This paper presents a novel acoustic wave-based method for the detection of leakage in downhole tubing of offshore gas wells. The localization model is developed on the basis of analyzing the propagation model of leakage acoustic waves and the critical factors in localization. The proposed method is validated using experimental laboratory investigations that are conducted to locate tubing leakage by setting five holes at different positions on the tubing wall. A detection system is developed for the leakage acoustic waves in the tubing-casing annulus, where one acoustic sensor is installed at the annulus top. Laboratory experimental results show that the depth of downhole leakage can be effectively located by using the proposed localization model. The localization errors are kept at a very low level, and are mainly generated from extracting the characteristic time and calculating annular acoustic velocity. A case study focusing on an offshore gas well is presented to illustrate the feasibility of the proposed method, and to demonstrate that the proposed model can locate the liquid level and leakage points under field conditions. The test can be performed without interrupting the production of gas wells.

To monitor fatigue crack initiation and propagation, and to judge the fatigue damage status of ferromagnetic material, fatigue bending tests of four-point single edge notch bend (SENB4) specimens were carried out. Metal magnetic memory signals were measured during the whole fatigue process. The results showed that the fatigue process could be divided into four stages by observing the morphology of the fracture surface. With the increase of fatigue loading cycles, the tangential component of the magnetic field (H_x) and the normal component of the magnetic field (H_y) increased. At the notch H_x presented a “trough” shape and had a maximum value at the midpoint, while H_y at the notch rotated clockwise around the midpoint. Compared with the tangential characteristic parameters, the variation of normal characteristic parameters (i.e., maximum gradient value of H_y (K_y-max) and the variation range of H_y at the notch (∆H_yn), with the fatigue loading cycles are more similar to the variation of fatigue crack length with loading cycles), both K_y-max and ∆H_yn had a good linear relationship with fatigue crack length. Plastic deformation accumulated on both sides of the fatigue crack, and metal magnetic memory (MMM) signals measured from the specimens were able to indicate the location of the fatigue crack and the variation of the fatigue crack length. Furthermore, the distribution of magnetic signals was analyzed according to the theories of stress magnetization and magnetic flux leakage.

Sliding-induced subsurface microstructure evolution is believed to be decisive for determining the friction and wear performance of metallic contacts as well as the development of tribo-magnetization. This expects to develop a new prediction method of wear state by elucidating the correlation between subsurface microstructure evolution and corresponding magnetic domain changes. Herein, subsurface microstructure evolution including crystal and magnetic domain under tribological action is investigated experimentally. Our results demonstrate that dislocation mediated plastic deformation decisively influences microstructural changes during tribological contact, further determining the magnetic domain structure. Specifically, sliding-induced plastic deformation causes an increase in the width of magnetic domains, but depth-dependent derived microstructure formed under severe plastic deformation such as the refined grains and sub-grains, in turn, promoted the refinement of magnetic domains and their discontinuity, forming depth-dependent magnetic domain structure. These results are helpful to clarify the evolution of tribo-magnetization and the pinning effect of dislocations on magnetic domains.

The blowout preventer (BOP) is the most important and the last line of safety defense in drilling engineering. Once a blowout occurs and the BOP fails, engineers will lose control of the entire wellbore pressure, and combustible fluids in the formation will continuously sprayed out, which can easily cause huge losses of life and property. At present, reliable and highly recognized emergency measures for BOP failure are lacking. Therefore, we propose a plugging method after the failure of the BOP that can maintain good control within the secondary well control. Numerical and experimental results indicate that using a small-to-medium displacement (1–2 m³/min) during the early stage of plugging and applying multiple plugging and killing cycles significantly improves plugging stability and killing efficiency. PEEK (polyether ether ketone) was selected as the bridging material for field plugging tests on full-scale blowout preventers, verifying its sealing effectiveness at pressures up to 80 MPa. Subsequently, the CFD–DEM was used to simulate the well killing process after plugging. This study mainly focused on the transportation of particles in a pipeline and the analysis of the process of well killing after plugging. The research results indicate that PEEK demonstrates sufficient pressure-bearing capacity under real blowout conditions. Also reveal that PEEK’s exceptional wear resistance and impact strength help maintain sealing stability during repeated particle–wall collisions, effectively reducing secondary erosion and prolonging the operational lifespan of temporary plugging structures. After undergoing six high-pressure tests of 70 MPa and two high-pressure tests of 80 MPa within 25 min, it remained intact. Both cylindrical and spherical particles can smoothly pass through the storage tank and double-bend pipeline at different displacements. Considering the retention effect of the plugging material, it is recommended to use 1–2 m³/min of pumping the plugging material at medium and small displacements in the early stage of plugging. During the process of plugging and killing, it is recommended to use alternating plugging and killing across multiple operations to prevent further blowouts to achieve the best plugging and killing effect.

To accommodate the extreme thermodynamic effects and erosion damage in throttling equipment for ultra-high-pressure natural gas wells (175 MPa), a coupled multiphase flow erosion numerical model for nozzles was established. This model incorporates a real gas compressibility factor correction and is based on the renormalized k-ε RNG (Renormalization Group k-epsilon model, a turbulence model that simulates the effects of vortices and rotation in the mean flow by modifying turbulent viscosity) turbulence model and the Discrete Phase Model (DPM, a multiphase flow model based on the Eulerian–Lagrangian framework). The study revealed that the nozzle flow characteristics follow an equal-percentage nonlinear regulation pattern. Choked flow occurs at the throttling orifice throat due to supersonic velocity (Ma ≈ 3.5), resulting in a mass flow rate governed solely by the upstream total pressure. The Joule–Thomson effect induces a drastic temperature drop of 273 K. The outlet temperature drops below the critical temperature for methane hydrate phase transition, thereby presenting a substantial risk of hydrate formation and ice blockage in the downstream outlet segment. Erosion analysis indicates that particles accumulate in the 180° backside region of the cage sleeve under the influence of secondary flow. At a 30% opening, micro-jet impact causes the maximum erosion rate to surge to 3.47 kg/(m²·s), while a minimum erosion rate is observed at a 50% opening. Across all opening levels, the maximum erosion rate consistently concentrates on the oblique section of the plunger front. Results demonstrate that removing the front chamfer of the plunger effectively improves the internal erosion profile. These findings provide a theoretical basis for the reliability design and risk prevention of surface equipment in deep ultra-high-pressure gas wells.