As the casing undergoes bending deformation along the well trajectory, the contact position and state between the casing and wellbore exhibit dynamic variations during casing running operations in extended-reach wells. Assessing random contact between the casing and wellbore becomes critical for studying the mechanical properties of running casings. Hence, we utilized the Universal Mechanical Tester (UMT) pin-on-disk friction system to assess the kinetic and static friction coefficients of casing with surface casing and formation rock in mud environments, and employed the stochastic contact approach to develop a 3D dynamic finite element model incorporating the actual well trajectory. We conducted an analysis of the mechanical characteristics of running casings and discussed how factors such as wall thickness, friction coefficients, running speeds, and wellbore fluid density affect the mechanical distribution of casing. Numerical results indicate that the substantial frictional contact between the casing and the wellbore wall appears in the positive inclined and horizontal sections, and the casing initiates buckling behavior at a running depth of 3000 m. With the increase in running depth, the casings buckling behavior gradually transitions from sinusoidal to helical, causing a sharp increase in additional resistance against casing running. Finally, the casing cannot continue running beyond 4022 m. In comparison to the effect of wellbore roughness, the wall thickness of the casing and the fluid density primarily influence the horizontal well section. Furthermore, higher running speeds exacerbate buckling behavior by delaying the efficient release of the casings accumulated elastic energy.

With the frequent occurrence of stuck pipe incidents during the ultra-deep well drilling operation, the hydraulic-while-drilling (HWD) jar has become a critical component of the bottom hole assembly (BHA). However, during jarring operations for stuck pipe release, the drill string experiences severe vibrations induced by the impact loads from the jar, which significantly alter the stress state and dynamic response of the threaded connections—the structurally weakest elements—under cyclic dynamic loading, often leading to fracture failures. here, a thread failure incident of a hydraulic jar in an ultra-deep well in the Tarim Basin, Xinjiang, is investigated. A drill string dynamic impact model incorporating the actual three-dimensional wellbore trajectory is established to capture the time-history characteristics of multi-axial loads at the threaded connection during up and down jarring. Meanwhile, a three-dimensional finite element model of a double-shouldered threaded connection with helix angle is developed, and the stress distribution of the joint thread is analyzed on the boundary condition acquired from the time-history characteristics of multi-axial loads. Numerical results indicate that the axial compression induces local bending of the drill string during down jarring, resulting in significantly greater bending moment fluctuations than in up jarring and a correspondingly higher amplitude of circumferential acceleration at the thread location. Among all thread positions, the first thread root at the pin end consistently experiences the highest average stress and stress variation, rendering it most susceptible to fatigue failure. This study provides theoretical and practical insights for optimizing drill string design and enhancing the reliability of threaded connections in deep and ultra-deep well drilling.

Summary: Proper use of drilling jars can effectively minimize nonproductive time resulting from pipe-sticking incidents during drilling operations. Due to factors such as temperature, friction, and viscous resistance, the jarring performance of drilling jars is generally poor during jarring operations in ultradeep wells. Existing theories and downhole tools are unable to accurately assess and measure the jarring performance of drilling jars. To address this issue, we designed and developed a high-frequency jarring force measurement sub capable of operating under downhole high-temperature and high-pressure (HTHP) conditions. The tool is characterized by a high sampling rate, high structural strength, strong vibration resistance, a wide measurement range, and stable operational performance, and it can be installed directly below the drilling jar as part of the bottomhole assembly. Finite element analysis indicates that the measurement sub remains structurally safe under rated loads. Combined with high-temperature bench testing and field experiments, the results demonstrate that the measurement sub can accurately capture instantaneous impact loads, axial acceleration, and downhole temperature and pressure during continuous jarring operations, with actual test results showing a relative error of less than 1%. This provides a direct and reliable technical means for quantitatively evaluating the jarring performance of drilling jars under ultradeep well conditions. The findings offer essential data support and experimental evidence for subsequent structural optimization of drilling jars, refinement of jarring operating parameters, and improvement of operational procedures.