated, and the predicted results are in good agreement with the experimental testing results. The maximum relative errors between the estimation and the experiment are mostly within the range of factor two to three for proportional, and 90° non-proportional tension–torsion loading. Meanwhile, the failure mechanism is also discussed through fracture analysis.

The strength and sealing failure of the connecting thread of the casing head mandrel hanger causes huge economic losses. One of the major challenges is the thread seal failure mechanism of the casing hanger in the wellhead during pressure testing in shale gas wells. In order to analyze the failure causes of connecting threads and put forward improvement measures, a typical case of a well accompanied by a hanger seal failure is analyzed in this paper, and a series of material tests are carried out. The microstructure and mechanical properties of casing materials and hanger materials could meet the field requirements. It is concluded that both the hanger material and casing material are characterized with significant ductile fracture. A three-dimensional model of the hanger and casing system is established, and the mechanical behavior is calculated for the connecting thread under different working conditions. The results showed that the connection degree of the hanger–casing is insufficient at the torque recommended by the manufacturer because of the difference in wall thickness between the box thread of the hanger and the box thread of the joint according to the connection degree of the coupling casing. It is seen that the high contact pressure ring of zone three on the sealing surface plays an effective sealing role under the manufacturer’s recommended torque (20,465 N·m). Finally, when the torque is increased by 25%, the maximum contact pressure between the pin thread of the casing and the box thread of the hanger can fully meet the internal pressure from the wellbore pressure test and the internal pressure strength required for subsequent operations.