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In order to prevent well leakage in high temperature and high pressure (HTHP) wells, pressure test is a very important leak prevention operation that causes artificial high-pressure environment in wellbore. However, failure of wellbore equipment due to harsh condition is a common problem that brings great economic losses. Ram blowout preventer (BOP) is an important component of wellbore integrity, which is subjected to heavy loads from the test gas and the push rod during the pressure test. Therefore, the serious stress concentration of ram BOP induced by heavy loads and complex geometry is a major reason of fracture. Hence, the focus is to find out the fracture mechanism of ram BOP with experimental method and finite elements (FE) model in this paper. A series of damage inspection and strength test of ram BOP is carried out to find out the fracture characteristics. Based on detection information, FE analysis is conducted to analysis stress distribution and fracture reason of ram BOP. The results show that the maximum stress exceeds the yield strength (705 MPa) of the material under the actual working condition. Serious stress concentration at the notch root of the ram BOP can be easily caused by test pressure (from test gas) and contact pressure (from push rod). Both experimental method and FE analysis show the characteristics of ductile fracture obviously. Stress concentration at notch root is very sensitive to the test pressure. The work presented in this paper can provide basis and reference for preventing failure of ram BOP and improve efficiency and production.

Co2C-based catalysts with SiO2, γ-Al2O3, and carbon nanotubes (CNTs) as support materials were prepared and evaluated for the Fischer-Tropsch to olefin (FTO) reaction. The combination of catalytic performance and structure characterization indicates that the cobalt-support interaction has a great influence on the Co2C morphology and catalytic performance. The CNT support facilitates the formation of a CoMn composite oxide during calcination, and Co2C nanoprisms were observed in the spent catalysts, resulting in a product distribution that greatly deviates from the classical Anderson-Schulz-Flory (ASF) distribution, where only 2.4 C% methane was generated. The Co3O4 phase for SiO2- and γ-Al2O3-supported catalysts was observed in the calcined sample. After reduction, CoO, MnO, and low-valence CoMn composite oxide were generated in the γ-Al2O3-supported sample, and both Co2C nanospheres and nanoprisms were identified in the corresponding spent catalyst. However, only separated phases of CoO and MnO were found in the reduced sample supported by SiO2, and Co2C nanospheres were detected in the spent catalyst without the evidence of any Co2C nanoprisms. The Co2C nanospheres led to a relatively high methane selectivity of 5.8 C% and 12.0 C% of the γ-Al2O3- and SiO2-supported catalysts, respectively. These results suggest that a relatively weak cobalt-support interaction is necessary for the formation of the CoMn composite oxide during calcination, which benefits the formation of Co2C nanoprisms with promising catalytic performance for the sustainable production of olefins via syngas.

Fischer-Tropsch to olefins (FTO) is recognized as a surface-catalyzed structure-sensitive reaction, and the catalytic performance is strongly influenced by the morphology and exposed facets of the active phase. Here we report the effect of the reduction process on the morphology of the active phase and the catalytic performance for FTO over the CoMn catalyst. For the catalysts reduced by 10% CO-300 °C, 10% H2-300 °C and 10% H2-250 °C, Co2C nanoprisms were formed after reaching the steady state. However, for the catalysts reduced by CO-300 °C and 10% H2-400 °C, Co2C nanospheres were found instead. Both Co2C nanoprisms and nanospheres were present for the spent sample reduced by 10% H2-350 °C. Kinetic study found Co2C nanospheres to possess higher activation energy, and are more sensitive to hydrogen than Co2C nanoprisms. Density functional theory (DFT) calculations were also performed to clarify the structure-performance relationship of Co2C nanostructures for syngas conversion.

The Fischer-Tropsch to olefins (FTO) process is a promising alternative non-petroleum route to produce value-added olefins. In this study, Co2C nanoprisms formed via the CoMn spinel structure were used as the active phase for the FTO reaction and the product distribution was thoroughly investigated. A break from the traditional ASF model was found with much lower methane selectivity than that predicted by the ideal ASF law. In addition, the as-obtained hydrocarbons were mainly concentrated in the range of C2 to C12 with the C2-12 selectivity as high as 91% while the C13+ selectivity as low as 5%, indicating a much narrower distribution, which was rarely observed in the FT reaction. Moreover, ∼86% of the hydrocarbon products were olefins with very high olefin to paraffin ratios, suggesting a promising route for the selective production of olefins directly from syngas. A modified ASF distribution model was proposed with three different chain growth probabilities to rationalize the non-ASF phenomenon.

Fischer-Tropsch synthesis (FTS) is a structure-sensitive reaction for sustainable production of green fuels and value-added chemicals via syngas derived from coal, biomass, shale gas and natural gas. The nanostructure of a Fischer-Tropsch (FT) catalyst plays a crucial role in its catalytic performance. This review summarizes recent advances in the investigation of nanoeffects of FT catalysts, especially the effects of the active phase, particle size and exposed facet on catalytic performance. Perspectives and challenges for further research in nanocatalysis for syngas conversion are also given.

Flowerlike ZIF-67 micro/nano composite structures were developed by the evolution of ZIF-67 rhombododecahedrons ZIF-67(r) via facile ion-assistant solvothermal treatment. The morphology evolution of ZIF-67 with time-on-stream was studied. The flowerlike Co3O4 micro/nano composite were formed under calcination at relatively low temperature. The flowerlike Co3O4 micro/nano composite structures showed good catalytic properties for photocatalytic degradation of RhB (83.2%), higher than that of Co3O4(p) (69.7%) and Co3O4(r) (75.7%) after 90 min irradiation.

Drillstring vibration is one of the primary causes of drillstring failure, well trajectory deterioration, excessive bit wear and low rate of penetration. In order to obtain an improved understanding of drillstring vibration when drilling horizontal wells, an experimental drillstring system is established in this paper on the basis of drillstring dynamics equations and similarity principle. The system is capable of simulating the nonlinear dynamic behaviors of horizontal drillstring. Axial, lateral and torsional vibration phenomena are investigated in steady state. Experimental results indicate that axial force fluctuates wildly when drillstring is buckled. The fluctuation of WOB (weight on bit) is a typical sine curve and the frequency of WOB increases linearly with rotary speed. The lateral vibration is much more severe than axial vibration for horizontal drillstring. According to similarity principle, in order to reduce lateral vibration, the proposed rotary speed range is 37.5 rpm–50 rpm for the actual horizontal drilling operation. The whirling speed amplitude and frequency of horizontal drillstring increase with rising rotary speed. Stick-slip can also occur in horizontal drilling where the rotary speed of bit involves periodic fluctuation varying almost from zero to about twice the surface rotary speed. The work presented in this paper can provide a technological basis for drillstring dynamics and vibration analysis of horizontal wells.

CoMn model catalysts were prepared by co-precipitation and evaluated for higher alcohol synthesis (HAS) via syngas. The selectivity to oxygenates (mainly alcohols and aldehydes) was found to be higher than 20 C% for the Na-promoted CoMn catalyst. Among the oxygenates, C2+ and C6+ fractions accounted for >90 wt% and ∼50 wt%, respectively. Metallic Co0 particles and MnCO3 were found in both Na-promoted and unpromoted CoMn catalysts after the reaction, whereas Co2C nanoparticles could only be observed in the spent Na-promoted catalyst. The addition of Na benefited the carbonization of cobalt and increased the selectivity to oxygenates and CO2. Our studies thus suggested that Co/Co2C were the bifunctional dual-sites for the oxygenates formation over the Na-promoted CoMn catalyst, where Co catalyzed CO dissociation and chain propagation, while Co2C was responsible for CO non-dissociative activation and subsequent insertion. In addition, the sole Co2C nanoparticles with certain exposed facets may also act as another kind of active dual-sites for oxygenates formation.

Co2C nanoprisms exhibit promising catalytic performance for Fischer-Tropsch synthesis to olefins (FTO) but with high CO2 selectivity (>40%). Herein, silica-coated CoMn-based catalyst was designed to limit CO2 production and remained Co2C nanoprisms as active sites unchanged. With a desired coating amount of silica, CO2 selectivity was significantly suppressed to 15.1 C% while enhancing olefins selectivity from 39.7 C% to 58.8 C%, which also shows at least 160 h of stability. It is suggested the silica-coating not only reduces the adsorption capacity of H2O, but also promotes the fast transfer of H2O away from active sites due to the higher adsorption energy of H2O on SiO2 surface, thus suppressing water-gas-shift-reaction (WGSR) activity. Moreover, the sodium promoter can counteract the H2-enrichment effect caused by SiO2-coating and largely restrain CH4 formation and olefins hydrogenation. This work provides an effective strategy to suppress CO2 formation and enhance the carbon efficiency of FTO process.

The wellhead Christmas tree uses top sand drainage during multi-stage sanding fracturing, and it fails at the four-way and gate valves. Due to the multi-stage sanding fracturing process of a wellhead Christmas tree in an oilfield, the connection between the four-way and the gate valve flange failed for many times during service. In this paper, the failure causes of four-way flange joints were analyzed by reading spectrometer, Rockwell hardness tester, optical microscope, scanning electron microscope, energy dispersion spectrometer and numerical simulation method. The results show that the chemical composition, hardness and grain size of the four-way flange and connecting gate valve meet the standard requirements; By microstructural characterization and energy spectrum analysis, it is confirmed that a large number of corrosion products are attached to the inner wall of the four-way valve and gate valve; During the wellhead Christmas tree fracturing, high-speed sand-carrying fluid flows into the pipeline through four-way, and high-frequency multiple impacts occur to the connection of the four way flange and gate valve. At the same time, a large number of eddy current are formed at the intersection of the four-way and the gate valve flow channel. A large number of corrosion products distributed in the passage of the four-way valve and gate valve cause corrosion accumulation under the action of eddy current, forming corrosion wear, and finally the connection between the four way and the gate valve is damaged by erosion wear and corrosion wear.

SiO2 supported Co2C-based catalysts were used for Fischer-Tropsch to olefins (FTO), and the effect of thermal pretreatment of SiO2 support under different temperatures on the Co2C morphology and catalytic performance was investigated. It was found that the interaction between cobalt and support was weakened when SiO2 was pretreated at high temperature (990 °C) due to the decreased content of surface Si OH groups. The relative weak interaction between cobalt and support benefited the formation of cobalt manganese composite oxide after calcination and reduction, and thus promoted the generation of Co2C nanoprisms with promising FTO performance. In contrast, for the SiO2 support pretreated at 350 °C or 650 °C, the strong interaction between cobalt and support led to phase separation of cobalt and manganese. As a result, only Co2C nanospheres were generated which displayed low activity and high methane selectivity.

Tandem hydroaminomethylation is an efficient and green route for one-pot synthesis of amines directly from olefins. Herein, heterogeneous hydroxyapatite (HAP) supported single-atom Rh catalyst was prepared and used for tandem hydroaminomethylation of olefins. Characterization techniques confirmed the atomic dispersion of Rh species on HAP. Up to 99% conversion of 1-hexene with high selectivity to the desired amines (93.2%) was obtained over 0.5Rh1/HAP catalyst. Mechanism study demonstrated that the first hydroformylation step during the tandem catalytic process was rate-determining. Compared with the Rh nanoparticles on other oxide supports (Mg3Al, MgO and Al2O3), the atomically dispersed Rh sites on HAP ensured the high hydroformylation activity, thereby guaranteed the outstanding catalytic performance for the total tandem process. Furthermore, various corresponding amines can be obtained with satisfactory yields over 0.5Rh1/HAP catalyst from a wide scope of olefins or amines substrates.

Higher alcohols synthesis (HAS) from syngas with high selectivity attracts great attention but remains challenging. Herein, we reported an effective strategy by tuning chemical environment and synergistic relay reaction to promote the production of higher alcohols. CO insertion rate was greatly enhanced by introducing Rh or Ru component to CoMn oxides. The catalytic activity and oxygenates selectivity increased dramatically over the as-obtained Rh-CoMn or Ru-CoMn catalyst, while the fraction of C2+OH in oxygenates maintained >92 %. Multiple studies demonstrated the highly dispersed Rhδ+ or Ruδ+ species not only effectively tuned the chemical environment and facilitated the stable existence of Co2C, but also catalyzed the coupling of syngas and in-situ generated olefins to produce extra oxygenates via hydroformylation route. The synergistic effect of Co0, Co2C and Rhδ+ (or Ruδ+) species, as well as the promotional effect of olefins relay reaction contributed to the enhancement in both higher alcohols selectivity and CO conversion.

In recent years, the stimulated reservoir volume is a new developing technology applied to the effective exploitation of low permeability shale gas reservoirs. Despite of its superiority and potential, the geostress field is under a complex mechanical environment during the volume fracturing process due to excessive stimulated stages, large fracturing volume, high injection capacity, and increasing dense areas of microseismic events. Based on the drilling and completion data and microseismic monitoring data of Sichuan shale gas horizontal well X201-H1, the three-dimensional finite element model of volume fracturing is established, combining fluid-solid interaction mechanics with the basic theory of rock damage mechanics. According to the established model, the finite element analysis on different fracturing conditions is carried out, which finally results in the stress distribution of near-wellbore area after each staged fracturing operation. The results show that: 1) The change of pore pressure caused by volume fracturing can generate induced stress field, which leads to the re-orientation of in-situ stress field and even the appearance of tension stress areas and zero stress areas within the region of volume fracturing stimulation. 2) The existence of stress field interference in different stages of fracturing operations leads to the change in the magnitude and direction of stress field after each staged fracturing. Research methods and results of the paper will provide guiding significance to the optimization design of staged fracturing of horizontal wells to some extent.

Casing wear during directional well drilling remains a prominent problem because it can cause casing strength degradation, casing deformation and even well abandonment. In order to obtain a better understanding of casing wear and quantify the wear amount in gas drilling of directional wells, theoretical and experimental study is carried out in this paper. Based on casing wear mechanism and energy principle, a prediction model of casing wear is investigated and programmed. Experimental study on casing wear is conducted and casing specimens are worn in air and mud respectively. According to wear morphology analysis, casing wear mechanisms can be recognized as adhesive wear and abrasive wear. Data processing and analysis reveals the influence factors of casing wear, including casing steel grade, rotary speed and contact force. Through linear fitting of test data, wear coefficients in air and mud are obtained and compared. Then the intermediate casing wear of an actual directional well is predicted using the methods and models proposed in this paper with special attention focused on the comparison of wear amount in mud drilling and gas drilling. Prediction results indicate that casing wear is not the major factor restricting the application of gas drilling in directional wells. The feasibility of gas drilling in directional wells is verified in the perspective of casing wear. Finally, optimization of drilling parameters and reducing friction coefficient are proposed for casing wear reduction. The work presented in this paper can provide theoretical foundation and technological basis for casing wear prediction and reduction in gas drilling.

Under the effect of in situ stress in a directional well, the casing wear problem is a highly nonlinear and complicated process. To address this problem, theoretical studies for casing wear in a directional well under in situ stress are studied. Then, casing friction and wear experiments are conducted to obtain the relevant parameters for simulation. Finally, based on the theoretical studies and experimental data, combined with the drilling and well completion data, a finite element model of joint-casing wear for the NP-1 directional well is established. By reprogramming and redeveloping the software with Fortran language, the dynamic simulation of the casing wear process in directional well is realized considering the effects of in situ stress on the wear process. The rules of casing wear depth and residual strength at different locations and different times are obtained, and a new prevention method different from previous measures is proposed. The work presented in this paper can provide a theoretical foundation and technological basis for casing integrity evaluation and casing design in directional wells under in situ stress.

Casing integrity is an important category of well integrity in drilling and well operations. Casing integrity defect due to wear and corrosion can cause casing strength degradation, casing deformation and even well abandonment. In this paper, a theoretical model for casing strength degradation due to wear is established in bipolar coordinate system. Another model is established to calculate stress concentration factor of casing with corrosion pit at inner wall. The effects of relevant parameters on residual strength of defective casing are analyzed according to parametric study. According to the stress distribution of casing after wear and corrosion under tri-axial stress, strength check is carried out based on Von Mises yield criterion and Lame thick-walled solution for the pipe. Then, in order to evaluate casing integrity of a real well, casing wear and corrosion experiments are conducted to study mechanisms and obtain relevant parameters for calculation. Finally, according to the established models and experimental results, a modified casing design of this well is proposed considering the effects of wear and corrosion on casing strength degradation. The work presented in this paper can provide a theoretical foundation and technological basis for casing integrity evaluation and casing design of highly-deviated wells or extended-reach wells in sour environment.

With its advantages of lost circulation prevention, drilling speed improvement and reservoir protection, gas drilling technology has been widely applied in Sichuan and Xinjiang oilfields in China . However, drill collar failures have often occurred under high weight on bit (WOB) during gas drilling. These incidents have not only caused serious economic loss but hampered the development and application of gas drilling. Finite element analysis is used to determine the drill collar thread stress distribution and, using simulation of drill string dynamics, multi-axial fatigue life theory is used to calculate the life of a standard API drill collar connection. The computed results reveal the early drill collar fatigue failures in gas drilling. Then, the multi-axial fatigue life of drill collar with double shoulder thread is analyzed under the same loads. Analysis shows that the fatigue life of a double shoulder thread is some thirty times that of the API thread. So drill collar with double shoulder thread could be an efficient way to solve the fatigue failure problems of drill collar in gas drilling. The work presented in this paper can provide theoretical foundations for safe and efficient drilling with gas.

Frequently happening drill pipe failure accidents in oil and gas wells not only affect drilling speed, but cause enormous economic losses and many safety issues. Most of these accidents are transverse cracking of drill pipe body and pin thread or axial cracking of box thread. Based on the axial cracking failures of drill pipe body in an ultra-deep well in China, this paper give a systematic analysis of axial cracking failure in consideration of service condition, material quality and stress corrosion mechanism. Measurement and inspection are performed on macroscopic and microscopic morphology of crack surface, corrosion products and circumferential residual stress. Then stress corrosion cracking experiments against hydrogen sulfide is conducted. Finally, the critical stress value for sulfide stress corrosion cracking of the drill pipe material is obtained, and the mechanisms of axial cracking failure and corresponding preventive measures are proposed.

Dual active sites play a vital role in higher alcohols synthesis (HAS) from syngas and the relative proportion of each active sites may greatly affect the contact boundaries between the dual active sites and the reaction network. Herein, we regulated the ratio of Co0/Co2C dual active sites by changing the content of Na promoter, and investigated the structure-performance relationship of Co0/Co2C for HAS. It was found that both of the catalytic activity and paraffins selectivity increased with the increase of Co0/Co2C ratio, while the formation of olefins was inhibited. However, the oxygenates selectivity exhibited a volcano variation trend. The results suggested that only when the Co0/Co2C ratio was in the appropriate range, the strong synergistic effect of Co0/Co2C dual-site could be obtained, and the CO insertion rate can thus match well with hydrogenation rate and chain-growth rate, which benefited for the oxygenates formation.

Pressure vessel is a primary barrier to gathering system and an important guarantee for normal production in the oilfield. Composite materials for pressure vessels are widely used to adapt to acidic media and temperature and pressure environment. Therefore, the stress corrosion cracking (SCC) is easily caused considering harsh working condition and multi-material structure. Pressure vessel defect due to SCC can cause material strength degradation, medium leakage and even explosion. In order to analyze the stress distribution of the whole equipment and verify the crack detection results in the field, a detailed finite elements model (FEM) of the absorber considering the composite material is established based on mechanical experiments and field data. On the other hand, a WOL specimen FEM is established according to the industry standard. Finally, twelve SCC evaluation experiments of composite materials are carried out based on the results of two established FEM, and the safety windows of 316L and SA516-70 are obtained in specific stress corrosion environments. The work presented in this paper can offer a technological basis and theoretical foundation for strength design of composite materials for pressure vessels in sour environment.

The effects of preparation methods for CoMn-based catalysts on the precursor phase of Co species, the morphology of Co2C nanostructures as well as the catalytic performance for Fischer-Tropsch to olefins were investigated. Five catalysts prepared by precipitation and impregnation methods were compared in detail. CoMn spinel oxide was found in the calcined catalysts prepared by co-precipitation or sequential precipitation starting from the Co salt. However, Co3O4 and MnO2 were the major phases for the calcined catalysts prepared by sequential precipitation starting from the Mn salt or impregnation methods. During the syngas conversion process, Co2C nanoprisms with exposed facets of (101) and (020) were generated from CoMn spinel phase, and exhibited high CO conversion, low methane selectivity and high olefins selectivity. In contrast, Co2C nanospheres carburized from metallic Co or CoO were observed for the other samples with poorer FTO catalytic performance.

The combination of gas drilling and horizontal well has been considered as an effective technique for the exploitation of low permeability reservoirs to protect reservoir, enlarge drainage area and increase production. Given the currently inadequate understanding about drill string dynamic characteristics in gas drilling of horizontal wells, a theoretical model of drill string dynamics is established in this paper. The nonlinear dynamics equations are derived to study the motion state of drill string. Meanwhile, an experimental apparatus is developed according to similarity principle, and the kinetic characteristic of drill string is investigated based on the simulation experiment. Particular attention is focused on the lateral vibration which results from the impact and frictional interaction with wellbore constraint. The effect of weight on bit and rotary speed on drill string motion pattern is also discussed based on experimental results. Finally, the buckling and contact of drill string are analyzed through finite element simulation study. The results indicates that the contact force between wellbore and drill string is relatively large and helical buckling of drill string can be caused without the lubrication and damping effects of drilling fluid in gas drilling. The work presented in this paper can provide theoretical foundation and technological basis for drill string dynamics analysis and drilling parameter optimization in horizontal wells drilled with gas.

Multi-stage SRV fracturing in horizontal wells is a new technology developed at home and abroad in recent years to effectively develop shale gas or low-permeability reservoirs, but on the other hand makes the mechanical environment of fracturing strings more complicated at the same time. In view of this, based on the loading features of tubing strings during the multi-stage fracturing of a horizontal well, mechanical models were established for three working cases of multiple packer setting, open differential-pressure sliding sleeve, and open ball-injection sliding sleeve under a hold-down packer. Moreover, mathematical models were respectively built for the above three cases. According to the Lame formula and Von Mises stress calculation formula for the thick-walled cylinder in the theory of elastic mechanics, a mathematical model was also established to calculate the equivalent stress for tubing string safety evaluation when the fracturing string was under the combined action of inner pressure, external squeezing force and axial stress, and another mathematical model was built for the mechanical strength and safety evaluation of multi-stage fracturing strings. In addition, a practical software was developed for the mechanical safety evaluation of horizontal well multi-stage fracturing strings according to the mathematical model developed for the mechanical calculation of the multi-packer string in horizontal wells. The research results were applied and verified in a gas well of Tahe Oilfield in the Tarim Basin with excellent effects, providing a theoretical basis and a simple and reliable technical means for optimal design and safety evaluation of safe operational parameters of multi-stage fracturing strings in horizontal wells.

Volume fracturing technique has effectively helped develop unconventional oil and gas reservoirs in recent years. At the same time, new problems of casing deformation failure occurred. Based on the drilling and well completion data, microseismic surveillance data, theories of fracture mechanics, rock damage mechanics and rock failure criterion, this paper established a finite element model of the formation of effective stimulated reservoir volume, including clustering perforation casing for X-1 h shale gas horizontal well, to address the problems. The research results indicate: 1) the stress deficit of zero stress areas and tension stress areas occurred within the range of stimulated reservoir volume during the process of volume fracturing. And, the state of this stress deficit, which would make clustering perforation casings of horizontal wells “hanging” in the formation to some extent, resulted in certain degree of deflection deformation radically and S-shape deformation axially. 2) the problem of casing deformation failure remains fundamentally unsolvable through simply improving casing grade and wall thickness to increase flexural strength. 3) the key to solve casing deformation failure is the reasonable spacing design of multi-stage fracturing. The methods and achievements in the paper provide theoretical supports for the popularization and application of shale stimulated reservoir volume and controlling the S-shape deformation failures of the horizontal multi-cluster perforation casing.

Radial spherical ZnO structures with nanorods grown on both inside and outside of a hollow sphere-like core along the c -axis direction have been produced by thermal evaporation of Zn powder in the absence of catalysts. Scanning electron microscopy and X-ray diffraction investigation show that the radial spherical ZnO structures are of high purity nanocrystal with a wurtzite structure. It is found that the formation of the sphere-shaped liquid Zn droplets before adding oxygen is a key factor to control the morphology of the radial spherical ZnO structures. The radial spherical structures could possibly be used as a building block and open up new opportunities for the self-assembly of functional nanodevices.