Various admixtures to mitigate the long-term strength retrogression of Portland cement cured under high pressure and high temperature conditions

In order to investigate the problem of long-term strength retrogression in oil well cement systems exposed to high pressure and high temperature(HPHT)curing conditions,various influencing factors,including cement sources,particle sizes of silica flour,and additions of silica fume,alumina,colloidal iron oxide and nano-graphene,were investigated.To simulate the environment of cementing geothermal wells and deep wells,cement slurries were directly cured at 50 MPa and 200?C.Mineral compositions(as determined by X-ray diffraction Rietveld refinement),water permeability,compressive strength and Young’s modulus were used to evaluate the qualities of the set cement.Short-term curing(2e30 d)test results indicated that the adoption of 6 m m ultrafine crystalline silica played the most important role in stabilizing the mechanical properties of oil well cement systems,while the addition of silica fume had a detrimental effect on strength stability.Long-term curing(2e180 d)test results indicated that nano-graphene could stabilize the Young’s modulus of oil well cement systems.However,none of the ad-mixtures studied here can completely prevent the strength retrogression phenomenon due to their inability to stop the conversion of amorphous to crystalline phases.

Growth and annealing study of hydrogen-doped single diamond crystals under high pressure and high temperature

A series of diamond crystals doped with hydrogen is successfully synthesized using LiH as the hydrogen source in a catalyst-carbon system at a pressure of 6.0 GPa and temperature ranging from 1255 C to 1350 C.It is shown that the high temperature plays a key role in the incorporation of hydrogen atoms during diamond crystallization.Fourier transform infrared micro-spectroscopy reveals that most of the hydrogen atoms in the synthesized diamond are incorporated into the crystal structure as sp 3-CH 2-symmetric（2850 cm-1） and sp 3 CH 2-antisymmetric vibrations（2920 cm-1）.The intensities of these peaks increase gradually with an increase in the content of the hydrogen source in the catalyst.The incorporation of hydrogen impurity leads to a significant shift towards higher frequencies of the Raman peak from 1332.06 cm-1 to 1333.05 cm-1 and gives rise to some compressive stress in the diamond crystal lattice.Furthermore,hydrogen to carbon bonds are evident in the annealed diamond,indicating that the bonds that remain throughout the annealing process and the vibration frequencies centred at 2850 and 2920 cm-1 have no observable shift.Therefore,we suggest that the sp 3 C-H bond is rather stable in diamond crystals.

B-C Bond in Diamond Single Crystal Synthesized with h-BN Additive at High Pressure and High Temperature

The synthesis of diamond single crystal in the Fe64Ni36-C system with h-BN additive is investigated at pressure 6.5 GPa and temperature range of 1300-1400℃. The color of the obtained diamond crystals translates from yellow to dark green with increasing the h-BN addition. Fourier-transform infrared （FTIR） results indicate that sp2 hybridization B-N-B and B-N structures generate when the additive content reaches a certain value in the system. The two peaks are located at 745 and 1425cm-1, respectively. Fhrthermore, the FTIR characteristic peak resulting from nitrogen pairs is noticed and it tends to vanish when the h-BN addition reaches 1.1 wt%. Furthermore, Raman peak of the synthesized diamond shifts down to a lower wavenumber with increasing the h-BN ~ddition content in the synthesis system.

Synthesis and characterizations of boron and nitrogen co-doped high pressure and high temperature large single-crystal diamonds with increased mobility

We synthesized and investigated the boron-doped and boron/nitrogen co-doped large single-crystal diamonds grown under high pressure and high temperature(HPHT) conditions(5.9 GPa and 1290℃). The optical and electrical properties and surface characterization of the synthetic diamonds were observed and studied. Incorporation of nitrogen significantly changed the growth trace on surface of boron-containing diamonds. X-ray photoelectron spectroscopy(XPS) measurements showed good evident that nitrogen atoms successfully incorporate into the boron-rich diamond lattice and bond with carbon atoms. Raman spectra showed differences on the as-grown surfaces and interior between boron-doped and boron/nitrogen co-doped diamonds. Fourier transform infrared spectroscopy(FTIR) measurements indicated that the nitrogen incorporation significantly decreases the boron acceptor concentration in diamonds. Hall measurements at room temperature showed that the carriers concentration of the co-doped diamonds decreases, and the mobility increases obviously. The highest hole mobility of sample BNDD-1 reached 980 cm^(2)·V^(-1)·s^(-1), possible reasons were discussed in the paper.

Effects of FeNi-phosphorus-carbon system on crystal growth of diamond under high pressure and high temperature conditions

This paper reports the crystal growth of diamond from the Fe Ni–Carbon system with additive phosphorus at high pressures and high temperatures of 5.4–5.8 GPa and 1280–1360°C. Attributed to the presence of additive phosphorus,the pressure and temperature condition, morphology, and color of diamond crystals change obviously. The pressure and temperature condition of diamond growth increases evidently with the increase of additive phosphorus content and results in the moving up of the V-shape region. The surfaces of the diamonds also become coarse as the additive phosphorus added in the growth system. Raman spectra indicate that diamonds grown from the Fe Ni-phosphorus-carbon system have more crystal defects and impurities. This work provides a new way to enrich the doping of diamond and improve the experimental exploration for future material applications.

Stability of Titanium-aluminium Nitride (Ti_2AlN) at High Pressure and High Temperatures

The stability of Ti2AlN at high pressure of 5 GPa and different temperatures of 700-1 600 ℃ was investigated using X-ray diffraction （XRD）,scanning electron microscopy （SEM） equipped with an energy dispersive spectrometer （EDS）.Ti2AlN was found to be stable at temperatures as high as 1 400 ℃under 5 GPa for 20 min,and was proved that it held better structure stability than Ti2AlC under 5 GPa through comparative experiments of Ti2AlN and Ti2AlC （representative compounds of M2AX phases （211 phase））.The reaction process at high pressure had some difference from that at ambient pressure/vacuum,and Ti2AlN directly decomposed to TiN and TiAl at 5 GPa and 1 500 ℃ for 20 min.Moreover,the mechanism of phase segregation was discussed.In addition,the behavior of Ti2AlN contacting with Zr at high pressure and high temperature （HPHT） was also studied.

Synergistic influences of titanium,boron,and oxygen on large-size single-crystal diamond growth at high pressure and high temperature

The synergistic influences of boron,oxygen,and titanium on growing large single-crystal diamonds are studied using different concentrations of B_(2)O_(3) in a solvent-carbon system under 5.5 GPa-5.7 GPa and 1300℃-1500℃.It is found that the boron atoms are difficult to enter into the crystal when boron and oxygen impurities are doped using B_(2)O_(3) without the addition of Ti atoms.However,high boron content is achieved in the doped diamonds that were synthesized with the addition of Ti.Additionally,boron-oxygen complexes are found on the surface of the crystal,and oxygen-related impurities appear in the crystal interior when Ti atoms are added into the FeNi-C system.The results show that the introduction of Ti atoms into the synthesis cavity can effectively control the number of boron atoms and the number of oxygen atoms in the crystal.This has important scientific significance not only for understanding the synergistic influence of boron,oxygen,and titanium atoms on the growth of diamond in the earth,but also for preparing the high-concentration boron or oxygen containing semiconductor diamond technologies.

Effects of Mg on diamond growth and properties in Fe–C system under high pressure and high temperature condition

Diamond crystal crystallized in Fe–Mg–C system with Archimedes buoyancy as a driving force is established under high pressure and high temperature conditions. The experimental results indicate that the addition of the Mg element results in the nitrogen concentration increasing from 87 ppm to 271 ppm in the diamond structure. The occurrence of the {100}plane reveals that the surface character is remarkably changed due to the addition of Mg. Micro-Raman spectra indicate that the half width of full maximum is in a range of 3.01 cm^-1–3.26 cm^-1, implying an extremely good quality of diamond specimens in crystallization.

Crystallization and characteristics of {100}-oriented diamond with CH4N2S additive under high pressure and high temperature

Diamond crystallization was carried out with CH4N2S additive in the FeNiCo-C system at pressure 6.0 GPa and temperature ranging from 1290 ℃ to 1300 ℃. The crystallization qualities of the synthetic crystals were characterized by Raman spectra and the Raman peaks located at 1331 cm-1. Fourier transform infrared (FTIR) results showed that the hydrogen-related absorption peak of the as-grown diamond was at 2920 cm-1, respectively. Interestingly, A-center nitrogen was observed in the obtained diamond and the characteristic absorption peaks located at 1095 cm-1 and 1282 cm-1. Especially, the absorption peak at 1426 cm-1 attributing to the aggregation B-center nitrogen defect was distinctly found when the CH4N2S content reached 0.3 mg in the synthesis system, which was extremely rare in synthetic diamond. Furthermore, optical color centers in the synthesized crystals were investigated by photoluminescence (PL).

The experimental studies on electrical con-ductivities and P-wave velocities of anortho-site at high pressure and high temperature

Results of P-wave velocity (vP) and electrical conductivity measurements on anorthosite are presented from room temperature to 880 C at 1.0 GPa using ultrasonic transmission technique and impedance spectra technique respec-tively. The experiments show that the P-wave velocities in anorthosite decrease markedly above 680 C following the dehydration of hydrous minerals in the rock, and the complex impedances collected from 12 Hz to 105 Hz only indicate the grain interior conduction mechanism at 1.0 GPa, from 410 C to 750 C. Because the fluids in the rock have not formed an interconnected network, the dehydration will not pronouncedly enhance the electrical conduc-tivity and change the electrical conduction mechanism. It is concluded that the formation and evolution of the low-velocity zones and high-conductivity layers in the crust may have no correlations, and the dehydration can result in the formation of the low-velocity zones, but cannot simultaneously result in the high-conductivity layers.

Ceramic synthesis of 0.08BiGaO_3–0.90BaTiO_3–0.02LiNbO_3 under high pressure and high temperature

In this paper, the preparation of 0.08BiGaO3–0.90BaTiO3–0.02LiNbO3 is investigated at pressure 3.8 GPa and temperature 1100–1200？C. Experimental results indicate that not only is the sintered rate more effective, but also the sintered temperature is lower under high pressure and high temperature than those of under normal pressure. It is thought that the adscititious pressure plays the key role in this process, which is discussed in detail. The composition and the structure of the as-prepared samples are recorded by XRD patterns. The result shows that the phases of Ba TiO3, BaBiO（2.77）, and Ba2Bi4Ti5O（18） with piezoelectric ceramic performance generate in the sintered samples. Furthermore, the surface morphology characteristics of the typical samples are also investigated using a scanning electron microscope. It indicates that the grain size and surface structure of the samples are closely related to the sintering temperature and sintering time. It is hoped that this study can provide a new train of thought for the preparation of lead-free piezoelectric ceramics with excellent performance.

Behaviors of Zn2GeO4 under high pressure and high temperature

The structural stability of Zn2GeO4 was investigated by in-situ synchrotron radiation angle dispersive x-ray diffraction. The pressure-induced amorphization is observed up to 10 GPa at room temperature. The high-pressure and hightemperature sintering experiments and the Raman spectrum measurement firstly were performed to suggest that the amorphization is caused by insufficient thermal energy and tilting Zn–O–Ge and Ge–O–Ge bond angles with increasing pressure,respectively. The calculated bulk modulus of Zn2GeO4 is 117.8 GPa from the pressure-volume data. In general, insights into the mechanical behavior and structure evolution of Zn2GeO4 will shed light on the micro-mechanism of the materials variation under high pressure and high temperature.

Reasons resulting in the collapsed tubing near wellhead in high pressure and high temperature deep well during well testing and measures to prevent the collapsing

Because various reasons, the tubing near wellhead was collapsed during well testing in high pressure and high temperature deep well when the outer pressure was less than collapsing strength. To find the reasons in the abnormally collapse and countermeasures, first the quality of the tubing was checked. It was founded that the collapse was not resulted from the defect of the tubing. Then, force and stress exerted in the tubing was analyzed taking XS2 well as an example. The analysis results were concluded as follows. The collapsing strength of tubing decreased due to the axial tensile, which is seriously at the upper tubing especially. During injecting, the additional axial force that was caused by the temperature effect increased the tubing near wellhead to suffer axial tensile and further reduced the collapsing strength of tubing near wellhead. Reinforcing defect, prohibiting defect tubing to trip in hole, according to the calculation to impose appropriate annular pressure, selecting size nozzle to reverse pumping and controlling the reverse pumping speed and pressure, prohibiting to be opened flow and reducing or releasing the annular pressure can prevent the well testing tubing down-hole being collapsed at the wellhead.

Growth and annealing study of hydrogen-doped single diamond crystals under high pressure and high temperature

A series of diamond crystals doped with hydrogen is successfully synthesized using LiH as the hydrogen source in a catalyst-carbon system at a pressure of 6.0 GPa and temperature ranging from 1255 C to 1350 C.It is shown that the high temperature plays a key role in the incorporation of hydrogen atoms during diamond crystallization.Fourier transform infrared micro-spectroscopy reveals that most of the hydrogen atoms in the synthesized diamond are incorporated into the crystal structure as sp 3-CH 2-symmetric(2850 cm-1) and sp 3 CH 2-antisymmetric vibrations(2920 cm-1).The intensities of these peaks increase gradually with an increase in the content of the hydrogen source in the catalyst.The incorporation of hydrogen impurity leads to a significant shift towards higher frequencies of the Raman peak from 1332.06 cm-1 to 1333.05 cm-1 and gives rise to some compressive stress in the diamond crystal lattice.Furthermore,hydrogen to carbon bonds are evident in the annealed diamond,indicating that the bonds that remain throughout the annealing process and the vibration frequencies centred at 2850 and 2920 cm-1 have no observable shift.Therefore,we suggest that the sp 3 C-H bond is rather stable in diamond crystals.

Polycrystalline cubic boron nitride prepared with cubic-hexagonal boron nitride under high pressure and high temperature

Polycrystalline cubic boron nitride(Pc BN)compacts,using the mixture of submicron cubic boron nitride(c BN)powder and hexagonal BN(h BN)powder as starting materials,were sintered at pressures of 6.5–10.0 GPa and temperature of1750℃without additives.In this paper,the sintering behavior and mechanical properties of samples were investigated.The XRD patterns of samples reveal that single cubic phase was observed when the sintering pressure exceeded 7.5 GPa and h BN contents ranged from 20 vol.%to 24 vol.%,which is ascribed to like-internal pressure generated at grain-to-grain contact under high pressure.Transmission electron microscopy(TEM)analysis shows that after high pressure and high temperature(HPHT)treatments,the submicron c BN grains abounded with high-density nanotwins and stacking faults,and this contributed to the outstanding mechanical properties of Pc BN.The pure bulk Pc BN that was obtained at 7.7 GPa/1750℃possessed the outstanding properties,including a high Vickers hardness(~61.5 GPa),thermal stability(~1290℃in air),and high density(~3.46 g/cm^(3)).

Synthesis of large diamond crystals containing high-nitrogen concentration at high pressure and high temperature using Ni-based solvent by temperature gradient method

This paper reprots that with Ni-based catalyst/solvent and with a dopant of NAN3, large green single crystal diamonds with perfect shape are successfully synthesized by temperature gradient method under high pressure and high temperature in a China-type cubic anvil high-pressure apparatus （SPD-6 × 1200）, and the highest nitrogen concentration reaches approximately 121-1257 ppm calculated by infrared absorption spectra. The synthesis conditions are about 5.5 CPa and 1240-1300 ℃. The growth behaviour of diamond with high-nitrogen concentration is investigated in detail. The results show that, with increasing the content of NaN3 added in synthesis system, the width of synthesis temperature region for growth high-quality diamonds becomes narrower, and the morphology of diamond crystal is changed from cube-octahedral to octahedral at same temperature and pressure, the crystal growth rate is slowed down, nevertheless, the nitrogen concentration doped in synthetic diamond increases.

Recension of boron nitride phase diagram based on high-pressure and high-temperature experiments

Cubic boron nitride and hexagonal boron nitride are the two predominant crystalline structures of boron nitride.They can interconvert under varying pressure and temperature conditions.However,this transformation requires overcoming significant potential barriers in dynamics,which poses great difficulty in determining the c-BN/h-BN phase boundary.This study used high-pressure in situ differential thermal measurements to ascertain the temperature of h-BN/c-BN conversion within the commonly used pressure range(3-6 GPa)for the industrial synthesis of c-BN to constrain the P-T phase boundary of h-BN/c-BN in the pressure-temperature range as much as possible.Based on the analysis of the experimental data,it is determined that the relationship between pressure and temperature conforms to the following equation:P=a+1/bT.Here,P denotes the pressure(GPa)and T is the temperature(K).The coefficients are a=-3.8±0.8 GPa and b=229.8±17.1 GPa/K.These findings call into question existing high-pressure and high-temperature phase diagrams of boron nitride,which seem to overstate the phase boundary temperature between c-BN and h-BN.The BN phase diagram obtained from this study can provide critical temperature and pressure condition guidance for the industrial synthesis of c-BN,thus optimizing synthesis efficiency and product performance.

High-pressure and high-temperature sintering of pure cubic silicon carbide:A study on stress-strain and densification

Silicon carbide(SiC)is a high-performance structural ceramic material with excellent comprehensive properties,and is unmatched by metals and other structural materials.In this paper,raw SiC powder with an average grain size of 5μm was sintered by an isothermal-compression process at 5.0 GPa and 1500?C;the maximum hardness of the sintered samples was31.3 GPa.Subsequently,scanning electron microscopy was used to observe the microscopic morphology of the recovered SiC samples treated in a temperature and extended pressure range of 0-1500?C and 0-16.0 GPa,respectively.Defects and plastic deformation in the SiC grains were further analyzed by transmission electron microscopy.Further,high-pressure in situ synchrotron radiation x-ray diffraction was used to study the intergranular stress distribution and yield strength under non-hydrostatic compression.This study provides a new viewpoint for the sintering of pure phase micron-sized SiC particles.

A Comprehensive Method for the Optimization of Cement Slurry and to Avoid Air Channeling in High Temperature and High-Pressure Conditions

Air channeling in the annulus between the casing and the cement sheath and/or between the cement sheath and formation is the main factor affecting the safe operation of natural gas wells at high temperatures and pressures.Prevention of this problem requires,in general,excellent anti-channeling performances of the cement sheath.Three methods to predict such anti-channeling performances are proposed here,which use the weightless pressure of cement slurry,the permeability of cement stone and the volume expansion rate of cement sheath as input parameters.Guided by this approach,the anti-channeling performances of the cement slurry are evaluated by means of indoor experiments,and the cement slurry is optimized accordingly.The results show that the dangerous transition time of the cement slurry with optimized dosage of admixture is only 76 min,the permeability of cement stone is 0.005 md,the volume shrinkage at final setting is only 0.72%,and the anti-channeling performances are therefore maximized.The effective utilization of the optimized cement slurry in some representative wells(LD10–1-A1 and LD10–1-A2 in LD10–1 gas field)is also discussed.

Mechanical Analysis of a Multi-Test String in High-Temperature and High-Pressure Deep Wells

The mechanical behavior of the test string in deep wells is generally relatively complex as a result of the high temperature and high pressure,severe dogleg and buckling effects,which in some circumstances can even lead to string failure.Traditional computational methods for the analysis of these behaviors are often inaccurate.For this reason,here a more accurate mechanical model of the test string is introduced by considering variables such as temperature,pressure,wellbore trajectory,and buckling,as well as combining them with the deformation and string constraint conditions brought in by changes in temperature and pressure during the tripping,setting,and test operations.The model is validated by applying it to a specific high-pressure gas well(located in Northeast Sichuan).