Full-aluminum alloy drill pipes with both pipe body and tool joints made of aluminum alloy can reduce the risks associated with cementing operations in oil and gas wells. However, no detailed failure analysis reports on their field applications have been published to date. Following the initial application of a 7075 series full-aluminum alloy drill pipe in an ultra-deep section of well SDCK1 during cementing operations, severe corrosion was observed on the pipe body, and fracture occurred in the joint during subsequent breakout operations. This paper presents a failure analysis of the full-aluminum alloy drill pipe under high-temperature oil-based drilling fluid conditions. The analysis employed chemical composition analysis, scanning electron microscopy (SEM), metallographic examination, x-ray diffraction (XRD) analysis, mechanical property testing, and energy-dispersive x-ray spectroscopy (EDS). The research indicates that the yield strength of the aluminum alloy tends to decrease with increasing temperature, with a more rapid decline observed when the temperature exceeds 80 °C. Furthermore, the yield strength of the aluminum alloy material in the drilling fluid environment is significantly lower compared to that in an air environment. Additionally, the hardness of both the failed aluminum alloy drill pipe body and the joint was significantly reduced. Corrosion is identified as the primary cause of failure in the aluminum alloy pipe body. The corrosion products mainly consist of powdery deposits and fibrous structures, both composed of AlO(OH) and Al2O3. The fibrous corrosion products are more detrimental to the aluminum alloy matrix than the powdery ones. Stress corrosion fatigue is the main failure mechanism for the aluminum alloy joint. The high-temperature downhole environment further accelerated the corrosion process and the reduction in matrix strength, while torsional stresses from makeup/breakout operations and tensile stresses accelerated the fracture process of the aluminum alloy joint. To prevent failure accidents involving full-aluminum alloy drill pipes, optimization measures are proposed from three aspects: enhancing the high-temperature resistance of the aluminum alloy material, refining its microstructure, and reducing the corrosion rate of the aluminum alloy drill pipe. The research results can provide a theoretical and practical basis for the application and optimization of full-aluminum alloy drill pipes.

This study addresses the insufficient torsional strength and fatigue life of conventional API NC50 threaded connections under extreme downhole conditions in deep and ultra-deep wells. Through three-dimensional finite element modeling validated against API standards, an orthogonal optimization scheme was applied to key geometric parameters—including pitch, taper, secondary shoulder clearance, secondary shoulder slope, and the thread height of the first and last engaged threads—resulting in the development of a novel variable-height-thread design, referred to herein as VHT516. The optimized structure incorporates a double shoulder and a refined thread profile to mitigate stress concentration in critical engagement zones. Comparative simulations demonstrate that the VHT516 connection exhibits a 62.5

Accurate prediction of oil-based drilling fluid (OBD) behavior under high-temperature, high-pressure (HTHP) conditions in ultra-deep wells is essential for ensuring well control safety. However, research on the volumetric expansion of OBD in HTHP wellbores remains limited. This study develops a mathematical model to predict the expansion characteristics of OBD in HTHP wellbores, incorporating temperature- and pressure-dependent variations in rheological properties, thermophysical parameters, and density. The model is solved using discretization and the successive over-relaxation (SOR) method. Laboratory experiments were conducted to measure the rheology and density of OBD at different initial densities under varying temperature and pressure conditions. Binary mathematical models describing density and rheology as functions of temperature and pressure were established. The results indicate that the rheological properties of OBD exhibit consistent trends across different initial densities. Apparent viscosity (AV) and plastic viscosity (PV) decrease with increasing temperature and gradually stabilize, while both increase with rising pressure. The density of OBD decreases due to thermal expansion at elevated temperatures and increases due to thermal contraction at lower temperatures. Temperature exerts a greater effect on OBD density than pressure. Model validation using field data from an ultra-deep well confirms the accuracy of the proposed approach. During drilling and tripping operations, changes in mud pit volume occur due to temperature fluctuations; however, these changes are relatively slow compared to the rapid volume changes associated with wellbore influx. It is recommended to suspend the use of drilling fluid cooling devices before pulling out of hole (POOH) operations to minimize mud pit volume fluctuations. These findings provide theoretical guidance for accurate bottom-hole temperature prediction and improved drilling strategy optimization.

The safe and efficient extraction of deep oil and gas resources demands improved well construction techniques. Frequent failures of drillstrings with industry standard American Petroleum Institute (API) rotary-shouldered connections (RSC) have been reported, and adopting double-shoulder connections (DSC) can significantly reduce such failures. Currently, there is a lack of precise methods for calculating the makeup torque of DSC, which affects the joint size design. To address this issue, a 3D finite element model of various types of connections is developed to simulate the screw-in process, analyze the average axial stress in critical sections under applied torque, and determine the appropriate makeup torque according to API standards. The method's effectiveness is validated through application to the NC38 joint. Unlike analytical solutions, this study provides a more accurate assessment of the average stress in the section. Additionally, for DSC, the secondary shoulder gap can be optimized during the makeup torque determination. The optimal gap should ensure that the torque ratio of the secondary shoulder after assembly remains between 8% and 10%. The joint with the optimized gap was then subjected to a torsion test. The shoulder was firmly joined, with no material loss due to overfitting at the thread contact surface. This workflow offers a cost-effective method applicable to existing drillstring threaded connections and new designs, aiding in determining the length of pin and box, manufacturing deviations, and providing operational parameter guidance for field applications.

The safety problem of large-size drilling tools in large-size boreholes has become increasingly prominent with the exploration and development of deep and ultradeep wells. This study analyzes the causes of large-size drilling tool failures from the engineering point of view via statistical analysis, experimental material test, and vibration and bending analyses. Results show that the violent downhole vibration changes the drilling tool's mechanical properties. These changes result in an uneven distribution of hardness and reduced impact work, finally leading to the initiation of fatigue cracks at stress concentration points. Drilling tool bending is closely related to drilling parameters and bottom hole assembly (BHA) configuration. Unreasonable BHA configuration and drilling parameters increase BHA bending and accelerate fatigue failure. Once a crack is generated, the corrosive ions in water-based drilling fluids invade the microcrack, causing the corrosion of the drilling tool material. As a result, the strength is reduced, and the fracture is aggravated. Therefore, measures for preventing the failure of large-size drilling tools are proposed. We hope that the results of this work can provide useful guidance for drilling engineers.

As an important equipment for deep-sea oil and gas development, riser is prone to failure due to external ocean environmental load and internal fluid in the process of oil and gas exploitation. In this paper, a dynamic analysis model of the riser system under the action of internal and external flow coupling is established. This model is discretized by using the finite element method and solved by Newmark-beta method. The vortex-induced vibration characteristics of the mining riser under the action of internal and external flow coupling are analyzed. Results show that compared with the traditional riser containing pure liquid flow, gas-liquid two-phase flow in pipe can reduce the natural frequency of the riser and increase the vibration amplitude and frequency. The natural and vibration response frequencies of the riser decrease with the increase in the discharge rate and the density of the two-phase mixture, and the modal response remains unchanged. With the increase in the air inlet ratio, the natural frequency of the riser decreases, but the vibration frequency increases, and induces the vibration of higher modes of the riser.

In order to simulate the influence of well inclination angle on cuttings transport in the whole inclined section and horizontal section, we use cubic spline interpolation method to carry out curve fitting on the well trajectory, and establish a two-layer dynamic model of cuttings transport in long horizontal wells considering the actual well trajectory, and use MATLAB to simulate it. This study simulates the process of drilling and washing, analyzes the influence of the flow rates, the drilling fluid density, and the rate of penetration (ROP). Research results demonstrate that, under drilling conditions, cuttings are difficult to deposit in the section with small deviation angle, the degree of hole cleaning increases with the flow rates and the drilling fluid density, and lower ROP is helpful in hole cleaning. Under washing conditions, the smaller the deviation angle is, the worse the erosion effect is; the degree of hole cleaning increases with the flow rates and the drilling fluid density. The research results have guiding significance for hole cleaning of long horizontal wells.

Grasping the dynamic characteristics of drill bit can help optimize drilling design and improve rate of penetration (ROP). This work focuses on the nonlinear dynamic characteristics of full-size polycrystalline diamond compact (PDC) bit. One of the most conspicuous features of this work is that the nonlinear dynamic model for drillstring-bit-rock system is established in actual drilling engineering scale. In the proposed model, the drillstring dynamic model is based on the FE method combined with beam theory, and PDC bit-rock dynamic model could take the actual bit structure and rock properties into consideration. A logical and feasible coupling method is proposed to realize the coupling of the drillstring dynamic model and PDC bit-rock dynamic model. A full-size 81/2″ PDC bit and three kinds of rock samples (limestone, sandstone and artificial stone) are used to conduct systematic experimental study on dynamic response of PDC bit. The simulated and experimental bit footage, WOB, TOB, three-way vibration acceleration, and bottomhole topography after drilling are obtained. Results showed that: PDC bit drills the fastest in artificial rock, followed by sandstone, and drills the slowest in limestone. With the increase of rock strength, the MV of WOB increases and the fluctuation becomes more severe. The TOB is larger when drilling in artificial rock, followed by sandstone and limestone. The bit footage increases as initial WOB increase. With the increase of the initial WOB, the MV of WOB increases, and the WOB and TOB fluctuation become more severe. Axial acceleration, radial acceleration and tangential acceleration all increase as initial WOB increase. The bit footage increases as rotation speed increase. With the increase of rotational speed, the fluctuation of TOB tends to decrease, and the tangential acceleration decreases obviously. When PDC bit is crossing different lithologically rock, a dramatic instantaneous fluctuation will appear in WOB, TOB and vibration acceleration. The research results of this paper can provide a valuable and useful guidance for drilling engineers to improve ROP and reduce downhole accidents.

Understanding the characteristics of downhole vibration helps optimize the drilling parameters and improve the rate of penetration (ROP). This work establishes a nonlinear drillstring–bit–rock model to predict the downhole vibration behavior of 3D curved wells. The main novelty of this model is to consider the complex nonlinear bit–rock interaction and well trajectory. The model is solved by the Newmark method and verified by an indoor experiment and field data. The drillstring dynamics and effects of the main working parameters on the vibration characteristics are obtained and analyzed. Results show that, in the horizontal section, the drillstring motion and shape deformation are stable, and radial vibration is dominant. In the buildup section, the drillstring motion is irregular and messy, shape deformation becomes large, and radial vibration is significant but tangential vibration starts to become intense. In the vertical section, the drillstring motion is small in wellbore center, shape deformation is stable, and vibration severity reduces. The weight on bit (WOB) only affects drillstring vibration near the bit, and radial, tangential, and axial accelerations intensify as WOB increases. RPM has little effect on the drillstring vibration in 3D curved wells. In the buildup and horizontal sections, as the coefficient of friction (COF) increases, the radial and tangential accelerations rise while the axial vibration obviously weakens. When drilling in 3D curved wells, small WOB and large RPM can be adopted to reduce downhole vibration and increase ROP. The safety of drillstring and downhole tool also needs more attention when drilling in formations with large COF, such as gravel stone and sandstone. Large oil/water ratio or lubricant is recommended to improve the lubricity of drilling fluid. The research results could provide a valuable and useful guidance for drilling engineers to improve ROP and reduce downhole accidents.

This study focuses on the nonlinear dynamic characteristics of the bottom hole assembly (BHA). One of the most important features of this work is that the nonlinear interaction model of polycrystalline diamond compact (PDC) bit and rock is proposed. The PDC bit dynamic characteristics are used as the boundary conditions of the drillstring dynamic model. To more realistically simulate the drillstring dynamics, the drillstring dynamic model is established with the beam finite element method and solved by the generalized-α method. The model is verified by the field data. The PDC bit dynamic characteristics are analyzed, and the effects of weight on bit, rotation speed and rock drillability on the drillstring dynamic behavior are discussed. We hope that our research will aid in the development of the drillstring dynamics in the future.

Casing integrity is significant for subsequent drilling and production and prediction of casing wear accurately helps to reduce casing damage. Thus, this study aims to represent the casing impact wear characteristics more intuitively and accurately during drilling. The vibrating drillstring when drilling will collide with casing frequently and increase the casing wear. The drillstring nonlinear dynamic model is established using beam finite element method in this work, and the drillstring dynamic characteristics are obtained using generalized-α method. Subsequently, the friction coefficient and wear factor are obtained through the casing wear test. On this basis, the casing impact wear characteristic combined with the casing wear efficiency model is described, and the residual strength of casing is analyzed. In addition, the effects of drilling depth, weight on bit (WOB), rotation speed and the eccentric distance of the drillstring on the casing wear are discussed. We hope that our research will aid in designing drilling parameters and reducing casing damage.

Fatigue is the main cause of drilling tool fracture. Accurate prediction of drilling tool fatigue life is challenging in drilling engineering. In this study, a drillstring dynamic model was established based on the finite element (FE) method, and the dynamic response of Bottom Hole Assembly (BHA) is obtained by numerical solution. Then the Mises equivalent stress of BHA was obtained using the fourth strength theory. Combined with Walker model, the fatigue life of drillstring with initial defects was predicted. The model was first verified by reproducing the experimental data. Then, the BHA dynamic characteristics were obtained and the effects of weight on bit (WOB), rotational speed, stabilizer (STB) location and crack size on the drillstring fatigue life were discussed. Results show that, increase in rotational speed leads to a small increase in the fatigue life below the neutral point. Decrease in WOB significantly prolongs the fatigue life, and adding STB can decrease the maximum equivalent stress and increase the drillstring fatigue life near the bit. The BHA fatigue life decreases as the size of initial crack increases. When the crack size increases to a critical value, the fatigue life tends to be stable and drilling tool may break instantly. This study could provide important guidance for drilling engineers in designing BHA.

Ocean thermal energy is clean and renewable. In recent years, many scholars have focused on improving the generation efficiency of the ocean thermal energy conversion system. To the best of our knowledge, the majority of the scholars focus on equipment improvement and working fluid optimization, and few study how to reduce the outlet temperature of cold-water pipes. This paper establishes a numerical model, which is solved by the finite volume method, to analyze the temperature distribution and heat transfer characteristics of cold-water pipes during pumping. The influence of cold-water pipe materials, wall thicknesses, flow rates, and insulation layer thicknesses on outlet temperature is explored. In addition, this paper qualitatively gives performance optimization recommendations based on technical and economic indicators. This study will be useful for the OTEC community.

Purpose The purpose of this paper is to study the multi-phase flow behaviors in solid fluidization exploitation of natural gas hydrate (NGH) and its effect on the engineering safety. Design/methodology/approach In this paper, a multi-phase flow model considering the endothermic decomposition of hydrate is established and finite difference method is used to solve the mathematical model. The model is validated by reproducing the field test data of a well in Shenhu Sea area. Besides, optimization of design parameters is presented to ensure engineering safety during the solid fluidization exploitation of NGH in South China Sea. Findings To ensure the engineering safety during solid fluidization exploitation of marine NGH, taking the test well as an example, a drilling flow rate range of 40–50 L/s, drilling fluid density range of 1.2–1.23 g/cm3 and rate of penetration (ROP) range of 10–20 m/h should be recommended. Besides, pre-cooled drilling fluid is also helpful for inhibiting hydrate decomposition. Originality/value Systematic research on the effect of multiphase flow behaviors on the engineering safety is scare, especially for the solid fluidization exploitation of NGH in South China Sea. With the growing demand for energy, it is of great significance to ensure the engineering safety before the large-scale extraction of commercial gas from hydrate deposits. The result of this study can provide profound theoretical bases and valuable technical guidance for the commercial solid fluidization exploitation of NGH in South China Sea.

This study focuses on the nonlinear lateral vibration of drillstring in curved wells. One of the most important features of this work is that the drillstring dynamics in curved wells (including both vertical, deflecting and horizontal well structures) are modeled and presented in actual drilling engineering scale. In the proposed model, beam finite element (FE) method is employed to calculate the drillstring dynamics in curved wells. The model is solved by the generalized-α method and verified by reproduction of the field data. The model has coupled the lateral, torsional and longitudinal vibrations of the drillstring, while we only pay our attention to the nonlinear lateral vibration of drillstring in curved wells. The efficient length of lateral vibration of a field curved well is firstly proposed, and the influences of rotation speed, weight on bit (WOB), coefficient of friction (COF) and stabilizer (STB) on the lateral vibration of drillstring in curved wells are discussed.

This study focuses on the nonlinear dynamic characteristics of the bottom hole assembly (BHA). One of the most important features of this work is that the digital model of polycrystalline diamond compact (PDC) bit is established. The PDC bit dynamic characteristics are used as the boundary conditions of the drillstring dynamic model. The drillstring dynamic model is established with the finite element method and solved by the generalized-α method to more realistically simulate the drillstring dynamics. The model is verified with the experiment and field data. The PDC bit dynamic characteristics are first analyzed, and the effects of weight on bit, rotation speed and rock drillability on the drillstring dynamic behavior are discussed. We hope that our research will aid in the development of the drillstring dynamics in the future.

Leaching tubing is an important tool for building underground gas storage. Its reliability and stability directly affect the building process. However, buckling deformation or fracture often occurs in leaching tubing due to severe vibration. In order to study the vibration characteristics of leaching tubing, a large experimental system is designed. Modal analysis is performed to examine characteristics of leaching tubing. The result of research shows that pressure pulsation of internal turbulence primarily induces the vibration of leaching tubing. Vibration response frequency of leaching tubing is similar to the turbulence pressure pulsation frequency. Meanwhile, amplitude at the bottom and middle of center tube is large. These locations are the risk points of vibration failure and need more attention. Furthermore, the response frequency and amplitude of center tube in reverse circulation are greater than those in direct circulation. Excessive vibration of center tube can be avoided by limiting water injection flow rate and two-port distance. The research results can provide some reference for construction engineers of gas storage and vibration researchers of suction pipe and discharge pipe.

With the wide application of extended reach wells and horizontal wells, the geological conditions faced during the drilling process have become increasingly complicated, and the resulting wellbole collapse problem has become serious. Based on an L-1 horizontal well, this study provides a method for calculating collapse pressure considering the influence of formation hydration. This method is used to obtain the stress distribution around the well and collapse situation at 4100 m underground. The wellbore stability can be evaluated by the method. The factors affecting the stress and collapse of the well are analyzed by changing the construction parameters and geological parameters. Studies have shown that the rock near the well has collapsed to a certain extent and shale hydration increases the formation Poisson's ratio, resulting in more serious collapse. The study also found that hydration, drilling fluid density, and borehole size have the greatest impact on the stability of the borehole wall, followed by the well deviation angle, and the bedding angle the least. Carefully exploring the situation of the shale gas reservoir in the drilling design stages, considering the impact of hydration, and selecting appropriate construction parameters are necessary to avoid the collapse of the shaft wall and affect the shale gas production process. This study can provide a reference for ensuring the stability of the wellbore.

为研究双层连续管内管举升水合物的井筒多相流动规律,建立了采用双层连续管固态流化开采天然气水合物的井筒多相流动模型,采用有限差分法对模型进行求解.基于以上模型,对南海某实例井采用双层连续管固态流化开采天然气水合物的井筒多相流动行为进行了模拟.研究结果表明,随着流体上升,双层连续管内管温度和压力逐渐降低;在海底1 600米至700米时,固相含量以及液相含量相对稳定,含气率为0;当流体上升至700多米的高度时,固相水合物开始逐渐分解.当钻井液密度与排量过高或者过低,都会使得井底压力脱离安全窗口,威胁到井控安全.因此,合理地选用钻井参数,对确保井控安全尤为重要.

通过用7075铝合金销盘和G105钢销钉在UTM摩擦试验机上研究了的实验接触载荷对铝合金隔水管摩擦学性能的影响机理.结果表明:随着接触载荷增加,摩擦系数先大幅减小后略有增大;磨损率先缓慢下降再急剧上升;在10～30 N接触载荷下,主要磨损机制为磨料磨损;在50 N接触载荷下,主要磨损机制为氧化磨损;在70 N接触载荷下,主要磨损机制为分层磨损;50 N为由轻微磨损向严重磨损转变的临界载荷.研究证明了在铝合金隔水管的运用过程中,应该避免高载接触引发严重磨损而导致材料失效.